49. 2. 2 Main service principles

1. The system must make provision for voice and non-voice transmission.

2. All radio channels are available to all users and applications, hence channel licensing and regulation is not needed.

3. The air interface between the handset and base has a common specification for telephony so that a common handset design can be used in all applications.

1. Speech quality equivalent to that of a wired telephone.

49. 2. 3 Traffic capacity

Research has shown (Swain, 1985) that a modest CT penetration of 7% of all telephone terminals in a city centre could produce mean traffic densities in excess of 800 Erlangs per square km. (Note an Erlang is the amount of traffic carried by one line operating for 100% of the time. See Chapter 5). Other studies (ETSI, 1992a) indicate the following traffic densities by application:

1. Residential suburban house, 150E per sq. km at 0. 05E per telephone and cordless penetration of 30%.

2. Residential apartment block, 200E per sq. km at 0. 05 E per telephone and cordless penetration of 30%.

3. Cordless business system, 10000 E per sq. km per floor at 0. 2E per terminal and cordless penetration of 100%.

4. Telepoint service (examples of railway stations and airports), 900-5500E per sq. km.

49. 2. 4 Digital cordless standards

The foregoing requirements clearly point towards a new digital cordless telecommunications standard and as early as 1981 studies were in progress to identify the technical options. Since that time two digital standards have been developed within Europe to the status of interim European Telecommunications Standards by the European Telecommunications Standards Institute (ETSI), as fol­lows:

1 CT2/CAI; which translates to second generation CT with a Common Air Interlace (ETSI. 1991).

2 DECT; which means Digital European Cordless Telecommunications (ETSI, 1992b).

The key factor in both standards, however, is that they offer a common air interface specification. This means that each can support a public access voice service in which the handset and base station may be manufactured by different companies and yet still signal and communicate to each other to establish the call. This is not a feature of the earlier analogue standards.

These two ETSI standards are considered by many to be complementary in terms of the market expected to be served and the entry time to that market. CT2/CAI has been optimised to serve the residential and small (business) cordless markets with potential to open up the high capacity business market. DECT, however, has been designed from the outset to meet the demands of the very high capacity cordless office with roaming and in-call handover as standard features of the specification. It also has the capability to be appropriately configured for the residential markets. Both have telepoint capability.

49. 3 Ct2/cai digital specification

These specifications are described in Swain (1990), Holmes and Swain (1990) and Evans (1940). The FDMA/TDD principle of operation of CT2/CA1 is shown in Figure 49. 1. Duplex transmission is provided by transmitting in time interleaved burst mode on the same carrier frequency for both directions. This technique was appropriate for a low cost instrument since both ends are identical. Furthermore the technique only required a single block of radio spectrum rather than the duplex bands favoured by other mobile radio systems. Hence the band 864-868MHz was allocated to the service to support forty 100 kHz duplex channels.

By adopting the CCITT standard 32kbit/s ADPCM speech coding algorithm it was possible to contain the transmitted symbol rate to 72kbit/s with some allowance for framing and signalling bits. In practice the corresponding B-channel capacity is 32kbit/s and that for the D-channel 1 kbit/s or 2 kbit/s depending on manufacturer's choice.

By adopting a common air interface the radio interface is stand­ardised in terms of:

1. Physical parameters of the radio link e. g. modulation, frame rate, data rate.

2. System and user signalling.

3. Speech transmission.