SSS SUBSCRIBER SWITCHING SUBSYSTEM

• 
  MTS MOBILE TELEPHONY SUBSYSTEM: Software and hardware. The subsystem handles traffic to and from mobile subscribers.
  
• 
  MNS NETWORK MANAGEMENT SUBSYSTEM: Only software. The subsystem contains functions for supervising the traffic flow through the exchange, and for introducing temporary changes in that flow.
  

As has been said, the control part consists of one central processor and a number of regional processors.

The task of the software allocated to a subsystem is to control the hardware of that subsystem.

Since the hardware (the telephony devices) is controlled by the regional processors, these must, of course, also contain programs belonging to the subsystem concerned. Consequently, the software for a subsystem can be divided into one central part (programs + data which are stored in the central processor) and one regional part (programs + data which are stored in the regional processors). Naturally, this applies only to subsystems containing hardware.

APT = Telephony Part of AXE

Each subsystem is in turn divided into a number of parts called FUNCTION BLOCKS. At this level, too, the division is function-related.

To illustrate this we are going to study the Trunk and Signalling Subsystem (TSS).

TSS contains a function block called BT (Both-way Trunk). The function of the BT function block is to handle both-way digital links between exchanges. (A both-way trunk is a trunk that can carry traffic in both directions). Of course, there is hardware to which the digital link is connected. In this case, the hardware consists of a printed board assembly containing circuits and logic for clocking the digital signals.

A regional processor contains software to control and supervise the hardware. The software belongs to the BT function block. If a change occurs in the hardware, this will be detected by the regional software, which scans the hardware at regular intervals.

The regional software (BTR) will then inform the central software (BTU) in the BT function block.

A
fter that, the central software can interwork with other function blocks in the central processor. The interworking between function blocks always takes place at the central level, i.e. in the central processor. See Figure 2.2.3.
BT = Bothway trunk

BTR = Regional software of block BT

BTU = Central software of block BT
Figure 2.2.3

Examples of Function Blocks

As shown in the figure, function block Y has neither hardware nor regional software, and this is just as frequent a solution as any other combination, taking into account that entire subsystems may consist exclusively of central software.

The data belonging to a function block can only be addressed by the block’s own programs. If a block needs data from some other block, it must make a “request”.

WHY FUNCTION BLOCKS?

The basic idea of function blocks can be explained as follows:

• 
  Well-defined processes with data of their “own”.
  
• 
  Borders between function blocks where the exchange of information is least frequent.
  
• 
  A function block need not know what other blocks do.
  
• 
  Standardized signals between the function blocks.
  

R
emember:

We are now going to have a closer look at some central system parts. To describe the operation of an AXE exchange we will study how TCS (Traffic Control Subsystem) interworks with the other subsystems.

As has been said TCS is the central part from the traffic-handling point of view. TCS in AXE corresponds to the operators in a manual system.

Remember that TCS consists only of central software.

T
he TCS subsystem consists of 9 important function blocks; see Figure 2.3.2.

• 
  RE REGISTER FUNCTION
  

• 
  CL CALL SUPERVISION
  

• 
  DA DIGIT ANALYSIS
  

• 
  RA ROUTE ANALYSIS
  

• 
  SC SUBSCRIBER CATEGORIES
  

• 
  TOM TRUNK OFFERING MANAGEMENT
  

• 
  TOD TRUNK OFFERING DATA
  

• 
  COF COORDINATION OF FLASH SERVICES
  

• 
  SECA SEMI-PERMANENT CONNECTIONS
  

A
s we can see, the TCS subsystem occupies a central position in the AXE system. As its name indicates (Traffic Control Subsystem), TCS’s tasks include controlling the set-up and clearing phases. Figure 2.3.3 shows where in the system TCS is positioned.

To set up a call to another exchange, the operator of an old-type manual system exchanged verbal information (“signals”) with other operators. When automatic exchanges were introduced, these, too, needed to exchange signals. Different electrical signals were given different meanings. Signalling can be divided into two main groups: line signalling and register signalling.

Line signals control the set-up and clearing of a speech connection. Register signals contain information such as the number to which a call is to be connected. Register signals are only used in the set-up phase. Let us compare automatic signalling with the operator’s way of communicating.

To set-up a call to another exchange the operator sends a current through the line by turning the handle of a generator. The current causes an indicator to react at the receiving operator’s desk, thus indicating that a call is coming. This is a line signal. The receiving operator connects her headset to the line and says, “Hello”. The other operator hears this and says, “Please connect me to number 1234”. These are examples of register signals.

This was one of the first procedures for interexchange signalling. During the hundred years of telephony history, a great many signalling systems have been developed. These systems have naturally been dependent on the technology available, and consequently the “history of signalling” covers a wide range of means - from uncomplicated currents and tones to today’s high-capacity digital signalling systems.

This development process has resulted in a mixture of new and old technology in telecom networks. An exchange must often be capable of handling many different signalling systems simultaneously.

I
n the AXE system, this problem has been solved by letting the TSS subsystem (Trunk and Signalling Subsystem) adapt different signalling systems to TCS. In other words, TCS can be said to be unchanging.

GSS = Group Switching Subsystem

RP = Regional Processor

TCS = Traffic Control Subsystem

TSS = Trunk and Signalling Subystem
Figure 2.3.4

Adaptation to Different Signalling Systems is made in TSS

To see how TCS works we will study a small portion of an incoming call to an AXE exchange.

T
he register signalling system used in our example is MFC (Multi Frequency Code). MFC sends register signals by combining two tones. A special piece of equipment is required to handle these tones. This equipment is called the CR (Code Receiver) and is connected via the group switch.

The sequence of events is as follows:

1. 
   The other exchange wants to set–up a call to “our” exchange, and selects a free line to interconnect the two exchanges.
   
2. 
   The other exchange sends a line signal to our exchange simultaneously with the sending of the first digit by means of MFC signals.
   
3. 
   The line signal is detected by the regional processor scanning the incoming line (IT, Incoming Trunk). The regional processor sends a message to the central software of the IT block, telling it that a call attempt is in progress.
   
4. 
   As IT’s central software (ITU) receives the message, it consults its data and finds that the line concerned uses MFC signalling. ITU now requests a CR from the central software (CRU) of the CR block.
   
5. 
   CRU selects a free CR device and orders GSS (Group Switching Subsystem) to connect the CR device to the IT device.
   
6. 
   ITU informs the RE block in TCS that a call is coming. RE reserves a data area in the memory to be used exclusively for this call.
   

7. 
   The first digit is received by the CR device. The regional processor scans the CR device and sends the digit to CRU. CRU sends the digit on to ITU, which forwards it to the register, RE.
   
8. 
   RE sends the digit to the DA block for analysis. The DA block contains a number of tables for digit analysis. The result of the analysis is stored in RE. Depending on the result of the analysis, the register can now take different kinds of action.
   

As has been said, it is the register that controls the set-up phase. This control is based on the result obtained in the digit analysis.

The following data may come from the DA block on completion of the digit analysis (one digit at a time is analysed - not the whole B-number in one go).

• 
  Send the next digit.
  
• 
  Routing case (the analysis in the Route Analysis Block, RA, indicates an outgoing route).
  
• 
  Charging case.
  
• 
  Number length.
  
• 
  Terminating Call.
  
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  Modification of B-number.
  
• 
  End of analysis.
  

We will now study some of the TSS and CCS hardware in APT. It is important to remember that all hardware is controlled by its own software both in the central processor and in the regional processors.

= Digital Signal

ETC (Exchange Terminal Circuit) is the hardware of the BT blocks. An ETC consists of a printed board assembly housed in a magazine. For examples of magazines, see Section 2.10, “Construction Practice”.

T
he printed board assembly is illustrated in Figure 2.3.8.

Figure 2.3.8

Exchange Terminal Circuit (ETC)

Each channel in the digital connection is regarded as a BT device. If a 32-channel system is used, only 30 of the channels can be utilized for speech. Channel 0 is always used for synchronization and alarm information while channel 16 is used for signalling (Channel 16 is primarily used for line signalling, but some signalling systems can also use it for register signals).

The USA and South Korea are examples of countries using 24-channel systems. In these systems, all 24 channels can be used for speech (Line signals are sent by “stealing” one bit from every six samples).

OT (Outgoing Trunk) is the block used to handle outgoing analog connections.

The hardware consists of a magazine containing 32 devices, and an analog-to-digital converter. The converter, which is called PCD (Pulse-Code Modulation Device), has no software and no signalling function.

IT (Incoming Trunk) is the block used to handle incoming analog connections. The hardware is almost identical with that of OT.

To distinguish between different variants, the “trunk blocks” are given numbers: BT1, BT2 …. Here the term “variant” refers to different signalling systems.

Exchanges installed today are almost exclusively equipped with ETCs. In applications with analog transmission, the digital signals sent by ETCs are converted into analog signals.

T
he equipment used to do the conversion is called a Multiplexer (MUX). A multiplexer thus converts signals from digital to analog form, but it can also multiplex several analog signals on one and the same line (FDM, Frequency Division Multiplex). Note that the MUX does not belong to the AXE system; it is transmission equipment.

ETC = Exchange Terminal Circuit

GSS = Group Switching Subsystem

MUX = Multiplexer

~ = Analog signal

= Digital signal
Figure 2.3.9

A Multiplexer (MUX)

CODE SENDERS and CODE RECEIVERS (TSS)





CR = Code Receiver

CS = Code Sender

CSR = Code Sender/Receiver

ETC = Exchange Terminal Circuit

GSS = Group Switching Subsystem

MUX = Multiplexer

~ = Analog Signal

= Digital Signal
Figure 2.3.10

Analog and Digital Code Senders/Receivers

Code Senders (CS) and Code Receivers (CR) are used for sending MFC register signals.

CR/CS are connected by means of the group switch when a device (IT, OT or BT) needs to send register signals by MFC.

AXE has two types of CR/CS:

1. 
   Analog Devices: 4 CR or 4 CS in each magazine. Analog-to-digital conversion takes place in the PCD (Pulse Code Modulation Device).
   
2. 
   Digital Devices: 16 devices in a magazine, CSR, that can be used on both CR and CS.
   

The announcing machine is a subscriber facility which uses recorded messages to inform calling subscribers why they cannot reach dialled numbers.

Announcing machines are also necessary in combination with certain subscriber facilities where the subscriber can control the facility by dialling predetermined codes (The announcing machines inform subscribers whether they have used the right or wrong procedure).

Two different types of announcing machine can be connected to AXE: a digital machine of recent design, or a “conventional” analog machine.

As its name indicates, the Digital Announcing Machine (DAM) is fully digital. Recorded verbal messages and tones are stored in digital form on two types of storage boards: one with PROMs and one with RAMs. The messages stored in PROMs are seldom changed and special external recording equipment is required to make changes in them. But no external equipment is needed to change messages stored in RAMs. In fact, uses can change them by dialling procedures on an ordinary telephone. Consequently, these messages are best suited for the Weather Line, sports results, news, etc.

The maximum message length is 32 seconds for “permanent” messages and 64 seconds for information that is frequently changed. Verbal messages from an external analog announcing machine can be connected to DAM, and external messages can be combined with messages stored in DAM. An example of how this type of message is used is the subscriber facility “automatic wake-up service”. When woken up by the ringing signal, the called subscriber hears a message, for example: “You have ordered automatic wake-up. The time is ………”. (Here a speaking clock can be activated to give the hour).

Verbal messages can also be combined with various types of tones.

As appears from Figure 2.3.11, the analog machine requires a great deal of peripheral equipment.

Announcing machine messages are recorded on magnetic disks, which repeat the message as the disk rotates. To prevent subscribers from being connected up in the middle of a message, the announcing machines send synchronizing pulses when a message starts. These pulses are sent to a magazine called RD (Recording Device). RD sees to it that ASD (Auxiliary Service Device) connects the subscriber at the right moment. The ASD magazine also operates as a “mini-switch”, as each input from the group switch must be connectable to any of the recorded messages.