Switched Multimegabit Data Service (SMDS) is a broadband networking technology developed by Bellcore based on the IEEE 802.6 DQDB (Distributed Queue Dual Bus) MAN technology.

SMDS can use fiber- or copper-based media; it supports speeds of 1.544 Mbps over DS-1, or 44.736 Mbps over DS-3. In addition, SMDS data units are large enough to encapsulate entire IEEE 802.3, IEEE 802.5, and Fiber Distributed Data Interface (FDDI) frames. SMDS operates by accepting high-speed customer data in increments of up to 9,188 octets, and divides it into 53-octet cells for transmission through the service provider's network. These cells are reassembled, at the receiving end, into the customer data.

The SMDS Interface Protocol (SIP) is a three-level protocol that controls the customer's access to the network, which is based on the IEEE 802.6 DQDB.

SIP Level 3 operates at the Media Access Control (MAC) sublayer of the data link layer of the OSI reference model. SIP Level 2 operates at the MAC sublayer of the data link layer. SIP Level 1 operates at the physical layer of the OSI reference model. SIP Level 3 receives and transports frames of the upper layer protocol information. SIP Level 2 controls access to the physical medium. SIP Level 1 includes the PLCP and the transmission system.

Protocol Structure

The SIP Level 3 PDU is shown in the following diagram.

The format of the level 3 header is as follows:

And the format of the level 3 trailer is as follows:

The Level 3 PDU fields are described as follows:

• Rsv - Reserved. A 1-octet field that the CPE and the SS fill with zeros.
• Betag - A 1-octet field that contains a beginning/end tag.
• Basize - A 2-octet field containing the length in octets of the Level 3 PDU from the beginning of the Destination Address field and including the CRC32 field, if present.
• Destination address - An 8-octet field containing the address of the intended recipient of this PDU.
• Source address - An 8-octet field containing the address of the sender of this PDU. This field contains Address Type and Address subfields as described for Destination Address.
• HLPI - Higher Layer Protocol Identifier. A 6-bit field that aligns the SIP and DQDB protocol formats.
• PL - PAD Length. A 2-bit field that indicates the number of octets in the PAD field, which aligns the Level 3 PDU on a 32-bit boundary.
• QoS - Quality of Service. A 4-bit field that aligns the SIP and DQDB protocol formats.
• CIB - CRC32 Indication Bit. A 1-bit field that indicates the presence (1) or absence (0) of the CRC32 field.
• HEL - Header Extension Length. A 3-bit field that indicates the number of 32-bit words in the Header Extension field.
• Bridging - A 2-octet field that aligns the SIP and DQDB Bridging protocol formats.
• HE - Header Extension. A 12-octet field that contains the version and carrier-selection information .
• Information field - Variable-length field, up to 9,188 octets in length, that contains user information.
• PAD - Variable-length field, 1-3 octets in length, filled with zeros aligning the entire PDU on a 32-bit boundary.
• CRC32 - 2-octet field that performs error detection on the PDU, beginning with the DA field, up to and including the CRC32 field.

The SIP Level 2 PDU contains a 5-octet header, a 44-octet Segmentation Unit (payload) and a 2-octet trailer as shown below.

The Level 2 PDU fields are described as follows:

• Access control - 8-bit field that indicates whether the Level 2 PDU Access Control contains information (1) or is empty (0).
• Network control info - 4-octet field that determines whether Network Control Information of the Level 2 PDU contains information (FFFFF022H) or is empty (0).
• Segment type - 2-bit field that indicates how the receiver should process non-empty Level 2 PDUs. Sequence number - 4-bit number that verifies that all the Level 2 PDUs belonging to a single Level 3 PDU have been received in the correct order.
• Message identifier - 10-bit number that allows the various segments to be associated with a single Level 3 PDU.
• Segmentation unit - 44-octet field that contains a portion of the Level 3 PDU.
• Payload length - 6-bit field that indicates which of the 44 octets in the Segmentation Unit contain actual data. BOM and COM segments always indicate 44 octets. EOM segments indicate between 4 and 44 octets, in multiples of 4 octets. SSM segments indicate between 28 and 44 octets, in multiples of 4 octets.
• Payload CRC - 10-bit field that performs error detection on the Segment Type, Sequence Number, Message Identifier, Segmentation Unit, Payload Length and Payload CRC fields.

Once assembled, SIP Level 2 PDUs are passed to the PLCP and physical functions within SIP Level 1 for transmission.

Related protocols: IEEE 802.6 (DQDB), ATM

Sponsor Source: SMDS is defined by Bellcore (http://www.telcordia.com)

Reference: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/smds.htm: Switched Multimegabit