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8. WAN NETWORK SERVICES

8.1. Frame Relay

Frame relay is a connection oriented services operating at n x 64 kbps or 2.048 Mbps. It has evolved from X.25 packet switching and aims to reduce network delays, protocol overheads and equipment cost. Error correction is done on an end to end basis rather than a link to link basis as in X.25 switching. Frame relay can support multiple users over the same line and can establish a permanent virtual circuit or a switched virtual circuit.

Like ATM it is a protocol which must be carried over a physical link such as a Kilostream or Megastream link. While useful for connection of LANs, the combination of low throughput, delay variation and frame discard when the link is congested will limit its usefulness to multimedia.

8.2. SMDS

The Switched Multi-megabit Data Service (SMDS) [King93] is a new switched broadband data service. One of the first users of the service in the UK have been SuperJANET sites. SMDS provides a switched connectionless data service at speeds of 34 Mbps (at present) for connection of LANs. It uses variable length packets up to 9188 bytes in length, each of which carries an address in the E.164 format (ISDN uses this address format too) SMDS packets are transported in the public network using the Distributed Queue Dual Bus (DQDB) IEE 802.6 standard which uses packets fixed at 53 bytes. There is some overhead from this conversion process which reduces the bandwidth available to users to about 75% of the line speed. There are several access classes that limit the sustained data rate and burst data rate that can be injected into the network by a user. These access restrictions may result in discard of packets that exceed a certain limit. SMDS does not support timing. The higher speeds of SMDS will be of benefit to multimedia applications seeking to transfer large volumes of data quickly, but the lack of a time structure will reduce the video conferencing quality obtainable.

8.3. The Integrated Services Digital Network (ISDN)

In the real world the delivery of multimedia requires a widespread network capable of delivering at high data rates. The current implementation of ISDN in the narrow band form is the best access and delivery medium available. ISDN is seen by many in the industry as the ramp through which multimedia networking will gain acceptance. The installed base of ISDN is growing rapidly (30,000 line per month in Germany). ISDN is able to provide connections throughout the world. In Europe the Euro-ISDN agreements between operators is valuable.

ISDN offers point to point delivery, network access, and network interconnection for multimedia. Different data rates from 64 kbps up to 2 Mbps are commercially available which can meet many needs for ransporting multimedia. Call set-up times are under one second.

ISDN will be the feeder network for broadband ISDN based on ATM standards. Initially the ISDN and ATM networks will be overlaid on top of each other, but users of ISDN will eventually be able to call an ATM user directly and be allocated an appropriate amount of bandwidth. The development of 'middleware' will enable applications to communicate over mixed networks.

Although ISDN could be cheaper, particularly in the UK (currently 300 to connect), it is likely to be cheaper than ATM connections and more widespread in availability for a long time. It is therefore an important tool in bringing multimedia applications to a wide range of users. The idea that multimedia can only be delivered on broadband networks is erroneous as the assertion that only a Macintosh can deliver multimedia.

The cost of ISDN hardware was high, but is now decreasing. Terminal adapters are available from 400 upwards, and PC cards for 300 upwards. Video conferencing cards cost around 3000, (BT's VC8000 card). Costs of ISDN equipment are much lower in Germany and some of these products are beginning to appear in the UK under the Euro-ISDN banner.

British Telecom are pursuing a strategy to make ISDN the preferred option for all multiple (2 or more) exchange line requests by the mid 1990s. ISDN is accessed through one of two services, named by the CCITT as Basic Rate Access (BRA) and Primary Rate Access (PRA).

Basic Rate Access (BRA) provides an ISDN user with simultaneous access to two 64 kbps data channels using the existing twisted pair copper telephone cable. The B.T. basic rate ISDN service is called ISDN2. The connection cost of ISDN2 is currently 300. Rental for the equivalent of two PSTN telephone lines is 384 per year.

Each data channel is referred to as a B-channel and can carry voice or data. Another channel, the D-channel, operates at 16 kbps and is used for signalling between user devices and the ISDN. The total data rate of BRA is therefore 144 kbps. The two B-channels and the single signalling channel give rise to the term '2B+D'. BRA is also referred to as I.420, after the CCITT recommendation. Basic rate ISDN is intended for low capacity usage, such as that required for small businesses.

British Telecom's primary rate ISDN service is known as ISDN30. This service is generally available throughout the UK and is based on the CCITT recommendations for primary rate ISDN. Mercury Communications Limited also offer a primary rate service known as 2100 Premier. Although this service is largely based on CCITT recommendations, it still utilises the some proprietary signalling.

Primary rate access can carry 30 independent voice or data channels, each at 64 kbps. The structure has a 64 kbps D-channel for signalling between devices and the network, and a 64 kbps channel for synchronisation and monitoring. The total data rate of PRA is 2.048 Mbps.

Primary rate access is often referred to as '30B+D' because of the number of B-channels and D-channels, or I.421 because of the CCITT recommendation from which it is taken. This form of access is primarily intended for use in situations which require a large transmission capacity, such as when organisations make voice and data calls through an Integrated Services PBX.

There are two standard ISDN connectors. For accessing basic rate ISDN, an RJ-45 type plug and socket (similar to a telephone plug) is used using unshielded twisted pair cable. Access to primary rate ISDN is through a coaxial cable.

The ISDN passive bus, which can be a maximum of 1 km in length, is a cable which in user premises. It enables up to eight user devices to be attached to the basic rate ISDN interface. Since there are only two B-channels, only two of the eight devices can communicate at any one time. For this reason, each device must contend for access to the passive bus.

ISDN signalling information, carried in the D-channel, is used to establish, monitor and control ISDN connections between users as well as instigating, the audible ringing or engaged tones.

The ISDN numbering system is similar to the contemporary telephone numbering system. Each B-channel has its own unique directory number which allows access to different terminal types (such as telex or facsimile devices). Each terminal type has an identity code which ensures that it only communicates with similar terminals.

The equipment available for ISDN includes Terminal Adapters, ISDN internal computer Terminal Adapter cards, Video Conferencing PC cards, and LAN access gateways or bridges, some of which are based on PC cards or stand alone boxes. Products are available from in this country from the USA, UK, France, and Germany. The market for ISDN is most developed in Germany.

Internal Terminal Adapters from Germany will all inter-work with each other, products developed in the UK are all totally and individually proprietary and will not inter-work in many cases. Many manufacturers are awaiting the dust to settle on the competing application programming interface standards from the European PTT body ETSI.

It is possible to avoid all the problems of API standards for internal computer adapters by using an external ISDN Terminal Adapter. Since the speed of most serial ports on a PC has been limited to about 19.2 kbps until recently, this approach has not been viable. However recently internal PC cards which will work asynchronously up to 115 kbps have appeared, which could have applications in multimedia work when used with an appropriate external Terminal Adapter.


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