GB2503676A

GB2503676A - Method for transmitting/receiving data comprising synchronisation information after a fixed delay

Info

Publication number: GB2503676A
Application number: GB201211784A
Authority: GB
Inventors: Norihito Aoki; Arnaud Closset
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2012-07-03
Filing date: 2012-07-03
Publication date: 2014-01-08
Anticipated expiration: 2032-07-03
Also published as: GB2503676B; GB201211784D0

Abstract

A method for communicating data between a transmitter device and a receiver device comprises the following steps: - detecting a time event (i.e. synchronisation information, Critical Time Event, CTE, S401) by the transmitter device; - transmitting a first group of data S408 comprising the time event (synchronisation information) after a first interval of time S405 elapses from the time at which the time event (synchronization information) is detected; - receiving the first group of data by the receiver device; and - generating the time event (synchronisation information) by the receiver device after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event (synchronisation information) is detected by the transmitter device to the time at which the time event (synchronisation information) is generated by the receiver device is a predefined value. The arrangement described is used particularly for transmission of video data comprising synchronization information between a transmitter and receiver to counter variable latencies in the transmission such as those attributable to variable buffering (FIFO) delays at either side. The first interval of time comprises a first sub-interval depending on the time that is necessary to transmit a group of data preceding the first group and a second sub-interval depending on a synchronisation interval of the physical layer. The first interval of time is a predetermined interval of time corresponding to the sum of maximum possible values of the first and second sub-intervals. The first group of data or a subsequent group of data may comprise information indicating the first interval of time to the receiver.

Description

Method and device for communicating data The present invention concerns a method and a device for communicating data between a transmitter device and a receiver device.

On communicating data between a transmitter device and a receiver device, transmission latency is not constant. This may cause inconvenience when transmitting data, for example video data, including synchronization information.

It should be noted that, for video data, synchronization information is essential for maintaining the cadence in displaying the video data received at the receiver device.

Figure 1 describes a communication system comprising a transmitter device 1 and a receiver device 2.

The transmitter device 1 comprises a Data Link Layer (DLL) device which transfers data from the Network Layer to the Physical Layer.

The receiver device 2 comprises a Data Link Layer device 150 which receives data from the Physical Layer and transfers data to the Network Layer.

Transmitter DLL device 100 transfers data to a transmitter Media Access Control (MAC) device 110 (the Media Access Control Layer is a sub-layer of the Data Link Control Layer).

Transmitter MAC device 110 makes CF (lsochronous Continuous Packet) packets from data (payload 101, configuration data 102 and critical time event datalo3) received from the transmitter DLL device 100.

Data received at transmitter MAC device 110 is stored. In this example, payload 101 is stored in a data FIFO (First Input First Output) 115, configuration data 102 is stored in a configuration FIFO 116 and critical time events 103 are stored in an Event FIFO 117. Once ICP packets have been built, they are stored in an intermediate FIFO 112 before being sent to the transmitter physical device 120.

The lOP packets are sent to a transmitter Physical Layer device 120 which transfers the lOP packets to a physical medium (here a Serial Link 160).

A receiver Physical Layer device 130 receives the ICP packets and transfers them to a receiver MAC device 140. The receiver MAC device 140 extracts data (payload 141, configuration data 142 and critical time event data 143) from ICP packets and transfers them to DLL receiver device 150.

Figure 2 is a timing diagram representing the transmission of data between the transmitter device 1 and the receiver device 2.

In Figure 2 the time line named DLL CTE IX 103 represents critical time events (CTE) 103 generated by the transmitter DLL device 100. The time line named DLL Cnfg TX 102 and DLL Data TX 101 represents respectively configuration data 102 and payload 101 generated by the transmitter DLL device 100. lOP IN represents the lOP packets that are formed in transmitter MAC device 110. SF 111/131 represents groups of bits formed from ICP packets and that are transmitted from the transmitter device 11 to the receiver device 12 by the serial link 160. ICP OUT represents the packets reconstructed in receiver MAC device 140 from the received group of bits 131. DLL Data RX 141, DLL Cnfg RX 142 and DLL OTE RX 143 represent respectively payload 141, configuration data 142 and CTE 143 received in the receiver DLL device 150.

A first critical time event (OTE) 303 is encapsulated in a first ICP packet 306. The transmitter DLL device 100 also generates configuration data 102, 302 and payload 101, 301 which are both encapsulated in lop packets.

A critical time event is synchronization information. For example, for video data, a critical time event may be the time at which a line or an image begins.

A second CTE 304 is encapsulated in a second lOP packet 307.

Next, the lOP packets 306, 307 are accommodated in groups of bits 111, 131 in order to be sent to the receiver device 2 by the serial link 160. The receiver MAC device 140 receives ICR packets 308, 309 and extracts from them payload 141, configuration data 142 and critical time event 31 0, 311.

The first 303 and second CTE 304 are received at the receiver DLL device 150 respectively with a first latency L362 and the second latency L363, these latencies L362, L363 being different.

It should be noted that once the lOP packets have been built, the latency for receiving the ICP packets at the receiver DLL depends particularly on the buffering status of transmitter MAC device 110 and on synchronization of the physical layer. The buffering status depends on features of intermediate FIFO 112, such as size, latency times required for reading and writing, and on the amount of data to be transmitted.

Thus, in the example of Figure 2, the latency L362 concerning the ICP packet 306 containing the first CTE 303 is particularly due to the buffering status of the transmitter MAC device 110 while the latency [363 concerning the ICP packet 307 containing the second CTE 304 is particularly due to the buffering status of the transmitter MAC device 110 and to the periodic self synchronization of the physical layer (latency 312).

Thus, each transmitted lOP packet may be affected by different and uncontrolled transmission latencies and synchronization information is not received periodically at the receiver. This may cause trouble when reconstructing the received data.

The present invention is directed to mitigating the aforesaid limitations and to providing a method for communicating data between a transmitter device and a receiver device and a transmitter device, a receiver device and a communicating system associated with that method, making it possible to control the reception of synchronization information.

To that end, according to a first aspect, the present invention concerns a method for communicating data between a transmitter device and a receiver device.

According to the invention, the method comprises the following steps: -detecting a time event by the transmitter device; -transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected; -receiving the first group of data by the receiver device; and -generating the time event by the receiver device after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.

Thus, the interval of time that elapses between a time event detection in the transmitter device and the generation of the time event in the receiver device does not exceed a predefined value. Therefore, the value of this interval of time is controlled and does not cause synchronization problems when data are reconstructed (for example video data).

In practice, the first interval of time may comprise a first sub-interval depending on the time that is necessary to transmit a group of data preceding the first group of data and a second sub-interval depending on a synchronization interval of the physical layer.

According to a feature, the method further comprises the steps of: -transmitting the group of data preceding the first group of data; and -stopping the transmission of data until the second sub-interval has elapsed.

According to one embodiment, the first interval of time is a predetermined interval of time corresponding to the sum of maximum values of the first sub-interval and the second sub-interval.

According to another embodiment, the first group of data further comprises data representing the first interval of time.

Thus, the receiver device is able to know from one group of data the interval of time that elapses from the time at which the time event is detected to the time at which the time event is transmitted.

According to another embodiment, the method comprises a second step of transmitting a second group of data, the second group of data comprising data representing the first interval of time.

According to a second aspect, the present invention concerns a device for transmitting data.

According to the invention, said transmitting device comprises the following means: -means for detecting a time event in the transmitter device; and -means for transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected.

According to a third aspect, the present invention concerns a device for receiving data.

According to the invention, said receiving device comprises the following means: -means for receiving the first group of data in the receiver device; and -means for generating the time event in the receiver device after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.

According to a fourth aspect, the present invention concerns a system for communicating data between a transmitter device and a receiver device.

According to the invention, said communicating system comprises the following means: -means for detecting a time event in the transmitter device; -means for transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected; -means for receiving the first group of data in the receiver device; and -means for generating in the receiver device the time event after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.

According to a feature, the device for communicating data further comprises means for: -transmitting the group of data preceding the first group of data; and -stopping the transmission of data until the second sub-interval has elapsed.

According to another feature, the communicating device comprises means for transmitting a second group of data, the second group of data comprising data representing the first interval of time.

According to a fifth aspect, the present invention concerns a means for storing information which can be read by a computer or a microprocessor holding instructions of a computer program, which is adapted to implement a method for communicating data according to the invention, when said information is read by said computer or said microprocessor.

In a particular embodiment, the means for storing information according to the invention is partially or totally removable.

According to a sixth aspect, the present invention concerns a computer program product which can be loaded into a programmable apparatus, characterized in that it comprises sequences of instructions for implementing a method for communicating data according to the invention, when said computer program product is loaded into and executed by said programmable apparatus.

The advantages and particular features of the transmitting device, of the receiving device, of the system for communicating data, of the storage means and of the computer program products are similar to those of the communicatting method that they implement.

Still other particularities and advantages of the invention will appear in the following description, made with reference to the accompanying drawings which are given by way of non-limiting example, and in which: -Figure 1 represents a communication system comprising a transmitter device and a receiver device of the prior art: -Figure 2 illustrates a timing diagram representing the transmission of data in a communication system of the prior art; -Figure 3 represents a communication system comprising a transmitter device and a receiver device in accordance with the invention; -Figure 4a is a flow chart representing a first and a second embodiment of a part of the method for communicating data implemented by a transmitter device in accordance with the invention; -Figure 4b is a flow chart representing a third embodiment of a part of the method for communicating data implemented by a transmitter device in accordance with the invention; -Figure 5 is a flow chart representing an embodiment of a part of the method for communicating data implemented by a receiver device in accordance with the invention: -Figure 6 illustrates a timing diagram representing the transmission of data in a communication system in accordance with the invention and -Figure 7 represents a particular hardware configuration of a device suitable for implementation of the method according to the invention.

With reference to Figure 3, a description will first of all be given of a communication system comprising a transmitter device and a receiver device in accordance with the invention.

The communication systems 10 represented in Figure 3 has the same structure as that represented in Figure 1. The communication system 10 comprises a transmitter device 11 and a receiver device 12. The transmitter device 11 comprises a Data Link Layer (DLL) device 200, a Media Access Control (MAC) device 210, and a physical device 220. The receiver device 12 comprises a DLL device 250, a MAC device 240 and a physical device 230.

The transmitter DLL device 200, the transmitter physical device 220, the receiver DLL device 250 and the receiver physical device 230 are similar to the transmitter DLL device 100, the transmitter physical device 120, the receiver DLL device 150 and the receiver physical device 130 represented in Figure 1 respectively.

Data received at transmitter MAC device 210 is stored. In this example, payload 201 is stored in a data FIFO (First Input First Output) 215, configuration data 202 is stored in a configuration FIFO 216 and critical time events 203 are stored in an Event FIFO 217. Once ICP packets have been built, they are stored in an intermediate FIFO 212 before being transmitted to the transmitter physical device 220.

The transmitter MAC device 210 according to an embodiment comprises means 210 for determining the latency from the time at which a time event is detected and the time at which the time event is transmitted to the receiver device 12. The transmitter MAC device 210 generates an output signal named Ready signal 205 which is adapted to either enable or disable the transferring of data from transmitter DLL device 200 to the transmitter MAC device 210.

The functioning of transmitter MAC device 210 is described with reference to Figures 4a and 4b.

The MAC receiver device 240 according to an embodiment comprises means 240 for capturing the latency from the time at which a time event is received at the receiver and the time at which the time event is generated in the receiver device 12.

The functioning of transmitter MAC device 210 is described with reference to Figure 5.

A first and a second embodiment of the functioning of the transmitter MAC device 210 will be described with reference to Figure 4a.

After an initialization step 340, the Ready signal 205 is enabled at an enabling step 3400, allowing the transmitter DLL device 200 to transmit data (payload 201, configuration data 202 and critical time event 203) to the transmitter MAC device 210.

If, at a detecting step S401, a critical time event 203 is detected, the Ready signal 205 is disabled at a disabling step 3402 and a Timer is started at a step 3403.

At a checking step S404, the filling of the intermediate FIFO 212 is checked. When the intermediate FIFO 212 is empty (the all ICF packets that have been stored therein are transmitted to the receiver device 12), the Timer has a value corresponding to a first-subinterval U, and a waiting step S405 is implemented.

At the waiting step S405, the transmitter device ii stops transmitting data for a period of time corresponding to a second sub-interval L2. This second sub-interval L2 depends on a synchronization interval of physical layer.

Once this period of time corresponding to the second sub-interval L2 has elapsed, the timer is stopped at a step S406.

Thus, the value of the timer corresponds to a first interval of time that corresponds to the sum of the first sub-interval Li and the second sub-interval L2.

Next, at a step S407 a control word is created in order to flag that a is CTE has been detected with a timing indication that corresponds to the value of the timer (Li+L2), that isto say to the first interval of time.

Next, at a creating step S408 a ICP packet containing a first group of data is created, the first group of data comprising the control word, the time event and the first interval of time (Li + L2). The ICP packet is then stored in the intermediate FIFO 212.

Of course, as known by a person skilled in the art, a header and a CRC (Cyclic Redundancy Check) word for example, are included in the ICP packet.

In a second embodiment, the ICP packet that was created at the creating step S408 only contains the control word flagging that a CTE has been detected, together with the CTE. The first interval of time (Li +L2) is included in a subsequent packet.

A third embodiment of the functioning of the transmitter MAC device 210 will be described with reference to Figure 4b.

Enabling step 3400', checking step S40i and disabling step 3402' are similar to enabling step S400, checking step 3401 and disabling step 3402 respectively, which were described with reference to Figure 4a.

In this embodiment, once the Ready signal 205 has been disabled at the disabling step S402', a step of waiting S403' is implemented in order to wait until a predetermined interval of time L has elapsed. This predetermined interval of time L corresponds to the addition of the maximum value of the first sub-interval Li and the second sub-interval L2.

In practice, the maximum value of the first sub-interval Li may depend on the filling of the intermediate FIFO 2i2. When the intermediate FIFO 212 is nearly full, the value of the first sub-interval Li is higher than when the intermediate FIFO 2i 2 is nearly empty.

Once the predetermined interval of time L has elapsed, at a step 3407' a control word is created in order to flag that a CTE has been detected and at a creating step S408' an ICP packet containing a first group of data is created, the first group of data comprising the control word and the time event.

The ICP packet is then stored in the intermediate FIFO 212.

It should be noted that in this embodiment, a timer for monitoring timing information is not necessary and no timing information is transmitted.

With reference to Figure 5, a description is now given of a part of the method implemented by the receiver device 12 according to an embodiment of the invention.

When the reception of an lOP packet is detected at a receiving step 3500, a first checking step S50i is implemented in order to verify if a CTE is included in the received packet. If a CTE is included in the received lOP packet, a parameter named here TimeOut is initialized with a predetermined value K at an initialization step 3502, and a Timer is started at a step 3507.

The predetermined value K represents the time elapsed from the time at which an lOP packet is received at the receiver device 12 and the time at which a OlE included in the ICP packet is generated in order to be sent to the next layer.

Next, a second checking step 3503 is implemented in order to verify if the lOP packet includes time information.

If lOP packet includes time information, the value of the parameter TimeOut is updated in an updated step S507, so that the time elapsed from the time at which a CTE is detected in the transmitter device 11 to the time at which the CTE is generated in the receiver device 12 is a predefined value (K÷L). This predefined value is K+L, where L corresponds to the first interval of time. Thus, in this example, TimeOut is updated with the value K+L-(Li +L2).

If at the first checking step S501, no GTE is included in the packet, a third checking step S508 is implemented in order to verify if the received ICP packet contains time information (this is the case of the second embodiment according to the invention). If the response is positive, the updating step S507 is implemented in order to update the value of TimeOut so that the time elapsed from the time at which a CTE is detected in the transmitter device ii to the time at which the CTE is generated in the receiver device 12 is the predefined value K+L.

In the meantime, at a verification step S505, the value of the timer started at the step S504 is verified. When the timer is equal to the value TimeOut, the GTE included in the received packet is generated at a generation step S506.

Figure 6 illustrates a timing diagram representing the transmission of data between the transmitter device 11 and the receiver device 12.

In Figure 6, the time line named DLL GTE TX 203 represents critical time events 203 generated by the transmitter DLL device 200. The time line named DLL Cnfg TX 202 and that named DLL Data TX 201 represent respectively configuration data 202 and payload 201 generated by the transmitter DLL device 200. ICP IN represents the ICP packets that are formed in transmitter MAC device 210. SF 211/231 represents groups of bits formed from ICP packets and that are transmitted from the transmitter device 11 to the receiver device 12 by a serial link 260. ICP OUT represents the packets reconstructed in receiver MAC device 240 from the received group of bits 231.

DLL Data RX 241, DLL Cnfg RX 242 and DLL CTE RX 243 represent respectively payload 241, configuration data 242 and CTE 243 received in the receiver DLL device 250.

In the example represented in Figure 6, a first critical time event 603 is encapsulated in a first group of data or ICP packet 609. The DLL transmitter device 200 also generates configuration data 602 and payload 604 which are both encapsulated in the ICP packet 609. Next, the OP packet 609 is accommodated in groups of bits 608 in order to be sent to the receiver device 12 by the serial link 260.

The time is detected that elapses from the time (ti) at which CTE 603 is detected to the time (t2) at which the group of data 607 preceding the first group of data 609 was transmitted to the receiver device 12 corresponds to the first sub-interval U. The second sub-interval L2 corresponds to a period of time in which the transmitter device 11 stops transmitting data and depends on a synchronization interval of physical layer (time elapsed from time t2 to time t3).

The sum of the first sub-interval Ll and the second sub-interval L2 corresponds to the first interval of time.

Once the second sub-interval L2 has elapsed, the ICP packet 613 is received at the receiver 12 after a transmission delay A (time t4). It should be noted that the value of this transmission delay A is negligible.

In this example, the CF packet 613 received at the receiver 12 contains the data representing the first interval of time. Thus, once the ICP packet 613 is received, the value TimeOut is updated in updating step S407 at the time t5.

In this example, the value TimeOut is updated to the value K+L- (LI÷L2). Thus, the CTE 614 included in the ICP packet 613 is generated at time t6, that is, once the period of time corresponding to the value TimeOut (K-FL- (L1+L2)) has elapsed from the time (t4) at which the ICR packet 613 is received.

Thus, the elapsed time from GTE detection (tl) in the transmitter device 11 to CTE generation (t6) in the receiver device 12 is the predefined value K+L.

An example of the structure of the transmitter device 11 or receiver device 12 to perform the method for communicating data is illustrated in Figure 7.

The structure of the transmitter device 11 or receiver device 12 is substantially identical and comprises among others a central processing unit 701 (designated CPU in the drawing) which executes the instructions relative to the implementation of the invention. The instructions are stored in a Read Only Memory (ROM) 702 or in other storage means. On powering up, the programs which are stored in a non-volatile memory, for example the ROM 702, are transferred into the Random Access Memory RAM 703, which will then contain the executable code of the invention as well as registers for storing the variables necessary for implementing the invention. In a variant, the program or programs may be received in order to be stored in an identical fashion to that described previously via the communication network 704.

In more general terms, an information storage means, which can be read by a computer or microprocessor, integrated or not into the device, and which may possibly be partially or totally removable stores a program implementing the method for communicating data according to the invention.

The communication bus 705 enables communication between the different elements included in the transmitter device 11 or receiver device 12.

The representation of the bus 705 is non-limiting and, in particular, the central processing unit 701 may communicate instructions to any element of the transmitter device 11 or receiver device 12 directly or by means of another element.

The transmitter device 11 or receiver device 12 also comprise a hard disk 706, used by the central processing unit 701 in a conventional manner via the bus 705.

The transmitter device 11 or receiver device 12 further comprise a communication interface 709 linked to the network 704 adapted to transmit digital data in the context of the implementation of the invention.

The transmitter device 11 or receiver device 12 integrates all the means of a device for communicating data according to the method for communication data which has been described above.

The transmitter device 11 or receiver device 12 further comprises elements for interface with the user and in particular a keyboard 707 as well as a screen 708.

Naturally various modifications can be made to the examples described above without departing from the scope of the invention.

Claims

CLAIMS1. Method for communicating data between a transmitter device and a receiver device comprising the following steps: -detecting a time event by the transmitter device; -transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected: -receiving the first group of data by the receiver device; and -generating the time event by the receiver device after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.
2. Method for communicating data according to claim 1, wherein the first interval of time comprises a first sub-interval depending on the time that is necessary to transmit a group of data preceding the first group of data and a second sub-interval depending on a synchronization interval of the physical layer.
3. Method for communicating data according to claim 2, further comprising the steps of: -transmiffing the group of data preceding the first group of data; and -stopping the transmission of data until the second sub-interval has elapsed.
4. Method for communicating data according to one of claims 2 or 3, wherein the first interval of time is a predetermined interval of time corresponding to the sum of maximum values of the first sub-interval and the second sub-interval.
5. Method for communicating data according to claim 1, wherein the first group of data further comprises data representing the first interval of time.
6. Method for communicating data according to claim 1, wherein the method comprises a second step of transmitting a second group of data, the second group of data comprising data representing the first interval of time.
7. Transmitter device comprising the following means: -means for detecting a time event in the transmitter device; and -means for transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected.
8. Receiver device comprising the following means: -means for receiving the first group of data in the receiver device; and -means for generating the time event in the receiver device after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapse from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.
9. System for communicating data between a transmitter device and a receiver device comprising the following means: -means for detecting a time event in the transmitter device; -means for transmitting a first group of data comprising the time event after a first interval of time elapses from the time at which the time event is detected; -means for receiving the first group of data in the receiver device; and -means for generating in the receiver device the time event after a second interval of time elapses from the time at which the first group of data is received, the value of the second interval of time being adapted such that the interval of time that elapses from the time at which the time event is detected by the transmitter device to the time at which the time event is generated by the receiver device is a predefined value.
10. Device for communicating data according to claim 9, wherein the first interval of time comprises a first sub-interval depending on the time that is necessary to transmit a group of data preceding the first group of data and a second sub-interval depending on a synchronization interval of the physical layer.
11. Device for communicating data according to claim 10, further comprising means for: -transmitting the group of data preceding the first group of data; and -stopping the transmission of data until the second sub-interval has elapsed.
12. Device for communicating data according to one of claims 10 or 11, wherein the first interval of time is a prefixed interval of time corresponding to the sum of maximum values of the first sub-interval and the second sub-interval.
13. Device for communicating data according to claim 9, wherein the first group of data further comprises data representing the first interval of time.
14. Device for communicating data according to claim 1, comprising means for transmitting a second group of data, the second group of data comprising data representing the first interval of time.
15. Means for storing information which can be read by a computer or a microprocessor holding instructions of a computer program, characterized in that it is adapted to implement a method for communicating data according to any one of claims 1 to 6, when said information is read by said computer or said microprocessor.
16. Means for storing information according to claim 15, characterized in that it is partially or totally removable.
17. Computer program product which can be loaded into a programmable apparatus, characterized in that it comprises sequences of instructions for implementing a method for communicating data according to any one of claims 1 to 6, when said computer program product is loaded into and executed by said programmable apparatus.19. A system for communicating data substantially as hereinbefore described with reference to, and as shown in, Figure 4a of the accompanying drawings.20. A system for communicating data substantially as hereinbefore described with reference to, and as shown in, Figure 4b of the accompanying drawings.21. A system for communicating data substantially as hereinbefore described with reference to, and as shown in, Figure 5 of the accompanying drawings.22. .A transmifter device substantially as hereinbefore described with reference to, and as shown in, Figure 4a of the accompanying drawings.23. A transmitter device substantially as hereinbefore described with reference to, and as shown in, Figure 4b of the accompanying drawings.24. A receiver device substantially as hereinbefore described with reference to, and as shown in, Figure 5 of the accompanying drawings.