Question

Classify data networks. Explain with the help of Nyquist theorem, the data rate limitations in PSTN’s. Give an account of modems used in data transfer. Explain their importance and list some of the V-series recommendations.

Answer 1

Classifications of Data Network:

 Data Networks are classified according to their geographical coverage:

• Wide area networks (WANs)

• Metropolitan area networks(MANs)

• Local area networks (LANs)

Data rates in PSTNs: A voice channel in a PSTN is band limited with a nominal

bandwidth of 3.1 kHz. A first-cut estimate of this can be obtained from Nyquist’s

theorem which applies to noiseless channels and states:

 R = 2H log V bps

 Where

 R = maximum data rate

 H = bandwidth of the channel

 V = number of discrete levels in the signal 

 For a 3-kHz channel, and a binary signal, the maximum data rate works out to be 600 bps, if the channel is ideal. In a practical channel, the maximum rate would come down. By increasing the number of levels used to represent the signal, the bit rate may be increased arbitrarily in a noiseless channel. It is important to recognize that the actual number of signal transitions is still limited to the binary level limit; the effective bit rate goes up with more than two signal levels as each signal level can now represent a group of two or more bits. The maximum rate of signal transitions that can be supported by a channel is known as baud rate or symbol rate. In a channel where noise is present, there is an absolute maximum limit for the bit rate. This limit arises because the difference between two signal levels becomes comparable to the noise level when the number of signal level is increased. Claude Shannon extended Nyquist’s work to the case of noisy channels affected by random or thermal noise. Shannon’s result states;

R = H log (1+S/N)

 Where R= the maximum bit rate obtainable

 H= bandwidth of the channel

 S/N = signal to noise ratio. 

Modem : Modems are generally provided by network operators (Department of Telecommunication in India) or by vendors who are not necessarily the manufacturers of computer systems. Hence, there is a need to standardize the interface between the modem and the computer equipment. CCITT terminology for the modem is data circuit terminating equipment (DCE), and for the computer it is data terminal equipment (DTE). DCE is often referred to as data communication equipment, outside CCITT. It is important to recognize that DCE and DTE are generic terms and may be applied to a variety of equipments not necessarily only to modems or computers. For example, a DTE may be a terminal, workstation or a computer, and a DCE may be a modem or a computer based node in a data network. A series of standards, known as V-series, has been defined by CCITT for interfacing DTEs to DCEs operating with PSTNs. The series also defines a variety of DCEs using different modulation techniques and operating at different speeds using either a leased PSTN or dial-up line Examples of DTE-DCE interface standards and other standards related to data transmission on PSTN are: V.5 Standardization of data signaling rates for synchronous data transmission in the PSTN 

V.24 DTE-DCE interface and control signals

 V.28 DTE-DCE electrical characteristics for unbalanced double-current interchange

circuits.

 V.53 Limits for the maintenance of telephone type circuits used for data

transmission.