Page 112 - DCAP406_DCAP_207_Computer Networks

Unit 7: Data Link Layer

Checksum is a simple type of redundancy check used to detect errors in data. Notes
Cyclic Redundancy Check is used widely in computer networks, is a technique of providing
a data string added to packets of information that can be used to detect errors in the data
packets.
Stop and Wait protocol is easiest to implement and proves to be the most efficient on an
error free communication channel. However, an error free communication channel is
practically not possible.

7.4 Keywords

Acknowledged Connection-oriented Service: The data link layer provides this service to the
network layer by establishing a connection between the source and destination hosts before any
transfer of data takes place.
Acknowledged Connectionless Service: Refers to delivery of each frame sent between two hosts
arrives correctly.

Checksum: Refers to an algorithm that calculates the binary values in a packet or other block of
data and stores the results with the data to compare with a new checksum at the other end.
Cyclic Redundancy Check: Refers to a technique of providing a data string added to packets of
information that can be used to detect errors in the data packets.
Error Control: Involves sequencing frames and sending control frames for acknowledgement.
Flow Control: Refers to control the rate of data transmission between two source and destination
hosts.
Framing: Provides a reliable transfer of bit streams to the network layer the data link layer
breaks the bit stream into frames.

Go Back N: The Go Back N protocol enables the source machine to have more than one outstanding
frame at a time by using buffers.
High-level Data Link Control: Refers to receipt of data that is checked after multiple frames are
sent for improved transmission efficiency. It also offers a form of advanced error control called
CRC (Cyclic Redundancy Check).
Parity Checks: Consists of even parity and odd parity method. The operation of receiver is
simple, as the receiver needs to count only the number of 1’s in the received data stream with
additional parity bit.
Unacknowledged Connectionless Service: Refers to the independent frames from source host to
the destination host without any acknowledgment mechanism.

7.5 Review Questions

1. What is the data link protocol?
2. What advantages does Selective Repeat sliding window protocol offer over Go Back N
protocol?

3. What is the purpose of flow control?
4. Describe how does finite state machine model carry out protocol verification?
5. What are different data link protocols available? Why does PPP have become popular?

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