Previous | Table of Contents | Next |
Switched Multimegabit Data Service (SMDS) is an offshoot of the Distributed Queue Dual Bus (DQDB) technology defined by the IEEE 802.6 standard for Metropolitan Area Networks (MANs), as a means of extending the reach of the LAN across a metropolitan area. The original work on the DQDB concept was done at the University of Western Australia where it was known as Queued Packet Synchronous Exchange (QPSX). Subsequently, the original technology was licensed to QPSX Ltd., formed by the University of Western Australia and Telecom Australia [11-8]. The real commercial success is attributable to further development work by Bellcore, at the request of the RBOCs. The first commercial offering was by Bell Atlantic in 1992, based on a test implementation at Temple University at 45 Mbps [11-29].
Since that time SMDS has enjoyed particular success in the U.S. through deployment by most of the RBOCs and GTE. As the RBOCs and their subsidiary BOCs primarily are LECs (Local Exchange Carriers) limited to providing service within the confines of the LATA, they have exhibited particular interest in network technologies that are appropriate for providing service in confined geographic areas. This geographical limitation of SMDS, therefore, did not pose a practical limitation for the LECs. Additionally, SMDS is extendible across the WAN.
While the RBOCs have deployed SMDS fairly aggressively in the past, their ardor has cooled lately. As the Telecommunications Act of 1996 promises to release them from the artificial LATA constraints, they currently seem to be lusting after ATM, which is not geographically restrained and which is the network technology of the future. By way of example, US West pulled the plug on SMDS on April 1, 1996, citing its long-term interest in ATM and the relative popularity of its competing Transparent LAN Service. NYNEX, by the way, decided early on to skip over SMDS in favor of ATM [11-30]. While other RBOCs have remained loyal to SMDS, they are also likely to have similar plans. Ameritech, for instance, has only 80 SMDS customers in its five-state region. MCI Communications has approximately 130 SMDS customers, and plans to offer SMDS-to-ATM internetworking in late 1996 [11-31]. If the above sounds negative, its because SMDS by no means is the ultimate answer for all networking requirements. The ultimate solution appears to be ATM, upon which B-ISDN is founded. SMDS does have a future, although perhaps in a niche market.
SMDS also has enjoyed moderate success in Western Europe, where the nations tend to be small in geographic terms and where population is dense in the major metropolitan areas. The European version of SMDS is known as CBDS (Connectionless Broadband Data Service). CBDS is available in Australia (Telecom Australia), Germany (DBP), and England (BT)numerous additional trials are underway.
According to Bellcore, SMDS is a high-speed, connectionless, public, packet switching service that will extend Local Area Network-like performance beyond the subscribers premises across a metropolitan or wide area. SMDS is a MAN network service based on cell-switching technology. Generally delivered over a SONET ring, SMDS has a maximum effective serving radius of approximately 30 miles (50 Km). SMDS is a connectionless service that accepts user data in the form of a Protocol Data Unit (PDU) up to 9,188 octets in length over access lines of < 45 Mbps (DS3); OC-3 (155 Mbps) access is planned, as are higher levels. The user data is segmented into 53-octet cells, 48 octets of which are payload and 5 octets of which are overhead. The segmentation process can occur either at the network node or in the user equipment.
SMDS is designed to support LAN-to-LAN traffic, under the IEEE 802.6 standards, although other data applications are supported effectively. SMDS offers excellent performance characteristics, including a guaranteed rate of access, transport and delivery. SMDS also provides a smooth migration path to ATM. In fact, the 53-octet cell format was chosen specifically for this reason.
The IEEE developed the original MAN standard 802.6. That standard was intended as a standard for the interconnection of DQDB subnetworks within metropolitan areas. The DQDB subnetworks can provide data concentration, routing, or switching functions; or can interconnect workstations, hosts and LANs over a metropolitan area [11-9]. According to the IEEE, the 802.6 DQDB standard is intended to provide a high-speed shared medium access protocol for use of a dual, counter-flowing, unidirectional bus subnetwork. Bellcore then modified that standard to develop SMDS, that is a carrier service based on the 802.6 concept and intent. The scope of the standard includes two layers, DQDB and Physical. Within the two layers are defined the functions of Medium Access Control (MAC), connection-oriented service and isochronous service.
Bellcore has issued a number of standards in the form of Technical References (TRs). The TRs cover Generic System Requirements, Access Requirements, Networking Requirements, Operations Interfaces, Frame Relay Access to SMDS, and a host of other issues relative to access, network management, billing, and operations. Bellcore also issues Technical Advisories (TAs), which are preliminary documents, ultimately becoming TRs.
The SMDS Interest Group (SIG), formed in 1991, has released a number of technical implementation specifications that address specifics such as AppleTalk and DECnet over SMDS networks. The European SIG has developed a similar set of specifications. The SMDS Interest Groups are not standards bodies. Rather they are voluntary organizations of manufacturers, carriers, consultants, and other interested parties.
European Telecommunications Standards Institute (ETSI) has adopted most 802.6 standards elements, including many SMDS features, for the European market.
Previous | Table of Contents | Next |