Computer Network Unit 1 q what are the topologies in computer n/w ?

Q 13. What is modem? Explain null modem?
Ans: A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from driven diodes to radio. Modems, as devices that can either initiate or terminate telecommunications. A modem connection is never an end in itself. Users make modem connections in order to access the Internet or other online services, or to perform a function by emulating some other equipment such as a standalone fax machine, video telephone, or voice telephone. This fact may bring into access considerations other applications that by themselves may not be considered telecommunications.


Null modem is a communication method to connect two DTEs (computer, terminal, printer etc.) directly using an RS-232 serial cable. The RS-232 standard is asymmetrical as to the definitions of the two ends of the communications link so it assumes that one end is a DTE and the other is a DCE e.g. a modem. With a null modem connection the transmit and receive lines are cross linked. Depending on the purpose, sometimes also one or more handshake lines are cross linked. Several wiring layouts are in use because the null modem connection is not covered by a standard.

Types of null modem

No hardware handshaking

The simplest type of serial cable has no hardware handshaking. This cable has only the data and signal ground wires connected. All of the other pins have no connection.

Loop back handshaking

Because of the compatibility issues and potential problems with a simple null modem cable, a solution was developed to trick the software into thinking there was handshaking available.