TRANSMISSION REQUIREMENTS

The solution to a given transmission problem is dependent on several geographical factors. The terrain, the distance involved, and the number of channels required will simultaneously influence the chosen transmission method. We shall now see how today's telephone networks, which are based on FDM techniques, have accommo-dated these variabl"es.

The transmission solution within a city, between a city and a town, and between two distant cities will all be different. In each case the distance of transmission and the number of telephone channels required will differ. Conventional telephony practice identifies the need for three types of link, and they are referred to as local, junction,3 and trunk⁴ connections, as shown in Fig. 2-3.

A local connection is one that joins a telephone subscriber to the local exchange.

The distances involved are short.

A junction connection is one that joins a local exchange to a switching center.

In Europe,⁵ the most usual distance for this type of route ranges between 20 and 50 km, while the maximum distance is about 80 km.

A trunk connection is one that interconnects two switching centers, both nationally and internationally. These are the long-distance (maximum: 25,000 km), high-capac-ity routes.

Two types of telephone exchange have been identified within the simplified network illustrated in Fig. 2-3. The local exchange switches all calls within its local network connections in addition. Clearly, careful attention must be given to the attenuation,

3 This is the British term. In America the term toll-connecting trunk is used.

4 This is the British term. In America the term intertoll trunk is used.

5 In America the most usual distance for a toll-connecting trunk is of order 20 to 50 km also, while the maximum is about 300 km.

Transmission Requirements and Methods 21

U :

CD :

Fig. 2-3 A simplified view of a national tele-phone network.

or gain, introduced by these routes, since we cannot allow the speech volumes received at the subscriber's handpiece to be dependent on the transmission distance.

Since any transmission system contains gains, due to amplification, and losses, due to cable attenuation, a signal will have different levels at different points within the system, as shown in Fig. 2-4. Conventionally, levels are expressed at different points with respect to a chosen position, known as the zero reference point. The relative level of all other points within the system may then be denoted by the suffix

Line

Local exchange (a) Line Amp

---'---·-..1- -I-

-I--10 dB +24dB

zero ref, point

-5dB

-24dB +24 dB

Line

-I- Local exchange (b)

-24dB

+24 dB -6dB

Fig. 2-4 Example of relative signal levels within a transmission network.

+3 dBr

22 Introduction

dBr, as shown in the diagram. This value is of course equal to the algebraic sum of the gains and losses between the point of interest and the reference point.

The signal seen at the reference point normally has a level of 1 milliwatt (m W), and the power of signals relative to this value is denoted by the symbol dBm.

Thus 1 watt (W) is equivalent to 30 dBm.

Frequently it is convenient to express signal levels in terms of the corresponding level at the reference point; this is denoted by dB mO. Therefore,

dBmO = dBm - dBr (2.1)

As an example, if a signal is found to have an absolute level of -4 dBm at a point where the relative level is -9 dBr, the signal at the reference point is +5 dBmO.

The junction and trunk routes are always arranged in such a way that they impose a-dB loss or gain from end-to-end. Small gains may be corrected by introducing a passive network of known attenuation. The loss provided by such a network is known as the insertion loss. Alternatively, if in a similar way a corrective amplifier had been added, the gain provided is referred to as the insertion gain.

Transmission equipment

Switching machine

Distribution frame

Cables

Fig. 2-5 Layout of a local telephone exchange.

Transmission Requirements and Methods 23

Only the local routes can significantly alter the overall attenuation, which is normally not allowed to exceed 5 dB per local connection. Therefore we can see that it is the local network which exerts the dominant effect on the quality of a telephone call, not the long-distance transmission systems. This is dictated by the economics of the situation.

The equipment required to provide long-distance communication, although expen-sive, is in continual usage, and may be optimized to advantage. On the other hand, the local network consisting of many individual telephone sets, connection wires be-tween telephone and exchange, and subscriber designated exchange plant is rarely used. Any improvement must be made on a per subscriber basis and is expensive. used must withstand a hostile environment, and have traditionally been provided with a protective lead cover. Cables of this type enter the exchange within the splicing vault, where they are joined to smaller, more manageable plastic-covered cables.

Each spliced pair is connected to a distribution frame, whose purpose it is to allocate a dialing number for each telephone within the local network. Physically, the distribu-tion frame is an extremely large matrix which has connecdistribu-tions to both the splicing vault and the switching machine.

The switching machine, under the direction of the dialing pulses, may either intercon-nect two cable pairs within the same exchange (local call) or conintercon-nect one cable regular intervals between the terminals (see Fig. 2-6b). The loading coils, or inductors, compensate for the capacitance of the cable, which typically is in the range 30 to