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CEPT-30 PCM and Multi-frame Concept Introduction

Human ear is sensitive to a limited frequency rage only and this range is approximately 0-20kHz. However it is seen that in usual telephone conversations, we do not need to transmit the total frequency range. Instead with only the 0-4kHz range, a good perception can be achieved at the receiver end. Therefore in speech signal communication, total signal bandwidth is low pass filtered to extract the 0-4kHz range.

We have already discussed the Nyquist criteria for sampling in the previous chapter. Therefore it’s obvious that the minimum sampling rate we need for the above filtered speech signal is 8 kHz. Usually in practical speech signal communication sampling is done at this minimum rate.

8 kHz corresponds to an inter-sample duration of 125 µs. Further, it is seen that with duration of 3.9µs, bits in an 8 bit sample can be effectively recovered. However with this setup, the transmission link will be idling for a period of (125-3.9) = 121.1 µs.

One genius design is to transmit the sampled data of 31 other users within this idling period so that the link is utilized for the total time. A 32 time slot data unit is known as a frame and there will be 8000 frames transmitted within every second. This time division multiplexing makes the use of precious band width very efficient.

This 32 time slot TDMA transmission system is a European standard system and is known as CEPT-30 PCM system. There is another 24 time slot system which is used in America and Japan. This American system is beyond the scope of our discussion.

CEPT-30 PCM

Fig1: CEPT-30 frame structure

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8 32 8000 bps

2048

8 8000

64

3.9/8 μ

488

Synchronization

Due to the fact that the bits belonging to multiple users are sent in the same PCM, it is of highest importance to identify them separately. If we identify the start of the frame properly, it is a matter of counting and grouping sets of 8 bits to separate the users.

In order to identify the start of the frame, a unique bit pattern is inserted to the start of the frame. In other words, time slot zero is occupied by this unique bit pattern.

This limits the number of multiple users to 31.

Signaling information in a CEPT-30 PCM

If we consider the communication between two voice telephones connected to two switches ( exchanges), we can see the following steps in generating a call.

1. ‘A’ party lift the receiver and switch ‘A’ sends the dial tone. 2. ‘A’ party dial the ‘B’ number, the originating exchange then finds the correct route and

sends the ‘B’ number information to the remote switch to find the destination party ‘B’. 3. When the terminating exchange finds the ‘B’ number free, it generates a ringing tone.

Further ring back tone is send to the ‘A’ party. 4. Upon lifting the terminating exchange informs the answer status to the originating

exchange and the real communication takes place.

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5. Whenever one party hangs up the call is terminated.

It is very important to note that there are two types of signaling that take place in this scenario. First for the establishment of the call some signals are being passed. Then after the establishment of the call, in order to inform the status of the call, there need another set of signals. First type is known as the register signals while the second type is known as supervisory signals.

Due to the fact that the register signals occur before the real voice traffic, the channels used for voice can be used for these too. However the supervisory signals need separate dedicated channels since they take place while the voice communication is going on.

In order to achieve this we need to dedicate some time slots for signaling in the interconnecting PCM. ITU has recommended a standard for this signaling time slots. In this standard, 4 bits in the 16th TS of a PCM frame is used to carry the signaling information of one subscriber. Therefore signaling information of two subscribers can be accommodated in one frame. 15 of such frames carry the signaling of all 30 subscribers in a PCM. Further to this another frame is carrying full synchronization information for this multi-frame scenario.

Fig 2: CAS PCM multiframe concept

As in fig 2, frame zero carries synchronization information and TS16 of frame 1 carries signaling corresponding to the traffic channel which uses TS1 and TS17 of each frame.

Because of the fact that the traffic and the signaling both are carried in the same frames, this signaling is known as Channel Associated Signaling (CAS).