views updated May 18 2018

SMDS Abbrev. for switched multimegabit data service. A switched broadband service that is being introduced by a number of public network operators (PNO). The intention is that SMDS will support data services at a range of bit rates from 1.536 Mbps (2.048 Mbps in Europe) up to 45 Mbps (34 Mbps in Europe) and subsequently at higher speeds, certainly as high as 155 Mbps. SMDS is a connectionless system, based on variable-length packets with a payload of up to 9188 bytes. Each packet contains both the source and destination addresses, the addresses being the same as ISDN addresses and globally unique.

SMDS offers the end-user access at a range of predetermined bit rates (e.g. 2, 4, 10, 16, and 25 Mbps), treating these as average bit rates that cannot be exceeded, while using a fixed bit rate bearer (in this case of 34 Mbps) between the SMDS access points. A control algorithm in the access unit ensures that the average rate at which the end-user can submit data for transmission cannot be exceeded. There are no restrictions on the rate at which the user can receive data, since it is possible for data to arrive from a number of separate subscribers at a rate that would exceed the average rate at which a user can transmit. This allows the PNO to offer a service tailored to the end-users' overall needs, together with the ability to upgrade between one bit rate and the next by simply resetting the values of the control parameters in the access unit.


views updated May 11 2018

SMDS switched multimegabit data service