C-2APPENDIX C: ATM AND VLAN MANAGEMENT BASICS
ATM Basics 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.
The fixed-length cells offer smaller and more predictable switching delays as cell switching is less complex than variable-length packets and can be accomplished in hardware for many cells in parallel. Having all data in the same cell format speeds transmission dramatically by obviating the need for protocol recognition and decoding. A good analogy is containerized shipping, where uniform shape and weight of containers and standardized labeling considerably ease and quicken processing.
The cell format also allows for multiprotocol 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.
The Role of Edge Devices
All ATM traffic-handling decisions are based on the information in the destination header, not on the content of the cell payloads. To move traffic through the ATM network, devices at the boundary or edge of the network convert non-ATM traffic streams into cells. The addition of new traffic types requires only a new edge device, deployed where the demand for such traffic exists.
ATM is a connection-oriented transport service. With only five bytes of header, an ATM cell cannot carry the full destination address for each cell. Instead it uses an abbreviated address, called a virtual channel identifier, that provides enough information to establish a connection between two ATM stations. Once a connection exists through the ATM network, communications can ensue.
Legacy LANs employ connectionless transmission technology based on 48-bit addressing. This edge devices must have some way of adapting existing network protocols, such as IP and IPX, to the connection-oriented cell-switching paradigm.
ATM provides 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)
