Implementation
Date: 2015-10-07; view: 406.
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.
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
| multiplex | объединение, уплотнение (сигналов, каналов); мультиплексирование | |
| to convey | перемещать; передавать; проводить | |
| OSI (open systems interconnection model) | модель взаимодействия открытых систем | |
| equalization | коррекция; компенсация; выравнивание | |
| dispersion | дисперсия (случайной величины); разброс (значений) | |
| time dispersion | временная (частотная) дисперсия | |
| to support | обеспечивать; обслуживать; поддерживать | |
| rate | скорость; отношение; коэффициент | |
| error rate | частота (появления) ошибок; коэффициент ошибок | |
| information rate | скорость передачи информации | |
| preamble | заголовок; вводное поле; преамбула сообщения | |
| to intercept | перехватывать; подслушивать | |
| interceptor | станция перехвата; подслушивающее устройство | |
| receive mode | режим приема | |
| search mode | режим поиска | |
| identity | идентичность, тождество | |
| throughput | пропускная способность | |
| bit timing | тактовая синхронизация | |
| reversals | обратное движение (ход), реверсирование | |
| burst synchronization | пакетная синхронизация | |
| loop delay | запаздывание в петле (шлейфе) | |
| adaptive differential pulse-code modulation | адаптивная дифференциальная импульсно-кодовая модуляция | |
| power requirement | потребность в электроэнергии | |
| radio frequency duplexer | радиочастотный дуплексер (перключатель) |
Exercise 2 Read the text
Exercise 3 Give the Russian equivalents for the following English ones:
| to employ three multiplex structures | |
| the call is in progress | |
| the base and handset identities | |
| the observed performance | |
| to be about half the basic throughput | |
| the channel marker word (CHM) | |
| to transmit the same frame structure | |
| call set up is abandoned | |
| the above mentioned multiplexes | |
| a number of message packages | |
| in the case of | |
| the overall speech transmission characteristic | |
| the digital speech codec algorithm | |
| a radio frequency duplexer |
Exercise 4 Answer the following questions:
| How many multiplex structures does CT2/CA1 employ in order for a call to be set up and maintained? | |
| What is multiplex 1 used for? | |
| In what cases is multiplex 2 used? | |
| When do we need multiplex 3? | |
| At various stages of a call set up three channel types are needed, aren't they? | |
| What does layer 1 ensure? | |
| What does a layer 2 message package consist of? | |
| What are the main characteristics of layer 3 messages? | |
| Why is it necessary to define the transmission characteristic of both the handset and base unit? | |
| Are the requirements the same for interworking with digital networks and in the case of analogue network interconnection? | |
| Why has the loop delay imposed on a CT2/CAI link been set less than 5 ms? | |
| How do basic principles of CT2/CA1 operation minimize the cost of the basic digital link? |
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