SMDS
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Switched multimegabit data service (SMDS) was a connectionless service used to connect LANs, MANs and WANs to exchange data. SMDS was based on the IEEE 802.6 DQDB standard. SMDS fragmented its datagrams into smaller "cells" for transport, and can be viewed as a technological precursor of ATM.
Increases in raw data rates removed the need for fragmentation into cells, and SMDS' niche market position ensured that it remained a high-priced service. As a result, SMDS has been supplanted by IP-based and Ethernet-based services and MPLS.
Switched Multimegabit Data Service (SMDS) is a telecommunications service that provides connectionless, high- performance, packet-switched data transport. Being neither a protocol nor a technology, it supports standard protocols and communications interfaces using current (and future) technology.
SMDS allows users to transparently extend their data communications capabilities over a wider geographical area. Since it is a service offered by the telephone companies, SMDS permits this expansion using existing Customer-premises equipment (CPE) and protocols, with minimal investment in dedicated leased lines as the number of line terminations increases.
SMDS has been defined by the IEEE 802.6 Metropolitan Area Network (MAN) standard, as implemented by Bellcore. It can use a variety of technologies, including Broadband ISDN (B-ISDN) and Distributed Queue Dual Bus (DQDB). Current North American implementations utilize DQDB with DS1 (1.5 Mbit/s) or DS3 (45 Mbit/s) lines. Other implementations utilize E1 lines at speeds in excess of 1.9 Mbit/s or E3 lines. Future SMDS networks will couple B-ISDN with SONET OC3 at 155 Mbit/s.
The development of this service has paralleled the emerging Asynchronous Transfer Mode (ATM) standards. Like ATM, SMDS uses cell relay TRANSPORT. Both services use 53 octet cells for transport and can accommodate packet lengths of 9188 octets (However, the maximum length for SMDS is 9188 octets and the maximum length for ATM is 65535 octets.) Because of this, SMDS is considered to be an intermediate between the packet-switched services offered today and the ATM service of the future.
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