by Dinesh Thakur Category: Computer Network

The Multimegabit Data Switching Service (SMDS) is not a protocol, but rather a “metropolitan area service (MAN).” In essence, it is a method of transmitting ATM cells (Asynchronous Transfer Mode) through a shared bus. First used in 1992, it obtained its support when the Regional Bell Operating Companies (RBOC) and other local communication service providers (LEC) began to feel competitive pressure from long-distance communication companies (IXC) in local markets and considered that the best defense against competition was a high-speed data transmission system. SMDS, thanks to its relatively low cost and high-speed data switching service, seemed the perfect solution and many of the LECs in large metropolitan areas began offering the service.

SMDS (Switched Multimegabit Data Service) is a high-speed, packet-based, WAN networking technology used for communications over public data networks (PDN).

SMDS can use copper or fiber optic-based media and supports speeds of 1,544 Mbps over DS-1 transmission facilities (Digital Signal level 1), or 44,736 Mbps speeds over DS-3 transmission facilities. Also, the SMDS data units have sufficient capacity to encapsulate frames or frames IEEE 802.3, IEEE 802.5, and FDDI integers.

The Multimegabit Data Switching Service is the combination of the characteristics of a shared media access LAN and ATM. It based on the IEEE 802.6 protocol that defines the transmission of ATM cells on a shared bus. However, SMDS cells are not identical to ATM cells: the former use an 8-bit access control field while ATM uses a generic 4-bit flow control field. It is because SMDS is a shared access service to the medium and not connection-oriented and, therefore, requires more detailed addressing than the ATM, switched, and connection-oriented service.

The SMDS is a service of local exchange companies that allows LAN networks to extended to MAN networks. Bellcore SMDS development that uses the same fixed-size cell transmission technology that ATM and telecommunications companies offer SMDS as a service that works on it.

• The SMDS is designed to connect the multiple LANs together. This is the first high speed broadband service offered to the public.

• Fig.(a) shows the interconnection of four LANs using six high speed leased lines. But this interconnection is expensive.

• The same LANs can be interconnected using SMDS as shown in Fig.(b). It allows the packets from one LAN to pass to any other LAN.

• The SMDS network is in the telephone company's office. SMDS is designed to handle bursty service.

• The type of traffic in interconnected LANs is not continuous but bursty type. i.e. once in a while a packet will be transferred from one LAN td other but otherwise there is no LAN to LAN traffic.

• So the option of leased lines will be expensive. It will result in high monthly bills.

• The SMDS is a much cheaper solution· to this problem.

• SMDS are supposed to be fast enough. Standard speed is 45 Mbps.

                         Switched Multimegabit Data Service (SMDS)

• With SMDS, each LAN gets connected to a telephone company switch which will route the packets through the SMDS network toward the destination.

• The SMDS is a connectionless packet delivery service with a packet format.

                        SMDS Packet Format

• The three fields in the packet are destination, source and user data. The destination address and source field are of 8 bytes whereas the user data field has a variable length (upto 9188 bytes).

• In the source and destination addresses contain a 4-bit code followed by a telephone number upto 15 decimal digits.

• As soon as a packet arrives at the SMDS network, the first router will check the source address. If it not correct, then the packet is discarded.

• If it is correct, then the packet is routed to the destination.

• It is possible to broadcast a packet to many members, by using SMDS in the broadcasting feature.

• One more important feature is address screening on both outgoing and incoming packets.

• If the outgoing screening is used, then the customer can list a few telephone numbers and specify that no packets should be sent to them.

• On the contrary, if the incoming screening is used, then packets from only certain predecided numbers will be accepted.

SMDS, being a switching technique, has advantages over private networks with dedicated digital lines, such as T1. It is a non-connection-oriented cell-based transport service, which can provide connections from anyone to anyone between different locations without the need to establish a calling system or any termination procedure. In this way, it is possible to extend the communication techniques of the LANs to metropolitan areas. Once the information reaches the SMDS switching network, it directed to any number of locations.

The connection from the user site to the SMDS network made through dedicated or switched circuits. The transmission speed starts from 16 Mbps and increases.

The cost of the service usually based on a fixed monthly rate, but it is ideal for customers who need to switch lines of communication with many different locations.

SMDS, together with ATM and Frame Relay, is one of the “fast packet” techniques that leave error checking and flow control in the hands of terminal nodes. If a packet is lost, it is the destination node that is responsible for requesting retransmission. The burden of error checking does not fall on the network. Although this assigns more work to the end systems, the advantage is that new transmission services have few errors.

SMDS is compatible with the MAN IEEE 802.6 standard, as well as with ISDN BA (Broadband). However, SMDS provides fee management and control services that not specified in the MAN standard. It uses the distributed-queue dual-bus network (DQDB) is a method of access and interface to the network. Physically, it has the form of a star network, like the ring network with a witness, where all the cables connected to a central point.

DQDB provides a right multi-access bus since all nodes can access the bus at any time they want, provided the network is not saturated. Access to the bus of up to 512 nodes, which can be 160 km away, is allowed to operate at 150 Mbps. Like ATM, it works with “time slots” (cells) where sites or devices that are connected can locate data to transport.

Many telecommunications companies are increasing their ATM services and are installing SMDS as a step for ATM. SMDS differs from ATM in the inclusion of any service improvements, such as usage-based billing or better network management facilities.

Some industry analysts believe that SMDS is not become popular due to ATM and Frame Relay techniques. Communications companies have been slow in adopting SMDS, and it is not yet available in all metropolitan areas and many parts of the world. At present, the cost of Frame Relay connections is one-sixth of the cost of comparable MSDS equipment.

The SMDS Interest Group (SIG) is an association that includes local and telecommunications communications companies, interested in SMDS, that works on the development of new specifications for SMDS, such as support for TCP / IP, SNA, IPX, and Frame Relay encapsulation among other things.

Advantages of SMDS

SMDS, as its switching technology, has several advantages over private networks limited to a building that use dedicated digital lines such as T1 or even ATM. These advantages include ease of installation and configuration, scalability, and cost.

Installation and configuration

As the figure shows, those, who install an SMDS network only need to establish a line with the local provider's SMDS network. In a T1 environment, users would have to establish lines between all the places that need interconnection. Its non-connection-oriented nature provides everyone's connections with everyone between a variety of locations without the delays that accompany the call establishment and release procedures. Therefore, SMDS offers a LAN type of communication in metropolitan areas. The information, once the SMDS network reached, is switched to one or multiple sites.

In addition, the non-connection oriented nature of the SMDS facilitates the addition and removal of sites in just a few minutes. Adding or removing locations in an ATM network, with its connection-oriented protocol, would require reconfiguring the entire network.

Ease of administration

SMDS offers some interesting management aspects that many other protocols, such as ATM, do not offer. For example, SMDS provides a usage-based fee, as well as the feature of controlling network access to prevent nodes from monopolizing the network. It also has a possibility of address shielding that increases security by limiting communication to a group of addresses and allows to build a logical private network, using the public SMDS network. SMDS also provides usage statistics and other network administration data directly to end-users. Finally, SMDS allows you to build data packets on the basic structure of ATM cells, thus allowing you to benefit from both the characteristics of the packet and the rapid cell switching.


SMDS is broad and easily scalable. Thanks to its ease in aggregating and removing connections (and because it only pays for the bandwidth used) SMDS can provide enough flexibility and connection options to accommodate changing needs quickly.

SMDS, moreover, is not just a MAN service. Despite its emphasis on the metropolitan area, SMDS is also a WAN service that can reach long distances. It means, in theory, that it is easy to expand an SMDS network throughout a city or country. However, there must be local and long-distance operators that offer this possibility.

SMDS also supplies a wide range of access speeds, from 56 Kbps to 45 Mbps, which gives many options for building high-speed segments. For example, a trunk segment of 45 Mbps could built, and the places of least traffic connected to the trunk segment at 56 Kbps, and remember only pay for the bandwidth used.


The Multimegabit Data Switching Service provides a reasonably high level of interoperability with existing network infrastructures. It supports most existing network environments, including TCP / IP, Novell IPX, AppleTalk, DECnet, SNA, and OSI. Also, as mentioned in the DQDB access method, it allows a data unit of up to 9,188 octets. Therefore, SMDS can encapsulate entire packets from most LANs.

Perhaps, those who have collaborated more significantly in the development of specifications for the interconnection of SMDS and ATM services have been the ATM Forum and the SMDS Interest Group. However, if SMDS considered as a means of migration to ATM, it should be borne in mind that SMDS supports the ATM adaptation layer. As seen, most LAN emulations based on ATM adaptation layer 4 (AAL4). It means that when it is time to integrate existing local area networks to the high-speed trunk, it may be necessary to convert to ATM to benefit from LAN Emulation (LAN-E, LAN Emulation).


As indicated, SMDS supports a wide variety of network and access speeds. Currently, for user access, it offers DS1 (1,544 Mbps) and DS3 (45 Mbps) rates. Access to the network requires a dedicated line at a speed DS1 or DS3.


In the metropolitan and extensive area (where the purchase, installation, and maintenance of the transmission medium is relatively expensive) the economic aspects of a shared medium, such as the one offered by SMDS, are seductive. Usually, the cost of the service is a flat monthly rate based on the bandwidth of the links. Also, SMDS offers expansion potential. You have to pay for the costs of another connection port and the associated utilization costs.

However, the buyer should be cautious. Because, as local communication service providers are the leading providers of SMDS, usually, in a given area, there is no competition for the service. Therefore, the cost of the SMDS should assess other services.

Note: it is difficult to predict the use of the network. Therefore, to prevent the costs of using the SMDS from ruining the budget, we must try to find a provider that offers a flat monthly utilization rate or a monthly limit on the expenses of SMDS.


The SMDS Interest Group (SIG) is the largest promoter of SMDS. It is an association of SMDS product vendors, service providers, operators, and end-users. The GIS has not only user groups but also workgroups that promote SMDS and work on specifications. The technical working group works on improvements to the IEEE 802.6 standard, while the interconnection working group suggests innovations to the standards governing the interconnection and management of SMDS interconnection. They also sponsor a group of users and, of course, have a public relations group that organizes seminars and disseminates information on the availability of SMDS.

Disadvantages of SMDS

Although SMDS seems a satisfactory and affordable solution to await the arrival of ATM, it has some significant disadvantages. These disadvantages must carefully assess before precipitating.

Limited multimedia compatibility

As mentioned at the beginning of this chapter, local service providers began implementing SMDS to help compete in the long-distance data transmission market. Therefore, for this service, they did not spend much time in development that supported voice and video. As a result, SMDS can only transmit data. It does not offer deterministic transmission (such as Frame Relay) or guaranteed and orderly delivery of packages (such as ATM).

Limited use

Currently, only about a couple of hundred companies in the United States are actively deploying SMDS. Even if SMDS expanded rapidly, such a small market might not be able to guarantee enough revenue to keep so many vendors in the market, which ultimately limits the choice of SMDS equipment and services, as well as increasing its cost. Although many SMDS vendors and industry analysts predict that this number grows substantially, many others are of the opposite opinion and believe that SMDS is losing its chance as ATM products get cheaper and more complete.

Limited technical support

As indicated, SMDS is far from achieving universal exploitation. Too many local communication providers do not offer it and, currently, there are very few long-distance operators offering SMDS (MCI is the only one in the US).

About Dinesh Thakur

Dinesh ThakurDinesh Thakur holds an B.C.A, MCSE, MCDBA, CCNA, CCNP, A+, SCJP certifications. Dinesh authors the hugely popular blog. Where he writes how-to guides around Computer fundamental , computer software, Computer programming, and web apps. For any type of query or something that you think is missing, please feel free to Contact us.

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