A low cost searching device for an ATM adapter

Abstract

A key function of an ATM adapter is its ability to sort the incoming traffic from the VPi/VCi field of the cell headers. This has to be done at media speed and requires a hardware assist to keep up with the high speed lines over which ATM cells are transported. Although the device of choice to perform a search is a content addressable memory (CAM) such devices are seldom used, especially in the cost sensitive part of the market, because they require a large die size to be built and are too expensive. However, numerous methods have been devised just using regular R/W RAMs. The most efficient, based on various tree structures, tend to be difficult to manage when updating is considered (especially deletions). Hashing, because of its simplicity, is often preferred. Although "no hash function is perfect" some are better adapted in a particular situation. The paper describes a hash device which makes use of the HEC header field as the hash function, requires a modest RAM space and is able to sort cell traffic at the ingress of an ATM adapter with predictable performance.