SUBJECT E1)

SMDS is the Switched Multi-megabit Data Service, a service offering interface from Bellcore. SMDS provides a datagram service, where a packet has about a 40-octet header plus up to 9188 octets of data. The packets themselves may or may not be transported within the network on top of a connection- oriented ATM service. SMDS uses E.164 (ISDN) addresses. Therefore SMDS is a connectionless packet switched *service*, not a cell-relay service.

However, the SMDS Subscriber Network Interface is currently defined to use IEEE 802.6 Distributed Queue Dual Bus (DQDB) access across the SMDS user-network interface. DQDB itself *is* a form of cell relay. The lower layers of SMDS fragment the packets into cells with a 5-octet header and 48-octet payload. The payload itself has a 2-octet header, 44-octets of data, plus a 2-octet trailer. An SMDS cell therefore is nearly identical in form to an AAL3/4 cell. Note that while DQDB is used as the access protocol, either DQDB or AAL3/4 may be used for the switch-to-switch interface.

Several have noted that the point to stress is that SMDS is a *service* rather than a technology. As such it can be accessed by multiple protocols, as long as those protocols support the features of SMDS. SIP based on 802.6 is one such a protocol. however, others have been defined and are being used, including:

• DXI based (HDLC based)
• Frame Relay based
• ATM based

Another way to look at SMDS is as an ATM application. A common approach is to have an SMDS server in an ATM network, thus creating a connectionless datagram service over ATM that provides all SMDS service features while utilizing the benefits of ATM.

One source of (readable) information on SMDS is probably the SMDS Interest Group (SIG), 480 San Antonio Road, Suite 100, Mountain View, California 94040, USA; Tel +1 415 962 2590; Fax +1 415 941 0849. This SIG is in many ways similar to the ATM Forum, and cooperates with it. Also, there is a European branch known as ESIG which is concerned with adapting the American SIG documents to fit European network architectures and regulations. SIG work is mostly based on Bellcore SMDS TAs and such like, while ESIG aligns with ITU and ETSI standards.

Obviously, Bellcore documentation will be an authoritative SMDS reference. (Contact Bellcore at (908) 699-5800 or 1-800-521-CORE.) Additionally there are SMDS references in section C1 of this FAQ.

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Last Changed 25 August 1996