A study on accommodation of TCP/IP best-effort traffic to wide area ATM network with VBR service category using selective cell discard

It is important to establish the technology to accommodate best-effort TCP/IP traffic over wide area ATM networks. The UBR (unspecified bit rate) service category is the most typical service category for the best-effort traffic, especially in the LAN environment. On the other hand, the VBR (variable bit rate) service category with SCD (selective cell discard) option is considered as the service category which is appropriate for wide area networks due to its fairness and minimum guarantee of the cell transmission using not only PCR (peak cell rate) but SCR (sustainable cell rate) and MBS (maximum burst size). However, there is no actual evaluation for such service. We have, therefore, performed the experimental studies on TCP/IP over VBR with SCD service along with UBR and VBR without SCD cases. Through these experiments, we measured the link utilization of the effective data and the fairness between each obtained TCP/IP throughput during congestion of the ATM switch. From the results of the link utilization, the value is always over 90% under the various conditions. Therefore, even in the case of cell losses due to SCD or buffer overflow in ATM switch congestion, the average throughput is almost the same as the value which equals the trunk line speed divided by the number of the accommodated TCP/IP connections. From the results of the fairness, VBR with SCD has almost the same characteristics as UBR and obtains better TCP/IP throughput than VBR without SCD. Finally, we discuss the effectiveness of VBR with SCD and the other service categories, such as UBR and ABR (available bit rate), and conclude that VBR with SCD is one of the most suitable ATM service categories for wide area ATM networks.