Switched Multimegabit Data Service

Description

Switched Multimegabit Data Service (SMDS) is a connectionless, public, packet-switched data service designed for wide-area networking. Technically, it is based on the IEEE 802.6 Distributed Queue Dual Bus (DQDB) standard for Metropolitan Area Networks (MANs). SMDS operates at Layer 3 (network layer) and provides a datagram service, meaning each data unit (an SMDS Protocol Data Unit) is independently routed from source to destination using a 10-digit E.164 address, similar to a telephone number. The service was designed to offer scalable bandwidth from 1.5 Mbps (T1) up to 45 Mbps (T3) in its North American implementation, with European implementations using E1 (2 Mbps) and E3 (34 Mbps) rates. It supported features like address screening (allowing closed user groups) and group addressing for multicast.

Within the 3GPP architecture, particularly in early releases, SMDS was relevant as one of the external Packet Data Networks (PDNs) that a GPRS or UMTS network could interconnect with. The Gateway GPRS Support Node (GGSN) would act as the interface point. The GGSN would use the Interworking Function (IWF) to adapt between the GPRS Tunneling Protocol (GTP) used within the 3GPP core and the SMDS interface protocol (SIP) used over the SMDS network. This allowed mobile subscribers to access corporate LANs or services offered over public SMDS networks. The data transfer was connectionless end-to-end; the mobile device's IP packets would be encapsulated within GTP tunnels to the GGSN, which would then forward them as SMDS frames.

The role of SMDS in 3GPP was primarily for backward compatibility and interworking during the transition from fixed data networks to ubiquitous IP-based mobile data. Its specifications in documents like TS 29.061 detail the protocol mapping and interworking procedures. As a technology, SMDS did not maintain state for individual user sessions, which simplified the network design but also meant it lacked built-in QoS mechanisms comparable to later technologies like ATM or MPLS. Its importance in 3GPP diminished rapidly with the global adoption of the Internet Protocol (IP) as the universal network layer, making IP-based PDNs the standard and obviating the need for complex interworking with legacy MAN technologies.

Purpose & Motivation

SMDS was developed in the late 1980s and early 1990s to meet the growing demand for high-speed, scalable data connectivity between geographically dispersed Local Area Networks (LANs). Prior to SMDS, organizations relied on dedicated, point-to-point leased lines (like T1) to connect sites, which was expensive and inflexible. SMDS provided a switched, on-demand service that was more cost-effective for meshed connectivity. It aimed to be the public data service equivalent of the telephone network, offering any-to-any connectivity without requiring permanent virtual circuits. Its creation was motivated by the need for a MAN standard that could bridge the gap between LAN speeds and the lower speeds of Wide Area Network (WAN) services available at the time.

3GPP's inclusion of SMDS interworking in Release 4 and beyond was driven by the practical need for GPRS and UMTS networks to provide access to a wide variety of existing corporate and public data networks. At the time of 3GPP's formation, SMDS was still a deployed service in some regions, particularly in North America and Europe. To ensure broad compatibility and commercial viability for early 3G data services, the standards had to define how the mobile packet core (GPRS) would interface with these legacy PDNs. SMDS interworking addressed the specific problem of allowing a mobile subscriber to appear as a node on a remote SMDS network, enabling access to pre-Internet era corporate data services. This capability became largely obsolete as IP became the dominant protocol for all data networking, both fixed and mobile.

Key Features

• Connectionless, datagram-based service using E.164 addressing
• Scalable bandwidth from 1.5/2 Mbps up to 34/45 Mbps
• Support for closed user groups and multicast group addressing
• Based on the IEEE 802.6 DQDB access protocol for MANs
• Interworking function defined for connectivity with 3GPP GPRS/UMTS networks
• No per-connection state maintained in the network core

Evolution Across Releases

Defining Specifications