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