Signalling

Frame alignment

The frame alignment or framing is typical of time division multi­plexing. It consists of synchronising the receiving equipment both in frequency and in phase to the stream of symbols it receives. This operation is obviously necessary each time the receiver is switched on, but also during normal operation. Indeed, once aligned, the receiver needs a periodic time reference in order to check its isochronism and detects eventual shifts.

This necessary time reference consists of a particular pattern of several bits, called the framing pattern. When the receiver has lost frame alignment, it searches for this pattern in order to realign itself with as short a delay as possible. The framing pattern is periodically carried by the frame according to one of the following organisa­tions:

1. Grouped framing pattern, which consists of a number of conse­cutive bits at the beginning of a frame.

2. Distributed framing pattern which, as the name suggests, is spread over the frame on a bit by it basis, or over several frames at one bit per frame.

There is of course a danger of simulating the framing pattern by chance combination of other information carrying bits. There are different ways to protect the system from this, such as:

1. One can choose a framing pattern with low autocorrelation, so that it is impossible to imitate by shifting and infringement on random neighbouring bits. Examples are the patterns 110 or 1110010 for for 3-bits or 7-bits grouped framing patterns re­spectively.

2. One can block all the channels of the frame when the framing is lost at reception. The frame would be replaced by the trans­ mission of a resynchronisation signal. This necessitates an
announcement warning in the opposite direction, which is as­sumed to be correctly aligned.

3. One could confirm correct framing by a criterion other than the presence of the framing pattern, e.g. absence of the pattern in one frame out of two.

In order to avoid reacting to each transmission error, the reaction to an incorrect reception of the framing pattern in an aligned situ­ation, i.e. during normal data transfer, must be delayed. The solution to the framing problems of both Tl and El are given in the respec­tive sections later.

The transmission of multiple telephone calls is not the only require­ment for a Tl or El network. The network also has to transfer signalling information. Signalling information is auxiliary informa­tion (digits, commands, acknowledgement signals, etc.) sent from exchange to exchange to control switching and management oper­ation of the network.

Aside from analog signalling by sinusoidal carriers (in-band or out-band) used in analog systems and translatable by PCM modula­tion, digital systems lend themselves by nature much better to direct digital transmission of signalling information. Several solutions can be considered:

1. In-octet signalling, which is also called bit robbing. In this the least significant bit (LSB) among the 8 bits that represent a coded sample of speech is periodically (typically every six frames) assigned to signalling. This results in an imperceptible degradation of the telephone transmission. However, when data are transferred over the same network, the impact of losing the LSB every sixth byte is not acceptable.

2. Out-of-octet signalling, also referred to as channel by channel signalling. In this system each digital channel arranges, besides its octet, one or several signalling bits. These signalling bits of the n channels of the frame can be distributed, i.e. juxtaposed at each octet as a supplementary bit per time slot (possible signalling rate being 8kbit/s per channel), or grouped in a time slot reserved for this purpose and of which the bits are assigned in turn, cyclically, to the n channels of the frame (possible signalling rate being 64/n Kbit/s per channel)

3. Common signalling, where one time slot per frame is reserved for signalling and assigned according to need to one channel, then to another. Signalling is done using labelled messages, of which the label indicates which channel the message belongs to. This technique allows an instantaneous signalling rate of 64kbit/s for one channel at a time.

1 Learn the words & word combinations:

Intermediary	Посредством
Correspond	Соответствовать
Interleave	Покрывать, прослаивать
Sample	Выборка
Shift	Сдвиг
With respect to…	Относительно
In order to…	Чтобы
Encounter	Встречать
Obtain	Получать
Carrier	Несущая
Medium	Среда
Delay	Задержка
Consider	Рассматривать
Sampling frequency	Частота выборки
Rate	Уровень, скорость, коэффициент
Time slot	Временной сдвиг
Word interleaved	Чередование слов
Frame alignment	Выравнивание структуры цикла
Grouped framing pattern	Модель групповой синхронизации цифровых или ИКМ сигналов
Distributed framing pattern	Модель (паттерн или образец) распределенной синхронизации
Infringe	Нарушать
Imperceptible	Незаметный
Juxtaposed	Сопоставленный

2 Find Russian equivalents:

ü one frame of a duration; ü pulse modulation; ü in the frame cycle; ü a rotating switch; ü by transmission facilities or systems; ü auxiliary bits; ü on a character by character basis;

ü a number of channels; ü a time division multiplexed signal; ü by means of; ü pulse amplitude modulated signals; ü incompatible; ü strictly repetitive; ü suffers less throughput delay; ü a periodic time reference.

3. Find English equivalents:

ü назначено; ü равный частоте выборки; ü скорость передачи информации канала в битах; ü временное разделение каналов; ü чередование слов; ü импульсно-кодовый модулятор; ü цифровое переключение каналов;

ü периодическая сверка времени; ü образец формирования кадра; ü как можно меньшая задержка; ü ряд последовательных битов; ü случайная комбинация; ü на приеме; ü правильное формирование кадра; ü передача кратных (множественных) телефонных звонков.

4 Answer the questions:

1 What is a time division multiplexer?

2 What does it mean: multiplexing time is a relative value?

3 What is the carrier?

4 When does the collection of the n words of b bits constitute a frame?

5 How can two types of frame organization be considered?

6 What does the frame alignment consist of?

7 What are the several solutions of signaling?

PART 2 (21.3 – 21.4; 4.1; 4.2, 4.3)

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