A Comparison of ATM Stream Merging Techniques

Multi-layer forwarding (a.k.a. multi-layer switching or routing) approaches, using ATM as transport technology have proven not to be sufficiently scaleable, if route aggregation is not performed. In ATM networks route aggregation means stream merging, that is cells from different incoming streams to be switched to the same outgoing link using the same stream identifier. This identifier could be either the whole VPI/VCI pair (VC merging) or only the VPI (VP merging). Stream merging approaches are quite often addressed as VC merging approaches and in this paper we follow this naming convention, unless otherwise specified. The standard way of carrying IP over ATM exploits AAL5 as the ATM Adaptation Layer, but AAL5 does not provide the native support for VC merging. On the other hand, AAL3/4 supports VC merging, but it introduces a considerable and unnecessary overhead when it is used to carry connectionless traffic. This paper provides an overview of the VC merging problem and presents a review of the most common approaches proposed so far. It also presents CLIMAX, a solution that could fit in different scenarios to solve the VC merging problem.