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JOLTASAURUS FEB 95 FILE 20
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BRITISH TELECOM DIGITAL SERVICES____________________________________________
The services offered by British Telecom are known as X-Stream Digital
Services. A brief description of these services follows.
KILOSTREAM
----------
This digital service provides full-duplex synchronous point-to-point links at
2400 bits/w, 4800 bit/s, 9600 bit/s, 48 kbit/s and 64 kbit/s. It is
available between approximately 200 telephone exchange areas throughout the
UK. The customer interface to Kilostream is via a Network Termination Unit
(NTU) installed on the customer's premises. An NTU physically resembles a
modem and is connected to a local Kilostream exchange by a 4-wire local
circuit. An NTU transmits and receives digital signals over a 4-wire local
circuit at 12.8 kbit/s or 64 kbit/s using Walsh 2 (WAL2) diphase
transmission. The customer's computer/terminal interface to an NTU is CCITT
X.21 or X.21bis.
The Kilostream X.21 interface operates at 2400 bit/s, 4800 bit/s, 9600 bit/s,
48 kbit/s or 64 kbit/s.
The Kilostream X.21bis interface provides V.24/V.28 (RS-232-C) compatibility
at 2400 bit/s, 4800 bit/s or 9600 bit/s and V.35 compatibility at 48 kbit/s.
The Kilostream network comprises 2 Mbit/s links connecting multiplexers at
nominated serving exchanges via cross-connect sites.
The equipment at a cross-connect site takes in a number of 2 Mbit/s signal
streams and demultiplexes them into individual 64 kbit/s timeslots which are
then routed to the required 64 kbit/s timeslots in the appropriate outgoing 2
Mbit/s signal streams. The equipment is microprocessor controlled and the
routing can be controlled from a VDU either locally or remotely from a
network control centre. The network is monitored by two Network Control
Centres located at Manchester and London. At transmission rates up to and
including 48 kbit/s an NTU adds two extra bits (alighnment bit and status
bit) to each 6 bits transmitted by a user. These two bits are used for
control and in-service monitoring purposes by network control. At
transmission rates of 64 kbit/s these two bits are required for data and
therefore monitoring of a circuit right up to the NTU (in-service monitoring)
is not possible.
MEGASTREAM
----------
This digital leased line service provides point-to-point synchronous 2048
kbit/s (2 Mbits/s) links which can be directly connected to a modern PBX for
voice transmission or can be used for very high speed data transmission.
Megastream also provides even higher speed point-to-point links of 8 Mbit/s,
34 Mbit/s or 144 Mbit/s.
A variety of time division multiplexers (TDMs) are available for use on
Megastream links. A TDM connected at each end of a 2 Mbit/s Megastream link
can provide a number of different multiplexer channel options:
30 analogue 2/4-wire channels, each channel can be used for analogue
speech or for data transmission using modems at data rates up to 9600
bit/s; A total of 30 analogue 2/4-wire channels and/or digital 64 kbit/s
channels (X.21);
Up to 64 synchronous or 54 asynchronous/synchronous data channels
(V.24/V.28, V.10, V.11, V.35) and optionally a number of 32/64 kbit/s
digital speech channels.
INTERNATIONAL KILOSTREAM-MEGASTREAM
-----------------------------------
Before this service was introduced, British Telecom International (BTI) used
analogue circuits to connect users to its satellite earth stations. BTI then
linked the inland Kilostream and Megastream networks to its satellite earth
stations enabling users to have digital access to international services.
For a typical International Kilostream circuit a Network Terminating Unit
(NTU) is located in the customer's UK premises connected by a 4-wire local
line to a nearby Kilostream exchange. From the exchange the circuit is
connected via BT's trunk network to the international gateway in London. At
the gateway the circuit is carrier by satellite link, cable or microwave
system to its international destination.
International Kilostream provides synchronous full-duplex data transmission
at 1200 bit/s, 2400 bit/s, 4800 bit/s 9600 bit/s or 56 kbit/s using an X.21
bis interface (V.24 or V.35).
International Megastream provides synchronous full-duplex data transmission
at 2.048 Mbit/s to CCITT standard G.732. Additional equipment can convert
this to 1.544 Mbit/s G.733 operation for North America. This rate is often
referred to as 2/1.5 Mbit/s.
For a typical International Megastream circuit, Line Terminating Equipment
(LTE) is located in the the customer's premises connected by a special cable
to the nearest digital exchange. From there the circuit is routed to the
international gateway and onward to the distant network using similar
transmission methods to International Kilostream.
BTI provides a "time assigned service" at 56 kbit/s (Kilostream) and 2/1.5
Mbit/s (Megastream) which offers a pre-bookable link between the UK and
distant end international gateways. For this service a full time digital
link to the UK international gateway is required which incurs connection and
annual rental charges.
SATSTREAM
---------
This is a British Telecom International Service which uses small-dish (3.7 to
5.5 m diameter) satellite earth stations ("terminals") to provide digital
links between the UK, Europe and North America. The service provides one-way
or both-way point-to-point (or point to multipoint) digital links which
operate at 64 kbit/s or selected multiples of 64 kbit/s up to 1920 kbit/s.
Multiplexer options are available for lower bit rates, and the service can be
provided on a full time, regular part time or ad hoc basis.
British Telecom "terminals" are sited in a number of fixed locations in the
UK and Satstream is provided as a dedicated link from a "terminal" to a
customer's site. Alternatively, where there is a need, British Telecom can
install on the customer's site a fixed "terminal" or a transporatble
"terminal" mounted on a trailer.
VIDEOSTREAM
-----------
This is a video-conferenceing service which offers full-motion monochrome or
colour video-conferencing between a customer's premises. Equipment is
supplied for installation in conference rooms and offices. One of the main
items of equipment is a CODEC (coder-decoder) which converts a black and
white or colour video signal into a 2 Mbits/s digital signal for transmission
over Megastream or an equivalent bandwidth link. Other items include TV
cameras, television type monitors, loudspeakers etc.
There are two other associated British Telecom services.
Confravision is similar to Videostream except that equipment is located
in a number of British Telecom studios. The studios are booked by
telephone and charged for by the half hour.
The International Video-conferencing service provides a
video-conferencing link between a British Telecom studeo in London and
studio locations in North America, Canada or Europe. A gateway will be
provided to allow Videostream customers to access International
Video-conferencing locations.
SWITCHSTREAM
------------
Packet Switchstream (PSS) is British Telecom's X.25 packet switched data
service. The international extension of PSS is called International PSS and
provides an X.25 gateway to X.25 services in other countries.
Interstream is an associated PSS service which provides a gateway between
PSS, the UK PSTM and UK Telex/Teletex Services.
USA DIGITAL SERVICES_________________________________________________________
The digital services provided by AT&T are known as Dataphone Digital Services
(DSS) and provide data transmission rates of 2400 bit/s, 4800 bit/s, 9600
bit/s, and 56 kbit/s. One of the drawbacks of this service is the lack of a
basic end to end diagnostic capability. This is to be improved with the
introduction of a new service known as DDS-2. This service will provide
secondary channels which will allow users to run their own end to end
diagnostics. The proposed DD-2 secondary channels rates are as follows:
---------------------------------------------------------------------------
Data Channel Secondary Channel Total Aggregate
---------------------------------------------------------------------------
2400 bit/s 133 bit/s 3200 bit/s
4800 bit/s 266 bit/s 6400 bit/s
9600 bit/s 533 bit/s 12800 bit/s
56 kbit/s 2666 bit/s 72 kbit/s
---------------------------------------------------------------------------
Every 8-bits (octet) transmitted or received over a DDS circuit consists of 6
bits of user data and 2 bits for AT&T framing/control functions.
With DDS-2, at data transmission rates of 2400, 4800 and 9600 bit/s the
secondary channel will be derived by using one of the two AT&T bits in every
third octet.
At 56 kbit/s, 9 bits will be used instead of 8 bits, and the 2666 bit/s
secondary channel will be derived by using the ninth bit in every third 9-bit
byte.
INTEGRATED DIGITAL NETWORK (IDN)
--------------------------------
In countries throughout the world, digital transmission and digital switching
equipments are gradually replacing analogue transmission and analogue
switching equipments. The impact has been strongly felt in domestic
telephone networks and in the UK System X digital exchanges are replacing
Strowger exchanges.
From the growth of digital transmission and switching, CCITT standards have
evolved for an Integrated Digital Network (IDN) in which the switching units
are linked by digital transmission paths providing a 64 kbit/s switched
digital network between local exchanges. For a telephony IDN, a telephone
user connects to the network by a conventional 2-wire analogue line. However
if the telephony IDN is extended to the customer's premises by digital
transmission links with enhanced customer to network signalling, then an
all-purpose network can be created. This conceptual network is known as an
Integrated Services Digital Netowrk (ISDN).
THE INTEGRATED SERVICES DIGITAL NETWORK (ISDN)
----------------------------------------------
This is a new network concept which has evolved from the present Integrated
Digital Network (IDN). It is being developed nationally and internationally
and ISDN recommendations (I-series) are being produced by CCITT Study Groups.
The majority of the ISDN (I-series) CCITT recommendations are still in draft
form. They are liable to change and this should be borne in mind when
reading this ISDN description.
The concept of an ISDN is to provide a network where the same digital
switches and paths are used to establish connections for a wide range of
services both analogue (voice) and digital (data). For users who require
access to a wide range of services the ISDN will be a natural successor in
the UK to the PSTN.
Interworking between ISDN and the following Networks/Services is being
considered. This is a general world-wide list and ignores national
variations.
Public Switched Telephone Network (PSTN)
Circuit-Switched Public Data Network (CSPDN)
Packet Switched Public Data Network (PSPDN)
Domestic Satellite System
Leased Digital Lines
Leased Analogue Lines
Switched private networks for PBX
Telex Network
Teletex Network
Videotext Communication
Facsimile Communication
ISDN Interface Structures
-------------------------
Two different interface structures are being defined for the ISDN
user-network interface.
The first is the "basic interface" structure which operatres at 144 kbit/s
and supports two "B" channels each operating at 64 kbit/s and one "D" channel
operating at 16 kbit/s. The "B" channels are used for data transmission and
the "D" channel is used for signalling and low-rate data.
The second is the "primary rate" interface which operates at 2048 kbit/s or
1544 kbit/s. At 2048 kbit/s it supports 30 "B" channels and one "D" channel
and at 1544 kbit/s is supports 23 "B" channels and one "D" channel; all
channels operate at 64 kbit/s. The primary rate interface is intended for
PBX connection.
INTEGRATED DIGITAL ACCESS (IDA)
-------------------------------
British Telecom is offering a Public Network using System X digital switching
and transmission which is the basis of an Integrated Services Digital Network
carrying speech and data in digital form.
To access the ISDN, BT provides an IDA services from customers' premises
which combines speech and data transmission over a single digital link. This
link is being provided in two forms as a Single Line IDA or a Multi-line IDA.
Single Line IDA
A single line IDA provides a customer with two digital channels, each with
its own network address and therefore capable of independent operation. In
the initial IDA pilot phase, a single line IDA is terminated in the
customer's permises on one of two Network Terminating Equipments called NTE1
and NTE3.
An NTE communicates at 80 kbit/s using full-duplex operation over an ordinary
2-wire line to a System X ISDN Exchange. The 80 kbit/s serial stream is
demultiplexed inside the NTE into a 64 kbit/s data/speech channel, an 8
kbit/s data channel and an 8 kbit/s signalling channel. THe 8 kbit/s and the
64 kbit/s channels can be independently routed to separate destinations under
the control of the 8 kbit/s signalling channel. The signalling channel uses
a protocol based on HDLC.
Multi-Line IDA
This IDA connection is designed to connect an ISDN System X exchange to a new
generation digital PBX. It uses a 2 Mbit/s digital link which carries up to
thirty information channels and a common signalling channel. Each
information channel operates at 64 kbit/s and can be used for voice or data.
This type of connection is very similar to that provided by the Megastream
Service which provides a private 2 Mbit/s connection between two PBX/s. In
the case of Megastream, a common channel signalling system has been approved
by British Telecom and the PBX Industry which is known as DPNSS (Digital
Private Network Signalling System). DPNSS is different from the IDA common
channel signalling system. However the Multi-line IDA common channel
signalling channel can support both DPNSS and DASS signalling.
A sophisticated and advanced PBX uses all of the Multi-line IDA features to
provide integrated voice and data services to its connected extensions. A
PBX that provides these functions is referred to as an Integrated Services
PBX (ISPBX). A less sophisticated PBX only provides enhanced telephone
facilities to its connected extensions; such a PBX is referred to as a
Digitally Connected PBX (DCPBX). It is envisaged the many PBX using
Multi-line IDA will start life as a DCPBX and with software enhancements will
eventually become an ISPBX.
Multi-line IDA is not limited to PBX connection only but can be used by any
customer installation which requires a large number of circuits, eg a LAN.
SOURCE_______________________________________________________________________
Practical Data Communications, Fred Jennings, Blackwell Scientific
Publications, UK, 1986
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