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Chapter: Computer Networks : Physical Layer

Dial-up Modems

Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz.

Dial-up Modems:


Traditional telephone lines can carry frequencies between 300 and 3300 Hz, giving them a bandwidth of 3000 Hz. All this range is used for transmitting voice, where a great deal of interference and distortion can be accepted without loss of intelligibility.

The term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator. A modulator creates a band pass analog signal from binary data. A demodulator recovers the binary data from the modulated signal.


Modem stands for modulator/demodulator.


Modem Standards

The most popular modems available are based on the V-series standards by the ITU-T.


a.  V.32 and V.32bis


The V.32 modem uses a combined modulation and encoding technique called trelliscoded modulation. Trellis is essentially QAM plus a redundant bit. The data stream is divided into 4-bit sections.


The V.32 calls for 32-QAM with a baud rate of 2400. Because only 4 bits of each pentabit represent data, the resulting data rate is 4 x 2400 = 9600 bps. The constellation diagram and bandwidth are shown in Figure 1.53


The V.32bis modem was the first of the ITU-T standards to support 14,400-bps transmission. The V.32bis uses 128-QAM transmission (7 bits/baud with I bit for error control) at a rate of 2400 baud (2400 x 6 = 14,400 bps).

b. V.34bis

The V.34bis modem provides a bit rate of 28,800 with a 960-point constellation and a bit rate of 33,600 bps with a 1664-point constellation.



Traditional modems have a data rate limitation of 33.6 kbps, as determined by the Shannon capacity. However, V.90 modems with a bit rate of 56,000 bps are available; these are called 56K modems. These modems may be used only if one party is using digital signaling.



The standard above V90 is called ~92. These modems can adjust their speed, and if the noise allows, they can upload data at the rate of 48 kbps. The downloading rate is still 56 kbps. The modem has additional features. For example, the modem can interrupt the Internet connection when there is an incoming call if the line has call-waiting service.

Figure 1.54 Uploading and downloading in 56K modems


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