Modem & Multiplexers



Modem is a short form for Modulator/Demodulator

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

Enables a computer to transfer data over the telephone or cable lines



Traditional telephone lines can carry frequencies between 300 and 3300 HZ, giving them BW 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 effective BW of a telephone line being used for data Transmission is 2400 Hz, covering the range from 600 to 3000 Hz.


 modulation and demodulation

Figure shows the relationship of modems to a communication link. The computer on the left sends binary data to the modulator portion of the modem; the data is sent as an analog signal on the telephone lines. The modem on the right receives the analog signal, demodulates it through its demodulator, and delivers data to the computer on the right.

The communication can be bidirectional, which means the computer on the right can also send data to the computer on the left using the same modulation and demodulation processes.

Modem standards

V-series standards published by the ITU-T.

  • V.32
  • V.32bis
  • V.34bis
  • V.90
  • V.92


This modem uses a combined modulation and demodulation encoding technique called trellis-coded modulation. Trellis is essentially QAM plus a redundant bit. The Data stream is divided into 4-bit sections.  Instead of a quad bit, however, a pentabit is transmitted. The value of the extra bit is calculated from the values of the data bits.

V.32 bis

The V.32 bis modem support 14,400-bps transmission. The V.32 uses 128-QAM transmission.

V.34 bis

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


Traditional modems have a limitations on the data rate.V.90 modems with a bit rate of 56,000 bps, called 56Kmodems, are available. Downloading rate is 56K, while the uploading rate is a maximum of 33.6 kbps.


                The standard above V.92 is called V.92. These modems can adjust their speed, and if the noise allows, they can upload data at the rate of 48 Kbps. The modem has additional features. For example, the modem can interrupt the internet connection when there is an incoming call if the lines  has call-waiting service


Features of Modem

  • Transmission speed
  • Voice or Data communication
  • Data compression
  • Auto answering
  • Fax capability

Working of a Modem

  • Software in computer activates modem, which listens for dial tone on phone line
  • Modem dials a specified number
  • Modem at receiving end answers call and sends out a signal
  • Modem at sending end converts digital signals to analog signals
  • Modem at receiving end converts analog signals back to digital signals


Basic Types of Modems

Internal - A modem card that you can plug into an expansion slot on the motherboard

External – Connected to the PC through a cable, which is plugged into serial port on the back of the system unit




A communications device that multiplexes (combines) several signals for transmission over a single medium. A demultiplexer completes the process by separating multiplexed signals from a transmission line. Frequently a multiplexer and demultiplexer are combined into a single device capable of processing both outgoing and incoming signals. A multiplexer is sometimes called a mux.



Type of Multiplexer


Time Division Multiplexer 

 Short for Time Division Multiplexing, a type of multiplexing that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel. Within T-Carrier systems, such as T-1 and T-3, TDM combines Pulse Code Modulated (PCM) streams created for each conversation or data stream.


Wavelength Division Multiplexing

Short for wavelength division multiplexing, a type of multiplexing developed for use on optical fiber. WDM modulates each of several data streams onto a different part of the light spectrum. WDM is the optical equivalent of FDM.


Dense Wavelength Division Multiplexer

Dense Wavelength Division Multiplexing, an optical technology used to increase bandwidth over existing fiber optic backbones.

DWDM works by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers. So, if you were to multiplex eight OC -48 signals into one fiber, you would increase the carrying capacity of that fiber from 2.5 Gb/s to 20 Gb/s. Currently, because of DWDM, single fibers have been able to transmit data at speeds up to 400Gb/s.

A key advantage to DWDM is that it’s protocol- and bit-rate-independent. DWDM-based networks can transmit data in IP, ATM, SONET /SDH, and Ethernet, and handle bit rates between 100 Mb/s and 2.5 Gb/s. Therefore, DWDM-based networks can carry different types of traffic at different speeds over an optical channel.


Frequency Division Multiplexing

frequency division multiplexing, a multiplexing technique that uses different frequencies to combine multiple streams of data for transmission over a communications medium. FDM assigns a discrete carrier frequency to each data stream and then combines many modulated carrier frequencies for transmission. For example, television transmitters use FDM to broadcast several channels at once.  


Statistical Multiplexer

Statistical multiplexers make it possible for multiple RS-232 devices to share a single data line. They also perform error correction to insure error-free transmissions. The term “statistical” refers to their ability to take advantage of the intermittent usage statics of most RS-232 devices (and all PC and terminal users).

Because keyboards are idle a large part of each second with no one typing and no data being sent from the computer, each PC or terminal often averages less than 5% of its potential data rate. Statistical multiplexers allow the sum of the PC and terminal rates to exceed the composite link speed between the multiplexers. Statistical multiplexer need Buffer.











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