83 CAC CONTROLLED SERVICES SHARING A LINK WITH CONNECTIONLESS DATA SERVICES IN AN ATMNETWORK: A PERFORMANCE EVALUATION S'IVDY R Fabregat *, J L Marzo *, J Sole **, J Ddngo ** * Universitat de Girona, Spain ** Universitat Politecnica de Catalunya, Spain ABSTRACT In the B-ISDN there is a provision for four classes of services, all of them supported by a single transport network (the ATM network). Three of these services, the Connected Oriented (CO) ones, permit Connection Access Control (CAC) but the fourth, the ConnectionLess Oriented (CLO) one, does not. Therefore, when CLO service and CO services have to share the same ATM link, a conflict may arise. This is because a bandwidth allocation to obtain maxlmum statistical gain can damage the contracted ATM-Quality of Service (QOS) ; and vice versa, in order to guarantee the contracted QOS, the statistical gain have to be sacrifice. This paper presents a performance evaluation study of the influence of the CLO service on a CO service (a Circuit Emulation service or a Variable Bit-Rate service) when sharing the same link. - In the Broadband Integrated Services Digital Network (8-ISDN) three main blocs may be distinguished: The services network (the applications), the adaptation platform, and the transport network (6). The services network is the side of the B-ISDN that may be seen by the end user. The adaptation platform adapts the characteristics of the different applications that form the services network to the offer of a single transport provider. The adaptation platform gives service to the services network and gets the carrier service from the transport network. The transport network works in the Asynchronous Transfer Mode (ATM), and the adaptation platform may be seen as performing the ATM Adaptation Layer (AAL) functions. In the B-ISDN there are four services declared on the AAL: Circuit hulation (Class A), Variable Bit Rate service (ClassB), Connection Oriented Data service (Class C) and ConnectionLess Data service (Class D) (1). A number of AAL protocols are being defined by the CCITT as type 1 to 5 which are in consonance with the service classes -type 1 for Class A service, type 2 for Class B, types 3 and 5 for Class C, and type 4 and also 5 for Class D (5). We will focus our attention on the services rather than the protocols. Class A serves constant bit-rate (CBR) sources that comunicate between each other in a connection oriented manner. Class A service generates a cell stream with a constant intercell distance towards the ATM network. From the CAC point of view, this traffic is very easy to deal with because only constant amounts of bandwidth have to be managed. Both Class B and Class C serve variable bit- rate (VBR) sources, The difference between Class B and Class C services is that Class B provides the destination with a timing reference of the source, and Class C does not. Class B is designed for managing VBR real time traffic (compressed coded voice, sound or image traffic) while Class C was designed for data comunications traffic. Optimum bandwidth allocation of the requirements of these two services is strongly dependent on the CAC mechanism selected. CAC can take advantage Of the statistical characteristics of the VBR traffic. Class D is similar to Class C service. Class D manages VBR traffic and does not provides timing reference between source and destination. But Class D differs from Class C service in offering a connectionless oriented service. Thus, admission control cannot be applied as usual. CAC can only be applied in the service contracting phase, but in addition admission control at the burst level may be applied. It works as follows: when contracting the Class D service a maximum bit-rate is negotiated, but this maximum bit-rate may be renegotiated at each activity period if necessary. Several protocols have been proposed for Boyer and Tranchier (2). Hui (3) and Ohnishi et a1 (4). The service offered by the ATM network is based on Virtual Circuits (VC) -in this paper no distinction between Virtual Channels and Virtual Path will be made-. In order to deal with the CO services (Class A, B and C), the AAL asks the ATM network to establish a VC when necessary. In contrast, to deal with the CLO service (Class D) the AAL uses permanently (or semi-permanently) established VCs (those VCs that were negotiated at the contracting phase). Some links of the ATM network, and in particular the ATM network access ones, may have to allocate both permanent and non- permanent (ephemeral) VCs. Permanent VC allocation implies permanently reserving part of the link capacity. Thus, VCs for CO service incoming demands have to be allocated in the remaining link capacity. This paper presents a performance evaluation study of the bandwidth use on links that have to allocate permanent and ephemeral VCs, that is, a performance evaluation study of bandwidth use on links shared by CLO and CO services. The paper is organized as follows: Section I1 reviews the most usual CAC algorithms discussed in the literature to be adopted in ATM networks. Section I11 gives an accurate formulation of the problem. Section IV describes the environment where the study is carried out. And in Section V the results are presented and discussed. - The ATM transport network is based on packet switching using small fixed-size packets. ATM permits flexible bandwidth allocation, so an important objective is to obtain the maximum statistical gain on a physical channel. A system of traffic control is therefore required This work has been supported by CICYT (Spanish Education Ministry) under contract TIC 1289t92. Authorized licensed use limited to: UNIVERSITAT DE GIRONA. Downloaded on April 23,2010 at 12:24:59 UTC from IEEE Xplore. Restrictions apply.