MXPA97008878A - Data communications network with highly efficient interrogation procedure - Google Patents

Abstract

The present invention relates to a method for interrogating a plurality of interconnected devices, the devices are each capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of devices, the groups Preferred and subordinate are mutually exclusive, wherein the devices of the preferred group are interrogated at least N / M times as frequently as the devices of the slave group, where N and M are positive integers with N > M; each of the devices of the preferred group corresponds to the interrogation by transmitting either a first type of response indicating that the device has no data to send, or a second type of response indicating that the device has data for send, and further comprising the steps of: determining how often each device of the preferred group transmits the first type of response, and reassigning the device of the preferred group to the subordinate group, if said device transmits the first type of response more frequently than a predetermined degree of frequency

Description

DATA COMMUNICATIONS NETWORK WITH HIGHLY EFFICIENT INTERROGATION PROCEDURE FIELD OF THE INVENTION This invention is directed to a data communication network and more particularly to a data communication network for use with a closed-circuit television surveillance system. BACKGROUND OF THE INVENTION It is known to provide data communication networks, whereby each of a number of devices are capable of sending and receiving data messages, from and each one.

A commonly used type of data network is a local area network (LAN) where all the devices for transmission and reception of data are connected to a common transmission medium such as a coaxial cable or twisted pair of wires. . The use of a common transmission medium in the data network makes some provision necessary to resolve or prevent attempts to access the transmission medium from conflict. A well-known technique for avoiding conflicts involves providing a master station that sequentially interrogates each of the other stations, with the other stations allowing access to the transmission medium only in response to an interrogation message directed to the particular device that wishes to access the middle. Networks of REF: 25929 conventional interrogation can provide satisfactory performance when each of the devices requires frequent access to the transmission medium, but if the message traffic is infrequent or in "bursts", an interrogation network does not use the transmission medium efficiently. Also, if the number of devices connected to the transmission medium is large, there may be unacceptably long delays to allow access to the transmission medium. It is also known to provide so-called random access control techniques, which are generally more suitable for "burst" traffic. For example, a known random access technique is referred to as "multiple access with a carrier sense" (CSMA = carrier sense multiple access). According to this technique, when a device has data to transmit, the device "listens" to the transmission medium, to determine if the medium is in use, and if not, proceeds to transmit data. If there is a collision, that is, if two devices transmit respective data packets that transpose in time, then the terminals retransmit their respective data packets in a manner designed to avoid future collisions, such as delaying a random period before retransmitting.

A disadvantage of the CSMA technique is that the probability of collisions increases as the number of devices connected to the transmission medium and the traffic frequency increase. This can result in unacceptable delays. In addition, this technique is "non-deterministic" in the sense that it can not be known with certainty that a device will be able to gain access to the medium of transmission within any finite period of time. This may not be an acceptable feature in the case of time-sensitive data. There are particularly difficult challenges in managing a local area network that is used to interconnect video surveillance cameras, camera control consoles and other devices that constitute a closed-circuit television surveillance system. This LAN may include a relatively large number of devices with only infrequent needs for network access and also a smaller number of devices with frequent need for access. In addition, there may be substantial differences in size between the data packets that are transmitted. With access control techniques that to date have been applied to LAN * s used in connection with video surveillance systems, it has not been possible to achieve satisfactory performance without severely limiting the number of devices (cameras and controllers) connected to the network. LAN.

It can be contemplated to "divide" the video surveillance system, by providing separate LANs for respective portions of the video surveillance system, but only at the cost of preventing certain control consoles from controlling video cameras not connected to the same LAN as the control consoles. In this way, dividing the system is inconsistent with the desirable feature of allowing every console to control every camera. OBJECTIVES AND COMPENDIUM OF THE INVENTION Accordingly, an object of the invention is to provide an improved local area data network for interconnecting video cameras, camera control consoles and other devices that constitute a video surveillance system. Another object of the invention is to provide a data communications network that exhibits both high performance and deterministic operation. Still a further objective of the invention is to provide a data communications network that efficiently serves more than one type of data transmission and reception device. Still a further object of the invention is to provide a data communications network that efficiently handles data packets of varying sizes.

According to a first aspect of the invention, a method for interrogating a plurality of interconnected devices is provided, with each of the devices capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a group of devices. subordinate of devices, with the preferred and subordinate groups that are mutually exclusive, and the preferred group devices interrogate at least N / M times as frequently as the devices of the subordinate group, where N and M are both positive integers with N > M. According to the additional practice according to this aspect of the invention, M can be equal to 1. According to a still further practice according to this aspect of the invention, each of the devices responds to interrogation by transmitting either a first type of response indicating that the device has no data to send, or a second type of response indicating that the device has data to send, and the method according to this aspect also includes the steps of determining how frequently each device of the preferred group transmits first type of response and reassign the device from the preferred group to the subordinate group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

According to further practice according to this aspect of the invention, the method can also include the steps of determining how frequently each subordinate group device transmits to the second type of response, and reassigning that device from the subordinate group to the preferred group if the device transmits the second type of response more frequently than a predetermined degree of frequency. In accordance with this aspect of the invention, more frequent access to the data network is provided to devices that have a more frequent need for access, and the frequency with which access to the network is allowed to be changed dynamically and adaptively, in accordance with the demonstrated needs of the devices. According to another aspect of the invention, there is provided a method for probing a plurality of interconnected devices, with devices that are capable of receiving and sending data messages, and the plurality of devices that include a first group and a second group of devices. devices, the two groups are mutually exclusive, and the method includes the steps of transmitting an interrogation message at least N times to each device of the first group of devices for a period of time, N is an integer greater than 2, and transmitting a interrogation message once and only once during the period of time to each device of the second group of devices.

According to yet another aspect of the invention, there is provided a method for interrogating a plurality of interconnected devices, with each of the devices that are capable of receiving and sending data messages and the plurality of devices include a preferred group and a group. subordinate of the devices, the preferred and subordinate groups are mutually exclusive, and the method includes the steps of first transmitting a first interrogation message once and only once in sequence to each device of the subordinate group of devices and second transmitting a message of interrogation at least N times to each device of the preferred group of devices after beginning the first transmission stage and before transmitting any different interrogation message to the first interrogation message to any device of the second subordinate group, with N being a larger integer 1. According to yet another aspect of the invention, a method for interrogating a plurality of interconnected devices is provided, with each of the devices that are capable of receiving and sending data messages and the plurality of devices includes a first preferred group of devices, a second preferred group of devices and a group subordinate of devices, the first preferred group, the second preferred group and the subordinate group, all are mutually exclusive, with the devices of the first preferred group being interrogated at least N times as frequently as the devices of the subordinate group, the devices of the second Preferred group interrogate at least N times as frequently as subordinate group devices and with N and M that both are integers greater than 1 and different from each other. According to further practice according to this aspect of the invention, N can be an integral multiple of M. According to yet another aspect of the invention, a method is provided for probing a plurality of interconnected devices, with each of the devices capable of of receiving and sending data messages, and the plurality of devices includes a first group and a second group of devices, the first and second groups each consisting of a respective plurality of devices, and the method includes the steps of first transmitting to each device of the first group of devices, according to a predetermined sequence, an interrogation message that is uniquely directed to the respective device of the first group of devices, after the first transmission stage, second to transmit an interrogation message of group simultaneously to all the devices of the second group of devices, detect if the If the contention is detected in the detection stage, resolve the contention detected when performing a polling algorithm between the devices of the second group responding to the group interrogation message. The practice in accordance with this aspect of the invention provides a relatively frequent network access to preferred devices through a dedicated opportunity to access the network, while less frequent access to other devices is efficiently provided. According to yet another aspect of the invention, a method for interrogating a plurality of interconnected devices is provided, with each of the devices being capable of receiving and sending data messages and the method includes the first stages of transmitting to each of the plurality of devices, according to a predetermined sequence, an interrogation message that is directed only to the respective device, then, after the first transmission stage, second transmission of a group interrogation message simultaneously to all the devices, detect if contention between the devices occurs response to the group interrogation message, and if containment is detected in the detection stage, resolving the contention detected when performing a polling algorithm. In accordance with this aspect of the invention, each device is assigned a predetermined opportunity to access the network while allowing more frequent network access to those devices that may occasionally require more frequent access. According to yet another aspect of the invention, there is provided a method for interrogating a plurality of interconnected devices, with each of the devices being able to receive and send data messages and the plurality of devices includes a preferred group of devices and a subordinate group of devices, the preferred and subordinate groups are mutually exclusive, and the method includes the steps of interrogating the subordinate group devices by means of interrogation messages, each directed to a different one of the subordinate group devices, with a first degree frequency, interrogate the preferred group devices by interrogation messages, each directed to a different one of the devices of the preferred group, and with the second frequency degree being N / M times as large as the first frequency degree, N and M both are positive integers with N >; M, transmitting a group interrogation message simultaneously to all devices of the subordinate group of devices with a third degree of frequency, detect whether contention occurs between the subordinate group devices in response to the group interrogation message, and if containment is detected In the detection stage, resolve the contention detected when performing a polling algorithm.

According to a further practice based on the last aspect of the invention, the third degree of frequency is the same as the first degree of frequency; M = 1; and N is greater than 2. According to a still further aspect of the invention, a method is provided for probing a plurality of interconnected devices, with each of the devices being capable of receiving and sending data messages and each of data messages other than interrogation messages that are in a first predetermined format consisting of at least N octets, N is an integer greater than one, and the method includes the steps of sending interrogation messages in sequence to each of the devices, wherein each of the interrogation messages is in a second predetermined format consisting of M octets, with M being a positive integer less than N. According to additional practice based on the last aspect of the invention , M = 5; N = 6; each of the interrogation messages consists of a starting flag octet and an address octet to identify a respective one of the devices to which the interrogation message is sent, an error detection code consisting of two octets, and a stop flag octet; while each of the data messages other than polling messages includes at least one starting flag octet, one address octet to identify a respective one of the devices to which the data message is sent, one type of octet for identify a type of data message, an error correction code consisting of two octets and a stop flag octet. According to additional practice, some of the data messages also include at least one octet of data being transferred, and none of the data messages includes more than 92 bytes of data to be transferred. According to a still further aspect of the invention, there is provided a method for operating a data network formed of a plurality of interconnected devices including a master interrogation device and other devices, and the method includes the steps of transmitting an interrogation message. respective in sequence from the master interrogation device to each of the other devices, and transmit data messages from the other devices in response to the respective interrogation messages, with the data messages that are transmitted from the other devices in response to respective interrogation messages, each consisting of at least N octets and the respective interrogation messages each consist of M octets, with N and M that are positive integers and M < N. In accordance with another aspect of the invention, there is provided a method for operating a data network formed of a plurality of interconnected devices including a master interrogation device and other devices and the method includes the steps of transmitting an interrogation message. from the master interrogation device to a first of the other devices, and in response to the interrogation message, transmitting data message directly from a first of the other devices to a second of the other devices. According to still another additional aspect of the invention, there is provided a method by which an interrogation device responds to an interrogation message received by the interrogation device from a data network, with the network including a plurality of devices that they are capable of receiving and sending data messages, network means for interconnecting the devices for data communications between the devices, and means for sending interrogation messages to the plurality of devices, wherein the method includes the steps of sending a message of data consisting of at least N octets if the interrogation device has data to send, N is a positive integer and transmitting an interrogation response signal that does not include any octets if the interrogation device has no data to send. According to further practice based on the last aspect of the invention, the interrogation response signal consists of a plurality of line transitions followed by an absence of line transitions for a period of at least two clock cycles and the plurality of line transitions does not include less than six line transitions. According to still further practice, the data message is transmitted by FM-0 coding and the interrogation response signal consists of four "0" bits followed by the absence of line transmissions. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic block diagram of a closed-circuit television video surveillance system, including a local area network that is provided according to the invention for data communications between the devices that they constitute the surveillance system. Figure 2 is a block diagram of a system control module for the surveillance system and the local area network of Figure 1. Figure 3 is a flow diagram illustrating an interrogation technique that is carried out by the system control module of Figure 2. Figure 4 is a flow diagram illustrating another interrogation technique that is carried out by the system control module of Figure 2.

Figure 5A is a waveform diagram illustrating a negative response signal that is transmitted by an interrogation device in the system of Figure 1 in response to an interrogation message; Figure 5B illustrates the format of an interrogation command that is transmitted by the system control module of Figure 2; and Figures 5C and 5D illustrate formats of other messages transmitted in the system of Figure 1. Figure 6 illustrates schematically an example of a sequence of interrogation messages and other signals that are transmitted in the system of Figure 1. Figure 7 is a simplified schematic diagram of a local area network constituted by sub-networks according to the invention. DESCRIPTION OF THE PREFERRED MOPAMPAPES The invention will now be described in greater detail with reference to the drawings, wherein like or similar elements are indicated therein by corresponding reference numbers. GENERAL VIEW OF THE SYSTEM Initially with reference to Figure 1, the reference number 10 generally indicates a surveillance system with closed circuit television. The system 10 includes a number of television cameras, represented by surveillance domes 12. Although only four surveillance domes 12 are explicitly illustrated in Figure 1, it will be understood that the surveillance system 10 may include a much larger amount of information. cameras, and that it is contemplated that some or all of the cameras may not be circumscribed within the dome. All cameras may be of conventional types such as those sold by the assignee of this application for video surveillance applications. The surveillance system 10 also includes a number of consoles for camera control 14 that are used to control panning, tilting, zooming in or out, focus, etc., of the cameras 12. Each console 14 can be conventional design it can be a compact console that incorporates an articulated work platform (joystick) and keypad such as those distributed by the assignee of this application under the brand "TOUCHTRACKER". The system 10 also includes a number of monitors 16 for displaying the video signals generated by the surveillance dome cameras 12. The selection of a particular camera to be the source of the signal displayed on a particular monitor can be controlled by a operator by actuating appropriate keys in a control console 14. Again, it should be understood that the number of control consoles 14 and monitors 16 constituting the viligance system 10 may be larger than the three consoles and three monitors illustrated in FIG. Figur 1. A system control module 18 is provided in the surveillance system 10 for distributing video signals from the cameras 12 to the monitors 16 according to the instructions received by the consoles 14. As will be described in more detail below , the system controller 18 also handles data communications between the consoles 14 and the cameras 12. The video signals generated by the systems 12 are transmitted to the system control module 18 by a conventional junction box 20 and a power video signal connection 22. After appropriate switching within the system control module 18, respective output video signals are provided. on the output video signal connections 24 to the monitors 16. A local area network connection 26 is provided for interconnecting data communications to the system control module 18, the cameras 12 and the control coneolae 14. The connection LAN 26 is preferably arranged in a conduit or star configuration or a combination thereof, and can advantageously be formed from a pair of twisted, shielded or unshielded wires. It will be noted that the connection to the LAN 26 for the cameras 12 passes through the junction box 20 and connectors 28, which preferably include separate wiring or for the video signals that are sent from and the data communications that are provided to and from the cameras 12. As illustrated in Figure 1, the LAN 26 also has one or more auxiliary devices connected to it. 30, which are neither cameras nor control consoles. Auxiliary devices may include, for example remotely located video switching devices, devices for sending an alarm message upon the occurrence of certain predetermined events (such as opening a door or a merchandise display cabinet) and devices for detect if the lights are off or on and / or to turn the lights on and off. The surveillance system 10 also includes a workstation 32 comprising a keyboard and a monitor and is used to configure the video surveillance system 10 and perform other administrative functions with respect to the seventh. The workstation 32 may be a conventional device such as the graphics-based terminal distributed by the ceeionary of the present invention under the VRS 2000 brand. The workstation 32 is connected to the seven-way control 18 by a video signal connection output 24 which allows the workstation 32 to receive and display video signals directed from the camera 12 by the system control module 18. The workstation 32 is also connected for data communications with the system control module. 18 by a separate local area network connection 34. According to a preferred embodiment of the invention, the LAN 34 is operated in accordance with the well-known ETHERNERT communications protocol. Although not specifically illustrated in Figure 1, it should be understood that the surveillance system 10 may include other components that are typically provided in such a system such as video cartridge recorders, and video signal multiplexer components that allow two or more Video signals are displayed in the form of a split screen on the video monitors. The surveillance system 10 can also be interconnected via ETHERNET LAN 34 to other systems, such as seven-mates for access control to the installations. PE CONTROL MODULE The system control module 18 will now be described in more detail with reference to Figure 2. The system control module 18 includes a seventh host processor 36 which controls the operations of the system control module 18 and also in a large proportion the operations of the video viligance system 10. The host of the system 36 can conveniently be implemented as a conventional personal computer card 386.

The seventh control module 18 includes a video switching component 38 which connects to the map host 36 and which operates under control of the output of control signals from the host system 36. The video switcher 38 receives video from power over the connections 22, switches the power video signals in accordance with the instructions received from the system guest 36 and outputs the switched video signals to the text overlay cards 40. The text overlay cards 40 also they are connected for control by the system guest 36 and produce text overlays appropriate in response to intructions that are received from the system guest 36. The switched video signals with added text overlays are output to the monitors 16 and the work station 32 over the aforementioned output video connections 24. The system guest 36 also connected A printer 42 is provided by which printed reports can be sent out, and is connected via circuit and communication interface 44 to the aforementioned ETHERNET LAN 34. Each of the elements 38, 40, 42 and 44 may be of a design known as the physical equipment for control of the surveillance system distributed by the assignee of the present invention under the trademark "VIDEO MANAGER".

The system control module 18 further includes a network communication card 46 that is connected to the seventh guest 36. The network communication card 46 allows the control module of the first 18 to receive and transmit data messages from and to the others devices connected to the seventh surveillance LAN 26, and also manages the LAN 26 administration in a manner that will be described below. The network communication card 46 may be constituted by communications circuitry provided in the well-known local area network "LOCALTALK", distributed by Apple Computer, Inc., except that in a preferred embodiment of the invention, the processor used in The LOCALTALK network communication card is replaced with a 64180 microprocessor available from Xilog or Hitachi. As will be seen, the communication card 46 operates according to the invention under the control of stored programs to carry out communication protocols different from those used in the LOCALTALK LAN. NETWORK PROTOCOLS The devices connected to the surveillance system LAN 26 are of different types and are required to transmit a variety of data types at varying and irregular time intervals. For example, control consoles 14 often transmit short data packets comprising signals to control cameras 12. On the other hand, cameras 12 are required, rarely, to transmit brief data packets indicative of border crossings or similarities. . The data packet tranemitted by the consoles 14 are critical in time, but the data transmitted by the cameras 12 usually does not. In addition, the system module 18 or other devices connected to the LAN 26 may sometimes be required to transmit significant amounts of configuration data and the like, although generally not in a critical time manner. As will be briefly described, the system control module 18 handles the access of all the devices to the LAN 26, in such a way that the various requirements of the devices can all be fulfilled over the common data connection. For this purpose, a variety of techniques are employed, separately or more preferably, all in combination. Among these techniques are interrogated different claees of devices with different degrees of frequency; dynamic displacement of a class to another to reflect changes from time to time in the needs for access of the devices; use of both interrogation messages as well as interrogation messages directed to particular devices, with procedures for resolving containment caused by multiple responses to the group of interrogation messages; an abbreviated negative response to interrogation messages; an interrogation message in a format that is shorter than the formats of other data messages; and direct peer-to-peer data transmission in response to interrogation messages. All of these techniques will be described in more detail below. The use of these techniques allows the system LAN 26 to accommodate a relatively large number of devices with satisfactory performance and response time. As previously implied, all the physical equipment constituting the LAN 26 and the associated interface circuits in the system control module 18 and the other devices connected to the LAN 26 can be constituted from standard physical equipment used for the LAN.

"LOCALTALK" conventional above mentioned distributed by Apple Computer. According to this, in a preferred embodiment, RS-422 signaling isolated from transformer and FM-0 coding are employed, with a clock or synchronization speed of 230.4 kilobits per second and framed SDLC (synchronized data link communications = synchronized data link Communications). SPECIFIC DEVICE INTERROGATION SPEEDS Turning now to the particular network access and interrogation management techniques that are provided in accordance with the invention, it is first considered that the dielectricities connecting to LAN 26 are to be divided into three classes , X, Y and Z, with 8 devices (ie 8 consoles 14) in class X, 96 devices (ie 96 cameras 12) in class Y and 10 devices (ie 10 auxiliary devices 30) in class Z Furthermore, it is considered desirable that at least on average, each dietary item in clause X is interrogated at least twenty times per second, each device in class Y is interrogated at least five times per second, and the devices in the class Z will be interrogated at least once per second. To accommodate the devices in class X, each basic interrogation cycle shall not be longer than 50 ms on average and shall include one interrogation for each device in class X. The number of devices of each of the classes X and Z to interrogate during the basic interrogation cycle, will be determined by dividing the device number in a class by the ratio of the interrogation speed for class X to the respective interrogation speed for classes Y or Z, and then round to the nearest whole. For example, there are 96 devices in class Y, and the desired interrogation frequency for class X is four times the desired frequency for class Y such that 24 (= 96 -5- 4) devices of class Y are they will interrogate during each 50 ms base interrogation period. Similarly, since the interrogation frequency is twenty times as great for class X as for class Z and class Z has 10 devices, the number of devices of class Z to be interrogated during each base interrogation period 50 ms ee 1 (rounded 10/20). With reference to the eight diepoeitives of clause X as diepoeitivoe XI, X2, ..., X8, the 96 diepoeitivos of class Y as Yl, Y2, ..., Y96 and the 10 devices of class Z as Zl , Z2, ..., Z10, an exemplary interrogation sequence according to the invention is established as follows: First cycle of interrogation interrogates devices XI to X8, Y1 to Y24 and Z1. Second interrogation cycle interrogates diepoeitives XI to X8, Y25 to Y48 and Z2. Third Circle interrogation interrogates devices XI through X8, Y49 through Y72 and Z3. Fourth interrogation cycle questions diepoeitivoe XI to X8, Y73 to Y96 and Z4. Fifth cycle _ interrogation interrogates devices XI to X8, Yl to Y24 and Z5. * * * * * * PégJH.9 C? Question CIP - interrogates devices from Xl to X8, Y25 to Y48 and Z10. Eleventh interrogation cycle - question dispoeitivoe Xl to X8, Y49 to Y72 and Zl. * * * * * * Twenty-sixth interrogation cycle - interrogates devices from XI to X8, Y73 to Y96 and Z10. * * * * * * It will be noted then that the necessary variants of the different types of devices are achieved by assigning the devices to the classrooms and then the interrogation devices in different classes with different degrees of frequency. The conformance interrogation method can be considered to be "device sensitive". It should be understood that instead of carrying out sensitive questioning with different device phases, it is possible to use the technical term with doe classes or with four or more classes. The most frequently interrogated classes may be considered "preferred" and the less frequently interrogated classes may be considered "subordinate." From eeta way, the example given above, lae claeee X and Y can referiree as repectively first and second preferred classes with class Z that is referred to as subordinate class. If only two kinds of dietary are formed, then one can simply say a preferred class and a subordinate one. Although it may be convenient to do integral eubmúltiploe of less frequent speeds of the highest interrogation speed, this is not required. Nor is it required that the highest interrogation speed be two or more times the lowest interrogation speed. For example, as for any two interrogation speeds, the upper interrogation speed may be N / M times the lower interrogation speed where N and M are positive integers with N >; M. Suppose, then, M = 1, then the lower interrogation speed is a whole submultiple of the upper interrogation speed. ADAPTIVE ALLOCATION TO INTERROGATION CLASSES In order to further improve the performance of LAN 26, the seven-eighth control module 18 can operate in such a way that the allocation of inputs to the different classes are not static but rather are adjusted dynamically according to the Demonstrated needs of the devices for network access. Figure 3 illustrates a procedure that will be carried out in the system control module 18 (particularly in a network communication card 46) wherein the devices are dynamically reassigned between the interrogation classes. For the purpose of Figure 3, it is found that all the devices have assigned either a preferred class X or a subordinate class Y. The addressing of Figure 3 begins with step 50, wherein the system control module 18 interrogates the diepoeitivoe of class X and class Y in one sequence at predetermined intervals based on a more frequent interrogation speed for devices of class X than for class Y devices. For example, the cycles of First to fourth interrogations described above can be carried out (by making the references to diepoeitivoe of class Z). When a device reeposes to interrogation indicating that it has data to transfer (the indication can be simply the data transfer), this fact is recorded by the system control module 18 (step 52). Then in step 54, it is determined by each die in class X whether the device has not responded to a consecutive series of predetermined number of interrogation messages (ie N interrogation messages). If in step 54 it is determined that the particular device of class X has not responded to the last N interrogations, then it may be considered that the device is inactive due to being turned off, or that it is not operated by a human operator, and thus hereinafter, and the particular compliance device is reassigned to class Y (step 56) and the routine then proceeds to the next step, which is step 58. Alternatively, if in step 54 the particular device is found to be has responded at least once to the last N interrogations, then the routine proceeds directly from step 54 to step 58. In step 58, it is determined by each device in class Y, if the particular device has responded more than one predetermined number of times (ie L) to the last K interrogations directed to the dietary class Y, where K is also a predetermined number. Otherwise, the routine enters a loop back to step 50. Otherwise, that is, if a particular device of class Y has exceeded the predetermined rule of activity for reassignment, then step 60 follows step 58 , and the device that reeposes frequently to claee Y is reassigned to class X. It will be noted that the routine enters a loopback to step 50 from step 60. The particular routine illustrated in Figure 3 suggests that the tests of steps 54 and 58 are to be performed at regular intervals, probably after each interrogation cycle or after each N * "1" or interrogation cycle, where N is the number of interrogation cycles required to interrogate all the devices in class Y. Of course, it is envisaged to perform the tests of stage 74 and 58 at shorter or longer intervals. It is also contemplated that the test of step 54 may be performed more or less frequently instead of equally frequent compared to the test of step 58. Of course, variations may be made in the tests of steps 54 and 58. For example, to retain assignment to class X, a device may have to respond twice or more to the last N interrogation messages directed to the device.

It is also within the scope of the invention to perform dynamic reassignment of devices to interrogation classes when more than two interrogation classes have been established. In this case, some of the interrogation classes can be excluded from the dynamic reassignment scheme. INTEGRATION OF GRUPQ X TCNICAS PE INTERROGATION INDIVIDUALIZED The performance of LAN 26 can be further improved according to the invention by combining group interrogation techniques with device-by-device interrogation previously described. Group questioning will first be discussed in a simplified form with reference to Figure 4, which illustrates a procedure to be carried out by the system control module 18. It will be initially considered for the purpose of the routine of Figure 4, that there is a classification of the devices connected to the LAN 26. The routine of Figure 4 begins with step 70, wherein the system control module 18 interrogates individually in sequence, each device connected to the LAN 26. Following step 70 is step 72 wherein the control module 18 broadcasts a group interrogation message to each device on the LAN 26. Any device that has data to transfer can then respond to the group interrogation message. There is no difficulty if none of the diepoeitivoe or just one device responds to the group interrogation message. However, if two or more devices repond to the group interrogation maneuver, then there is containment, which must be re-addressed. According to this, in step 74 (which follows step 72) there is determined containment. If not, the routine simply loop back to step 70. Otherwise, step 76 proceeds to step 74. In step 76, the contention detected in step 74 is resolved in some way. According to a preferred embodiment of the invention, a containment is solved by carrying out a known technique referred to as "eondeo". An example of the sounding technique is described in "Multiple Access Protocols in Packet Communication Systems" (Multiaccese Protocols in Packet Communication Systems), FA Tobagi, IEEE Transactions on Communications, Vol. COM-28, No. 4, April 1980, pgs. 468-488 and particularly page 478. Briefly, during a polling algorithm, the interrogation group is composed in eubgrupoe according to a tree structure and the group interrogation message is repeated to each subgroup. For example, the eubgroups can be formed in a binary form, such that the main group first decomposes into halves, then into quarters, then eighths and so on until the contention resolves.

After the contention has been resolved in such a way that each of the contending devices has had an opportunity to transfer information, the routine loops back to step 70. It is contemplated by the invention to apply the technique of Figure 4 in a number of different ways. For example, step 72 may be performed several times interspersed with step 70, such that a group interrogation message is supplied several times during the period in which the devices interrogate individually in sequence. For example, a quarter of the devices can be interrogated individually and then a group interrogation message be sent, with contention resolution if required, then the fourth client of the devices is interrogated individually and another group message is sent and so on. ahead. In this way, the minimum waiting period for each device is reduced by a large proportion without greatly contributing to the general management costs of the survey. As another variation, the devices connected to LAN 26 can be divided into classes with individual interrogation of the steps 70 applied to the devices of a preferred class and then with group interrogation applied to the subordinate class with containment resolution as required. In this caseeither the subordinate class may not be individually interrogated in fact (unless required for contention reolution) or the subordinate clause may also be interrogated individually but less frequently than the dispositions of the preferred class. Of course, you can also contemplate individually questioning diepoeitivoe of doe classes with equal frequency, but then to periodically use group questioning only for one of the doe claeee. Group questioning may also apply to two or more groups, of which at least one group is individually questioned. Furthermore, group interrogation can be applied in cases where tree or device clauses are defined for different interrogation treatment. It will be recognized that the group interrogation technique illustrated in basic terms in Figure 4 can also be applied together with the dynamic re-assignment procedure of Figure 3, although some coefficient in complexity in terms of the polling technique used to resolve containment when it is necessary. Addition of techniques to improve the performance of LAN 26 will be written with reference to Figure 5A-5D. NEGATIVE RESPONSE ABBREVIATED TO INTERROGATION Part of the general management expenses present in a question mark is the time that elapses while a response from the interrogation device is expected. When the interrogation device has no information to transmit, this fact can be indicated in one of two ways. First, the interrogation device may simply not take action, in which case the polling device "terminates its synchronization", eeto ee, the polling device waits a predetermined period of time and if no signal is received from the polling device within that period of time, then the interrogation device continues to transmit the next interrogation in the sequence. As a second alternative, which may be more effecient in synchronization terms, the polling diepoeitive may transmit a signal indicating that it has no information to send. Typically, these negative responses have been made up of at least a few octets of data. However, according to the invention, the total time required for interrogation can be reduced by providing that the negative response provided by the interrogation devices is shorter than one octet. A negative response of this type is illustrated in Figure 5A, and considers a series of line transitions, for example at least six line transitions, which occur over several synchronization cycles, followed by at least two synchronization cycles in each case. no line transition. In the FM-0 coding scheme, the signal illustrated in Figure 5A is constituted by four "0" bits followed by two synchronization periods in which none of a 0 bit or a 1 bit is produced. When the interrogation device (in this case the system control module 18) detects the response signal as illustrated in FIG. 5A, that the interrogation device has tranemitted in a reeppette to an interrogation line sent by the control module 18, the module 18 immediately proceeds to send the subsequent interrogation message in the interrogation message sequence. In this way, greater efficiency is obtained than with any of the conventional termination technique or with the use of negative interrogation responses constituted of several octetoe. It will be appreciated that the negative interrogation reeplet shown in Figure 5A is suitable for use with interrogation messages directed to individual devices, but will not be satisfactory as a means of responding to group interrogation messages. In the case of group interrogation messages, a termination technique must be employed. ABBREVIATED INTERROGATION COMMAND According to another technique of the present invention, the general interrogation geetión expenditures can be reduced in addition by shortening the length of the interrogation command with respect to other types of data messages that are transmitted over the LAN 26. This technique will be described in more detail with reference to Figures 5B-5D . Figure 5B schematically illustrates a format for an interrogation command that will be transmitted by the system control module 18. Figure 5C shows a format for any of a number of commands other than an interrogation command that can be transmitted on the LAN 26 by the control module 18 or by other devices connected to the LAN. Figure 5D shows a format for various types of data packet messages that can be transmitted on the LAN 26 by various types of devices connected to it. Examination of the formats illustrated in Figures 5B-5D will indicate that the format for the interrogation command is shorter than the formats for the other types of data to be transmitted on LAN 26. As a result, the interrogation command can be recognized. simply by its length and without specifying the type of command in the case of an interrogation command. In addition, because the po3 command is transmitted so frequently, the reduction in size as compared to the next shorter message type provides significant savings in general gaetoe geetión. In particular, you will notice that the interrogation command shown in Figure 5B is composed of one start flag octet, one address octet, two error detection code bytes, and one stop flag octet, for five octetoe total. In contrast, the shortest format for another type of maneuver includes a byte of type to specify the type of maneuver in addition to the five octets in the format for the interrogation command. Also, in the data packet format illustrated in Figure 5D, each data packet includes a start flag octet, a destination address byte, a source address byte, an octet type that identifies the packet type data, a destination "plug" address byte, an address byte of "plug" source, at least one and not more than 92 bytes of data constituting the data to traneferiree through the datoe packet, two octets of data error correction code and one stop flag octet. In this way, each data packet contains at least 10 octets and possibly as many as 101 octets. It will be recognized that, according to these formats, interrogation commands are constituted by a number smaller than octets than any other command or data message that is transmitted on the LAN 26. According to this, interrogation devices can recognize the command of interrogation simply by its unique length, which is shorter than the length of any other message, and the type byte can therefore be omitted from the interrogation command thereby saving interrogation time and reducing overall geetio expenditure.

The "type" octet in the format for other types of commands or in the data packet message format, identifies the type of command or data packet. For example, the numeric value contained in the "type" octet can identify a command as a recognition response, or as a reset command. Similarly, the numerical value in the "type" byte in a data packet can identify the packet as either the type that requires recognition or as the type that does not require recognition. The plug addressing provided by the plug address address, address and source allows separate management of access points or functions within the operating processes that are provided in each of the devices connected to the LAN 26. There will be it should be noted that the destination address "255" ("FF" in hexadecimal notation) is preferably reserved for broadcasting such that any device connected to the LAN 26 will be considered a intended recipient of a command addressed to "255". Although not illustrated separately, it will be understood that the command format illustrated in Figure 5 can be easily adapted to group interrogation and polling operations such as those discussed above. For example, a convenient type designation would be indicative of a group command for a given diepoeitivoe class and a format for polling commands can include a type octet, indicating that the ee command is a polling command, with address doe octetoe deetino indicativoe of start and end values of a range of addresses that define a subkey to which the polling command is directed. Also, for convenient group interrogation structures, it is envisaged to use a five octet format for the initial group interrogation command, with a destination address octet of "broadcast" but not a type octet. PAR-TO-PAR COMMUNICATION IN RESPONSE TO INTERROGATION Figure 6 illustrates in schematic terms a sequence of network access operations that are carried out in LAN 26 according to the invention. Block 18 illustrated in Figure 6 represents the control module of sevenma that acts as a master interrogation device to control access to LAN 26 and each of the nodes labeled NI to N6 represents a repective of the various types of different devices. to module 18 that are connected to LAN 26. In addition, Figure 6 shows the tagged Sl to S12, all of which are tracked in sequence on LAN 26.

More specifically, the first signal in the sequence in an interrogation command Sl is transmitted by the control module 18 and addressed to the node labeled NI. It is considered that the diepoeitive that corresponds to node NI has no data to bring and in accordance with signal S2, which is the negative response that has the format illustrated in Figure 5A. The next signal in the sequence S3, which is an interrogation message transmitted by the control module 18 and directed to the device corresponding to the node N2. It is again considered that the device in the node N2 has data to transfer and in fact diffuses a data packet (signal S4) to every device connected to the LAN 26. The next signal S5, which is another interrogation message transmitted from the control module 18, and this time directed to the device at node N3. It is found that the dietary node N3 has data to be transferred to another device (ie, node N6) in such a way that the dietary node N3 returns to interrogation S5 when transmitting a packet of data conveniently directed S6 on LAN 26 directly to the device at node N6. The next signal in the illustrated sequence is an interrogation command S7 which is transmitted by the control module 18 and directed to the device at the node N4. It is considered that this device has no data to transfer, and therefore transmits an S8 signal which is the negative response as illustrated in Figure 5A. The next signal in the sequence is an interrogation command S8 tracked by the control module 18 and directed to the device at node N5. It is now considered that the device in the node N5 has data to transfer to the dietary node N2 and in conformity with the command S9 when tracing an SIO data packet directed to the device in node N2. The next signal tracked by another interrogation command Sil is transmitted by the control module 18 and directed to the device at the node N6. In this case, it is considered that the device at the node N6 has data to transfer to the control module 18, such that the device at the node N6 responds to the interrogation command Sil when transmitting a data packet S12 addressed to the module control 18. A striking feature of the sequence illustrated in Figure 6 is that devices are allowed to reeponate to interrogation messages by transmitting data directly to another device (other than module 18), instead of transmitting the data to the dietary master interrogation to transfer the final recipient. This direct data transfer in response to interrogation provides additional improvement in the performance of the LAN 26.

Examples of this direct data transfer are data packets S4, S6 and SIO. In general, no device other than control module 18 is allowed access to LAN 26 except in response to an interrogation command that is directed, either individually or by group to the particular device. However, the exception to this rule is a device that is a recipient of a data packet that requests recognition is allowed to respond to this data packet by immediately transmitting a message of recognition or non-recognition. SURVEILLANCE SYSTEM FORMED SUBSYSTEMS According to a preferred embodiment of the invention, a video surveillance system 10 and its LAN 26 are set to accommodate up to 20 or 30 control consoles and approximately 100 cameras. However, it is contemplated to form a still larger video surveillance camera when interconnecting an amount of network 10, as schematically illustrated in Figure 7. In Figure 7, the reference number 100 generally indicates a video surveillance system. constituted by eubeymes 10-1, 10-2, 10-3 and 10-4. The video and data connections 102-1, 102-2, 102-3 and 102-4 ee provide for linking the respective control modules of the subsystems. One particular of the control modules, let's say module 18-1, is designated as a master module to handle the interconnection between eietemas. For purposes of illustration, only four eveieyemae are illustrated in Figure 7, but it is contemplated to interconnect forty or more subsietes, to produce a very large surveillance suite that probably includes 4,000 cameras and a large number of camera control consoles. For the most part, the interrogation techniques that will be provided according to the invention have been described here independently of each other and with certainty it is contemplated to use some instead of all the techniques described herein. However, it should be recognized that all the techniques described herein can be implemented in a protocol for simple LAN administration, and this is in fact done in a preferred embodiment of the invention. Thus, in a single preferred embodiment, the negative response, interrogation command, and other message formats of FIGS. 5A-5D are all used, along with diepoeitive classification to interrogate different interrogation speed, the dynamic assignment of diepoeitivoe between interrogation clause, group interrogation for at least some diepoeitive claee, with contention resolution as required, and peer-to-peer messaging in response to interrogation command. As a result, all the devices that constitute a large video surveillance network can be accommodated by a local area data network with satisfactory performance in terms of performance and average delay time for network access. Although the network administration techniques provided in accordance with the invention have been described in connection with a LAN to serve a video surveillance system, it will be recognized that the inventive techniques are applicable to other environments. Also, although the techniques have been established in connection with a transmission medium consisting of a pair of twisted cables, it should be noted that these techniques can be applied to other types of trance transmission medium, such as coaxial cable or optical fibers, as well as infrared wireless communications and aei onwards. Although the present invention has been made with reference to the currently preferred modality, it will be understood that various changes may be made without departing from the real spirit of the invention as defined in the appended claims. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, which is clear from the present invention's decree. Having described the invention as above, the following content is claimed as property:

Claims (64)

1. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of devices, the preferred and subordinate groups are mutually exclusive. , wherein the devices of the preferred group are interrogated at least N / M times as frequently as the subordinate group devices, N and M both are positive integers with N > M; each of the devices of the preferred group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to be sent, and further comprises the steps of : determining how often each device of the preferred group transmits a first type of response; and reassigning the device of the preferred group to the subordinate group, if the device transmits the first type of response more frequently than a predetermined degree of frequency.

2. A method according to claim 1, characterized in that M = 1.

3. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of devices, the preferred and subordinate groups are mutually exclusive, wherein the devices of the preferred group are interrogated at least N / M times as frequently as the subordinate group devices, N and M are both integers positive with N > M; Each of the devices responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises the steps of: determining that so frequently each device of the subordinate group transmits the second type of response; and reassigning the subordinate group device to the preferred group if the device transmits the second type of response more frequently than a predetermined degree of frequency.

4. A method according to claim 3, characterized in that it further comprises the steps of: determining how frequently each device of the preferred group transmits the first type of response; and reassigning the device from the preferred group to the subordinate group if the device transmits the first type of response more frequently than another predetermined degree of frequency.

5. A method according to claim 4, characterized in that the second type of response includes at least a predetermined plurality of octets and the first type of response is shorter in duration than the predetermined plurality of octets.

6. A method according to claim 5, characterized in that the first type of response is shorter in duration than one octet.

7. A method according to claim 4, characterized in that M = 1.

8. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a first group of devices and a second group of devices, the first and second groups are mutually exclusive, the method is characterized in that it comprises the steps of: transmitting an interrogation message at least N times to each device of the first group of devices during a period of time, N is an integer greater than 2; and transmitting an interrogation message once and only once during the time period to each device of the second group of devices; Each of the devices responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises the steps of: determining that so often every device in the first group transmits the first type of response; and reassigning the device from the first group to the second group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

9. A method according to claim 8, characterized in that it further comprises the steps of: determining how frequently each device of the second group transmits the second type of response; and reassigning the device of the second group to the first group if the device transmits the second type of response more frequently than another predetermined degree of frequency.

10. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a first group of devices and a second group of diepoeitivoe, the first and second. groups are mutually exclusive, the method is characterized in that it comprises the steps of: transmitting an interrogation message at least N times to each device of the first group of devices over a period of time, N is an integer greater than two; and transmitting an interrogation message once and only once during the time period to each device of the second group of devices; each of the devices of the second group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send and also comprises the steps of: determine how often each device in the second group transmits the second type of response; and reassigning the device of the second group to the first group if the device transmits the second type of response more frequently than a predetermined degree of frequency.

11. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of devices, the preferred and preferred groups. subordinate are mutually exclusive, the method is characterized in that it comprises the steps of: first transmitting a first interrogation message once and only once in sequence to each device of the subordinate group of devices; second transmitting an interrogation message at least N times to each device of the preferred group of devices after initiating the first transmission stage and before transmitting any interrogation message different to the first interrogation message to any device of the subordinate group, wherein N it is an integer greater than one; wherein each of the devices responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to be sent, and also comprises the steps of: determine how often each device of the preferred group transmits the first type of response; and reassigning the device of the preferred group to the subordinate group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

12. A method in accordance with the claim 11, characterized in that it also comprises the steps of: determining how frequently each device of the subordinate group transmits the second type of response; and reassigning the device of the subordinate group to the preferred group if the device transmits to the second type of response more frequently than a predetermined degree of frequency.

13. An interrogation method a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of the devices, the preferred and preferred groups. subordinate are mutually exclusive, the method is characterized in that it comprises the steps of: first transmitting a first interrogation message once and only once in sequence to each device of the subordinate group of devices; and second transmitting an interrogation message at least N times to each device of the preferred group of devices after initiating the first transmission stage and before transmitting any different interrogation message to the first interrogation message to any device of the subordinate group, wherein N is an integer greater than one; wherein each of the devices of the subordinate group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises the stages of: determining how frequently each device of the subordinate group transmits the second type of response; reassigning the device of the subordinate group to the preferred group if the device transmits the second type of response more frequently than a predetermined degree of frequency.

14. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a first preferred group of devices, a second group of preferred devices and a subordinate group of devices, the first preferred group, the second preferred group and the subordinate group are all mutually exclusive, characterized in that: the devices of the first preferred group are interrogated at least N times as frequently as the devices of the subordinate group, N is an integer greater than 1; and the devices of the second preferred group are interrogated at least M times as frequently as the devices of the subordinate group, M is an integer greater than one and different from N.

15. A method according to the claim 14, characterized in that N is an integer multiple of M.

16. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a first group of devices and a second group of devices, each of the first and second groups of devices consists of a respective plurality of the devices, the method is characterized in that it comprises the steps of: first transmitting to each device of the first group of devices, in accordance with a predetermined sequence, an interrogation message that addresses only the respective device of the first group of devices; after the first transmission stage, secondly transmitting a group interrogation message simultaneously to all the devices of the second group of devices; detect whether contention between the devices of the second group occurs in response to the group's interrogation message; and if containment is detected in the detection stage, resolve the contention detected by performing a polling algorithm; each of the devices of the first group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send and also comprises the steps of: determine how often each device in the first group transmits the first response group; and reassigning the device from the first group to the second group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

17. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices and a subordinate group of devices, the preferred and subordinate groups are mutually exclusive, the method is characterized in that it comprises the steps of: interrogating the devices of the subordinate group by means of interrogation messages each addressed to a different one of the devices of the subordinate group with a first degree of frequency; interrogate one of the devices of the preferred group by means of interrogation messages, each directed to a different one of the devices of the preferred group with a second degree of frequency that is N / M times as large as the first degree of frequency, N and M both are positive integers with N > M; transmitting a group interrogation message simultaneously to all devices of the subordinate group of devices with a third degree of frequency; detect if contention occurs between the subordinate group devices in response to the group interrogation message; and if containment is detected in the detection stage, resolve the contention detected by performing a polling algorithm; wherein each of the devices of the preferred group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and further comprises the stages of: determining how often each device of the preferred group transmits the first type of response; and reassigning the device of the preferred group to the subordinate group, if the device transmits the first type of response more frequently than a predetermined degree of frequency.

18. A method according to claim 17, characterized in that the third degree of frequency is the same as the first degree of frequency.

19. A method according to claim 17, characterized in that M = 1.

20. A method according to claim 19, characterized in that N is greater than two.

21. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, each of the different data messages to the interrogation messages is in a first predetermined format consisting of minus N octets, N is an integer greater than one, the method comprises the steps of sending the interrogation messages in sequence to each of the devices, each of the messages is in a second predetermined format consisting of M octets, in where M is a positive integer that is less than N.

22. A method in accordance with the claim 21, characterized in that M = 5 and N = 6.

23. A method according to the claim 22, characterized in that each of the interrogation messages consists of a stop flag octet, an address octet to identify a respective one of the devices to which an interrogation message is sent, an error detection code consisting of two octets and a stop flag octet.

24. A method in accordance with the claim 21, characterized in that each of the data messages that different from interrogation messages includes at least one start flag octet, one address octet to identify a respective one of the devices to which the data message is sent, one octet of type to identify a type of data message, an error detection code consisting of two octets and a stop flag octet.

25. A method in accordance with the claim 24, characterized in that some of the data messages also include at least one octet of data to be transferred.

26. A method of compliance with the claim 25, characterized in that none of the data messages includes more than 92 octets of data to be transferred.

27. A method for operating a data network formed of a plurality of interconnected devices including a master interrogation device and other devices, the method is characterized in that it comprises the steps of: transmitting a respective interrogation message in sequence from the master interrogation device to each of the other devices; and transmitting data messages from the other devices in response to the respective interrogation messages; wherein the data messages transmitted from the other devices in response to the respective interrogation messages, each consist of at least N octets and the respective interrogation messages each consist of M octets, with N and M being positive integers and M less than N.

28. A method in accordance with the claim 27, characterized in that M = 5 and N = 6.

29. A method according to the claim 28, characterized in that each of the interrogation messages consists of a stop flag octet, an address octet to identify a respective one of the devices to which the interrogation message is sent, an error detection code consisting of two octets and one stop flag octet.

30. A method according to claim 29, characterized in that each of the data messages that is not an interrogation message includes at least one stop flag octet, one address octet to identify a respective one of the devices to which the message data is sent, a byte of type to identify a type of data message, an error detection code consisting of two octets and a stop flag octet.

31. A method for operating a data network formed of a plurality of interconnected devices, including a master interrogation device and other devices, the method is characterized in that it comprises the steps of: transmitting an interrogation message from the master interrogation device to a first of the other devices; and in response to the interrogation message transmitting a data message directly from a first one from the other devices to a second one of the other devices.

32. A method in accordance with the claim 31, characterized in that the data message is transmitted simultaneously to a plurality of the other devices:

33. A method in accordance with the claim 32, characterized in that the data message is transmitted to the master interrogation device at the same time that the data message is transmitted to the plurality of other devices.

34. A method by which an interrogation device responds to an interrogation message that is received by the interrogation device from a data network, the network includes a plurality of devices that are capable of receiving and sending data messages, network means for interconnecting the devices for data communications between the devices and means for sending interrogation messages to the plurality of devices, the method is characterized because it comprises the steps of: sending a data message consisting of at least N octets if the interrogation device has data to send; N is a positive integer; and transmitting an interrogation response signal that does not include any octet if the polling device has no data to send; the interrogation response signal consists of a plurality of line transitions followed by the absence of line transitions for a period of at least two clock cycles.

35. A method in accordance with the claim 34, characterized in that the plurality of line transitions includes not less than six line transitions.

36. A method in accordance with the claim 35, characterized in that the data message is transmitted by FM-0 coding and the interrogation response signal consists of four "0" bits followed by the absence of line transitions.

37. A data communications network characterized in that it comprises: a first group of devices that includes a first plurality of devices capable of receiving and sending data messages; a second group of devices that includes a second plurality of devices capable of receiving and sending data messages, the first and second groups are mutually exclusive; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending interrogation messages to the devices via the signal path, the interrogation means sends a respective interrogation message to each device of the first group at least N / M times as frequently to each device of the second group, N and M both are positive integers with N > M; Each of the devices of the first group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises: means for determine how frequently each device of the first group transmits to the first type of response; and means for reassigning the device from the first group to the second group, if the device transmits the first type of response more frequently than a predetermined degree of frequency.

38. A data communications network according to claim 37, characterized in that M = 1.

39. A data communications network according to claim 37, characterized in that the means for connecting include a pair of twisted wires connected to each of the devices and the interrogation means.

40. A data communications network characterized in that it comprises: a first group of devices that includes a first plurality of devices capable of receiving and sending data messages, the first and second groups are mutually exclusive; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending interrogation messages to the devices via the signal path, the interrogation means sends a respective interrogation message to each device of the first group at least N / M times as frequently to each device of the second group, N and M both are positive integers with N > M; wherein each of the devices of the second group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises: means for determining how frequently each device of the second group transmits the second type of response; and means for reassigning the device from the second group to the first group if the device transmits to the second type of response more frequently than a predetermined degree of frequency.

41. A data communications network according to claim 40, characterized in that the means for connecting include a pair of twisted wires connected to each of the devices and the interrogation means.

42. A data communications network characterized in that it comprises: a first group of devices that includes a first plurality of devices capable of receiving and sending data messages; a second group of devices that includes a second plurality of devices capable of receiving and sending data messages, the first and second groups are mutually exclusive; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending interrogation messages to the devices via the signal path, the interrogation means sends a respective interrogation message to each device of the first group at least N times over a period of time, N is a greater integer than two, and send a respective interrogation message once and only once during the time period for each device of the second group of devices; wherein each of the devices of the first group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises: means for determining how frequently each device of the first group transmits the first type of response; and means for reassigning the device from the first group to the second group if the device transmits to the first type of response more frequently than a predetermined degree of frequency.

43. A data communications network characterized in that it comprises: a first group of devices that includes a first plurality of devices capable of receiving and sending data messages; a second group of devices that include a second plurality of devices capable of receiving and sending data messages, the first and second groups are mutually exclusive; means for interconnecting all devices, to provide a signal path between them; interrogation means for sending interrogation messages to the devices by means of the signal path, the interrogation means sends to each device of the first group, according to a predetermined sequence, an interrogation message which is uniquely directed to the respective device of the interrogation. first group of devices, and then send a group interrogation message simultaneously to all devices; means for detecting if contention occurs between the devices of the second group in response to the group interrogation message and means for resolving containment detected by the means to detect when performing a polling algorithm; wherein each of the devices of the first group responds to interrogation by transmitting either a first type of response indicating that the device has no data to send to a second type of response indicating that the device has data to send, and also comprises: means for determining how frequently each device of the first group transmits the first type of response; and means for assigning the device of the first group to the second group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

44. A data communications network characterized in that it comprises: a first group of devices that includes a first plurality of devices capable of receiving and sending data messages; a second group of devices that includes a second plurality of devices capable of receiving and sending data messages, the first and second groups are mutually exclusive; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending interrogation messages to the devices by means of the signal path, the interrogation means sends to each device of the second group, with a first frequency degree, a respective interrogation message that is uniquely directed to the device of the second group, the polling means also send to each device of the first group, with a second degree of frequency, a respective interrogation message that is uniquely directed to the device of the first group, the second degree of frequency is N / M times as large as the first degree of frequency (N, M are positive integers with N >; M), and the interrogation means also sends a group interrogation message simultaneously to all the devices of the second group with a third degree of frequency; means for detecting if contention occurs between the devices of the second group in response to the group interrogation message; and means for resolving contention detected by the means to detect when performing a polling algorithm; wherein each of the devices of the first group responds to interrogation, by transmitting either a first type of response indicating that the device has no data to send or a second type of response indicating that the device has data to send, and also comprises : means to determine how frequently each device of the first group transmits the first type of response; and means for assigning the device of the first group to the second group if the device transmits the first type of response more frequently than a predetermined degree of frequency.

45. A data communications network according to claim 44, characterized in that the third degree of frequency is the same as the first frequency degree.

46. A data communications network according to claim 44, characterized in that M = 1.

47. A data communications network according to claim 46, characterized in that N is greater than two.

48. A data communications network, characterized in that it comprises: a plurality of devices capable of receiving and sending data messages; means for interconnecting all devices to provide a signal path between them; interrogation means for sending interrogation messages to the devices via the signal path, each of the data messages is different from interrogation messages that are in a first predetermined format consisting of at least N octets, N is an integer greater than one, each of the interrogation messages is in a second predetermined format consisting of M octets, where M is a positive integer that is less than N.

49. A data communications network according to claim 48 , characterized in that M = 5 and N = 6.

50. A data communications network according to claim 49, characterized in that each of the interrogation messages consists of a start flag octet, an address octet that identifies in a unique form a respective of the devices to which the interrogation message is sent, an error detection code consisting of two octets and an octet or stop flag.

51. A data communications network according to claim 50, characterized in that each of the data messages that is not an interrogation message includes at least one stop flag octet, one address octet to identify a respective one of the devices to which the data message is sent, a byte of type to identify a type of data message, an error detection code consisting of two octets, and a stop flag byte.

52. A data communications network characterized in that it comprises: a plurality of devices capable of receiving and sending data messages; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending an interrogation message to a first of the plurality of devices; wherein the first of the plurality of devices selectively responds to the interrogation message by sending a data message by the signal path directly to one second of the plurality of devices, the second being different from the interrogation means.

53. A data communications network according to claim 52, characterized in that the first of the plurality of devices selectively sends the data message simultaneously to more than one of the plurality of devices.

54. A data communications network according to claim 53, characterized in that the first of the plurality of devices selectively sends the data message simultaneously to the interrogation means and to every device of the plurality of devices except for the first of the plurality of devices. dispositives.

55. A data communications network according to claim 54, characterized in that the means for interconnecting comprises a pair of twisted wires connected to all the plurality of devices and to the interrogation means.

56. A data communications network characterized in that it comprises: a plurality of devices capable of receiving and sending data messages; means for interconnecting all devices to provide a signal path between them; and interrogation means for sending interrogation messages to the devices via the signal path; in each device each responds to a respective interrogation message addressed to the device by selectively sending one of a data message consisting of at least N octets, N is a positive integer and an interrogation response signal that does not include any octet; the interrogation response signal consists of a plurality of line transitions followed by the absence of line transitions for a period of at least two synchronization cycles.

57. A data communications network according to claim 56, characterized in that the data message is transmitted by FM-0 coding and the interrogation response signal consists of four "0" bits followed by the absence of line transitions. .

58. A data communications network according to claim 57, characterized in that the plurality of line transitions does not include less than six line transitions.

59. A data communications network according to claim 58, characterized in that the means for connecting include a pair of twisted wires connected to each of the devices and the interrogation means.

60. A method for interrogating a plurality of interconnected devices, each of the devices is capable of receiving and sending data messages, the plurality of devices includes a preferred group of devices, a subordinate group of devices and a master interrogation device, the preferred and subordinate groups are mutually exclusive, the method is characterized by comprising the steps of: interrogating the devices of the subordinate group by means of interrogation messages each addressed to a different one of the subordinate group devices with a first frequency degree; interrogate the preferred group devices by interrogation messages each directed to a different one of the preferred group devices with a second degree of frequency that is N / M times as large as the first degree of frequency, N and M both are positive integers , with N > M; transmitting a group interrogation message simultaneously to all devices of at least one of the subordinate group of devices and the preferred group of devices with a third degree of frequency; detect if contention occurs between the devices of at least one group in response to the group interrogation message; if contention is detected in the detection stage, resolve the contention detected to perform a polling algorithm; determining how often each preferred group device responds to interrogation by indicating that the device has no data to send; and reassigning the device of the preferred group to the subordinate group if the device indicates that it has no data to send more frequently than a predetermined degree of frequency; and wherein: each of the devices of the preferred and subordinate groups responds to interrogation messages directed only to the interrogation device by transmitting a negative interrogation response signal that does not include any octet if the interrogation device has no data to send; each of the data messages other than interrogation messages is in a first predetermined format consisting of at least K octets and each of the interrogation messages is in a second predetermined format consisting of L octets, where K and L are positive integers with K > L; and at least some of the devices of the preferred and subordinate groups selectively respond to interrogation messages directed to them by transmitting a data message directly from the interrogation device to another of the devices that is not the master interrogation device.

61. a method in accordance with the claim 59, characterized in that the third degree of frequency is the same as the first degree of frequency and the second degree of frequency.

62. A method according to claim 59, characterized in that K = 6 and L = 5.

63. A method according to claim 59, characterized in that the negative interrogation response signal consists of a plurality of line transitions followed. by an absence of line transitions for a period of at least two synchronization cycles.

64. A method according to claim 59, characterized in that the subordinate group of devices includes a plurality of remotely controllable video cameras and the preferred group of devices includes a plurality of control consoles for the video cameras.