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Modems Modern modems use combinations of amplitude, frequency, and phase modulation to achieve high data rates. The fastest dial-up modem at the moment is 56 Kbps. Modems can support auto answer, auto dial, auto disconnect, and auto redial. Connection negotiation is the ability of a modem to automatically fall forward or fallback to faster or slower speeds, respectively. Modems can perform data compression and error correction and support the MNP 1-10 protocols. Most modern modems can support the fax standards.

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Modems Modems for laptop computers are typically of two forms. The first type of laptop modem is completely inside the laptop. A user plugs a modular phone cord (RJ-11) into a slot which is attached to the modem. The second type of laptop modem is about the size of a credit card and plugs into a special connector. The modular phone cord then plugs into this card.

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The 56K Digital Modem A 56K modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems. A 56K modem would actually achieve 64K except: 1. the local loop is still analog, thus analog signaling 2. the analog to digital conversion at the local modem introduces noise/error Combined, these shortcomings drop the speed to at best 56K. A 56K modem does not achieve 56K either because the FCC will not let the modem transmit at the power level necessary to support 56K, so the best the modem can do is approximately 53K A 56K modem will not even achieve 53K if the connection between your modem and the remote computer contains an additional analog to digital conversion, or if there is significant noise on the line.

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Alternatives to Traditional Modems A T-1 line is a digital service offered by telephone companies and can transfer data as fast as 1.544 Mbps (both voice and computer data). To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection. Cable modems allow high speed access to wide area networks such as the Internet. Most cable modems are external devices that connect to the personal computer through a common Ethernet card. Cable modems can provide data transfer speeds between 500 Kbps and 2.5 Mbps. A few cable modem services require a telephone line for the upstream connection.

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Data Communications & Computer Networks, Second Edition 9

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Modem Pools A relatively inexpensive technique that allows multiple workstations to access a modem without placing a separate modem on each workstation. Modem pools can also be used to allow external users to dial into a business or corporate network via a modem in the modem pool.

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Interfacing a Computer to Modems and Other Devices Connecting a device such as a modem (or DCE - data circuit-terminating equipment or data communicating equipment) to a computer (or DTE - data terminal equipment). The connections between the DTE and DCE are the interchange circuits.

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