GB2254982A - Data networks. - Google Patents

Abstract

A broadcast tree network comprises a master hub 22, and a series of other hubs 12 interconnected by bidirectional data links 16. Users access the network by Network Interface Circuits (NICs) 14. The NICs 14 implement a Network Access Protocol whereby, following a successful attempt to access the network, further access is denied until the network has been silent for a preset period. In its basic form, the protocol guarantees access to the network within a pre-defined period. The protocol may be developed to provide prioritisation of users and also an acknowledgement mechanism. <IMAGE>

Description

DATA NETWORKS This invention relates to data networks and in particular but not exclusively to Local Area Networks having a broadcast tree architecture.

A typical example of a broadcast "rooted" tree network is illustrated in Figure 1 of the accompanying drawings. The network 10 consists of one or more hubs 12 and a plurality of Network Interface Circuits (NICs) 14 by which users are attached to the network and through which users gain access to the network. The hubs are interconnected to form the "rooted" topology by point to point data links 16 which may be optical, electrical or a mixture of both. Each hub has several (typically three to twelve) child interfaces 18 and one parent interface 20. Each of these interfaces contains a receiver and a transmitter, allowing bidirectional serial data transfer along the interface. A unique central hub 22 (variously referred to as the parent hub, master hub or root hub) has the output link from its parent interface looped back internally or externally to the input at the same interface.

The child interfaces 18 connect down the tree to parent interfaces 20 of other hubs (referred to as sub hubs or child hubs) or to users of the network via the NICs 14. The parent interface 20 connects up the tree to a child interface of a hub 12 closer to the central hub 22 or to the central hub itself.

Data is transmitted through the network in the form of packets. A NIC 14 which has a packet ready transmits it to the local hub 20. If the selection side of the hub is receptive, and there are no other outstanding packets, this packet is selected by the hub. This packet gets the undivided attention of the hub 20 and any other packets arriving are ignored until the transmission of the selected packet is complete. If more than one packet arrives simultaneously, one is selected arbitrarily and the other ignored.

The hub 20 transmits the selected packet to its parent hub where a similar arbitration process is implemented if there are two or more contending packets. The process is repeated as necessary until the selected packet reaches and is accepted by the root hub 22. This packet is then said to have "captured" or "gained access" to the network.

The root hub broadcasts the selected packet down all the output channels at its child interfaces so that the selected packet is propagated throughout the whole network and each NIC receives the packet. If the originating NIC receives an echo of its own packet within a known time then it is deemed to have successfully gained access to the network.

The architecture of a system as described above is set out in more detail in "Fault Tolerant Data Network for Use in Space", R.P. Mathur, Journal of the British Interplanetary Society, Vol.42, pp 27-34, 1989, the contents of which are incorporated herein by reference.

In the above and many other types of network, a user cannot be assured access to the network within a known time.

Users in such networks transmit data as required and, if they are unsuccessful in gaining access, they repeat the process. Thus a user cannot be guaranteed data delivery as it could repeatedly be denied access by other network users.

Accordingly, in one aspect, this invention provides a network comprising at least two users each capable of transmitting and receiving data via the network and access control means for controlling access to the network by the users such that on successfully accessing the network each user is prevented from further access thereto until the network has been silent for a preset arming delay.

In a non-prioritised example of this aspect of the invention, each user may expect to gain access within a period which can be calculated for a given network.

The term "silent" when applied to the network is used to mean that the network is not carrying data between users; it does not however exclude the possibility of the network carrying other data such as clock signals etc.

Another problem with the basic networks described above is that a user originating a data transmission - e.g. a data packet - cannot be certain that the data transmission has been received at the intended destination, even though the originating source has received the echo of the data. A need therefore exists for an acknowledgement scheme which allows prompt acknowledgement of a data transmission.

Accordingly, in another aspect, this invention provides a network comprising at least two users and wherein a user receiving data returns an acknowledgement to the user sending the data.

Guaranteed access to a network is important in a real time environment as users need to have information or react to events within a finite time. The first-mentioned aspect of the invention may be operated to provide all users with a set access time which can be calculated for a given network. However, for some purposes, this access time may be too long; for example for control data or packets which need a response within a "short" time or data sources such as video which must be synchronous in order to be meaningful.

Accordingly in another aspect of this invention, there is provided a network comprising at least two users and including means for assigning priorities to the users and/or data transmitted thereby, whereby higher priority users are allowed to transmit data before lower priority users in a situation where all users have data for transmission.

If a network is silent, then any user can gain access provided its arming conditions are met.

Whilst the invention has been described above it extends to any and every inventive combination of features defined above or disclosed herein.

The invention may be performed in various ways and an embodiment with certain developments thereof will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a broadcast tree network; Figure 2 is a block diagram representing either the uplink or downlink part of a hub circuit suitable for use in a fault-tolerant interface in accordance with the invention; Figure 3 is a block diagram representing a Network Interface Circuit (NIC) in accordance with the invention; Figure 4 is a flow chart showing the operation of a network access protocol in accordance with the invention; Figure 5 is a diagram representing continued operation of the network access protocol of Figure 4;; Figure 6a is a flow chart showing the operation at the interface, via which a source user is attached, of a packet acknowledgement mechanism in accordance with the invention; Figure 6b is a flow chart showing the operation at the interface, via which the destination user is attached of the packet acknowledgement mechanism, and Figures 7a and 7b are tables showing the preferred "arming" conditions for providing a packet prioritisation mechanism in accordance with the invention.

Referring initially to Figures 2 and 3, embodiments of a hub and an NIC for use in the network of Figure 1 and developments thereof will now be described.

In Figure 2, each hub has an uplink part for transmitting packets up the tree and a downlink part for transmitting packets down the tree. The diagram shown in Figure 2 could be used for either the uplink or the downlink function. Where the network is designed as a redundant system the uplink part includes more than one transmitter to provide a choice of routes up the tree. In the simplest, non-redundant case as shown in Figure 1, the uplink requires only one transmitter. The downlink part of the hub is similar to the uplink part and so will not be described in detail. It will be understood though that a redundant system will require more than one receiver in the downlink part whereas a non-redundant system will require only one.

If the hub shown in Figure 2 is performing the uplink function, it comprises four optical receivers 52 each serving one child interface. The outputs from the receivers 52 pass to receiver packet detect circuits 54 which both signal the arrival of a packet to an arbitrator 56 and operate as gates in response to control signals from the arbitrator. The function of the receiver packet detect circuits 54 and the arbitrator 56 is to select only one of the packets presented at the child interface and to pass that packet forward for transmission up the tree. The packet selected by the arbitrator passes via an OR gate 58 to a FIFO store 60. The FIFO store is controlled by a controller 62 which passes data from the store 60 to a transmit control circuit 64 which supplies the data to a series of four transmitters 66 at the parent interface of the hub for passing the selected packet up the tree.The arbitrator also passes a channel select signal to a packet overrun detect circuit 68 which detects whether a packet is too long. If an overlength packet is detected, then hub operation in that direction is reset and this hub is then available to receive incoming packets. Upon completion of the packet transmission, the arbitration and selection logic is re-initialised and the hub is ready to accept a further packet from one of its child interfaces.

The downlink part performs essentially the same functions as the uplink part except in the opposite direction. Any packet selected at the parent interface is broadcast along all the output channels at the child interfaces.

Referring to Figure 3 there is shown a NIC 14. Each NIC is connected to the child interface of a hub via two optical fibre or electrical links, one carrying data from the NIC to the hub and the other carrying data in the opposite direction. The NIC is also connected to a user of the network via a user interface 70, shown on the right hand side of Figure 3, which includes various electrical control and data lines. Each NIC 14 has a unique address which is used for addressing packets to the attached user by other users in the network.

The NIC 14 continually monitors the input line from the local hub via a receiver 72 and receiver packet detect circuit 74. If an incoming packet is addressed to the NIC (or is a broadcast packet) the NIC goes into a packet reception sequence. It strips the start and stop fields from the incoming packet and converts the data into bytes for storage in the FIFO store 75. The NIC performs validity checks on the packet and, if the packet is correct, the user is informed. Data is then transmitted from the FIFO store to the user using lines 76.

In the transmit mode, the NIC performs all the tasks associated with following the network access protocol. This is supervised by the protocol controller 78 and the network access protocol will be further described below. The NIC formats the data from the user into a serial packet form for transmission on the network via transmitter 79 by inserting the start and stop fields and appending any CRC check field etc. The transmit channel includes an acknowledge register 80 for storing a syntactic acknowledgement for return to the source NIC of a packet addressed to this user as described below. The transmit channel also includes a transmit packet counter 82.

The receive channel includes a receive counter 84. The protocol control 78 includes an access counter 86, a response and echo counter 88 and an elapsed counter 90.

The following examples of network access protocol, packet acknowledgement mechanism and packet prioritisation mechanism are applied to the broadcast tree network illustrated in Figure 1 and using the NICs and hubs described above, or in other ways.

Referring to Figures 4 and 5, the network access protocol is implemented by each of the NICs 14 and is set up for packet operation although the invention is not so limited. A NIC operates the network access protocol and allows an attached user to access the network if certain conditions are fulfilled for that user.

On successfully capturing or gaining access to the network on behalf of its attached user, an NIC 14 must wait until the network has been silent for greater than a minimum period of time before it attempts to transmit again. This minimum period is equivalent to the time taken for a packet to be transmitted from a NIC via the root hub and back down to any NIC (including itself) where that route has the longest propagation delay in the network. This period is referred to herein as "one network round trip delay", or, in abbreviated form, 1RT.

Following an unsuccessful attempt to access the network the NIC is allowed to retransmit the data as soon as the network is silent. Thus an unsuccessful user must be able to access the network following a successful user.

In other words, certain network conditions, so called "arming conditions" must exist before a NIC can access the network on behalf of its attached user. The conditions are (a) if the last attempt was successful, there must have been a gap in the network traffic of at least 1RT and (b) if the last attempt was unsuccessful, the network must be silent.

These conditions are assessed and access granted or denied by the protocol controller 78 at each NIC or other suitable processing. Conditions (a) and (b) may be restated in terms of "arming delays". If the previous attempt was unsuccessful then the arming delay is zero; if the previous attempt was successful the arming delay is 1RT.

The flow chart for the protocol implemented in each NIC is shown in Figure 4. Points 'a' and 'b' will be referred to below.

If all users continually have data for transmission, network access will be gained in a cyclical manner as shown in Figure 5. The maximum access time for a user within a cycle may be calculated as NL knowing the total number of users (N), the time to transmit a maximum length packet (L), and the longest round trip delay (RT) for the network. When all users have gained access for transmission of their first packet there will be a gap of 1RT between cycles before users contend again for network access. The order of user access within cycles is not fixed but access is guaranteed by the protocol.

The network access protocol also indicates various failure conditions. If for a given NIC the arming conditions are not satisfied within the expected access time limit (NL) an access failure alarm is generated at 20.

If a period of greater than 1RT elapses between a given NIC sending a packet and receiving an echo of either its own or another user's packet, a network failure alarm is generated at 22.

If, following transmission, the start of the first echo received does not correspond to the transmitted packet, the transmission is deemed unsuccessful and repeated at the earliest opportunity as indicated at 24.

If the whole of the received echo does not correspond to the transmitted packet, a data error alarm is generated at 26 and a further attempt to transmit the packet is made.

If all these conditions are satisfied positively, the transmission is deemed successful as indicated at 28.

Referring to Figures 6a and 6b, in addition to the network access protocol represented in Figure 4, the protocol controller 78 in the NICs 14 may also implement a syntactic Packet Acknowledgement Mechanism which confirms to the source originating a packet transmission on a broadcast tree network that its packet has been received at its intended destination. Acknowledgements can take two forms, either syntactic, as considered here, or semantic. A syntactic acknowledgement signals to a packet source that its packet has been correctly received, but not interpreted.

A semantic acknowledgement signals that received information has been interpreted, for example that following processing of received data a particular course of action will be taken.

As shown in Figures 6a and 6b the source and destination NICs 14 of each packet operate an agreed mechanism whereby an acknowledgement packet is sent back to the source to indicate that the packet was received correctly. However all other users could be contending for network access at that time and thereby stop the syntactic acknowledgement packet from being transmitted immediately. Immediate transmission of acknowledgements is often required in order to make efficient use of resources at a packet source and ensure the fastest flow of data through a system. Thus it is necessary to guarantee that the syntactic acknowledgement can be transmitted over the network to the source before other data packets. This is implemented by reserving a time period corresponding to the length of an acknowledgement packet immediately following transmission of an information packet.In other words, the arming conditions are modified thus: for an unsuccessful previous attempt the arming delay is 1RT + ACKNOWLEDGEMENT~LENGTH; for a successful previous attempt, the arming delay is 2RT + ACKNOWLEDGEMENT~LENGTH.

The acknowledgement packet is generated by the destination NIC and addressed to the source. On receipt of the acknowledgement packet the source is certain that its packet was correctly received.

If all users continually have data for transmission access will again be gained in a cyclical manner as shown in Figure 5. If acknowledgements are used the maximum access time for a user within a cycle is N(L+RT+ACKNOWLEDGEMENT~ LENGTH). When all users have gained access for transmission of their first packet there will be a gap of 1RT between cycles.

The manner of operation is shown in more detail in Figure 6. Figure 6a shows the Packet Acknowledgement Mechanism at the source NIC; if performed in conjunction with the Network Access Protocol these steps would be executed at Point a in Figure 4. The packet acknowledgement mechanism also indicates various failure conditions. If a packet is not received within the time period reserved for receipt of an acknowledgement an acknowledgement failure alarm is generated at 30. If, when received, the acknowledgement is invalid or other than expected a data or network error alarm is generated at 32. If a valid acknowledgement is received in time the packet transmission is deemed complete, as indicated at 34. The system also detects at 36 whether a broadcast packet has been sent, in which case the transmitting NIC does not look for an acknowledgement.

Figure 6b shows the Packet Acknowledgement Mechanism at the destination NIC; if performed in conjunction with the Network Access Protocol these steps would be executed at Point b in Figure 4.

In addition to the failure conditions noted in relation to Figure 6a, the packet acknowledgement mechanism indicates further failure conditions as seen in Figure 6b. Thus, if a NIC receiving data transmits an acknowledgement and the NIC does not receive a return or echo of the acknowledgement within one round trip time a network failure alarm is generated at 38. Likewise, if the NIC receives an echo which does not correspond to the acknowledgement which it has first sent, it generates a network failure alarm indicated at 40. If the complete echo when received does not agree with the acknowledgement then a data error alarm is generated at 42. If the correct acknowledgement is received in time then the packet reception is deemed complete as shown at 44. If the received packet is a broadcast packet, this is detected at 46 and no acknowledgement is sent.

Referring now to Figures 7a and 7b, in addition to the network access protocol, the NICs 14 may also operate a Packet Prioritisation Mechanism which assigns different priorities to packets and operates a generic mechanism by which all packets at a higher priority level are transmitted and received before all packets at the next lower level.

This allows the maximum access time for all users at the highest priority level to be calculated and thus access guaranteed within a time shorter than would have been the case without the prioritisation mechanism. The formula for the guaranteed maximum access time of users at the highest priority level is N,L, where No is the number of users at the highest priority and L is the time taken to transmit the maximum length packet.

The Packet Prioritisation Mechanism relies on modifying arming delays observed by the Network Access Protocol in accordance with the users' priority so that the network access protocol allows the higher priority users access in preference to lower priority users if all users have data for transmission at the same time. Thus, the sets of arming delays for all NICs carrying packets at a given priority level are the same but shorter than for NICs carrying packets at the next lower priority level.

As set out in Figures 7a and 7b, within each priority level there are two arming conditions which ensure that a successful NIC cannot retransmit until all other NICs with packets for transmission at the same, or higher, level which wish to transmit have done so. The arming delays at a NIC following successful packet transmission are longer than those following unsuccessful transmission and all are based on the length of quiet period detected on the network following transmission of a packet from any source. The quiet period is measured as multiples of the network round trip time (RT). Figure 7a shows the arming conditions implemented by the network access protocol for packet prioritisation where there is no acknowledgement and Figure 7b shows these conditions where acknowledgements are included.

In conclusion, the network access protocol described above operates at each NIC and in its basic form guarantees access to all users within a known time. This concept can be extended to provide packet acknowledgement and packet prioritisation mechanisms which can be used in conjunction with the network access protocol either individually or jointly.

Claims (19)

1. A network comprising at least two users each capable of transmitting and receiving data via the network and access control means for controlling access to the network by the users such that on successfully accessing the network each user is prevented from further access thereto until the network has been silent for a preset arming delay.

2. A network according to Claim 1, configured as a broadcast tree network comprising at least one hub, a plurality of network interface means for connecting the users to the network, and bidirectional data transfer links interconnecting said at least one hub and said network interface means, wherein each of said network interface means includes access control means.

3. A network according to Claim 1 or Claim 2, including at least one master or root hub and arranged such that data introduced into the network following a successful access to the network is transmitted along the network to at least one master hub and thence transmitted throughout the whole network.

4. A network according to Claim 3, wherein a successful access by a user transmitting data is determined on the basis of whether that user receives an echo of the data introduced into the network.

5. A network according to Claim 4, including means for detecting the arrival at a user of an echo of a signal transmitted by said user and for generating a network failure alarm if said echo is not detected within a preset echo time limit.

6. A network according to Claim 3,4 or 5, wherein said preset arming delay is set on the basis of the maximum round trip time (RT) for the network.

7. A network according to any preceding claim including means for monitoring access by said users and for signalling an access failure alarm if a user attempting to gain access is not successful within a preset access time limit.

8. A network according to Claim 7, wherein the access time limit is set on the basis of the number of users (N), the maximum time to transmit a length packet (L) and the maximum round trip time (RT).

9. A network according to any preceding claim, wherein a user receiving data returns an acknowledgement to the user sending the data.

10. A network according to Claim 9, wherein a user receiving data transmits an acknowledgement before any other data is transmitted in the network.

11. A network according to Claim 9 or 10, wherein each user is prevented from accessing the network until the network has been silent for at least a preset acknowledgement period.

12. A network according to Claim 11, including means for generating an acknowledgement failure alarm if the user which transmitted the data has not received an acknowledgement within said preset acknowledgement period.

13. A network according to any one of Claims 9 to 12, including means for monitoring the acknowledgement received by the user which transmitted the data and for generating a data/network failure alarm if the acknowledgement is incorrect.

14. A network according to any of Claims 9 to 13, wherein said acknowledgement is syntactic.

15. A network comprising at least two users each capable of transmitting and receiving data via the network and access control means for controlling access by the users so that each user may only attempt to access the network if one of the following "arming" conditions has been satisfied: (a) if the previous attempt to access the network by said user was unsuccessful, the network has been silent for at least a first preset arming period; (b) if the previous attempt to access the network was successful, the network has been silent for at least a second preset arming period.

16. A network according to Claim 15, including means for setting for each user the lengths of said first and second preset arming periods.

for setting for each user the lengths of said first and second preset arming periods.

17. A network according to Claim 16, wherein said users are assigned priorities and the arming periods for a higher priority user are shorter than those for a lower priority user.

18. A network comprising at least two users wherein a user receiving data returns an acknowledgement to the user sending the data.

19. A network substantially as hereinbefore described with reference to and as illustrated in any of Figures 2 to 7 inclusive.