CRC-4 cyclic redundancy check

Frame composition

The CEPT PCM-30 framing format

The CEPT PCM-30 is a PCM format used for time division multi­plexing of 30 voice or data circuits onto a single twisted pair cable using digital repeaters. As already mentioned, the delay with which CEPT has undertaken the definition of a primary digital PCM system has allowed it to profit from the experience of the American systems.

The basic characteristics of this primary PCM multiplex equip­ment are described in CCITT Recommendation G.732. Each voice circuit is sampled at 8 kHz using an 8-bit A-law companding analog to digital converter as specified in CCITT Recommendation G.711, and multiplexed with 29 other sampled channels plus one alignment and one signalling channel, resulting in 32 multiplexed channels.

The standard CEPT frame contains 32 channels of 8 bits each, or 256 bits. With 8000 samples per seconds, the CEPT data rate becomes 8000 x 256, or 2.048Mbit/s.

As with the DS1 D4 framing format, the CEPT PCM-30 frame structure is subject of CCITT Recommendation G.704. Each CEPT PCM-30 frame consists of 32 time slots to include 30 voice chan­nels, one alignment signal and one signalling channel. Time slots 1 to 15 and 17 to 31 are assigned to the 30 voice or data channels. This frame composition is depicted in Figure 21.7.

The frame alignment signal FAS (0011011) is transmitted in bit positions 2 to 8 of time slot 0 of every other frame. Bit position 1 carries the International bit, while frames not containing the frame alignment signal, i.e. the odd frames, are used to carry National and International signalling bits and alarm indication for loss of frame alignment. The loss of frame alignment alarm is announced by alarm bit A, which is the third bit of the odd. To avoid imitation of the frame alignment signal, bit 2 of the odd frames is set to 1.

Time slot 16 in each CEPT 30 frame is used to transmit such signalling data as on-hook and off-hook conditions, dialling digits and call progress. Since a common channel is dedicated for the signalling data of all voice circuits, this method of signalling is referred to as common channel signalling. The signalling consists of four bits per channel, grouped in the two halves of time slot 16. It therefore requires 15 frames to carry the information of the 30 channels. Completed by a 16th frame (frame 0) they constitute a multiframe. Time slot 16 of this 16th frame contains a multiframe alignment pattern MAS which allows unambiguous numbering of the frames within the multiframe. Each channel thus arranges the signalling of 4 bits every 16 frames, i.e. every 2 milliseconds.

Where there is a need to provide additional protection against simulation of the frame alignment signal, and/or where there is a need for an enhanced error monitoring capability, then bit 1 of the frame, i.e. the International bit, is used for the 4-bit cyclic redun­dancy check described below.

Each CRC-4 multiframe, which is composed of 16 frames num­bered from 0 to 15, is divided into two 8 frame sub-multiframes or SMF, designated SMFI and SMFII which indicates their respective order of occurrence within the CRC-4 multiframe structure. The SMF is the Cyclic Redundancy Check-4 block size, i.e. 2048 bits.

In the frames containing the frame alignment signal, bit 1 is used to transmit the CRC-4 bits. There are four CRC-4 bits, referred to as Cl, C2, C3, and C4 in each SMF. In the frames not containing the frame alignment signal, bit 1 is used to to transmit the CRC-4 multiframe alignment signal and two CRC-4 error indication bits (E). The CRC-4 multiframe alignment signal is a 7-bit sequence having the form 0011011. The E-bits are used to indicate received errored sub-multiframes by changing the binary state of one E-bit from 1 to 0 for each errored sub-mulnframe. Recommendation G.704 requires the delay between the detection of an errored sub-multiframe, and the setting of the E-bit that indicates the error state, to be less than 1 second. Table 21.3 shows the allocation of bits 1 to 8 of the frames for a complete CRC-4 multiframe.

1 Learn the words & word combinations:

Twisted pair	Витая пара
frame composition	Структура цикла
frame alignment signal	Сигнал выравнивания цикла
common channel signaling	Синхронизация по общему каналу
multiframe	Сверхцикл
multiframe alignment pattern	Модель (паттерн или образец) выравнивания сверхцикла
unambiguous numbering	Сокращенная нумерация
СКС – 4 cyclic redundancy check	Контроль циклическим избыточным кодом в 4 бита
enhanced error monitoring capability	Возможность усиленного контроля ошибок

2 Find Russian equivalents:

ü Time division multiplexing; ü A single twisted pair cable; ü Digital repeaters;

ü loss of frame alignment alarm; ü Frame composition; ü On-hook & off-hook conditions.

3 Find English equivalents:

ü Цепи данных; ü Основные характеристики; ü Мультиплексное оборудование; ü Структура цикла; ü Сигнал цикловой синхронизации;

ü Нечетный; ü Общий канал сигнализации; ü Потеря; ü Обнаружение ошибки; ü Сверхцикловая синхронизация; ü Дополнительная защита.

4 Answer the questions:

1 What is the CEPT PCM – 30?

2 What are the basic characteristics of the primary PCM multiplex equipment?

3 What does the standard CEPT frame contain?

4 When is the 4 – bit cyclic redundancy check used?

PART 4 (21.6; 6.1; 6.2; 7)

Дата добавления: 2015-03-31