Chapter 13 - Switched Multimegabit Data Service (SMDS)

Abstract

This chapter explores the switched Multimegabit Data Service (SMDS). It is based on IEEE 802.6, a data link layer protocol for metropolitan area networks (MANs). Called “Distributed Queue Dual Bus (DQDB),” this protocol is quite different from other protocols we have considered and merits examination. SMDS is a cell-based, connectionless, high-speed, public, packet-switched, broadband, metropolitan area data network service designed for LAN-to-LAN connectivity. It is also a data service, which means that it can only transmit data. SMDS uses fixed-length cells as its basic unit for transmitting data. These cells are similar to asynchronous transfer mode (ATM) cells; they contain 53 bytes—a 44-byte payload plus a 7-byte header and a 2-byte trailer. As a connectionless data service, SMDS does not establish a connection between sending and receiving hosts prior to data transmission. Instead, an SMDS switch establishes a virtual circuit between sending and receiving machines. Cells are placed on the medium when they are ready for transmission and then transmitted independently of each other. Thus, data cells are transmitted without delay and in no particular order. This provides bandwidth on demand for the kind of bursty traffic inherent in LAN applications. SMDS is available from telco providers as a shared, public network that uses an international standard-based addressing plan known as E. 164. Operating like a shared LAN (e.g., Ethernet/802.3), each SMDS cell contains a destination address enabling any SMDS subscriber to exchange data with any other SMDS subscriber—only those nodes with the correct destination address respond to the transmission.