MODEM & ASYNCHRONOUS COMMUNICATION

A modem is a device that makes it possible for computer to communicate over a tele-phone line. In a network environment, modems serve as a means of communicating between networks or connecting to the world beyond the local network. A modem at the sending computer converts the computer?s digital signals into analog and transmits the analog signals onto telephone line.  A modem at the receiving end converts the incoming analog signals back into digital signals.  In other words, a sending modem MOdulates digital signals into analog signals, and a receiving modem DEModu-lates analog signals back into digital signals. There are two types of carriers for modem communications are: 1. Public dial network lines (dial-up lines) These are common telephone lines. They require users to manually make a connect-ion for each communication session, and are slow and not totally reliable for transmitting data. However, for some companies it may be practical to temporarily dial up a communication link between sites for a certain amount of time each day to transfer files or update databases.  Carriers are continually improving their dial-up line services. Some digital lines claim data transmission speeds up to 56 Kbps can be achieved using error correction, data compression, and async modems. 2. Leased (dedicated) lines These provide full-time dedicated connections that do not use a series of switches to complete the connection. The quality of the line is often higher than the quality of a telephone line which was only designed for voice transmissions. They typically range in speed from 56 Kbps to 45 Mbps or more.

Network Introduction Network Cabling Wireless Network Network Interface Card OSI Model Protocols Ethernet - Network architecture Token Ring - Network architecture AppleTalk and ArcNet - Network architectures Repeaters, Bridges, Routers, and Gateways E-mail Standards Modem & Async Comm. T1, T3, and Switched 56 Digital Connectivities Packet-Switching Network,Frame Relay and ATM ISDN & FDDI

Asynchronous, or async, may be the most widespread form of connectivity in the world. This is because async was developed so it could use common telephone lines.

In the async environment, data is transmitted in a serial stream. Each character?letter, number, or symbol?is turned into a string of bits. Each of these strings is separated form the other strings by a start-of-character bit and a stop bit. Both the sending and receiving devices must agree on the start and stop bit sequence. The receiving computer uses the start and stop bit markers to schedule its timing functions so it is ready to receive the next byte of data.

Communication is not synchronized. There is no clocking device or nethod to coordinate transmission between the sender and the receiver. The sending computer just sends data, and the receiving computer just receives data. The receiving computer then checks to make sure that the received data matches what was sent. Twnety-five percent of the data traffic in async communications consists of data traffic control and coordination.

Async transmission over telephone lines can happen at up to 28,800 bps. However, the latest data compression methods can boost the 28,800 bps to 115,200 bps over directly connected system.

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