1.0 Introduction to Telecommunication Switching
1.1 Principles of Switching Systems
1.2 Folded Switching Network
1.3 Non-Folded Switching Networks
1.4 Traffic in Switching Networks
1.5 Video on Telecommunication Switching

2.0 Multiplexing in Telecommunication
2.1 Frequency Division Multiplexing
2.2  Time Division Multiplexing




Key components of a switching system or an exchange are the  set of input and output circuits called inlets and outlets, respectively. The primary function of a switching system is to establish an electrical path between a given inlet-outlet pair. The hardware used for establishing such a connection is called the switching matrix or the switching network. It is important to note that the switching network is a component of the switching system and should not be confused with telecommunication network. Figure 1 shows a model of a switching network with N inlets and M outlets.

Figure 1. Switching Network with N-Inputs and M-Outputs


When N = M, the switching network is called a symmetric network. The inlets/outlets may be connected to local subscriber lines or to trunks from or to other exchanges as illustrated in Figure 2. In the figure, four types of connections can  be established:

1. Local call connection between two subscribers in the same exchange

2. Outgoing call connection between a subscriber and an outgoing trunk, i.e to another exchange

3. Incoming call connection between an incoming trunk (from another exchange) and a local subscriber

4. Transit call connection between an incoming trunk and an outgoing trunk.

Figure 2. Model of a telephone switching Network


1.2 Folded Switching Network


When all the inlets/outlets are connected to the subscriber lines, the logical connection appears as shown in Figure 3 In this case, the output lines are folded back to the input and hence the network is called a folded network.



Figure 3. A folded Communication Network


In a folded network with N subscribers, there can be a maximum of N/2 simultaneous calls or information interchanges. The switching network may be designed to provide N/2 simultaneous switching paths, in which case the network is said to be nonblocking. In a nonblocking network, as long as a called subscriber is free, a calling subscriber will always be able to establish a connection to the called subscriber. In other words, a subscriber will not be denied a connection for want of switching resources. But, in general, it rarely happens that all the possible conversations take place simultaneously. It may, hence, be economical to design a switching network that has as many simultaneous switching paths as the average number of conversations expected. In this case, it may occasionally happen that when a subscriber requests a connection, there are no switching paths free in the network, and hence he is denied connection. In such an event, the subscriber is said to be blocked, and the switching network is called a blocking network. In a blocking network, the number of simultaneous switching paths is less than the maximum number of simultaneous conversations that can take place. The probability that a user may get blocked is called blocking probability.



1.3 Non-Folded Networks

In a switching network, all the inlet/outlet connections may be used for inter-exchange transmission. In such a case, the exchange does not support local subscribers and is called a transit (or tandem)  exchange. A switching network of this kind is shown in Figure 4 and is called a nonfolded network. In a non-folded network with N inlets and N outlets, N simultaneous information transfers are possible. Consequently, for a nonfolded network to be nonblocking, the network should support N simultaneous switching paths.


Figure 4. Block Diagram of a Non-folded Network


1.4 Traffic in Switching Networks

All the switching exchanges are designed to meet an estimated maximum average simultaneous traffic, usually known as busy hour traffic. Past records of the telephone traffic indicate that even in a busy exchange, not more than 20-30 per cent of the subscribers are active at the same time. Hence, switching systems are designed such that all the resources in a system are treated as common resources and the required resources are allocated to a conversation as long as it lasts. The quantum of common resources is determined based on the estimated busy hour traffic. When the traffic exceeds the limit to which the switching system is designed, a subscriber experiences blocking. A good design generally ensures a low blocking probability.


The traffic in 'a telecommunication network is measured by an internationally accepted unit of traffic intensity known as Erlang (E), named after an illustrious early contributor to traffic theory. A switching resource is said to carry one Erlang of traffic if it is continuously occupied throughout a given period of observation.

1.5 Control System


The switching network provides the switching paths while the control subsystem of the switching system establishes the path. The switching network does not distinguish between inlets/outlets that are connected to the subscribers or to the trunks. This is carried out by the control subsystem which not only distinguishes between the different categories of connections but also  interprets correctly the signalling information received on the  lines. It senses the end of information transfer and releases connections.


The control system establishes connections based on the signalling information received on the inlet lines. The system sends out signalling information to the subscriber and other exchanges connected to the outgoing trunks. In addition, signalling is also involved between different subsystems within an exchange. The signalling formats and requirements for the subscriber, the trunks and the subsystems differ significantly. Generally, a switching system provides for three different forms of signaling, i.e


1. Subscriber loop signalling

2. Inter-exchange signalling

3. Intra-exchange or register signalling.


1.6 Components of a Switching System

A switching system is composed of subsystems that perform switching, control and signalling functions. Figure 5 shows the different elements of a switching system and their logical interconnections. The subscriber lines are terminated at the subscriber line interface circuits, and trunks at the trunk interface circuits. There are some service lines used for maintenance and testing purposes. Junctor circuits imply a folded connection for the local subscribers and the service circuits.

Figure 5. Components of a Switching System


Some switching systems may provide an internal mechanism for local connections without using the junctor circuits. Line scanning units sense and obtain signalling information from the respective lines. Distributor units send out signalling information on the respective lines. Operator console permits interaction with the switching system for maintenance and administrative purposes.


1.7 Direct and Common Control


In some switching systems, the control subsystem may be an integral part of the switching network itself. Such systems are known as direct control switching systems. Those systems in which the control subsystem is outside the switching network are known as common control switching systems. An example of direct control systems is the Strowger Switch, whereas crossbar and electronic exchanges are common control systems. In general, all stored program control systems are common control systems. Common control is also sometimes referred to as indirect control or register control.


1.8  Video Lecture on Telecommunication Switching
Switching Techniques using Circuit Switching, Lecture @ Indian Institute of Technology


Multiplexing is the reversible operation of combining several information-bearing signals to form a single, more complex signal. The signals which are combined in a multiplexer usually come from independent sources, such as subscribers in a telephone network. Prior to multiplexing, each signal travels over a separate electrical path, such as a pair of wires or a cable, whereas the multiplexed signal can be transmitted over a single communication medium of sufficient capacity. The reversibility of the multiplexing operation permits recovery of the original signals which often have different final destinations at the receiving end of the transmission link. This inverse operation whereby the original signals are recovered is called demultiplexing.

Multiple access may be considered as a special kind of multiplexing in which two or more signals, each from a different location, are sent by radio to a single transponder. The term "multiplexing", however, is usually restricted to describe those other situations where the signals to be combined arrive over electrical circuits. The most commonly used techniques for both analogue multiplexing and multiple access are based on the common principles of Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM) which are discussed in detail below.


In a frequency division multiplex (FDM) system, a number of channels are arranged side by side in the baseband, and thus can be accommodated by a single wide bandwidth transmission system. The actual multiplexing process is as follows.


  1. Each telephony channel transmits audio frequencies from 0.3 to 3.4 kHz, the baseband signals being in the form of single-sideband (SSB) signals with suppressed carriers at 4 lcHz spacing;


  1. Twelve telephone channels are frequency-converted to compose a basic group in the frequency range from 60 to 108 kHz;


  1. Five basic groups are again frequency-converted to compose a basic supergroup in the frequency range from 312 to 552 kHz;


  1. Frequency-conversion of this basic supergroup can be realized to multiplex many further telephone channels. Supergroups are often packed to create  Mastergroups (10 supergroups). Similarly 6 master groups can be multiplexed to yield a jumbogroup  as shown in Figure 6.


Figure 6. Multiplexing 3,600 voice channels into one Jumbo Group in Analogue Telephony Systems

FDM is used for terrestrial communications, and conventionally the multiplexed telephone channels are arranged at baseband frequencies above 60 kHz. However, in satellite communications, one basic group is arranged in the frequency band of 12 to 60 kHz so as to use the baseband frequency bandwidth more effectively.


At the receiving earth station, the FDM signal is demultiplexed through a sequence of filtering and SSB-demodulation steps. Separation of supergroups, and individual channels by filters is possible with minimal impairment because of guardbands left in the FDM signal. It should be recalled that the voice signal occupies only 3 100 Hz of the 4 kHz channel. An important technical requirement of FDM concerns the accuracy and coherence of SSB camer frequencies, which are usually derived from stable master oscillators.



Time division multiplexing (TDM) is by far the most commonly used means of subdividing the capacity of a digital transmission facility among a number of sources. Figure 7 shows the basic principles of time division multiplexing using space switching.


There are two basic modes of operation for TDM is as shown in Figure 7.


Figure 7. Illustration of TDM using a space switch

In practice, Time Division Multiplexing is achieved by using digital circuitry. Figure 8 shows logic gate circuit that can be used to multiplex and demultiplex four digital channels (D0,D1,D2,D3 ) by using a common clock signal to drive inputs S1 and S2.

 Figure 8. Four input digital multiplexer and demultiplexer.

There exist many  integrated circuits for carrying out Analogue to digital conversion (ADC) then multiplexing several channels, e.g. ADC0808 shown in  Figure 9.


Figure 9. Digital multiplexing of 8 analog channels by ADC0808 Integrated circuit.

(1) Synchronous Time division multiplexing  (STDM)  that repeatedly assigns a portion of the transmission capacity to each source; and

(2) Asynchronous Time division multiplexing  (ATDM) or Statistical Multiplexing (Stat-Mux) that assigns capacity as and when needed.


3.1 Synchronous Time division multiplexing 

In Synchronous TDM, a time slot is assigned to a channel irrespective of wheter thae channel has anything to transmit. Capacity allocation may be done either bitwise or word-wise. In bitwise allocation, each source is assigned a time slot corresponding to a single bit and in word-wise allocation, a time slot corresponds to some larger number of bits (often 8 bits) referred to as a word.

In synchronous TDM, a frame refers to a complete cycle of time slots. Therefore the number of time slots in a frame is equal to the number of inputs. Figure 10 shows the operation of synchronous TDM.

Figure 10. Synchronous Time Division Multiplexing of four inputs A(four bytes), B(two bytes), C(one byte) and D (three bytes).

3.1 Plesiochronous TDM

Plesio is a Greek word meaning 'nearly.' Plesiochronous TDM refers to a TDM system used in telecommunications where timing is achieved by a locally generated clock. Since clocks can differ, the systems is called plesiochronous TDM.

Figure 11 shows the standard Plesiochronous TDM standard used in telephony. 30 digital channels (with 2 for synchronization and signaling) are combined to create a 2.040 Mbps pulse train usually referred to as E1. Four E1s are combined to create E2. Further four E2s are combined to create E4.

Figure 11. ITU Plesiochronous Digital Hierarchy.

E1 Frame Structure

The frame structure of E1 shown in Figure 12. Each channel in a frame has 8 bits and is called a time slot, TS. A frame contains a total of 256 bits, 8 bits x 32 channels. Time slots in a frame are numbered from 0 to 31. Each time slot corresponds to a 64Kbps channel carrying 8 bits of either data or an 8 kHz digitized voice sample. Bits in a time slot are numbered from 1 to 8. Time slots are combined using Timing Division Multiplexing (TDM) at 2,048MHz. Thus, a frame is transmitted each 125µs (i.e 1/8,000).

Figure 12. Frame structure of E1



                                                                                                                                                         ©  Prof. James Kulubi May-Aug 2015