Switched Multi-megabit Data Service (SMDS) is defined as a high speed packet switching data service. It is a high-speed, connectionless, public data service that will extend Local Area Network (LAN), Metropolitan or Wide Area Network.

SMDS uses fiber or copper-based media for data transmission. SMDS data units encapsulate entire IEEE 802.3, IEEE 802.5 and Fiber Distributed Data Interface (FDDI) frames.

SMDS provides data service features which include:
1) High-speed, low-delay connectionless data transport: For data transmission there is no call set up required and packets are sent out immediately when they received it.
2) Multicasting: SMDS handles group addressing.
3) CCITT E.164 addressing: Standard telephone numbers are used for SMDS address.
4) It support for important protocols used in LAN, WAN.
5) It provides Network scalability and SNMP based network management
6) It validates source/destination address for the secure interconnection of LANs and distributed client-server applications

High speed:
When SMDS is offered over ATM networks, customer can access over 2 or 34 Mbit/s access lines attached to MAN and at upto 155 Mbit/s. MAN backbones work at 140 Mbit/s. User does not have to pay for the full 34 Mbit/s bandwidth as 'Access Classes' feature allows them to specify their average transmitted data rate over time to certain sub rates namely 4, 10, 16 and 25 Mbit/s.

Extending LANs:
SMDS is mainly used to connect multiple LAN. Metropolitan Area Network provides interconnection of Token Ring and Ethernet networks. FDDI, Local Frame Relay networks are linked together by using SMDS.

SMDS Network Components:
To provide high-speed data service SMDS networks consist of several underlying devices. These devices are customer premises equipment (CPE), carrier equipment and the subscriber network interface (SNI). CPE is owned and maintained by the customer. CPE includes devices such as personal computers, terminals and intermediate nodes like routers, modems and multiplexers. SMDS carrier provides Intermediate nodes. Carrier equipment consists of high-speed WAN switches that must support to network equipment specifications. These specifications involve network operations, the interface between a local carrier network and a long-distance carrier network.

SNI acts as interface between CPE and carrier equipment. From this interface customer network ends and the carrier network begin. SNI makes the technology and operation of the carrier SMDS network transparent to the user.

SMDS Interface Protocol :
For communications between CPE and SMDS carrier equipment the SMDS Interface Protocol (SIP) is used. SIP offers connectionless service over the subscriber network interface (SNI) and allows the CPE to access the SMDS network. SIP is based on the Distributed Queue Dual Bus IEEE 802.6 standard across metropolitan-area networks. DQDB support SIP because it is an open standard for all the SMDS features. In addition, DQDB designed for compatibility with current carrier transmission standards and it is aligned with emerging standards for Broadband ISDN, which interoperates with broadband video and voice services.

SIP Levels
There are three levels for SIP. SIP Level 3 works at the MAC (Media Access Control) sublayer of the data link layer of the OSI reference model. SIP Level 2 works at the MAC sublayer of the data link layer. SIP Level 1 works at the physical layer of the OSI reference model.

SIP Level 3 starts its operation when user information is passed through it in the form of SMDS service data units (SDUs). SIP Level 3 header and trailer then encapsulate SMDS SDUs. The encapsulated frame is called a Level 3 protocol data unit (PDU). SIP Level 3 PDUs then passed to SIP Level 2.
SIP Level 2 begins operating when it receives SIP Level 3 PDUs. The PDUs are segmented into uniformly sized cells. Then these cells are passed to SIP Level 1 to place them on the physical medium.

SIP Level 1 operates at DS-1 or DS-3 rates between network and CPE devices. SIP Level 1 includes transmission system and Physical Layer Convergency Protocol (PLCP) sublayer. The transmission system sublayer provides method of attachment to a DS-1 or DS-3 transmission link. The PLCP defines how SIP Level 2 cells are to be arranged relative to the DS-1 or DS-3 frame. PLCP also contains other management information.

SMDS Addressing
Both source and a destination address are present in data units (PDUs). SMDS addresses are 10 digit values like telephone numbers.
The SMDS addressing provides group addressing and security features. In SMDS group addressing, multiple CPE stations refer to a single address, which defines the group address in the Destination Address field of the PDU. The network makes number of copies of the PDU, which are delivered to each group member. For distributing routing information, resolving addresses and dynamically discovering network resources the group addresses reduce the amount of network resources required. SMDS group addressing is similar to multicasting on LANs.

SMDS implements two security features:
The two security features are sources address validation and address screening. The Source address validation checks that the PDU source address is assigned to the SNI. Address spoofing is prevented by Source address validation. Address screening provides private virtual network to establish to subscriber that excludes unwanted traffic.