A connectionless, packet-switched telecommunications service with speeds ranging from 56 Kbps to 34 Mbps. Switched Multimegabit Data Services (SMDS) was designed by Bellcore in the 1980s for high-speed wide area network (WAN) communication. It was the first high-speed broadband networking technology offered to subscribers and was a precursor to Asynchronous Transfer Mode (ATM) networking. Most carriers are now phasing out SMDS.
SMDS is based on a packet-switching technology similar to frame relay networks. A subscriber’s local area network (LAN) typically connects to the SMDS service through a router using a RS-449 interface and a Channel Service Unit/Data Service Unit (CSU/DSU) using a copper DS1 connection (1.544 Mbps) for low-speed access or a fiber DS3 connection (44.736 Mbps) to achieve the highest possible transmission speeds. This point of connection between the subscriber’s LAN and the telco’s central office (CO) is called the Subscriber Network Interface (SNI). The CO provides a gateway to the SMDS packet-switching network, which consists of high-speed switches joined by trunk lines connecting different telco COs.
Graphic S-22. Switched Multimegabit Data Services (SMDS).
An SMDS packet consists of a header with the source address, destination address, and a payload of up to 9188 bytes. The SMDS payload is large so that SMDS can easily encapsulate Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) frames for WAN transmission. The E.164 addressing scheme uses decimal numbers up to 15 digits long and includes a country code, area code, and subscriber ID number (similar to ordinary telephone numbers). Different address classes support different data transfer speeds. The serial protocol used for communication between the customer premises equipment and SMDS equipment at the telco’s CO is called the SMDS Interface Protocol (SIP), which is based on the IEEE 802.6 standard for metropolitan area networks (MANs). The primary function of SIP is to provide encapsulation of the LAN protocol. (Internet Protocol, Internetwork Packet Exchange, AppleTalk, and just about anything else is supported.) Higher-layer protocols support processes such as address resolution and source address screening.
SMDS supports ATM communication and is suitable for use in high-capacity mesh topology WANs. SMDS is offered by long-distance carriers such as AT&T and by some Regional Bell Operating Companies (RBOCs). SMDS is not as widely supported as frame relay even though it uses similar packet-switching technology.