Switched Multimegabit Data Service (SMDS)

Switched Multimegabit Data Service is a packet swiched Cell-based service that can use the public telephone network. It uses the IEEE 802.6 Distributed Queue Dual Bus (DQDB) standard and an SMDS specific Digital Service Unit (DSU) or Channel Service Unit (CSU) takes frames from a router and breaks it up into 53-byte cells, the service is connectionless so no data-link acknowledgements are sent and there is no windowing capability. In addition, SMDS requires no virtual circuit.

SMDS supports two Access Paths DS1 - T1/E1 and DS3 - T3/E3 and for DS3 there are Access Classes of 4/10/16/25 & 34Mb/s which determine the limit of sustained information rate available.

Providing addressing, framing and error-detection, SIP has the following three levels:

Even at Level 3, switching is carried out at the MAC layer. The Data Exchange Interface (DXI) protocol defines the interface between the router and the DSU/CSU, the router performs SIP level 3 functions whilst the DSU/CSU performs level 2 functions. The DXI version 3.2 defines a heartbeat poll between router and DSU/CSU on Synchronous links only and this is similar to the Breath of Life (BOFL) implemented on Nortel routers except that the signal is different on its return than on the way. The DXI Local Management Interface (DXI LMI) on the router converts SNMP commands into DXI LMI commands which are; GetRequest, GetNextRequest, SetRequest, GetResponse and Trap.

The Subscriber Network Interface (SNI) is the entry point into the SMDS network where SIP is first implemented, sometimes it is seen as the output of the DSU/CSU if the customer has one.

The L3_PDU header contains a Source Address and a Destination Address, the SMDS address is 64 bits long made up of a 4 bit field indicating an Individual address (0xC) or a Group address (0xE) and an up to 60 bit variable length address containing the country code and national number. The address is in E.164 format. The group addresses are used for multicasting and the individual addresses for unicasting. In order to be able to operate the SMDS as a 'LAN' the service provider will be required to give you a group address as well as the individual addresses. The Service Provider can also undertake address filtering to provide a secure network at the SMDS level. The SNI treats the SMDS Individual address as a MAC address of the SNI, of which up to 16 can be assigned to the SNI. The rest of the 60 bits are padded out with 0xFs. Group addresses are used for broadcasts/multicasts for uses such as RIP.

When encapsulating IP, an 8-bit header is inserted between the IP datagram and the L3_PDU header, and this contains a Higher-Layer Protocol ID field (HLPI) and a SNAP header (DSAP, SSAP, Control, OUI and PID). Other protocols such as IPX, AppleTalk, DECnet, XNS and Vines are encapsulated in a similar manner.

RFC 1209 describes IP over SMDS.
RFC 1232 describes the DS1 interface.
RFC 1233 describe the DS3 interface.

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