Overview

In Chapter 7, we gave an overview of Switched Multimegabit Data Service (SMDS) as part of our discussion of various WAN technologies and services. In this chapter we present more specific information about SMDS.

1. What is SMDS?

   Switched Multimegabit Data Service is a cell-based, connectionless, high-speed, public, packet-switched, broadband, metropolitan area data network.

2. That's a mouthful. Can you break it down for me?

   Sure. SMDS is a wide area network service designed for LAN-to-LAN connectivity. It is not a technology like Ethernet or ATM. SMDS is also a data service, which means that it can only transmit data (although it is capable of transmitting voice and video). SMDS uses fixed-length cells as its basic unit for transmitting data. These cells are similar to ATM cells, namely, they contain 53 bytes a 44-byte payload plus a 7-byte header and a 2-byte trailer. (See Chapter 14 for more information about ATM.) 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 another. 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. (In contrast, ATM and frame relay are connection-oriented.) SMDS is available from telco providers as a shared, public network that uses...