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8 Cordless Telephone Systems Cordless telephone systems are mobile radio systems that, although their transmission range is not suitable for wide-area coverage, are attractive to many users because of the services offered and their low cost. Cordless telephony (CT) can be used within the area of a subscriber line, and provides a radio link within a radius of approximately 50 m inside a build- ing or 300 m outdoors between the fixed terminal and a mobile part. This requires the fixed terminal, as the base station, and the mobile part each to be equipped with transmitting and receiving capabilities. Along with their suitability for home use (a base station, a mobile part), CT systems lend themselves to other uses: • Microcellular cordless private branch exchanges, e.g., for use in offices and industrial plants. • Public cellular systems with local coverage and as regional or nation- wide networks for public cordless coin-operated telephones from which calls can be made from appropriate portable terminals without use of previously conventional telephone cells (Telepoint service). • Wireless access technology for stationary users of telecommunications networks. The first cordless telephones in Europe came from the USA and the Far East. This CT0-designated equipment, which was not authorized in the Eu- ropean countries, used analogue transmission technology at 1.6 or 4.7 MHz with eight channels (each with 25 kHz), and because of the lack of security provisions was particularly susceptible to eavesdropping (see Table 8.1). Fur- thermore, the equipment could be used (fraudulantly) as a telephone handset to access other base stations and their lines. The disadvantages of CT0 equipment led to the development of the CT1 standard for cordless telephony in 1983 under the aegis of CEPT. Two bands with a 2 MHz bandwidth and 45 MHz duplex spacing at 900 MHz were stipu- lated as the frequency bands for these systems, which incorporated analogue transmission technology (see Table 8.1). The frequencies from 914 to 916 MHz were designated for the link between the mobile and the fixed parts, and the frequencies from 959 to 961 MHz for the transmission from the fixed part to the cordless telephone. In accordance with the CT1 standard, FDMA tech- nology is used to access the total 40 channels available, each with a 25 kHz Mobile Radio Networks: Networking and Protocols. Bernhard H. Walke Copyright © 1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic) 454 8 Cordless Telephone Systems Table 8.1: The main parameters of cordless analogue telephone systems System CT0 CT1 CT1+ Signal transmission Analogue Analogue Analogue Frequency band [MHz] 1.6/4.7 914–915 885–887 959–961 930–932 No. of channels 8.0 40 80 Bandwidth [MHz] 0.4 2 4 Channel spacing [kHz] 25 12.5 Access procedure FDMA FDMA FDMA Duplex procedure FDD FDD FDD Channel allocation Fixed Dynamic Dynamic Cellular networks No Limited Limited Max. transmitter power [mW] 10 10 Range [m] < 1000 < 300 < 300 Handover No No No Capacity [Erl./km➨] 1.0 200 200 bandwidth. These channels are allocated dynamically; there is a fixed alloca- tion of frequencies to individual devices or local areas. It soon emerged that the number of channels available with the CT1 stan- dard was inadequate for metropolitan areas, and this led to the decision by different countries, including Germany, to introduce a system with 80 chan- nels in accordance with the CT1+ standard in 1989. With CT1+ the mobile part transmits to the fixed part in the 885–887 MHz frequency range, whereas the frequencies between 930 and 932 MHz are used for the link between the fixed part and the mobile device (see Table 8.1). An organizational channel is being planned for CT1+. Unauthorized access to the base station has been almost completely elimi- nated with the CT1 and CT1+ standards because of a number of codes that provide a clear identification between the mobile and fixed parts; however, these systems are not fully protected from eavesdropping. 8.1 CT2/CAI and Telepoint In contrast to other European countries, in the mid-1980s Great Britain started using a version of cordless telephony incorporating the T-standard from the USA. Because these systems did not provide adequate capacity (eight channels) and there was concern about a foreseeable frequency collision with GSM, Great Britain was not keen to continue with the CT1-standard, and a digital standard for cordless telephony was developed at the initiative of the network operator British Telecom. With the aim of making cordless telephony attractive to a broad range of customers, it was planned that CT2 terminals should also be used for Tele- 8.2 Technical Parameters of CT2/CAI 455 point applications. The Telepoint concept allows users with the appropriate equipment to set up a connection to the public telephone network over a Telepoint base station within a radius of up to 300 m of a public and highly frequented area (pedestrian area, railway station, airport, shopping centre, etc.), but with no facilities for receiving calls [1]. The CT2 standard is frequently supplemented with the abbreviation CAI (Common Air Interface), which denotes a radio interface between the fixed part and the mobile part developed by the Department of Trade and Indus- try (DTI) in Great Britain with the participation of British industry. The interface was required to enable existing non-compatible terminal equipment to access the Telepoint service. The CT2/CAI standard was designed with enough flexibility so that the same handset can be used to make calls at an office, at home and to the public network. CAI basically allows incoming as well as outgoing calls. The restriction that exists with Telepoint, i.e., that only outgoing calls are possible, is only due to the terms of the licence and not to any technological limitations. CT2/CAI has now been adopted as an ETSI Interim-Standard. 8.2 Technical Parameters of CT2/CAI Technically, CT2 is based on the CT1 standard; however, digital transmission has been introduced (see Table 8.2). The system has more than 40 frequency channels, each with a 100 kHz bandwidth, in the frequency range between 864 Table 8.2: The main parameters of cordless digital telephone systems System CT2/CAI DECT Signal transmission Digital Digital Frequency band [MHz] 864–868 1880–1900 No. of channels 40 120 Bandwidth [MHz] 4 20 Channel separation [kHz] 100 1.728 Access procedure FDMA FDMA/TDMA Duplex procedure TDD TDD Channel allocation Dynamic Dynamic Voice channels/carriers 1 12 Coding ADPCM 32 kbit/s ADPCM 32 kbit/s Modulation data rate 72 kbit/s 1.152 Mbit/s Modulation Two-level GFSK GFSK Cellular networks Limited Yes Max. transmitter power [mW] 10 250 Range < 300 < 300 Handover No Yes Capacity [Erl./km➨] 250 10 000 456 8 Cordless Telephone Systems and 868 MHz, which, unlike CT1, does not collide with other standardized mobile radio services in Europe. Two-level GFSK (Gaussian Frequency Shift Keying) is used for modula- tion. The nominal bit rate per channel is 72 kbit/s, which means that data transmission is also possible, albeit at a relatively low net rate. CT2/CAI is the first ever mobile radio system in which the base station as well as the mobile terminal transmit on the same radio channel. Yet the two transmission directions are not separated by different frequencies; instead the pingpong technique is employed, in which the transmission direction is changed every millisecond on the same frequency. With this procedure, called Time-Division Duplex (TDD), the expensive filters required with Frequency- Division Duplexing (FDD) for switching transmission direction are replaced by simple switches. The functions of the protocols of layers 1 to 3 specified for the air interface in the CAI standard correspond to those of the ISO/OSI reference model: Layer 1 The specifications apply to the physical transmission system, includ- ing modulation procedure, frame structure, synchronization, time be- haviour and bit rate, along with channel selection and link control. Layer 2 The tasks of this layer include error detection, error correction, mes- sage acknowledgement, connection control and identification. Layer 3 The functions and message elements of this layer are used in the sig- nalling for connection control, similarly to the ISDN-D-channel protocol. It is responsible for recognizing the type of messages, for setting up and terminating calls and for maintaining connections. Three logical subchannels are defined by the standard: • The B-channel, in which speech and, with the use of a modem, data can be transmitted. • The D-channel, for signalling. • The SYN-channel, which transmits information on bit and burst syn- chronization. Depending on the application, these subchannels are allocated different channel capacities in four different socalled multiplex frames (see Figure 8.1) [2]. Multiplex 1.2 This defines a 66-bit long burst of duration 1 ms that contains 64 B-channel bits and 2 D-channel bits. Since this frame is used with an existing connection, there is no need for an SYN channel. If the synchronism is lost when this operating mode is used, the connection must be reinitialized. Because this burst is sent every 2 ms, the data rate in the B-channel is 32 kbit/s and that in the D-channel 1 kbit/s. 8.2 Technical Parameters of CT2/CAI 457 Multiplex 2 24 bit 16 bit10 bit16 bit D-Channel CHMP or Sync. D-ChannelPreamble 66 bit (1 ms) D-Channel B-Channel D-Channel 1 bit 64 bit 1 bit 66 bit (1 ms) 68 bit (1 ms) D-Channel 2 bit64 bit B-Channel 2 bit D-Channel Multiplex 1.4 Multiplex 1.2 D + 10 P 8 10 DP 810 DP 8 D + 10 P 810 DP 8 D + 10 P 810 DP 8 D + 10 CHMP 2412 PCHMP 2412 P 12 P CHMP 24 12 P CHMP 24 P 6 Frame No. 1 2 3 4 5 6 7 144-bit Frame (2 ms) Sub-MUX 1 Sub-MUX 2 Sub-MUX 3 Sub-MUX 4 Like Frame1; Different Data in Channel D Like Frame1; Different Data in Channel D Like Frame1; Different Data in Channel D Receive Receive CHMP = Channel Marker for Portable P = Preamble on Synchronization-Channel The first 20 bits of the D-Channel are repeated in each Sub-Mux period In slots marked with ’+’ the D-Channel starts with a Synchronization Word Multiplex 3 (Portable to Base) Figure 8.1: The structures of the multiplex frames with CAI Multiplex 1.4 This is set up similarly to the Multiplex 1.2 burst, although a total of 68 bits are transmitted in 1 ms, of which four are D-channel bits. Therefore the data rate on the D-channel is 2 kbit/s. This multiplex frame is used with an existing connection if the base station and the mobile device have indicated during call setup that they are able to support the 68-bit long burst. Multiplex 2 This is a 66-bit long burst consisting of 32 D-channel bits and 34 SYN-channel bits, and is used during normal call setup and to restore an interrupted connection. A total of 16 kbit/s are available to the D- channel and 17 kbit/s for the SYN-channel. The SYN-field consists of a 10-bit long preamble, followed by one of three different 24-bit long synchronization patterns—a so-called channel marker for portable. Multiplex 3 This defines a 10 ms long burst that is used to set up and restore a connection that has been initiated by a handset. Here 10 ms are divided into five 2 ms long frames, each of which is subdivided into four identical subframes. The first four frames contain 20 D-channel bits and 16 preamble bits in each subframe, whereas the fifth frame only contains SYN-channel bits. Since the B-channel is used for transmitting speech, the data rate is 32 kbit/s. Speech is coded through Adaptive Differential Pulse Code Mod- ulation (ADPCM). 458 8 Cordless Telephone Systems The transmitter power of cordless CT2 equipment is a maximum of 10 mW, which provides a range outdoors of up to 200 m between the mobile part and the base station. Since distances are rarely this great in practice, there are plans in the CAI standard to reduce the transmitter power of the mobile parts dynamically in order to reduce interference. This is carried out by the base station, which measures the received field strength and, if necessary, signals the handset to reduce its transmitter power. The CAI standard contains a number of security measures. It supports the exchange of identifiers between the fixed and mobile parts, with the cordless CT2 terminal of a subscriber not being available for use until a personal identification number (PIN) has been entered. Authentication between the terminal and the fixed part is supported through the appropriate allocation of serial numbers. Because of the use of digital transmission technology, voice encryption is easy but efficient, thereby making CT2/CAI systems safe from eavesdropping. The CAI specifications allow a subscriber to have charges transferred to other telephone numbers (e.g., of his company) and billed there. Compared with CT1+ with the same bandwidth, these systems have a higher capacity of around 250 Erl./km➨ because of digital transmission. References [1] M. P. Clark. Networks and Telecommunications: Design and Operation. Teubner, 1991. [2] W. Tuttlebee, editor. Cordless Telecommunications in Europe. Springer, Berlin, 1990. . ever mobile radio system in which the base station as well as the mobile terminal transmit on the same radio channel. Yet the two transmission directions. Table 8.2: The main parameters of cordless digital telephone systems System CT2/CAI DECT Signal transmission Digital Digital Frequency band [MHz] 864–868 1880–1900