#### Data communication - Question and answer - 2

6. Explain what is Digital Signals ?
A digital signal is a signal that is being used to represent data as a sequence of discrete values; at any given time it can only take on one of a finite number of values. This contrasts with an analog signal, which represents continuous values; at any given time it represents a real number within a continuous range of values.

7. Explain different types of Multiplexing ?
Time division multiplexing is classifieds into four types:
Synchronous time-division multiplexing.
Asynchronous time-division multiplexing.
Interleaving time-division multiplexing.
Statistical time-division multiplexing.

8. Explain how the errors are detected and corrected using Block coding ?
If the following two conditions are met, the receiver can detect a change in the original code word by using Block coding technique.
1. The receiver has (or can find) a list of valid code words.
2. The original code word has changed to an invalid one.
In error detection, the receiver needs to know only that the received code word is invalid, in error correction the receiver needs to find (or guess) the original code word sent. So, we need more redundant bits for error correction than for error detection.

9. What is Circuit Switched Network ?
Circuit-Switched Network – a type of network where the communications between end devices (nodes) must be set up before they can communicate. Once set up, the “circuit” is dedicated to the two nodes it connects for the duration of that connection. An example of a circuit-switched network is an analog telephone network.

10. What is a Datagram Network ?
In data communications, we need to send messages from one end system to another. If the message is going to pass through a packet-switched network, it needs to be divided into packets of fixed or variable size. In a datagram network, each packet is treated independently of all others. A datagram is a basic transfer unit associated with a packet-switched network. Datagrams provide a connectionless communication service across a packet-switched network.

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