------------------------------------------------------------------------------- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::88888 8888888 888 888 ::888888 ::88888 :888 888 ::888888 ::88888 ::88888 :::: :::8888 888 888 888 888 ::88 8888 888 888 888 888 ::888 888 888 888 888 888 ::: :::8888 888 : 888 888 ::88 8888 888 888 888 888 ::888 888 888 888 888 888 ::: :::8888 888 88888888888 ::888888 :8888888 8888888 ::888 888 888 888 888 888 ::: :::8888 888 888 888 888 ::8888888 888 888 888 888 ::888888 888 888 888 888 ::: :::8888 888 888 888 888 ::88 8888 888 888 888 888 ::8888 ::888 888 888 888 ::: :::8888 888 888 888 888 ::88 8888 888 888 888 888 ::8888 :::888 888 888 888 ::: :::8888 888 888 888 888 ::88 8888 888 888 888 888 ::8888 :::888 888 888 888 ::: ::88888 8888888 888 888 ::888888 888 888 888 888 ::8888 ::::88888 ::88888 :::: :::.....:.......:...:...:::......::...:...:...:...:::....:::::....::::....::::: ------------------------------------------------------------------------------- JOLTASAURUS FEB 95 FILE 20 ------------------------------------------------------------------------------- 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 ############################################################################## #contact: jolt@cyberspace.org :::::::::::::::::::::::::::::::::::::::::::::::# ##############################################################################