Abstract:
A master-slave communication system for a communication switch is provided. The system comprises a master controller generating commands and receiving status signals and slave devices associated with the master controller. Each slave receives commands, executes local commands responsive to the commands and generates status signals for the master controller. Each slave has a communication arrangement for signals transmitted between it and the master controller. The arrangement comprises a communication controller associated with the master controller. The communication controller receives commands, transmits commands to each slave, receives status signals and provides information relating to the status signals to the master controller; also the controller has a communication link which transmits commands to each slave and the status signals to the controller. The system allows local commands executed by the slaves to replace other commands directed by the master controller to the slave. Further, each slave communicates independently with the master controller.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a system and method providing a master-slave communication system for a network element of a communication network.  
         BACKGROUND OF INVENTION  
         [0002]    Many communication switch and router systems architecture enable a service to be selected from a plurality of sources utilizing a master-slave arrangement of a master controller providing resources to, or access by, one of a plurality of slave devices. However, prior art systems lack a mechanism to provide a guaranteed bandwidth of access for each slave device to the master unit where there is significant amount of communication sent between the two entities in the switch. As such, in communication systems, for example, prior art master-slave systems, cannot provide maximum latency guarantees for transmissions therethrough.  
           [0003]    There is a need for a system and method providing minimum bandwidth access for master-slave systems that improves upon prior art systems.  
         SUMMARY OF INVENTION  
         [0004]    In a first aspect, a master-slave communication system for a communication switch is provided. The master-slave system comprises a master controller which generates commands and receives status signals and slave devices associated with the master controller. Each slave device receives the commands, executes local commands responsive to the commands and generates the status signals for the master controller. For each slave device, a communication arrangement for signals transmitted between the master controller and the each slave device is provided. It comprises a communication controller associated with the master controller. The communication controller receives commands, transmits the commands to each slave device, receives the status signals and provides information relating to the status signals to the master controller. The communication controller also has a communication link which transmits the commands to each slave device and the status signals to the communication controller. The master-slave communication system allows local commands executed by the slave devices to replace other commands directed by the master controller to the slave devices. Further, each slave device communicates independently with the master controller.  
           [0005]    The system may comprise a timing arrangement controlling transmission times of the signals.  
           [0006]    The system may have the timing arrangement utilizing a time division multiplex scheme.  
           [0007]    The system may have the communication arrangement providing a downstream communication link comprising a multiplexed signal gathering communications from each communication controller into a single multiplexed stream and a demultiplexed signals split from the single multiplexed stream where the signals are provided to each slave device.  
           [0008]    The system may have the communication arrangement providing an upstream communication link comprising a multiplexed signal gathering communications from each slave device into a second single multiplexed stream and a second demultiplexed signal split from the second single multiplexed stream which is provided to each communication controller.  
           [0009]    The system may have the slave devices each locatable on a separate shelf from the master controller.  
           [0010]    The system may have the master controller associated with a control card for the communication switch. The system may have at least one of the slave devices as a fabric interface card. Alternatively, the system may have at least one of the slave devices as a line card.  
           [0011]    The system may synchronize communications carried on the downstream communication link and the upstream communication link.  
           [0012]    In a second embodiment, a master-slave control system for a communication switch is provided. The system comprises a master controller operable to generate commands for controlling at least one slave device, communications controllers associated with the master controller, a time division multiplexer (TDM) coupled to each communications controller, a time division demultiplexer coupled to the time division multiplexer by a serial link and slave devices coupled to the time division demultiplexer. Each communication controller corresponds to a respective slave device and can send commands thereto according to a predetermined protocol. The multiplexer can form a TDM stream from the commands. The demultiplexer can receive the TDM stream and send commands from a communications controller. Each slave device can receive commands according to the predetermined protocol and respond to the commands.  
           [0013]    In a third embodiment, a master-slave control system for a communication switch is provided. It comprises a master controller which generates commands for controlling at least one slave device and a communication link associated with the communication controller and the slave device. The slave device can respond to the commands. The slave device has a communication controller which receives the commands from the master controller and generates a message embodying the command for transmission to the slave device. The communication link receives the message from the communication controller and transmits the message to the slave device.  
           [0014]    The system may have the communication link comprising a TDM arrangement associated with the communication controller. The TDM arrangement forms a TDM stream from the commands for a serial link, the TDM stream has a time slot assigned to a communication pair comprising the communication control and the slave device. The TDM arrangement also has a TDM demultiplexer associated with the serial link and the slave device. The TDM demultiplexer receives the TDM stream, extracts message from the stream and transmits the message to the slave device.  
           [0015]    The system may have a second communications link between the slave device and the communication controller which transmits data from the slave device to the communication controller. The communication controller receives the transmit data. The master controller may receive the transmit data from the communication controller.  
           [0016]    In other aspects of the invention, various combinations and subset of the above aspects are provided. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The foregoing and other aspects of the invention will become more apparent from the following description of specific embodiments thereof and the accompanying drawings which illustrate, by way of example only, the principles of the invention. In the drawings, where like elements feature like reference numerals (and wherein individual elements bear unique alphabetical suffixes):  
         [0018]    [0018]FIG. 1 is a block diagram of elements of a switch of an embodiment of the invention;  
         [0019]    [0019]FIG. 2 is a block diagram of components and connections of the switch of FIG. 1;  
         [0020]    [0020]FIG. 3 is a block diagram of midplane connection of the switch of FIG. 1;  
         [0021]    [0021]FIG. 4A is a block diagram of a controller unit and shelf units of a further embodiment of the switch of FIG. 1;  
         [0022]    [0022]FIG. 4B is a block diagram of a controller unit and shelf units of a further embodiment the switch of FIG. 1;  
         [0023]    [0023]FIG. 5 is a block diagram of a cabling and interface arrangement for the controller and shelf units of the switch of FIG. 4B;  
         [0024]    [0024]FIG. 6 is a timing diagram of time slots for the communication protocol used between the controller and shelf units of the switch of FIG. 4B; and  
         [0025]    [0025]FIG. 7 is a block diagram of multiplexing system for ingress transmissions associated with the switch of FIG. 4B. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0026]    The description which follows, and the embodiments described therein, are provided by way of illustration of an example, or examples, of particular embodiments of the principles of the present invention. These examples are provided for the purposes of explanation, and not limitation, of those principles and of the invention. In the description which follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals.  
         [0027]    Basic Features of System  
         [0028]    The following is a description of a system associated with the embodiment. Briefly, the system provides a master-slave arrangement of devices in a communication switch where a controller is provided as the master controller and a plurality of devices are the slave devices.  
         [0029]    Referring to FIG. 1, switch  100  is a multi-protocol backbone system, which can process both of ATM cells IP traffic through its same switching fabric for customer premise equipment (CPE)  102  connected thereto. Through a plurality of cards and processing modules, switch  100  provides CPEs  102  with access to its switching fabric  104  which is the core of switch  100 . The switching fabric  104  provides a matrix allowing each CPE  102  to be connected to other devices connected to the switch  100 . In the present embodiment, switch  100  allows scaling of the switching fabric capacity from 50 Gbps to 450 Gbps in increments of 14.4 Gbps by the insertion of additional shelves into the switch  100 .  
         [0030]    CPEs  102  are connected to switch  100  via optical links  106  to I/O cards  108 . I/O cards  108  provide the main input and output interface for conversion of communications between CPEs  102  and switch  100 . I/O cards  108  provide minimal intelligent processing of communications passed therethrough. I/O cards  108  are connected to line cards  110  via midplane connections  112 . Each line card  110  provides OC-192 functionality, bandwidth provisioning and ATM processing of cells between core of switch  100  and each CPE  102 . Each line card is also connected to a fabric interface card (FIC)  114 , which converts the signal to an optical signal and provides an interface for the communications with core  104 .  
         [0031]    Accordingly the FIC can monitor and react to conditions reported by the line card  110 . For example, the FIC  114  may analyze and respond to failures reported by its line card  110 , conduct sanity checks on data received from its line card  110  and send reporting messages to upstream shelf controller (described later).  
         [0032]    FICs  114  communicate with LPC  110  via midplane connections  116  and with core  104  via connections  118 . The interface to core  104  for each FIC  114  is a switch access card (SAC)  120 .  
         [0033]    For improved reliability switch  100  is designed as a redundant source system. Accordingly, each I/O card  108 , line card  110  and FIC  114  has a redundant counterpart, which is noted with the ‘b’ suffix. Accordingly, midplane connections  112  and  116  provide cross connections between the redundant and primary devices. For example, I/O cards  108  and  108   b  are connected to line cards  110  and  110   b  and line cards  110  and  110   b  are connected to FICs  114  and  114   b.    
         [0034]    To provide modular physical grouping of components, I/O card  108 , line card  110  and FIC card  114  are grouped together in a single high speed peripheral shelf (HSPS)  122 . Each HSPS  122  has two sets of I/O card groupings in slots  126  to provide redundancy between the groups of shelves. Switch  100  enables the use of multiple HSPSs  122  to provide enhanced expandability for the switch. Accordingly, with components grouped into shelves, a number of individual shelves can populate a switch  100  to provide modular functionality for switch  100 . However, the use of a modular system requires that control signals for each shelf are also provided in modules, as necessary. This entails separate cabling of bundled control signals to each shelf at a communications point on each shelf. From the communication point, individual signals for individual components in the shelf are isolated and forwarded accordingly.  
         [0035]    Each I/O card  108  grouping in HSPS  122  must be controlled and coordinated with the other I/O cards  108  in HSPS  122 . Accordingly the embodiment provides a shelf controller  124  which controls operating aspects of shelves  122  connected to it. Such control operations include managing control and status functions for the shelf (such as slot monitoring and fan unit control), controlling FIC configuration for each line card  108 , power rail monitoring and clock signal monitoring.  
         [0036]    Shelf controller  124  provides control connectivity via a specialized control service link (not shown). Data carried in the control service link controls downstream configuration and software downloading, time stamping, and synchronization of clocks.  
         [0037]    A terminal  128  is connected to switch  100  and runs controlling software which allows an operator to modify, and control the operation of, switch  100 .  
         [0038]    Referring to FIG. 2, switch  100  physically comprises a chassis  200 , which houses HSPS  122  in cavity  202 . HSPS  122  is contained in housing  204 , which sits in a section of cavity  202 . Shelf controller  124  is located above cavity  202 . Each housing  204  contains a midplane  206 , which is a physical support structure having connectors allowing line cards  110 , FICs  114  and I/O cards  108  to be connected thereto. Connections  112  and  116  (see, FIG. 1) are provided by appropriate electrical connections between connectors in midplane  206 .  
         [0039]    Referring to FIG. 3, view  300  illustrates line card  110 , I/O card  104 , and FIC card  108  and midplane  206  for housing  204 . Cards that have optical interfaces, namely the I/O card  104  and FIC card  108 , are located on one side of the midplane  206  and line card  110  is located on the other side of the midplane  206 . Connectors  208  provide the physical interface for the cards to midplane  206 . Specific connections between I/O card  104  and line card  110  and FIC card  108  are provided from the pins of various connectors  208  through midplane  206 .  
         [0040]    It will be appreciated that terms such as “routing switch”, “communication switch”, “communication device”, “switch”, “network element” and other terms known in the art may be used to describe switch  100 . Further, while the embodiment is described for switch  100 , it will be appreciated that the system and method described herein may be adapted to any switching system.  
         [0041]    Referring to FIG. 1, with a large number of I/O cards  108 , there is a need to have a mechanism for providing instructions from the shelf controller  124  to each line card  110 . Traditionally, either the remote line card was dumb, having no processing capabilities, e.g. a typical I/O card, or alternatively, all of the intelligence was placed on the line card, e.g. a typical line card or a FIC. However, by migrating the intelligence of the processing from either fully on the card or filly off the card, the computing power required at the processing end becomes too large for the processing entity.  
         [0042]    Accordingly, the embodiment utilizes a system wherein computing is distributed between the FIC  114  and the shelf controller  124 . At a broad level, the shelf controller  124  identifies what actions need to be taken by a FIC  114  and sends an appropriate instruction to the FIC  114 . Each FIC  114  receives and processes its instruction and provides a suitable response to the shelf controller  124 . In this view, the “master” element is the operative element in the shelf controller  124  and the “slave” element is the FIC  114 . The term “master” is used interchangeably with “shelf controller” and the terms “slave” and “FIC” are also interchangeable for this specification. It will be appreciated that in other embodiments, the slave may be line card  110  or any other downstream device to the master.  
         [0043]    Referring to FIGS. 4A, 4B and  5 , the embodiment provides an egress communication system  400  for each HSPS  122  and the shelf controller  124 . In the shelf controller  124  master controller  402  produces individual commands for each FIC  114  in each subshelf  122 . Communication controllers  404  in shelf controller  124  receive each command for each FIC, or slave  114  and has them sent to each slave  114 . Each HDLC communication controller  404  communicates with the FIC cards in slave  114  to request read/write access to FIC registers (not shown). For example, on a “read” command, master controller  402  may require status data about slave device  114   a . In the distributed system, master controller  402  generates a read command for a particular flag of slave device  114   a . Communication controller  404   a  receives the command from master controller  402  and has the command sent, ultimately, to slave device  114   a , which receives the read command and processes it. After the read command is processed by slave device  114   a , a response is generated and is sent back to master controller  402  through an ingress communication system  500 , which provides an ingress communication link from each slave device  114   a  to controller  404   a.    
         [0044]    Each controller  402  uses HDLC (High Level Data Link Control) protocol. HDLC is a known ISO and ITU-T standaridized link layer protocol used in point-to-point and multi-point communications. HDLC provides bit-oriented synchronous transmission of variable length frames. In the embodiment, master  124  has unbalanced links with slaves  114 . Accordingly, master  124  polls each slave  114  as necessary, and each polled slave  124  responds with information frames. The master  124  then acknowledges receipt of the frames from the slave. It will be appreciated that other communication protocols may be used. It will be appreciated that as there is a dedicated master for each slave, collectively, polling amongst all slaves can be done concurrently.  
         [0045]    Shown below is an HDLC frame used in the embodiment by the egress system of FIG. 4A.  
                                               Start   HDLC           End       Flag   Cntrl   Data Field   CRC   Flag                   8   8   X   16   8 bits                  
 
         [0046]    The field length (in bits) is variable, depending on the HDLC control field. As an example, master  404  may request to a slave  114  to respond with a report of the status of all interrupts on slave card  114 . Accordingly, the slave  114  would read all its registers that contain an interrupt status. An interrupt status may, for example, store the change of state information of an optical signal received by a pin diode. The slave  114  collects the register information and transmits it to master  402  per the designed communication protocol. It will be appreciated that this distributed messaging system overall provides a faster response time than have a master communicate with each slave device individually to and read their register status. Further, as each slave  114  only has knowledge of its local status, the master can collect all slave  114  information, then provide a response based on the net status of all slave registers. Referring to the earlier example of a read cycle, in the embodiment when master controller  402  requires data from a particular slave  114   a , the control field is set to 00000000 by software in master controller  402  and the data field is defined as 32 bits containing an embedded 16 bit slave address as shown below:  
                                             Data field Structure            Read/   Address   Data       Write   Bus   Bus               1   15   16                  
 
         [0047]    Referring to FIG. 4A, in one embodiment, it will be appreciated that for the master-slave system, it is possible to have a communication system where each communication controller  404  is individually hardwired to each slave  114  with links  405 . In another embodiment, in order to reduce the number of physical communication links between the communication controllers  402  and the slaves  114 , multiplexing of signalling links is provided on both the ingress and egress directions. This is shown in FIG. 4B.  
         [0048]    Accordingly, referring to FIGS. 4B and 5, for multiplexing signals, each communications controller  404  receives instructions from master controller  402 ; each HDLC controller  404  is connected to multiplexer  406 , producing one serial stream of data containing N channels of data on serial link  408 .  
         [0049]    Each communication controller  404  and master controller  402  is contained within a microprocessor  420 . In the embodiment, microprocessor  420  is a MPC 8260 Power PC PowerQUICC II programmable processor, available from Motorola, Inc. Microprocessor  420  has a programmable multichannel controller (MCC). The embodiment configures the MCC to provide the 16 communication controllers  404 . Microprocessor  420  also has an internal multiplexer  406  to produce single datastream  408  from the datastreams produced by the communication controllers  404 . Also, microprocessor  420  has a time slot assignor  421  which assigns a 8-bit timeslot from the TDM stream  408  to each of the controllers  404 . The TDM stream contains sixteen 8 bits slots and operates at 8.25 MHz. Accordingly, the TDM stream in link  408  comprises  16  serial packets as shown below:  
                                                           Ch 0   Ch 1       Ch 16                           HDLC 1   HDLC 2   . . .    HDLC 16                      
 
         [0050]    It is desirable to have the HDLC timeslot at a minimum length (and thus the TDM stream at a minimum length) to decrease the latency on time-sensitive information in the TDM stream (such as interrupt status).  
         [0051]    Serial link  408  is provided to a group demultiplexer  410  which collectively groups the N channels into M channels  412 . The demultiplexer  410  is embodied in a field programmable gate array (FPGA)  410 .  
         [0052]    Control for demultiplexer  410  is fixed and the demultiplexing does not change on different conditions. As will be further described later, a bit counter signal and a channel counter signal are associated with the TDM stream. The bit counter signal and the channel counter signal are used by demultiplexer  410  to identify which bits from controllers  404  (or which bits from registers within FPGA  410 ) are inserted into which channel  412  at the correct frame.  
         [0053]    The FPGA  410  provides the following functions for microprocessor  406 . First, the TDM stream  408  between the microprocessor  420  and FPGA  410  contains HDLC interfaces for FIC communications. The FPGA splits out TDM stream  408  into individual MTDM streams  412  for each of the HSPS sub-shelves  122 . Control signals are embedded into the TDM stream  408  by FPGA  410 . Second, control signals for a FIC, such as Line Card Presence, sub-shelf Number, FIC Interrupt Status, etc. may be transmitted between microprocessor  420  and slave  110  using the signal multiplexing scheme and FPGA  410 . Microprocessor  420  provides a request for control signals for a FIC to FPGA  410  sent via  60   x  bus  422 . FPGA  410  inserts an appropriate request in the appropriate timeslot for the requested slave  114  in the appropriate egress datastream  412 . The targetted slave responds to the request and transmits the status to FPGA  410  via the ingress multiplexed stream. The results are stored in FPGA registers, which can be accessed by microprocessor  420  over bus  422 . Also, FPGA  420  may send a (maskable) interrupt to microprocessor  420  upon a status change of a control signal. Third, FPGA  410  also performs a digital phase comparisons of the selected sources of timing from the shelf  124  and compares it with the system source sent to the shelf.  
         [0054]    From the FPGA  410 , four TDM streams  412  connect the shelf controller to each of the four subshelves. In the embodiment, the second TDM stream has sixteen timeslots operating at 8.25 MHz for each subshelf  122 . Each M channel  412  is provided to each subself  122 . Each of the four TDM substreams  412  (one to each sub-shelf) is a 16 timeslot frame operating at 8.25 MHz.  
         [0055]    Similar to demultiplexer  410 , TDM demultiplexer  414  utilizes the bit counter signal and the channel counter signal to determine which incoming part of the datastream on channel  412  is sent on which outgoing channel  416 .  
         [0056]    In each subshelf  122 , demultiplexer  414  receives each channel  412  and produces N/M separate communication links  416 , each of which is provided to each slave  114 . Each slave device  114  has a HDLC interface module  418  which translates the HDLC encoded datastream  416  into a format which can be used by each slave  114 . Each communication controller  404  has a timeslot in the TDM stream assigned to it. Similarly, each slave device  114  has a timeslot assigned to it for sending information to the master controller. Also, slave devices  114  can interrupt the master controller  124  at any time, if required.  
         [0057]    Having a dedicated communications controller  124  and corresponding control bandwidth for each slave device  114  ensures that control commands from the master controller  402  will be received by the slave devices  114  within a deterministic amount of time.  
         [0058]    Referring to FIG. 7, for multiplexing signals in the ingress direction, system  700  is shown. Therein, each slave  114  generates a response or a signal destined for master controller  402 ; each slave  114  is to multiplexer  702 , producing one serial stream of data containing N/M channels of data on serial link  704 .  
         [0059]    Serial link  704  is provided to FPGA  410  which processes the information in the N/M channels  704  and provides an appropriate response, if necessary to master controller  402  via  60   x  bus  422 .  
         [0060]    Since each slave device  114  has its own timeslot during which it can communicate with the controller  402 , information from the slave devices  110  will reach the master controller  402  within a defined amount of time. This allows bidirectional communications between the slave devices and the master controller to occur within a guaranteed latency. Accordingly, the embodiment allows a multishelf platform to detect a fault within  10  ms re-route around the fault within 50 ms, thereby conforming with requirements of a carrier-grade system.  
         [0061]    It will be appreciated that ingress multiplexing system  700  shares functional similarities with egress system  400 . However, in addition, line cards  110  and I/O cards  108  generate some status signals as dc signals (not shown) which are provided to their CPLD  702 . Each CPLD may embed these signals into the datastreams of its respective channel  704 . At FPGA  706 , these embedded signals may be extracted and processed locally as needed. For example, they may be provided to other cards and systems associated with the FPGA  706 .  
         [0062]    In the embodiment, an ingress signalling system is also provided, which is similar to egress system  400 , and is described later.  
         [0063]    Referring to FIG. 6, each TDM bus is configured according to the following timing parameters. Each multiplexer has access to these timing signals. A common clock  602  operates at 8.25 MHz and a frame pulse (FP)  604  operates at 64.45 KHz. The rising edge of FP  604  is aligned to the rising edge of clock  602 . The FP defines a frame for a byte of transmitted information.  
         [0064]    Within each frame pulse, there are  16  timeslots, one slot for each slave device. The current timeslot number in the TDM stream is indicated by timeslot signal  608 . In order to provide the system with an earlier indication of the arrival of the next timeslot, timeslot count signal  608  in generated which is the same count signal as timeslot signal  606 , but it is generated half a clock cycle earlier.  
         [0065]    Within each timeslot there are eight bit positions. The current bit position is indicated by bit position signal  610 . As with the timeslot signal  606 , as a mate to bit position signal  610 , bit  10  position count signal  612  is generated to provide the system with an earlier indication of the arrival of the next bit position.  
         [0066]    These signals are generated by the FPGA  410  (not shown). The first bit of the first timeslot (bit  7  of timeslot  0 ) is the MSB and will be coincident with the rising edge of FP  406 . As there are 8 bits of data per timeslot, for data transactions involving data fields of more than 8 bits requires more than 1 TDM slot. Successive required slots are provided in the next TDM superframe.  
         [0067]    Also, the timing of signals sent between shelf controller  124  to each of subshelf  104  requires that no cells be dropped. Timing is handled in the following manner.  
         [0068]    Referring to FIG. 4, for each controller  404 , each HDLC stream is transmitted at a clocking rate of 8.25/16 MHz, i.e. approximately 516 kHz, (or “R” for “Rate”) to multiplexer  406 . Once all of the 16 TDM streams are combined into a single TDM stream at multiplexer  406 , the collective datastream is clocked at  16   x  R on serial link  408  to ensure that successive packets from each controller  404  in successive frames are not lost. The collective datastream on link  408  is provided to FPGA  410  which splits datastream into four separate datastreams on channels  412 . Each separate datastream on each channel  412  contains datastreams for 4 HDLC slots destined for demultiplexers  414  associated with each subshelf  122 . The clocking rate for each datastream on each channel  412  is still  16 R. Accordingly, there is additional bandwidth available in each datastream in each channel  412 , as only four slots are needed in the time frame which contains 16 time slots. Accordingly,  12  control slots are added to each datastream in each channel  412  by FPGA  410 . The control slots contain information embedded into them by FPGA  410 .  
         [0069]    From each demultiplexer  414 , each datastream is then passed to a CPLD within demultiplexer  414 , which can extract some of the control information from the datastream for the FIC  114  or line card  110 . The CPLD is located on midplane  206 . The CPLD  414  further splits the datastream into four sub datastreams on channels  416 , 1 channel  416  per slave device  114 . At each slave device  114 , a second CPLD (#2) can extract further control information from the received datastream. The received HDLC datastream is then clocked-down to the original clocking rate of 8.25 MHz/16, i.e. approximately 516 kHz (R). The clocked-down data for data transmissions received by a slave device  114  contains the original information embedded in the TDM stream from its corresponding controller  404   a.    
         [0070]    It will be appreciated that in the above timing arrangement, timing is maintained for the data rate and additional control information is provided in each datastream without occupying “true” bandwidth from the master-slave communication link.  
         [0071]    Following is an example of latency aspects of the system. In the embodiment there are 16 timeslots in the TDM stream  408 , which is clocked at 8.25 MHz. Accordingly it takes 15.5 us to transmit the whole TDM stream  408 . An average read or write cycle for microprocessor  420  on the FIC is 200 ns (4-clock cycle access at 20 MHz). When the FIC microprocessor gets a local interrupt it performs  11  reads (in the worst case) to determine the source (1 interrupt cause register, then 10 registers). Accordingly the processing time is:  
         11×200  ns= 2.2  us    
         [0072]    The microprocessor must also write the contents of these 10 registers into the HDLC FIFOs, thereby requiring  
         10×200  ns= 2  us    
         [0073]    For a worst-case scenario of a 120-bit HDLC frame, there are 120 bits required for the HDLC frame (see frame below) and there are 8 bits of the HDLC frame transmitted each TDM stream, it takes 15 TDM streams to transport this HDLC frame back to the microprocessor  420 , i.e. 15×15.5 us=232.5 us.  
         [0074]    If a factor for receiver latency of 2 TDM frames is 2×15.5=31 us, it takes 2.2+2+232.5+31=267.7 us.  
         [0075]    As noted earlier, each HDLC link is dedicated, so if all 16 FIC  114  were reporting to their respective masters  404 , the total maximum service time is still 267.7 us.  
         [0076]    It is noted that those skilled in the art will appreciate that various modifications of detail may be made to the present embodiment, all of which would come within the scope of the invention.