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//        Keltic Phr0st / Fit-Of-Boredom Productions Inc. Presents:        //
//                          AXE 10 : Architecture                          //
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//  An Overview of the AXE 10 System Architecture and functional Structure //
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 INTRODUCTION
 ============

 In march 1985, after a detailed evaluation of digital switching systems, BT
 placed a contract with Ericsson Limited for the development of AXE10 for the
 BT Network and for the supply of a quantity of exchanges.

        AXE10 is a duplicated-processor stored-program controlled digital
 switching system developed by Telefon AB LM Ericsson of Sweden. The system
 is structured for local, tandem, transit and combined exchange applications.
 It can also be configured as a cewllular mobile or as an international
 switching centre.

 SWITCHING ARCHITECTURE
 ======================

 The system architecture is logically divided into two main parts, the
 switching system (APT) and the control system (APZ). See Figure 1.

        The switching system (APT) performs traffic and operation/
 maintennance functions. It comprises four main hardware subsystems:

        Subscriber Switching SubSystem      (SSS)
        Group Switching SubSystem           (GSS)
        Trunk and Signalling SubSystem      (TSS)
        Common-Channel Signalling SubSystem (CCS)

 and the following software subsystems :

        Traffic Control SubSystems              (TCS)
        Charging SubSystem                      (CHS)
        Operation and Maintennance SubSystem    (OMS)
        Subscriber Services SubSystem           (SUS)
        Network Management SubSystem            (NMS)

 All Hardware subsystems have a hardware component.

        The control system (APZ) is made up of centralised and distributed
 logic achieved through the following subsystems:

        Central Processor SubSystem         (CPS)
        Regional Processor SubSystem        (RPS)
        Maintennance SubSystem              (MAS)
        Support Processor SubSystem         (SPS)
        Data Communication SubSystem        (DCS)
        File Management SubSystem           (FMS)
        Man-Machine Communication Subsystem (MCS)
 
 In Earlier Exchanges the input/output subsystem was used instead of SPS, FMS,
 MCS and DCS. See Figure 2 for the Subsystem structure.


 FUNCTIONAL STRUCTURE
 ====================
 The division of the systems (APT and APZ) into subsystems is determined by
 conditions and requirements that arise from features, traffic handling and
 operations and maintennance functions. See Figure 3.

        The subsystems, including their interfaces, are designed such that
 they can be used for different applications with the minimum of adaptation.
 Each subsystem is built from a number of function blocks which in themselves
 comprise hardware, central software, regional sofware and data components,
 or just central software and data components. Each function block is designed
 to execute a specific set of functions or sub-functions. A library of
 several hundred functional blocks exists to satisfy all applications and
 individual exchange requirements. Standardised interface signalling is
 extensively used between blocks not only to achieve the neccesary flexibility
 to satisfy customer's requirements, but to also enable new technology to be
 introduced within the system in an efficient manner. See Figure 4.

        To meet the twin needs of high capacity and low cost, the complex
 executove tasks neccesary to operate the system are executed by the
 duplicated central processor (CP) working in a paralell synchronous mode.
 The routine, high volume tasks, such as scanning operations and for the
 direct control of hardware, are distributed across a dimensionable number
 of small regional processors (RPs). Regional Processors controlling several
 hardware units are configured in a load sharing mode. The software driving
 the APT functions, therefore, has both APT and APZ elements.

 THE APT SUBSYSTEMS
 ==================

 Subscriber Switching SubSystem (SSS)
 ------------------------------------

 The subscriber switching subsystem contains the digital subscriber switch
 and is built up of 16 line switch modules (LSMs). An LSM serves 128 analogue
 subscribers or can support four 30-Channel systems for ISDN Customers.
 Its principal function is to supervise the state of connected subscriber
 lines, and to set up release connections by sending and receiving signals
 to and from subscribers. The SSS comprises both hardware and Software. To
 allow for the possibility of reducing line-plant costs, the local exchange
 may be engineered with distributed switching in the form of a remote
 subscribers switch (RSS).

 Trunk and Signalling SubSystem (TSS)
 ------------------------------------

 The trunk and signalling subsystem includes the equipment for connecting
 trunks to the group switch. It supervises the state of trunks to other
 exchanges by measn of the signals it receives and sends. The TSS consists
 of both hardware and software.

 Group Switching SubSystem (GSS)
 -------------------------------

 The group switching subsystem houses a time-space-time digital switch built
 up of duplicated time switch modules (TSMs) and duplicated space switch
 modules (SPMs). Its primary purpose is to set yp a 64Kbit/s path between
 SSS and TSS devices through the group switching network. It is also
 responsible for network synchronisation functions. The GSS comprises
 hardware and software.

 Traffic Control SubSystem (TCS)
 -------------------------------

 The Traffic Control SubSystem is wholly software, and controls and supervises
 the set-up and release of connections. The TCS stores and analyses digit
 information received from the SSS and TSS, and then, after checking against
 previously recorded information, for example, subscriber categories, routing
 and tariff classes, decides how the call should be handled.

 Charging SubSystem (CHS)
 ------------------------

 The Charging SubSystem consists of central software blocks and is responsible
 for the task of charging calls by means of very accurate pulse metering
 techniques. In addition to normal charging, special subscribers charging
 facilities are available such as itemisation and advice duration and charge
 (AD&C) information. The CHS also provides the administration with charging
 statistics and per-event charging information. It functions by monitoring and
 then analysing call information from the TCS or SUS (for service information).
 Output to the FMS follows execution of the charging function.

 Operation and Maintennance SubSystem (OMS)
 ------------------------------------------

 The Operation and maintennance SubSystem mainly comprises software blocks.
 Its function is one of system supervision, fault location, collection of
 statistics and for dealing with the administration's operation and
 maintennance functions. It can be operated either in the local or in remote
 mode, say from a remote operation and maintennance centre.

 Subscriber Services SubSystem (SUS)
 -----------------------------------
 The subscriber services system consists entirely of central software. Its
 function blocks provide a variety of subscriber services. Typical SUS
 services include:

        Abbreviated Dialling
        Three-Party Services (Enquiry, hold and transfer)
        Diverion (Immediate, On Busy and and on no-reply)
        Malicious Call Trace
        Call Barring
        Automatic Alarm Call

 Common Channel Signalling SubSystem (CCS)
 -----------------------------------------
 The Common-channel signalling subsystem implements the message transfer part
 (MTP) of CCITT Signalling Systems No. 6 and No. 7. The subsystem consists
 of hardware and software blocks, the hardware devices (signalling terminals)
 being connected to the external signalling links via a semi-permanent
 connection through the group switch.

 Network Management Subsystem (NMS)
 ----------------------------------
 The functions of the network management subsystem are implemented in
 software. The role of NMS is to provide access via the normal input/output
 devices for the administration to monitor continuously the state of the
 network.


 THE APZ SUBSYSTEMS
 ==================

 Central Processor SubSystem (CPS)
 ---------------------------------
 The Central Processor subsystem is realised in both hardware and software.
 The CPS executes the complex software tasks which are stored in the various
 APT blocks. The main hardware parts of the central processor, which is
 duplicated and runs in the synchronous mode, are the central processing unit
 (CPU) and the memory stores. These stores comprise the main store (MS) or
 program store (PS), reference store (RS) and the data store (DS).
        The purpose of the CPS is to execute the following functions:

 o Program Control including supervision of functions and for measuring
   processor load.

 o Loading and storage of tasks

 o Output and updating reloading information

 o Controlling fault tracing programs resident in the MAS or RPs.


 Regional Processor SubSystem (RPS)
 ----------------------------------
 The regional processor subsystem consists of both hardware and software
 blocks. The purpose of the RPS is to run the simple, routine and very
 frequent tasks to drive the RP part of the APT software and hardware. The
 number of functions performed by an RP pair depends upon the complexity
 of these functions. The number of RP pairs required for a given exchange
 depends upon its size and the complexity of its signalling systems.

 Maintennance SubSystem (MAS)
 ----------------------------
 The maintennance subsystem consists of both hardware and software. The major
 role is to supervise the operation of the APZ control system and takes the
 appropriate action should a malfunction occur.

 Support Processor SubSystem (SPS)
 ---------------------------------
 The support processor subsystem consists of one or more independent
 processors which drive input/output equipment such as personal computers,
 visual display units and disc drives which are connected to the subsystem.

 Data Communications System (DCS)
 --------------------------------
 The software for the data communications system resides in the SPS. It also
 has a hardware element to support protocols such as X.25. Its function is to
 support remote operation and maintennance, and transfer of itemised call
 accounting data.

 File Management SubSystem (FMS)
 -------------------------------
 The file management subsystem consists wholly of software. It provides
 storage in the event that a data link failure occurs.

 Man-Machine Communications SubSystem (MCS)
 ------------------------------------------
 The man-machine communications subsystem consists of software. It provides
 security checking and authorises I/O devices and the operator for a
 particular function. It routes data output from the exchange to the presel-
 -ected terminal(s). In addition, the MCS controls the generation of alarm
 printouts or displays.

 RELIABILITY
 ===========
 System Performance is determined by both hardware and software reliability.
 Hardware reliability is achieved by choice of components and by duplication
 of units which perform a task affecting a proportion of the exchange
 function.

        Software reliability exists through system recovery functions which
 rely on three restart levels namely:

 o Small Restart - The APZ clears all jobs in process of being established.
   The status of all existing calls is maintained.

 o Large Restart - If the smll restart is rapidly followed by a new software
   error then a more extensive restart involving a reset of all dynamic data
   will take place. All existing calls are cleared.

 o Restart with reload - The highest level of restart is enacted when the
   restart fails to lead to a succesful program execution. The system then
   automatically reloads programs and data from an external store. All
   existing calls are cleared.