ayer applications such as Media Access Control (MAC) table consultation or layer 2 security-related VPNs segregation. The latter is by far the most frequently used category higher on the protocol stack, as searches for quality of service (QoS)- and class of service (CoS)-inspired clas- sification based on layers 3 and 4 must be performed with the use of wildcard characters. Therefore, the use of TCAM is predominant now in the industry. In terms of available sizes, TCAMs come in 1Mb, 2Mb, 4.7Mb, 9.4Mb, and 18.8Mb chips. Like ordinary memory chips, they are measured in megabits. Unlike ordinary memory chips, the actual capacity of CAM chips is slightly higher than the corresponding powers of 2 found in traditional mem- ory. This is because CAM entries are structured as multiples of 36 bits instead of 32 bits or even 8-bit bytes. Capacity figures are 4.5Mb instead of 4Mb, for example. One advantage of CAMs is that they can deliver a lot of productive work per input/output (I/O) pin, especially compared to regular memory. This is because CAMs produce a result with fewer mem- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SEARCH ENGINES SEARCH ENGINES ory accesses compared to algorithmic approaches, which must use regular SRAM or dynamic random access memory (DRAM). Pins are a scarce resource on an NPU because more pins translate into larger NPU packages and the corresponding board size increases; therefore, the cost is rapidly escalating. Newer NPU products from large and well-established companies such as Agere and even from a few of the more recent startups such as EZchip do not require CAMs or SRAM for lookups even when operating at 10 Gbps line rates. This means that after all has been said about them, CAMs do have some competition. Pros and Cons CAMs have the following powerful capabilities: They associate the input (comparand) with their 