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The evolution of communication networks from the current mix of analogue and digital access to full digital high capacity or broadband systems will occur. What is not clear is the exact path likely to be followed. Important networking issues will arise. With the availability at some sites of ATM technology, the use of FDDI for local networks at some institutions, and a large installed base of twisted pair Ethernet or Token Ring networks the migration path is not clear.

Access to ISDN, the availability of ISDN on PABX switches for internal use, the use of Metropolitan are networks to carry voice telephony and data traffic and the use of LANs for telephony all throw up important issues of network integration.

12.1. Broadband Network Services

B-ISDN (Broadband ISDN) with its ATM technology, going through the process of standardisation, is likely to be the most important candidate for the broadband multiservice network of the future. Much effort, worldwide, is being concentrated on building an understanding, through pilots projects, of this new technology. One of the main achievements of the ATM technology is its flexibility. It does not differentiate between various information characteristics, nor does it deal directly with end user time characteristics.

Users may not be satisfied with a low level direct access to the service offered by the ATM layer. To remedy this point an ATM (AAL) adaptation layer has been proposed. This layer will sit outside the pure ATM network and will provide the functionality not provided by the ATM network. These services are summarised below.

12.1.1. ATM Adaptation Layer Services

Currently there are four types of adaptation protocols progressing through the standardisation bodies: ITU-T, ETSI, and ATM forum. These are AAL-1, AAL-2, AAL-3/4 and AAL-5. Each AAL type is aimed at supporting specific communication requirements such as Connection Oriented, real-time issues, Variable Bit Rate etc.

The AAL-1 [ETS93c, ETS93e] offers a service that accepts Service Data Units (SDUs) at a fixed clock rate for transmission over the network and delivers them at the same clock rate in a Connection Oriented mode, also called isochronous service. The basic characteristic of the AAL-1 is the ability to offer a Constant Bit Rate (CBR) service. In addition it performs some basic error control mechanisms are performed which include sequence numbering in order to detect lost or mis-inserted ATM cells and Forward Error Correction (FEC). There are, currently, 3 services by AAL-1:

The AAL-2 protocol type should support Connection Oriented Variable Bit Rate (VBR) traffic and is intended to support transmission of VBR video codecs signal as an example. However, the specification process is at its early stages.

The AAL-3/4 protocol [ETS94b] offers a connection oriented service, not including any timing aspects, and is intended to support general data transmission applications. The basic functionality of the AAL-3/4 is SDU delimiting and multiplexing of higher layer SDUs on the ATM connection. From its incept the AAL-3/4 has been seen a provider of a Connectionless (CL) data service over the Connection Oriented ATM. Since it is an architectural requirement that the AAl should not deal with network layer issues, an additional layer is expected above the AAL-3/4 to offer a CL service. This CL protocol layer mainly provides the addressing functionalities in order to offer a Broadband Connectionless Data Service (BCDS) in ITU-T terminology or CBDS/SMDs in ETSI terminology. Therefore it makes no sense to consider the AAL type-3/4 service on its own.

The prime objective of the AAL-5 [ETS93d] is high speed transmission with reduced overhead. It is a compromise between overhead and functionality. The strong wish to develop a high speed AAL protocol type, which should support existing protocols, was the reason behind the specification of a reduced AAAL protocol type. From a service point of view AAL-3/4 and AAL-5 offer the same layer functionality. The main differences between these two protocol types are: the AAL-5 performs minimum error control mechanisms in comparison to the AAL-3/4; they perform different mechanisms for SDU delimiting; and the AAL-5 does not offer a higher layer SDU multiplexing capability.

Currently AAL-5 offers a service for the transport of B-ISDN signalling information, and a Frame Relay service. AAL-5 is being considered in the ATM for possible use to transport real-time multimedia information.

The following B-ISDN bearer services are currently under standardisation: Virtual Path (VP) ATM bearer service, Broadband Connection Oriented Bearer Service, Connectionless Broadband Data Service (CBDS), and Frame Relay service.

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