High bit rate digital subscriber line

RECENTLY MAJOR thrust has been given for modernization of the 'access network' of the subscriber line in the Indian Telecom, after the successful upgradation of switching and transmission networks to digital in the last decade. One of the methods for improving the performance of the existing cable pair in the local telephone system is by converting it into a virtual digital pipe with enhanced capability to carry higher bandwidths

ISDN (Integrated Services Digital Network) is one such technique, where a single copper cable is made to work as two 64 kbps circuits using suitable interfaces. But this only can serve as a connection to a single subscriber. When a group of subscribers in a single location wants higher bit rate of transmission, or one subscriber wants more number of 64 kbps channels, or an EPABX connectivity is to be given with more channels at higher rates of transmission using the same local cable pair a new standard has been evolved by ETSI (European Telecom Standard Institute) and ITU (International Telecom Union) known as HDSL (High Bit-rate Subscriber Line).

The economic efficiency contributed by such a system can be manifold such as, Saving in copper, improved quality of service, reliable and high speed transmission, dispensing with additional cable laying process, repeaterless transport over unconditioned twisted pairs and centralized network management system for maintenance of large number of subscribers.

E1- E1 Digital Link: This is used for extending 2 Mbps (E1) bandwidth to the customer premises from the Network Provider Location through 2 pairs of ordinary twisted pairs of copper cable. Some examples are: E1 Link of 30 Ports to the ISP and ISDN Primary Rate Interface (PRI) connection to subscribers in the local network.

Pair-gain system:

HDSL serves as a pair-gain system between local exchange and any location where there is a concentration of 30 or group of 30 subscribers including local PCO and Hotline, by using 2 pairs copper cable.

Junction lines to DID (Direct Inward Dialing) EPABX: Electronic PABX having direct inward dialing facility catering to more number of subscribers in a locality can be served by HDSL System for the incoming junctions from local exchange.

Upgrading Junction Network:

Inter-exchange junction network can be upgraded and also augmented without laying additional cable by using the HDSL technology.

The E1 line operates at a nominal rate of 2.048 Mbps using the line code HDB-3. the data transfer takes place @ 1168 Kbps in each of the two loops formed, using advanced equalization, adaptive filtering, and echo cancellation techniques.

E1 line is organized in frames.One frame of 125 microseconds contains 32 Time Slots (TS) of 8 bits each.

The interfacing functions:

Line Terminal Unit (LTU) or Central Office Terminal (COT) is the interface equipment provided at the Network Provider end manages the payload distribution, controls the system startup procedure, and provides timing reference for HDSL line transmission.

Other operational functions include, mapping of E1 data bits into HDSL frame, frame alignment functions, Loop identification and correction, system management and collection of performance data.2B1Q (2 binary and 1Quaternary) is the code used.The advantage of the COT\RT type HDSL interface is it facilitates modular growth and we can go up to 6 racks.

It has an in-built Network Management system connected through RS-485 to a PC for carrying out the following management - configuration, security, performance and fault tracing and clearing.

A.Ganesan

Divisional Engineer BSNL, ChennaiTelephones

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