Performance Evaluation of TCP over ABT Protocols Shingo Ata, Masayuki Murata, and Hideo Miyahara Department of Informatics and Mathematical Science, Graduate School of Engineering Science, Osaka University, Osaka, 560-8531 Japan SUMMARY ABT (ATM block transfer) has been considered an efficient protocol for high-burst data transmission in ATM networks. The characteristics of ABT in the ATM service class have already been investigated. For practical applica- tions, however, it is necessary to consider an upper layer having its own congestion-control mechanism, apart from the ABT. This paper analyzes the characteristics of a TCP (transmission-control protocol) for the Internet which has become very popular recently, when this is used for the upper layer of the ABT. It is shown to be possible to separate the congestion control and the error-recovery service, since the congestion control is carried out effectively in the ABT by the retransmission mechanism of TCP, without using the congestion control of TCP. However, for high performance it is necessary to tune the retransmission timer of the TCP as discussed in this paper. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 86(1): 97–104, 2003; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/ecja.1150 Key words: ATM; ABT; data-transfer protocol; congestion control; TCP; backoff. 1. Introduction In an ATM, service classes which satisfy the quality of service required by the customers have been established by standardization authorities such as the ATM Forum and ITU-T [1–3]. Among them, the ABT (ATM Block Transfer Service) [1–5] has mainly been used for data transfers. Among other data-service classes, the ABT Service Class sets the route alone when it sets the connection, without reserving its bandwidth. If an actual burst (PDU at ABT) occurs, however, the reservation of a band is indicated, and if this succeeds, then the burst can be transferred without cell loss. As a result, it is not necessary to know the traffic characteristics at the time of connection setting, and this improves the network utility. The indication of a bandwidth reservation in the ABT is similar to the CBR/VBR Service Classes, but unlike the CBR/VBR (which carries out band- width reservation with the connection as the unit), ABT carries it out with the burst as the unit. The ABT service class is divided into two subclasses depending on the difference of their bandwidth reservation systems: immediate transmission (ABT/IT) and delayed transmission (ABT/DT). The former transfers its burst when a bandwidth is reserved; and the latter transfers its burst after the bandwidth reservation of the whole system has been acknowledged. As a result, the ABT/IT is little influenced by transmission delay, but its hardware becomes more complicated since bursts must be discarded selec- tively. The characteristics of the ABT have recently been investigated. For example, Ref. 6 suggests that the charac- teristics of ABT/DT are very sensitive to its transmission delay time, and that the delay characteristics of the burst transmission can be improved by dynamically changing the required bandwidth after a backoff, depending on the net- work load. Reference 7 has compared the basic charac- teristics of the ABT/IT and the ABT/DT in a network, taking account of the influence of propagation delay. This study suggests that ABT/DT is easily influenced by the propagation delay, and that ABT/IT is not easily influenced. However, when a network has a heavy load, even ABT/IT increases the burst discard rate, and the characteristics of a © 2002 Wiley Periodicals, Inc. Electronics and Communications in Japan, Part 1, Vol. 86, No. 1, 2003 Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J82-B, No. 5, May 1999, pp. 951–959 97