NTP
                           NTP 297-1001-100                  PAGE 12
                          PREL., ISSUE 01D03
                               79 09 21



  4.   MESSAGE SYSTEM


  PURPOSE

  4.01   The Message System (MS) provides the media and protocol for
  the transmission of inter-module control messages.


  TRANSMISSION MEDIA

  4.02    The MS  consists of  hardware units,  within each  module,
  which are inter-connected via the  message channels in the message
  and speech  links.These units,  at the  terminating points  of the
  message  channels between  the  modules,  have the  capability  of
  accepting and/or originating  messages, and thus control  the flow
  of messages throughout the MS.


  MESSAGE PROTOCOL

  4.03    Control messages  contain a  fixed-length header  section,
  followed by  a data section of  variable length, depending  on the
  complexity of the message.   Message transmission uses "handshake"
  protocol (two-way sequences).  No message sequence is sent without
  receiving an acknowledgement from the receiver that it is ready to
  receive,   and   no   sequence  is   completed   without   another
  acknowledgement  after   transmission  that   the  reception   was
  error-free.


  OPERATION

  4.04   Via the message system, the  CC controls the logic of calls
  and  directs  the action  of  NM,  PM  and  IOC.  The  DMS  System
  structure is  based on  distributed processing.   Microprocessors,
  located  in the  NM, PM,  and IOC,  relieve the  CC of  repetitive
  real-time  consuming   functions  such   as  scanning   and  digit
  collection.

  4.05   The two  CPU are linked to the  Central Message Controllers
  via the  data port bus  (which also gives  each CPU access  to its
  private Data  Store).  CMC connect  to NMC  and IOC via  2.56 Mb/s
  serial data channels  on the message links.   NMC communicate with
  PM via  dedicated signalling  channels on the  speech links  at 64
  Kb/s per channel.

  4.06   The  CPU contains firmware  which controls the  movement of
  messages over  the data port  bus between  the CMC and  the active
  CPU.  The CMC scan Network Message Controllers and I/O Controllers
  for incoming messages to the CPU and direct outgoing messages from
  the CPU to the appropriate controller.




                  DMS-100 FAMILY SYSTEM DESCRIPTION
                  (c) NORTHERN TELECOM LIMITED, 1978


                           NTP 297-1001-100                  PAGE 13
                          PREL., ISSUE 01D03
                               79 09 21



  4.07   Network Message Controllers (NMC)  scan the PM for incoming
  messages  which they  pass  on to  the  CMC  and receive  outgoing
  messages  from  CMC for  transfer  to  the  PM.  The  IOC  provide
  interfaces between CMC and visual display units, tape units, etc.

  4.08   Peripheral Modules  have message-handling hardware/firmware
  to transfer  messages to the NMC  and to accept CPU  messages from
  the NMC.

  4.09   All elements of  the Message System collection/distribution
  system are duplicated.  Links also exist  so that the failure of a
  single   unit  does   not  force   other   units  into   'simplex'
  (non-duplicated)  operation.   The  path taken  by  a  message  is
  governed by  the CPU master  routing algorithm which  ensures that
  all paths are used. In this way, failures are quickly detected and
  the routing modified to bypass the  faulty unit or link.  Messages
  between the CPU and PM normally  follow only one path per message.
  However, because the duplicate networks  operate in step, messages
  originating  or  terminating  in one  (e.g.:  Plane-0)  NMC,  also
  involve the other NMC associated with Plane 1.




































                  DMS-100 FAMILY SYSTEM DESCRIPTION
                  (c) NORTHERN TELECOM LIMITED, 1978