21.2.3 Signalling

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:

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

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

3. Find English equivalents:

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

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