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At the ATM layer, there are several possible cell formats including the QoS (Quality of Service) test cell, UNI (Unified Network Interface) cell for end ATM devices, NNI (Network-to-Network Interface) cell for ATM switches and routers. There also are several predefined ATM cells, such as the OAM (Operations Administration and Operations Maintenance), PNNI (Private Network-to-Network) and LANE (LAN Emulation) control cells, and idle cells that are continuously transmitted when there's no data.

The AAL (ATM Adaptation Layer) defines the way in which data is broken up (if necessary) and placed into a series of 53-byte cells with predefined headers depending on the adaptation layer. For example, AAL 5 consists of a 5-byte header and 48-byte payload and, in one of its applications, is used to carry LANE information and data.

All the analyzers in our review excelled in decoding the relatively simple 53-byte cell. Putting cells back together for an upper-layer protocol is a bit more complex, however. In order for an ATM analyzer to capture and piece back together cells that make up a LAN packet being transported over ATM, a standard way must exist for sending LAN traffic over ATM. Using a switch or router vendor's proprietary methods will render your analyzer useless for reassembling captured cells back into packets.

RFC 1483, entitled "Multiprotocol Encapsulation over ATM Adaptation Layer 5," defines a service layer for sending Ethernet, token-ring or LLC frames over ATM. There are also a number of RFCs (check out RFC 1577, "Classical IP and ARP over ATM") regarding IP over ATM.