49. 3. 3 Protocol structure

Signalling in СT2/СА1 is similar in form to that adopted in Layers 1, 2 and 3 of the Open Systems Interconnection (OSI) model. Layer 1 ensures that communication channels can be established and maintained. To achieve this three channels types are needed:

1. B-channel for speech or data.

2. D-channel for signalling and control.

3. SYN-channel for bit and burst synchronisation.

At various stages of a call set up these three are conveyed by the above mentioned multiplexes.

Layer 2 supports the ability of CT2/CA1 to communicate mess­ages between end points of the link. Such messages include those associated with the D-channel error detection and correction, mess­age acknowledgement, call set up and clear down messages etc. A Layer 2 message package consists of up to six words each formed from eight 8-bit bytes. The first word is the address code word and the remainder are data code words. Messages may be formed from a number of message packages.

Layer 3 messages are the information elements delivered error free by layer 2 to the end of the CT2/CAI radio link. These elements have meaning and can be translated into specific responses. For example keyed digits 0-9, star and square, handset display elements and many other messages that a manufacturer may wish to employ in his product.

49.3. 4 Transmission plan and digital codec

To meet the European standard objectives it is necessary to define the transmission characteristics of both the handset and base unit. For interworking with digital networks the requirements of E'I'SI draft recommendation NET33 are met. In the case of analogue network interconnection, for historical reasons, the requirements change from country to country throughout Europe. So to ensure a common handset specification, particularly for telepoint application, any necessary country specific corrections to the overall speech transmission characteristic is built in to the base unit. To avoid problems arising from terminal speech echo in the trans­mission network the loop delay imposed on a CT2/CAl link has been set to less than 5 ms.

The digital speech codec algorithm used by CT2/CAI has been taken from the CCITT Recommendation G.721 (Blue Book) which defines a 32 kbit/s adaptive differential pulse code modulation codec. Some, strictly controlled, licence is given to manufacturers to simplify the resulting codec in order to limit power requirements and complexity.

49. 3. 5 Implementation

The basic principles of CT2/CAl operation offer significant oppor­tunities to minimise the inherent cost of the basic digital link. For example the 100 kHz channelling does not require time dispersion equalisation even when operating in a multipath environment with no line-of-sight signal component. Furthermore there is no need for a radio frequency duplexer and less carrier frequency sources are required. From a speech transmission point of view the moderate processing delays involved do not require echo control circuitry or raise absolute network delay issues. With these benefits the radio circuit design is simplified and the complexity of implementation is concentrated at baseband which can made using low power consumption, large scale integration CMOS technology.

Figure 49.7 is an example of the degree of integration that is possible and which leads towards a common set of components to support all applications. This is a key objective in realising the benefits of large volume production in terms of minimised compo­nent costs.

Exercise 1 Learn the following words and word combinations

1	multiplex	объединение, уплотнение (сигналов, каналов); мультиплексирование
2	to convey	перемещать; передавать; проводить
3	OSI (open systems interconnection model)	модель взаимодействия открытых систем
4	equalization	коррекция; компенсация; выравнивание
5	dispersion	дисперсия (случайной величины); разброс (значений)
6	time dispersion	временная (частотная) дисперсия
7	to support	обеспечивать; обслуживать; поддерживать
8	rate	скорость; отношение; коэффициент
9	error rate	частота (появления) ошибок; коэффициент ошибок
10	information rate	скорость передачи информации
11	preamble	заголовок; вводное поле; преамбула сообщения
12	to intercept	перехватывать; подслушивать
13	interceptor	станция перехвата; подслушивающее устройство
14	receive mode	режим приема
15	search mode	режим поиска
16	identity	идентичность, тождество
17	throughput	пропускная способность
18	bit timing	тактовая синхронизация
19	reversals	обратное движение (ход), реверсирование
20	burst synchronization	пакетная синхронизация
21	loop delay	запаздывание в петле (шлейфе)
22	adaptive differential pulse-code modulation	адаптивная дифференциальная импульсно-кодовая модуляция
23	power requirement	потребность в электроэнергии
24	radio frequency duplexer	радиочастотный дуплексер (перключатель)

Exercise 2 Read the text

Exercise 3 Give the Russian equivalents for the following English ones:

1	to employ three multiplex structures
2	the call is in progress
3	the base and handset identities
4	the observed performance
5	to be about half the basic throughput
6	the channel marker word (CHM)
7	to transmit the same frame structure
8	call set up is abandoned
9	the above mentioned multiplexes
10	a number of message packages
11	in the case of
12	the overall speech transmission characteristic
13	the digital speech codec algorithm
14	a radio frequency duplexer

Exercise 4 Answer the following questions:

1	How many multiplex structures does CT2/CA1 employ in order for a call to be set up and maintained?
2	What is multiplex 1 used for?
3	In what cases is multiplex 2 used?
4	When do we need multiplex 3?
5	At various stages of a call set up three channel types are needed, aren’t they?
6	What does layer 1 ensure?
7	What does a layer 2 message package consist of?
8	What are the main characteristics of layer 3 messages?
9	Why is it necessary to define the transmission characteristic of both the handset and base unit?
10	Are the requirements the same for interworking with digital networks and in the case of analogue network interconnection?
11	Why has the loop delay imposed on a CT2/CAI link been set less than 5 ms?
12	How do basic principles of CT2/CA1 operation minimize the cost of the basic digital link?

Part III (49.4 – 49.4.3)