Transit Networking in ATM/B-ISDN

The ATM Forum and ITU-T specify multiple service categories of B-ISDN, such as CBR (constant bit rate), RT-VBR (realtime variable bit rate), NRT-VBR-COD/CLD (non-realtime variable bit rate connection-oriented data/connectionless data), ABR (available bit rate), and UBR (unspecified bit rate). Each service category specifies different traffic parameters and QoS parameters : bandwidth, end-to-end transfer delay, cell transfer delay variance, bit error rate, and cell loss rate. In this paper, we propose a transit networking scheme in ATM/B-ISDN based on the service category. In the proposed scheme the end-to-end virtual channel/virtual path connections are routed according to the required traffic characteristics, differently. The major purpose of the service category-based transit networking is to provide the required QoS to end users efficiently, while maintaining the network utilization level high. We also propose a restoration strategy with 5 restoration classes. In the proposed restoration strategy the user selects a restoration class for his specific application at the connection setup phase. 1. B-ISDN service categories ATM Forum defines 5 B-ISDN service categories [1]: CBR (constant bit rate), RT-VBR (realtime variable bit rate), NRT-VBR (non-realtime variable bit rate), ABR (available bit rate), and UBR (unspecified bit rate). The examples of CBR service are plain old telephone service (POTS), circuit emulation for DS1 (1.544/2.048 Mbps) and DS3 (45 Mbps), and CBR-Video. CBR service requires constant transfer rate that is specified by its end-to-end cell transfer delay (CTD) and peak cell rate (PCR). The examples of RT-VBR service include the packetvideo and audio in teleconferencing and in multimedia applications. In RT-VBR services, the data transfer rate varies according to the amount of information that is usually specified by the source coding scheme. Thus the RT-VBR traffic is defined by the set of traffic/QoS parameters, such as end-to-end CTD, PCR, sustainable cell rate (SCR) with maximum burst size (MBS). The NRT-VBR service category is intended for nonreal-time applications which have bursty traffic characteristics. NRT-VBR-COD (connection-oriented data) is defined for the connection-oriented data services, such as X.25 PSDN or Frame relay. NRT-VBR-CLD (connectionless data) is defined for the connectionless data services, such as IP datagram service and SMDS (switched multi-megabit data service). For NRT-VBRCLD, the transport network provides several connectionless service function (CLSF) nodes in order to provide the store-and-forward data service. Both NRTVBR-COD and NRT-VBR-CLD do not specify the timing requirements, such as end-to-end CTD and CDV tolerance. ABR service category is defined for the Internetinterconnection service through ATM/B-ISDN. The ABR traffic defines the required bandwidth by MCR (minimum cell rate) that is the minimum bandwidth that the transport network must guarantee. UBR service category does not specify any traffic/QoS parameter. It only uses the currently available bandwidth of the transport network. The example traffic/QoS parameters for each service are as shown in the table. In the 64 kbps CBR voice telephony service, the end-to-end cell transfer delay should be kept below 300 ms for most live conversational type application [1-2]. The DS1/DS3 CBR circuit emulation requires the cell transfer delay