SMDS

By Bell communication SMDS is developed, its whole called exchange more Multimegabit Switched gigabit Data Service (SMDS) Data Service. SMDS in 1990 as connection FDDI network to MAN (a MAN) based on telecom system, this is released its first publication. Based on letter SMDS is a $data transmission technology, the transmission rate in T carrier line can be as high as 155Mbps, in Europe, there are a wide range of applications.

Already can and B - ISDN compatible

Along with the development of SMDS, it has can do it and B - ISDN compatible, once B - ISD N can widely used, can use long super-fast SMDS for data transmission. By SMDS SMDS letter yuan is processed, switches SMDS switches through high-speed DS - 1, ISDN and S O N E T link connection. SMDS is a connectionless transmission system, to reduce expenses, so will the intelligent error check to the terminal equipment, such as switches and routers.

In the U.S. the

In the United States, RBOC SMDS first into the Atlantic and Pacific, for example Telesis Bell. The company now has turned many SONET, because the more rapid SONET can provide data transmission.

Technology purpose

SMDS is to provide high-speed MAN data connection and development, its purpose is to:

For regional network to provide high-speed j the link.

Transfer image file j, such as medical X-ray pictures.

. Transmission building drawings and other CAD graphics.

To libraries and electronic databases j fast access.

SMDS was originally designed for the MAN realize, its purpose is to provide various rate from low to high communication, now it can also used conveniently WAN realize, as in the realization of the use of that. A MAN Another important SMDS the characteristic is that it can be used together and many of the agreement, including TCP/IP, S N A, IPX/the SPX, DECnet and AppleTalk. Because SMDS use letter yuan to transmit data, so it can need not get big frame into smaller units can be carried on WAN transmission.

SMDS system structure

SMDS interface called distribution queue dual bus (DQDB), consists of two sharing of optical fiber medium composition. Two fiber-optic cables are connected to the user and device one end and the other end and manufacturers, connected. Switch In a cable transmission is one-sided, user equipment to switch from another cable in the transmission is one-sided, switch to user equipments from. Use two one-way bus will eliminate the possibility of a conflict exists. In each a bus DQDB the transmission are divided into the gap. Any of the equipment connected with in time of need can access this bus, until the bus so the saturation of data transmission. Time buttress access through the time between devices distribution method for management, so no equipment can get 100% of the time. In most SMDS bus can be connected to 512 equipment, SMDS bus can cover 160 km (100 miles) away.

SMDS bus usually use T carrier lines, but, in these lines than this circuit transmission speeds of the highest transmission speed to slightly lower, because some bandwidth is used to control and signaling. For example, T - 1 Mbps, but 1.544 for speed in T - 1 line SMDS transmission rate of only 1.14 Mbps. When T - 3 lines using the DS - 3 visit rate, the lines SMDS into different service category, transmission different rate combination: 10Mbps, 16Mbps 4Mbps, 25Mbps and 3, 4 M b p s.

Mainly used to support data transmission SMDS from LAN receives the frame converted to believe yuan. The exception is routing network function frame not by conversion, these frames SMDS data exchange interface (SMDS - DXI) for processing. DXI SMDS - is used instead of letter yuan and frame format to senior link control (in HDLC), this format and x.25 LAPD LAPB and ISDN the format of the similar.

SMDS layered communication and letter yuan structure

SMDS hierarchy and the OSI model of the physical layer, data link layer and network layer is the corresponding. In the physical 802.6 standards used IEEE communications, a MAN in the data link layer LLC son layer communications. The network layer used for the transmission of data by communication path.

SMDS letter yuan for fixed length of 53 bytes, end-to-end, subsection unit and tail constitutes. One head comprised

Parts:

. Access control: contain information indicating whether letter by the user and device $send, such as a router or the merchant sites SMDS switches.

. Network control: that the information contained in the letter yuan is to control information or carry data.

. Piecewise type: indicating whether letter yuan includes a message, or the beginning of the sequence contains the entire sequence

Listed in the middle section, or the end of the sequence, or in the letter included in the whole message $sequence.

The message ID: contains j assigned to a message is a sequence of all the only Numbers, $letter indicating these letters yuan need unity explanation.

The subsection units contains letter yuan of the letter is loaded, $SMDS Internet users in the transmission of data. Finally, the letter by two domain $tail length and load composed: load CRC. Load the length of the instructions are segmented unit load length and null bytes length. If no load, load length field is 0. Load CRC is verified that receive node received data and the data sent by whether the same, these data including subsection type, message, ID and subsection unit and load length. All these domain of information contained in load with the recipient of receiving and explain about accurately. CRC is will says each domain of the presidents of the numerical addition get.