Introduction To Line Telecommunications

Return to:
Index or _
Products _
6. Private Circuit Links
  6.1 - Private Wires And Access Lines
  6.2 - Leased Lines
  6.3 - Digital Leased Lines

6.3 - Digital Leased Lines

With the advent of digital communication technology the service providers were able to offer digital leased line services to customers. These links are able to transmit voice and data and offer high capacity connections to be used to join sites where traffic is heavy. Since they are leased, calls over them are free, however they are only economic when the cost of leasing is less than the cost of dialling each call over the public network.

Specialist equipment called multiplexers are required at each end of the link to convert signals for transmission and to decode it again. These are usually separate units, but in a large installation may be part of a larger communication system.

6.3.1 Kilostream

Kilostream is a fixed point to point link for the exclusive use of the leasing customer. It is a 4 wire system operating at 64 Kbps (64 x 1024 data bits per second). At the local exchange the data is combined with other traffic and sent via the telephone network to the exchange closest to its destination where it is separated and passed to the customer site.

The 64Kbps capacity can be subdivided using multiplexing equipment to provide voice and data channels simultaneously over the link. These divisions of capacity are usually 8Kbps, 16Kbps or 32Kbps blocks dependant upon the bandwidth or capacity required by each person or device using it.

Speech can be transmitted using an 8Kbps channel, but the reduced bandwidth means that the quality of transmission is low. 16 Kbps is a better compromise between capacity and quality. Compression and coding techniques can now overcome these quality issues.

Data transmission speeds are increased with larger bandwidth, so the widest channels possible should be used to increase efficiency. Kilostream services are now offered in blocks of 64Kbps channels called a Kilostream-N, where N denotes multiple channels usually between 2 and 30.

6.3.2 - Megastream

The principle behind Megastream is similar to that of Kilostream. Megastream is a point to point link with a data rate of 2Mbps (32 x 64 x 1024bps = 2048Kbps = 2Mbps). It comprises 32 Kilostream channels, 30 for transmission and reception of data and 2 for signalling and control of the link. This allows far higher data transmission speeds and larger capacity channels to be created for the customer. Since Megastream is 30 bearer channels, it is in effect a Kilostream-N service with 30 channels.

Megastreams are used within the network by the service providers to pass call traffic between exchanges and they form the basic platform on which ISDN an its associated protocols are based. In a similar way to the combination of Kilostreams providing higher bandwidth, combining Megastreams also provides higher bandwidth. These types of grouping are used by the network providers and very heavy-duty users to create even faster links.

6.3.3 - X.21

X.21 is a data protocol, which can be implemented over a digital private circuit. It is used to connect local and wide area networks for data transfer and is not used in itself for voice traffic. The interconnected data networks may offer a voice encoding option such as Voice Over IP, but these types of link are beyond the scope of this book.




Return to top of page

Extracted from

Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000