Telephone standards

Digital telephones

PART 4 (58.9 - 58.16)

Digital telephones are characterised by having the codec in the telephone to convert the speech from analogue to digital and vice versa. The PCM encoded speech is then sent to line-at a rate of 64kbit/s. In addition a 16kbit/s signalling channel is also present to send address and alerting signals to the exchange. CCITT have now defined standard interfaces for digital operation in the I series of recommendations, but many proprietary interfaces exist.

The I series specifies a 192kbit/s full duplex digital path between the digital telephone (terminal) located in the user's premises and the local exchange. The 192kbit/s is divided into a Bl channel (64kbit/s), a B2 channel (64kbit/s) and a D channel (16kbit/s). The В channels can be used for either voice or data. A digital telephone will consist of a handset and possibly handsfree loudspeaker and microphone connected to an integrated circuit, which will provide the functions of send, receive gain, frequency response, sidetone generation and the codec functions. Additional circuits are necess­ary to provide the physical drive to the line, extract power from the line and operate the link access procedure on the D channel (LAPD).

The functionality of the telephone can be built into the micropro­cessor software (functional model) or respond to stimulus signalling from the exchange (stimulus model). In the former all the call processing logic is designed into the telephone, whilst in the latter it is held centrally in the exchange and the telephone responds only to the messages from the exchange.

European standards for telephones are country specific and contain many national peculiarities, which prevent the production of a single harmonised telephone suitable for use anywhere in Europe. NET 4 (Norme European de Telecommunication) for basic PSTN access was produced under an EEC directive in 1986 and covers all the essential parameters of signalling, transmission and safety. A new EEC directive in 1991 established the concept of CTRs (Com­mon Technical Regulations) which will supersede the NETs with the aim of establishing the essential requirements to be regulated. The harmonised standards give the specifications and tests to be applied to ensure conformity with the requirements. The CTRs will not duplicate other requirements already in existence, such as the LVD (Low Voltage Directive). The objective is, some time in the future, to be able to test and approve telephones in one country in Europe, apply the EC Mark, and sell that telephone in any other country without further approval testing. At present each country issues its own national standards for telephones, either via the PTT as the controlling authority or via a national standards organisation such as BSI (British Standards Institute) in the UK.

In North America the position on standards for telephones is much simpler. The FCC (Federal Communications Commission) issues the basic standards for all of the USA and in Canada the DOC (Department of Communications) issues similar standards.

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