AMU = Automatic Maintenance Unit (APZ 211)

What is an I/O system used for in a telephone exchange?

This question is frequently asked by persons with no experience of SPC exchanges.

The “I/O System” in mechanical exchanges was made up of a few lamps and perhaps some test equipment. In an SPC exchange, most of the internal work is done via the I/O system. Below follows a list of some of the activities performed with the aid of the I/O system.

• 
  Connection of subscribers.
  
• 
  Change of subscriber categories.
  
• 
  Output of charging (call metering) data.
  
• 
  Automatic printout of alarms.
  
• 
  Fault tracing (in software and hardware).
  
• 
  Measurements (e.g. traffic recording).
  
• 
  Storage of backup software for automatic reloading of the system as a result of serious faults.
  
• 
  Communication over data links with operation and maintenance centres, etc.
  

This subsystem includes a powerful processor for communication with all I/O devices. This processor, which was developed by Ericsson, is also used in other applications. The processor is generally referred to as “APN 167”. The SPS also handles functions for blocking, deblocking and supervision of I/O devices.

This subsystem handles all types of files used in the system. (The term “file” denotes all data stored on tape, floppy disks and Winchester disks). The data blocks of the system must always “consult” FMS before storage of information in external storage media (output of charging data, etc.).

This subsystem handles communication between the I/O devices and the rest of the system. The I/O devices can be in the form of display units, paper-copy printers, alarm panels, or intelligent personal computers (intelligent terminals using simple menu system instead of “regular” AXE commands).

This subsystem handles the communication between blocks in CP and SP. The subsystem structure is in accordance with international standards (ISO) for I/O systems: Open Systems Interconnection (OSI). DCS also handles the communication over data links according to CCITT’s standardized data protocols X.25, X.75 and X.28.

S
o far we have only discussed some of the functions included in the I/O system. Now, we are going to get acquainted with the actual hardware. As has been said, the nucleus of the I/O system is the Support Processor (SP). For capacity and reliability reasons, several SPs can be connected in parallel. Figure 2.7.1 shows some of the I/O devices that can be connected.

The operation and maintenance personnel use commands to make modifications in an exchange. The AXE command language is in accordance with CCITT’s recommendations.

A command consists of five letters which indicate - in abbreviated form - the task to be performed by the command concerned. This five-letter code is followed by a number of parameters which describe - in more specific terms - the work to be done. Let us study the following example:

The system has found a regional processor to be faulty and blocked it automatically. After repair (change of printed board assemblies) the regional processor is to resume service. The equipment is to be deblocked. The corresponding command is as follows:

The command code “BLRPE” is the abbreviated form of “BLocking of Regional Processor, End”. The parameter after the command code indicates the identity of the regional processor involved, in this case RP number 45 (all RPs have individual numbers). A command always ends with a semi-colon.

The number of commands in AXE is very large (about 1000), and some activities require several commands. Interventions of a frequent nature - such as changing subscriber categories - can be simplified by using a Personal Computer (PC).

The software in a personal computer makes it easier for the exchange personnel to communicate with AXE.

Instead of keying a large number of commands (each of which may contain several parameters), we compose a “form” on the graphic display unit of the PC. By filling in the desired values in the empty squares of the form, we can cover several commands. The forms are tailor-made for different types of job. Figure 2.7.2 shows a
n example of such a form.

This simplified method of communicating with the system offers many advantages to personnel and administrations, for instance:

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  Reduced risk of mistakes in handling procedures.
  
• 
  The operator need not possess a detailed knowledge of the system.
  
• 
  The forms can be written in the local language.
  
• 
  New forms can easily be tailor-made for new jobs.
  

If the system detects abnormalities in the hardware or software, it initiates an alarm. The alarm is automatically printed out on a typewriter and indicated visually on an alarm panel. (We will revert to this in Chapter 5).

Alarms can be classified in different types, classes and categories. In a given alarm situation, this classification enables the system to alert the right personnel and to indicate the degree of emergency.

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  Alarm Type: There are two alarm types:
  

1. 
   Automatically initiated alarms, and
   
2. 
   Observation alarms generated as a result of the exchange personnel blocking equipment units.
   

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  Alarm Class: A class indicating priority is assigned to all alarms. The system has three classes called A1 (top priority), A2 and A3.
  
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  Alarm Category: One out of 16 alarm categories is assigned to each alarm. The purpose of these categories is to indicate the type of equipment which caused the alarm. Examples: processors, power units, subscriber lines.
  

We will revert to this in Chapter 5, which deals with the operation and maintenance of AXE exchanges.

As we know, the AXE system is divided into function blocks, all interworking between the blocks taking place via the central processor.

To facilitate location and identification, each block has been given a unique number. (An AXE exchange contains between 300 and 500 function blocks).

T
he program store positions of all blocks are written in a store called the Reference Store (RS). Each block in the system is represented by a number of data words in RS. One of these words indicates where in the Program Store (PS) the block is stored. In APZ 212 these stores are physically separated, whereas in APZ 211, they are both contained in MS (Main Store). See Figure 2.8.1.

This word in the reference store is called Program Start Address (PSA) and indicates the address at which the block begins. The block number (BN) is used for addressing in RS.

We can thus say that each block in the system has an area of its own in the reference store: a table called the Reference Table. A function block can usually perform a large number of tasks or JOBS. Each job is initiated by sending a signal to the block. To find the wanted job within the block, each signal has been given an individual number, the Signal Number (SN). (A synonymous term is “signal location”).

To initiate a job within a block we thus use a block number and a signal number. These two elements form what is usually called a SOFTWARE SIGNAL and are normally abbreviated BN + SN.

How is the signal number used in the block?

SIGNAL DISTRIBUTION TABLE

At the beginning of each block there is a table called the Signal Distribution Table (SDT). This table, which starts at PSA, is reversed as shown in Figure 2.8.2.

PS = Program Store

An address is written in each position of the signal distribution table. This address, which is always relative to the start address of the block (PSA), indicates where in the program the different jobs begin.

Consequently, the absolute address of a job will be as follows: The start address of the block + the address given in the signal distribution table. See Figure 2.8.3.

We have now studied the addressing of programs, or rather: how a job in a block is addressed.

The text that follows describes how a block addresses its data. (A block can only address its “own” data).

PS = Program Store

First, we will use a simplified example to explain the term “data”. Assume that we have 4 devices of a given type, and that these devices are located in (connected to) the group switch. The devices form part of block X, whose data contains certain information about the 4 devices.

The following information must be stored for each device:

1. 
   The state of the device (free, engaged, blocked, etc.).
   
2. 
   Where the device is connected to the group switch, i.e. the inlet or multiple position (MUP).
   
3. 
   The value of a disturbance counter (for supervision).
   

GSS = Group Switching Subsystem

This way of illustrating data is not quite in accordance with the facts.

L
ooking at the way in which the data is stored in the data store we find that all variables of the same types are stored together. See Figure 2.8.5.

DS = Data Store

MUP = Multiple Position
Figure 2.8.5

Data Stored in the Data Store

What must the program do to find these variables (which are dispersed in different places in the store)?

The answer is the REFERENCE STORE (RS).

In the reference store all variables of a block have a table called the Base Address Table. The functions of this table include storage of information indicating where in the data store the variables are located.

E
ach variable within a block has been given a number which is used as a pointer within the table of the block. To indicate where in RS this Base Address Table is to be found, a word called Base Start Address (BSA) is given in the Reference Table. The word that indicates the storage position of the variable is called a Word Address (WA). See Figure 2.8.6.

BSA = Base Start Address

DS = Data Store

MUP = Multiple Position

PS = Program Store

PSA = Program Start Address

RS = Reference Store

SDT = Signal Distribution Table

WA = Word Address
Figure 2.8.6

Addressing of Program and Data for One Function Block

At first sight this may seem an unnecessarily complicated way of addressing. But, as we will see below, the method has obvious advantages, for instance:

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  A program can be stored anywhere in the program store without affecting anything except the PSA in the reference store.
  
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  A program sequence (a job) can be placed anywhere within a block without affecting anything except the address in the signal distribution table.
  
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  The number of variables per block is completely flexible. Only the size of the Base Address Table of the block is affected.
  
• 
  The number of devices can easily be increased or decreased because the variable block can be moved to a vacant area in the data store. Only the Word Address (WA) is affected.
  

One of the variables of block X is “disturbance counter”. (There is one counter for each device). Some other block will request - at regular intervals - the values of the counters. (For example, a supervisory block in the OMS subsystem. Preset limit values have been specified in this block).

The other block - here called Y - will request information by sending a software signal. Block X will read off the values of the counters and place them in a register called the Register Memory (RM).

This register, which is located in CPU, is used for standardized transfer of data between the blocks.

The block number and the signal number of block Y are stored in block X in a table called the Signal Sending Table (SST). This information, too, is placed in RM. See Figure 2.8.7.

RM now contains a software signal and its associated data, and these contents will initiate a program sequence in block Y. How important is it, that this must be done immediately? Are there other, more urgent tasks that the processor should attend to? Yes, probably.

BN = Block Number

We thus realize that the processor must have some kind of PRIORITY SYSTEM.

During the design phases it has been decided what signals should have priority over other signals. In other words, there is a signal hierarchy. The priority of a given signal determines the JOB BUFFER in which the signal will be placed.

In APZ 211 a job buffer is a separate area in MS, while in APZ 212, it is a separate store in SPU.

Two job buffers are provided for traffic-handling programs, and another two for maintenance programs. The four job buffers A-D are named JBA, JBB, JBC and JBD.

JBA has the highest priority and JBD the lowest. This means that the processor will execute jobs in JBA until that job buffer is empty. Only then can it start executing jobs in the other buffers.

If we send our signal to block Y, it will be placed in JBC. See Figure 2.8.8.

BN = Block Number

Our signal to block Y will thus have to wait for a short moment: a job with higher priority must be executed first.

In some cases it may be convenient to delay (queue) a signal for a given period of time or until a given point in time.

The central processor has 4 time queues. The operating principle of these queues is that a counter is decremented by 1 at regular intervals, and when the counter reaches zero, a software signal is sent to one of the blocks.

One of the time queues is an ABSOLUTE QUEUE, which means that the delay will last until a certain date and a certain hour.

Figure 2.8.9 shows the four time queues and their maximum delay.

Time queues are used to delay signals that do not appear at regular intervals. A so-called JOB TABLE is used if a block requires regular reception of a signal (for instance, to perform time supervision functions). The principle is the same as for the time queues (a counter is decremented to zero). The decrementation takes place every 10 ms, and to enable the processor to know when 10 ms have elapsed, a f
unction called Clock Interrupt is provided.

Figure 2.8.9

The Time Queues

This is a counter which sends a Clock Interrupt Signal (CIS) every 10 ms and initiates decrementation of the job table counters.

We say that the processor has a primary interval of 10 ms. Note that this has nothing to do with the speed at which the processor operates: it makes the processor capable of measuring time.

2.9 TRAFFIC HANDLING

As we know, the AXE system is made up of a number of function blocks. Since several functions take part in the set-up, supervision and clearing of call facilities must be provided for block interwork. In this section, we are going to have a closer look at such interblock communication.

The participants in the interwork are the central software units of the function blocks, which communicate with the aid of software signals (See Figures 2.2.3 and 2.8.7). How and in what order the blocks should exchange signals is determined in the design phase (We will return to this later in the book).

Since the actual number of signals required to set-up and clear a call is quite large (between 150 and 200 for an ordinary local call), our presentation will be somewhat simplified.

T
he model we are going to use when studying the various call phases is found in Figure 2.9.1.

BT = Bothway Trunk JT/RT = Junctor Terminal/Remote Terminal

CA = Charging Analysis KRC = Keyset code Reception Circuit

CCS = Common Channel Signalling Subsystem KR2 = (Digital) Keyset code Receiver

CHS = Charging Subsystem LIC = Line Interface Circuit

CJ = Combined Junctor LI2 = (Digital) Line Circuit

CL = Call supervision PD = Pulse Distribution

C7LABT = CCITT No. 7 Label Translation RA = Route Analysis

C7DR = CCITT No. 7 Distribution and Routing RE = Register functions

C7ST = CCITT No. 7 Signal Terminal SC = Subscriber Categories

DA = Digit Analysis SSS = Subscriber Switching Subsystem

EMTS = Extension Module Time Switch ST-7 = Signalling Terminal for CCITT No.7

ETC = Exchange Terminal Circuit TCS = Traffic Control Subsystem

GS = Group Switch TS = Time Switch

GSS = Group Switching Subsystem TSS = Trunk and Signalling Subsystem
Figure 2.9.1

Model for Describing the various Call Phases

Let us start by identifying the component parts of the figure. The upper section represents the hardware which takes part in the connections we are going to study, while the lower section represents the central software of the different blocks.

Since we are already familiar with most of the blocks, we will only comment on the ones that have been added.

• 
  CA CHARGING ANALYSIS
  

• 
  PD PULSE DISTRIBUTION
  

• 
  C7LABT CCITT No.7 LABEL TRANSLATION
  

• 
  C7DR CCITT No. 7 DISTRIBUTION and ROUTING
  

In the following we will study an outgoing call from subscriber “A” to subscriber “B”. (In telephone switching, “A-subscriber” and “B-subscriber” are accepted terms denoting the calling and the called subscriber, respectively).

In each of the figures below, the blocks that take part in the respective sequences are shaded. The arrows between the blocks indicate software signals. After each figure follows a text which describes the events illustrated in the figure.

BT = Bothway Trunk JT/RT = Junctor Terminal/Remote Terminal

C
A = Charging Analysis KRC = Keyset code Reception Circuit

CCS = Common Channel Signalling Subsystem KR2 = (Digital) Keyset code Receiver

CHS = Charging Subsystem LIC = Line Interface Circuit