Idea Transcript
PCM-Connections Contents 1 General 2
Coaxial cable
3
Twisted paired cable and optical fibre cable
4
Mini-Link
1 General Pulse Code Modulation, PCM, is a modulation method used to convert analogue information into digital form. The conversion of an analogue signal into a digital signal is based on three main principles: sampling, quantization and coding. PCM is also suitable for Time Division Multiplex, TDM, which facilitates economical transmission systems. In the telephony field CCITT has recommended two different PCM-systems of the first order, namely a system proposed by CEPT and one by American AT&T. The CEPT-system contains 32 time slots, 30 of which are channel time slots, one a signal time slot and one is for link synchronization, whereas the American system contains 24 time slots. These systems constitute the base for two digital transmission system hierarchies. Like Frequency Division Multiplex, FDM systems, by bringing a number of PCM-systems of the first order together it is possible to form a second order PCM-system and so on. The outgoing bit frequency for a 30/32-channel system is 2048 Kbit/s. In ASB 501 04 PCM-connections are used between LIMs and GS and between the PABX and public exchange or private exchanges. Coaxial cables, twisted paired cables or optical fibre cables are used as transmission media. Which transmission medium is to be used depends on the number of PCM-connections, environmental prerequisites and distance.
3 Coaxial cable Transmission via standard coaxial cable is used for short distances of about 40 metres (131 feet), e.g. between LIMs and GS or between LIMs. The maximum distance with special coaxial cable is about 350 metres (1148 feet).
4 Twisted paired cable and optical fibre cable Transmission via twisted paired cables can be used for distances of up to 2000 metres, (6520 feet), without recourse to intermediate repeaters. Long distances require intermediate repeaters for every 2000 metres. Optical fibre cable is less sensitive to disturbances than e.g. twisted paired cable ad distances of up to 7000 metres (22966 feet) can be used. Connection to optical fibre cable and twisted paired cable demands special line terminal equipment, LTU, see figure 3-1 and 3-2. The main tasks of the latter are: signal adaptation between the digital interfare and line interface power supply to terminal equipment and the intermediate repeaters monitoring and indication of alarm states fault location by own regenerators. For connection to optical fibre cable the LTU can be placed in a LIM-magazine if room exists. For connection to twisted paired cable used for distances more than 2000 metres, intermediate repeaters are necessary for every 2000 metres. If more than seven intermediate repeaters are used between the LTUs this will necessitate a special Power Feeding Unit, PFU. Note: A PFU occupies one LTU-board position.
Figure 6-1. Using twisted paired cable to interconnect LIM/TLU or GJUL4 with public exchange, GSM/GJUG5 or LIM/GJUL4
Figure 6-2. Using optical fibre cable to interconnect LIM/TLU or GJUL4 with public exchange, GSM/GJUG5 or LIM/GJUL4
7 Mini-Link Radio link systems exist as an alternative to cable systems for digital transmission. ERICSSON has designed a system family, MINI-LINK, for private networks. This family comprises five main types: MINI-LINK 7-E, 15-E, 23-E, 26-E AND 38-E. MINI-LINK data: MINI-LINK 7-E 15-E 23-E 26-E 38-E Capacity (Mbit/s) 2x2- 2x2- 2x2- 2x2- 2x2 17x2 17x2 17x2 17x2 17x2 Max. jump length (km) 40 30 15 12 10 Frequency range (GHz) 7.125- 14.5- 21.6- 24.5- 37.0 7.725 15.35 23.6 26.5 39.5 A MINI-LINK terminal is a complete duplex radio link with antenna and terminal equipment in one unit. It is designed for direct connection to the PABX from which it also receives its power, see figure 4-1.
Figure 7-1. Using radio link system, MINI-LINK, to Interconnect a remote LIM and GSM