How does ATM differ from SMDS?

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.

The best 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.

The 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.