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TUCoPS :: Phreaking Technical System Info :: axe10sss.txt

The AXE 10 Subscriber Switching Subsystem





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 //                The AXE 10 Subscriber Switching Subsystem              //
 //     Excerpted from Introduction to Digital Communications Switching   //
 //               Expunged into Digital Form by Keltic Phr0st             //
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 (I've often been told it was lame to type up files of already present info.
  In which case, fuck off - how is anyone supposed to learn, and how is the
  info supposed to spread??? And how do you know its accurate??? Anyway -
  hope you enjoy this release - There's Plenty more to come)

 The AXE System will serve well as an introduction to the functions of the
 subscriber's concentration stage of a PCM Local Exchange. Figure 5.20
 illustrated that, as analog-to-digital conversion is moved to the periphery
 of the exchange, many other functions are moved to the line interface also.

 The resulting line interface is shown in functional terms in fig 5.21.
 These functions will be discussed more fully in Chapter 8; It suffices now
 to indicate that the elements of the mnemonic BORSCHT are evident in fig
 5.21:

        B Battery Feed
        O OverVoltage protection
        R Ringing
        S Supervision (Detection of seizure and release) or Supervision
          and Signalling
        C Coding (Omitted within analogue enviroment - KpT)
        H Hybrid or 2-wire to 4-wire conversion
        T Test

 The principles of subscriber switching in the AXE system are illustrated
 in fig 5.22. Subscriber modules of 2048 subscribers are divided into sub
 modules of 128 subscribers, each with its own time switch, key sender
 receivers, test circuits and an optional 2Mb/s link to the group switching
 stage. All 16 sub modules have access to a common time switch bus haing
 512 Channels. Thus, connection to the group stage is effected via a channel
 of the sub-module's 2Mb/s link or, if all these are busy or the link is not
 fitted, via one of the 512 Channels linking to other sub-modules over the
 time switch bus. There is thus a time division bus interconnecting the lines
 within each sub-module called the device speech bus (DEVSB) and a single
 512 channel bus interconnecting all 16 modules called the Time Switch Bus
 (TSB). The speech store in each time switch therefore has 768 storage
 locations.  This is more adventurous than the devices described earlier
 in this chapter.

        There is capacity, therefore, to provide a normal minimum
 concentration of 4 to 1 (2048 subscriber's lines to sixteen 32-Channel links
 to the group stage). Only sufficient group stage links will be provided
 to carry the traffic offered by subscribers on the particular exchange and,
 because of the time switch bus (TSB) Link, traffic from all subscribers has
 full availability to al channels of all the links fitted.

        Figure 5.23 shows a block diagram of the time switch in concept only,
 illustrating the relationship of channel capacities of the various inlets
 to DEVSB and TSB. At any time slot, the time switch will read out a sample
 from the speech store on to the DEVSB destined for a subscriber or a
 channel to the group switch or a VF receiver or to line test and at the
 same time it will read a sample on to the the TSB Bus detsined for the
 group switch or for a subscriber via another subscriber stage time switch.
 The AXE10 subscriber's stage time switch is therefore performing space
 switching by performing more than one time-slot interchange at the same
 channel time-slot.

        The Control memory contains the data on the destination of the
 sample stored in the speech memory. This data is loaded by the device
 processor as a result of instructions received on the DEVSB Bus. Note
 that the speech memory is loaded at each channel time by a sample from
 DEVSB and a sample from TSB.

        The speech memory capacity includes space for two 32-Channel Links
 to the group switch, JTC, although the normal maximum is one JTC per LSM.
 There is also spare capacity for more than the normal 8 VF receivers
 the normal 4 Line test facilities.

        The diagram of the line interface (fig 5.21) Indicates that there
 two further time division buses involved; the control bus DEVCB, and the
 test Bus TEST B. The Control Bus provides acces from the regional processor
 of the line module EMRP to the device controllers of the constituent
 elements of the line module. The Control structure of the line module is
 illustrated in fig 5.24.

        The provision of solid state electronic switching right out to the
 subscriber's line interface, partcularly in TDM form, makes physical access
 to the subscriber's line for testing purposes impossible. Line and Trunk
 testing is a subject that will be returned to in Chapter 8. At this stage
 it is neccessary to point to the need to provide other means, in TDM
 exchanges, to gain access to the lines. The access method employed has
 become almost classical in its universal adoption. Figure 5.25 illustrates
 the contents of the LIC block labelled Test Access in fig 5.21. Test access
 relays are provided which normally (when released) provide a through
 connection from the line to the line interface. Operation of one test access
 relay connects the line to a test bus for outward testing, whereas operation
 of the other relay provides access from the test bus to the line interface
 for inwards testing of the interface and other exchange functions. In some
 systems (notable ITT System 12) the relays are also used to switch in a
 spare line circuit in place of a faulty line interface.

        The interface arrangements with these three bus systems, DEVSB,
 DEVCB and TEST B, are shown in fig 5.26. This introduces a point, expanded
 in the next chapter, that time division relates, not only to the circuit
 switching function, but also to the control functions of the telephone
 exchange.


 REMOTE CONCENTRATORS
 ====================

        The migration of the bulk of the cost of a telephone exchange to the
 periphery, caused, in part, by PCM TDM and illustrated in fig 5.20, has
 been encouraged to proceed still further. One aspect of this is the advent
 of ISDN where the migration reaches the subscriber's instrument which
 becomes a communications terminal and work-station as a result. Another,
 less drastic aspect is the attractiveness of locating the subscriber's
 concentration stage remote from the telephone exchange. There was always
 a requirement for remote concentrators and suitable space division
 equipments were designed, mainly for rural application. The costs involved
 never justified such concentrators in more urban locations since a reduction
 in the local cable of between 80% and 90% (One link to the exchange for
 every 10-20 subscribers instead of one per subscriber) could not be made
 to pay for the cost of the remote switch plus its secured power supply
 and ancilliaries. Added to this was the extra maintenance effort involved
 in routine maintenance of remote electro-mechanical devices.

        With PCM TDM, the reduction in links to the main exchange is much
 greater (two channels for every 10 to 20 subscribers), the power requirement
 is lower (although seldom low enough for power to be supplied down the line
 from the parent exchange), and the necessarily complicated signalling can be
 accomodated on common channel signalling links. Almost all practical PCM
 local switching systems therefore include remote multiplexor, remote
 concentrator, and possibly, remote simplified exchange options. Because
 of TDM, these options can also accomodate diversity of routings to the
 parent exchange, or even exchanges, thus overcoming the other disadvantage
 of concentrators that service is lost to large numbers of providers should
 the exchange link be broken. At the least, concentrators with just one link
 to the parent continue to provide local service when that link is broken.
 Because the concentrator is cut off from the processing power of the parent
 exchange, this residual local service is probably not charged. This is small
 comfort, however, to the subscriber, accustomed to access to only a few
 hundreds of nearby subscribers.

        The AXE10 concentrtaor is similar to the subscribers switching stage
 just described except that the links to the group stage now become 30 channel
 CEPT Systems, and the control links from the regional processor to the
 central control are formed by using common channel signalling via time-slot
 16 of two of the 30 channel links to the parent exchange. This concentrator
 arrangement is shown as part of the diagram of a full AXE 10 local exchange
 in fig 5.27. The diagram illustrates that the only difference in the
 remotely located subscriber stage is the use of common channel signalling
 to communicate between the remote regional processors and the exchange
 central processor.


 CHANNEL MODULARITY AND SUBSCRIBER SIGNALLING
 ============================================

        In this Brief survey of a practical subscriber's switching system
 several concepts have been introduced without detailed discussion. One, which
 is evident from the diagrams, is that a modularity of 30 Channels has been
 increased to 32. Similarly, the diagrams reveal units devoted to signalling:
 KRD (MF Receivers) and ST-C and ST-R (Common channel signalling equipment).

 Channel Modularity
 ------------------

        In describing the 30-Channel system, channel 0 was identified as
 being reserved for synchronisation, frame alignment and other link related
 functions, and channel 16 for signalling. However, these functions, possibly
 required outside the exchange, are certainly not neccesary within the
 exchange. Most practical switching systems, therefore, utilise all 32
 channels for traffic carrying connections. Channel 16 is identified in the
 CEPT system as a signalling channel and a quartet system is defined for
 channel associated signalling. It is often appropriate, therefore, to
 utilise channel 16 as a first choice for common channel signalling in which
 case the quartet protocol is abandoned and the channel becomes a normal
 communications channel of 64 kbit/sec capacity. This is the arrangement
 chosen for the AXE10 system. Throughout the book there is a distinction
 drawn between channel 16 signalling (Channel associated in Quartets), and
 signalling over channel 16, normally probably common channel but on occaison
 the channel may be used for ordinary traffic carrying circuits. As an
 example, were the remote concentrator of fig 5.27 to require 3 or more "30"
 channel links to the parent exchange, then the remaining channel 16's could
 be devoted to ordinary traffic.

 Subscriber Signalling
 ---------------------

 Signalling from the subscriber to the exchange is normally loop seizure and
 release, with either loop-disconnect dial impulses or multi frequency key
 sender signalling for routing information. In either case the digital
 exchange must provide means for detecting the routing request information
 and passing it on to control. If loop disconnect is used then this can
 be detected, in the PCM line circuit scanning process, by (in the case of
 AXE 10) The LIC device controller and passed on to the regional processor.
 MF Signalling will pass through the line circuit and be PCM - encoded.
 There must, therefore, be a MF receiver associated with the line circuit
 when it is seized and a TDM connection completed to this receiver which
 detects the digitised MF information, translates it and passes the resulting
 numerical information to the regional processor.

 /////  /////   /////   /////   // Phr0st //   /////   /////   /////   /////



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