by Dinesh Thakur Category: Computer Network

• 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.


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