SPC Exchance

Centalized & Distributed SPC

Abu Hanifa Safi
183 -19-2057
B.Sc in ETE.
Dept.of ETE/ICE.
 In 1965, Bell system installed the first computer controlled switching system which uses a
stored program digital computer for its control functions. The SPC concepts permitts the
features like abbreviated dialing, call forwarding, call waiting etc. The SPC provides
significant advantages to end users. The SPC enables easier number changes, automated call
tracing message unit accounting (for billing) etc.

In SPC, a programe or a set of instructions are stored in its memory and executed
automatically one by one by the processor. Carrying out the exchange control functions
through programs stored in the memory of a computer led to the name stored program
control. A computer can be programmed to test the conditions of the inputs and last states
and decide on new outputs and states. The decisions are expressed as programs which can be
rewritten to modify or extend the functions of control system. All switching systems
manufactured for use as public switching systems now use computers and software
programming to control the switching of calls. Using SPC, 20 mA transmitter (old transmitter
need 23 mA) with 52 V battery feed and longer subscriber loop can be achieved.

Basic view of SPC telephony switch. Fig. 1 shows a basic control structure of a SPC telephony
exchange.
Fig.1.Basic control
structure of SPC.
The SPC uses processors designed to meet the various requirements of the exchange. More than
one processors are used for the reliability. Normally these processors are duplicated. Also the SPC
system uses distributed software and hardware architectures. To carry over the maintenance
functions of the switching system, a separate processor is used.

Using the above setup, the SPC performs trunk routing to other control or tandem offices. Special
features and functions are also enabled with sophisticated equipments and in compact form.

There are two types in SPC exchanges, namely centralised SPC and distributed SPC. In the
following sections, both types are described.
Fig. 02
In the figure registers are connected to the processor with input/output interfaces to the register access
switch. Each interface has an address and may be connected to the input/ output buses by loading that
address into the address buffer. Programming of the computer is such that its memory is divided into two:

• Program memory, for the sequence of instructions.

• Call memory, for the state of each register.

There is a third memory called translation memory, which translate the information dialed by the
subscriber into specific actions. It is part of the program but is treated independently, so that the program
may be used for any similar system. The register interfaces have one or more inputs and are scanned at
regular intervals. These scan points are accessed by loading the address of the required register into the
address buffer. Decoding the address returns the state of the required scan points. The outputs of the
interfaces are treated similarly.
In operation, an external clock pulse triggers the computer. An overall control program, after loading the
address and setting one of the call memories to point to the block of the memory associated with the
first register, then hands over to the register program that decides whether any action is required for the
current state and input. The next register is addressed, when the register program has completed its
actions and the overall control memory modifies the addresses of the input/output buffer and call
memory index. When all the registers have been dealt with, the computer waits for the next clock pulse
before restarting a new scan. The SPC has the following advantages over other types of control:

• By changing the program, changes in the working of the exchange

can be made at any time. Thus new services for the used and new
maintenance and monitoring procedures can be introduced.
• It provides cheaper inter-exchange signaling, i.e., Common Channel
Signaling.
• No compromise between the efficiency of the switching network and
the cost of a marker.

The SPC exchange does introduce certain problems also. For example, its cost is independent of the size
of the exchange and in case of break down of the computer ihe exchange will stop functioning.
 Centralised SPC

Early electronic switching systems are centralised SPC exchanges and used a single processor to perform the
exchange functions. Presently centralised exchanges uses dual processor for high reliability.

Fig. 3 Centralised SPC

All the control equipments are replaced by the processors. A dual processor architecture may be
configured to operate in (a) stand by mode (b) synchronous duplex mode and (c) Load sharing mode.

Standby mode. In this mode, any one of the processors will be active and the rest is standby. The
standby processor is brought online only when the active processor fail. This mode of exchange uses a
secondary storage common to both processors. The active processor copies the status of the system
periodically and stores in axis secondary storage. In this mode the processors are not connected
directly. In secondary storage, programs and instructions related to the control functions, routine
programs and other required informations are stored.

Synchronous duplex mode. In this mode, the processors p1 and p2 are connected together to exchange
instructions and controls. Instead of a secondary storage common to P1 and P2, separate memory M1
and M2 are used. These processors are coupled to exchange stored data. This mode of operation also
uses a comparator in between p1 and p2.
The comparator compares the result of the processors. During normal operation, both processors
receives all the information from the exchange and receives related data from their memories. Although
only one processor actually controls the exchange and remaining is in synchronism with first one. If a
mismatch occurs, the fault is identified by the comparator, and the faulty processor is identified by
operating both individually. After the rectification of fault, the processor is brought into service.

Load sharing mode. In this mode, the comparator is removed and alternatively an exclusion device (ED) is
used. The processors calls for ED to share the resources, so that both the processors do not seek the
same resource at the same time. In this mode, both the processor are active simultaneously and share
the resources of exchange and the load dynamically. If one processor fails, with the help of ED, the other
processor takes over the entire load of the exchange. Under normal operation, each processor handles
one half of the calls on a statistical basis. However the exchange operator can vary the processor load for
maintenance purpose.
 Distributed SPC

The introduction of distributed SPC enabled customers to be provided with a wider range of services
than those available with centralised and electromechanical switching system. Instead of all
processing being performed by a one or two processor in centralised switching, functions are
delegated to separate small processors (referred as regional processors). But central processors is
still required to direct the regional processors and to perform more complex tasks.

The distributed SPC offers better availability and reliability than the centralised SPC. Entire exchange
control functions may be decomposed either horizontally or vertically for distributed processing.

In vertical decomposition, the exchange environment is divided into several blocks and each block is
assigned to a processor that performs all control functions related to that block of equipments. In
horizontal decomposition, each processor performs one or some of the exchange control functions.
Figure shows the distributed control where switching equipment is divided into parts, each of which
have its own processor.
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