3Com 7600 manual ATM Basics

12-2CHAPTER 12: ATM, LAN EMULATION, AND VIRTUAL LANS

ATM Basics

Cell Switching ATM uses short, fixed length packets called cells. The first five bytes of each cell, the header, contain the information necessary to deliver the cell to its destination.

Fixed-length cells offer the following advantages:

QNetwork and switching queueing delays are more predictable with fixed data cells than for variable-length packets.

QCell switching is less complex and more reliable. ATM hardware can be implemented more efficiently because control structures, buffers, and buffer management schemes can be designed to known size criteria.

QCell-relay switches can process cells in parallel, achieving speeds that far exceed the limitations of packet switch architectures.

QHaving all data in the same cell format speeds transmission dramatically by eliminating the need for protocol recognition and decoding. A good analogy is containerised shipping where uniform shape and weight of containers and standardized labelling considerably ease and quicken processing.

QThe cell format also allows for multi-protocol transmissions. Since ATM is protocol transparent, the various protocols can be transferred at the same time. With ATM, one line can carry phone, fax, video, data and other information simultaneously. This multiprotocol advantage also offers scalability, greatly reducing the configuration changes necessary for adding a new traffic type to your network.

Network interfaces ATM establishes the User Network Interface (UNI) which is typically used to interconnect an ATM user with an ATM switch that is managed as part of the same network, as well as the Network to Network Interface (NNI) which is typically used to interconnect two ATM switches managed as part of the same network.

Layered architecture ATM is based on a layered architecture. In the protocol stack, the ATM layer sits directly above the physical layer. Many physical layers can be specified, including several for 100 to 155 Mbps. The ATM adaptation layer (AAL) sits above the ATM layer. LAN Emulation (LE) sits above the AAL5 in the protocol hierarchy.