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
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
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.
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
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.