data link layer of computer networks, Slides of Computer Networks

Download data link layer of computer networks and more Slides Computer Networks in PDF only on Docsity!

Multiple Access

Protocols in

Computer

Networks

Data Link + MAC (Media Access Control)

Def

5/9/

• The Data Link Layer is responsible for transmission of data between two nodes. Its main functions are-
• Data Link Control
• Multiple Access Control

• A sequence of 56 bits (7 bytes) having alternating 1 and 0 values

0) that are used for synchronization. Preamble allows receiving

computer to synchronize timing and to reach a steady state before

actual ethernet frame arrives.

• SFD (Start Frame Delimiter) is a sequence of 8 bits having the bit

configuration 10101011 that indicates the start of the frame. The

difference between a Preamble byte and SFD byte is that the last bit of a

Preamble byte is "0" (10101010) and last bit of SFD byte is "1" (10101011).

The SFD (Start Frame Delimiter) byte indicates the receiving computer that

the next part of Ethernet Frame transmission is the destination MAC

address.

• LAN Vs WAN Vs MAN: Exact Difference Between Types Of Network

(softwaretestinghelp.com)

Two Parts

• The data link layer is used in a computer network to transmit the data between two devices or nodes. It divides the layer into parts such as data link control and the multiple access resolution/protocol. The upper layer has the responsibility to flow control and the error control in the data link layer, and hence it is termed as logical of data link control.
• Whereas the lower sub-layer is used to handle and reduce the collision or multiple access on a channel. Hence it is termed as media access control or the multiple access resolutions.

Explanation

• In CSMA, when two or more data packets are sent at the same time on a shared channel, the chances of collision occurred. Due to the collision, the receiver does not get any information regarding the sender's data packets. And the lost information needs to be resented so that the receiver can get it. Therefore we need to sense the channel before transmitting data packets on a network. It is divided into two parts, CSMA CA (Collision Avoidance) and CSMA CD (Collision Detection).

Let’s

understand

the meaning

of the

individual

component

of CSMA/CD

CS – It stands for Carrier Sensing. It implies that before sending data, a station first senses the carrier. If the carrier is found free, then the station transmits data else it refrains. MA – Stands for Multiple Access i.e. if there’s a channel, then there are many stations that are trying to access it. CD – Stands for Collision Detection. It also guides to proceed in case of packet data collision.

CSMA

• Carrier Sense Multiple Access ensures fewer collisions as the station is required to first sense the medium (for idle or busy) before transmitting data. If it is idle then it sends data, otherwise it waits till the channel becomes idle.
• However there is still chance of collision in CSMA due to propagation delay. For example, if station A wants to send data, it will first sense the medium. If it finds the channel idle, it will start sending data. However, by the time the first bit of data is transmitted (delayed due to propagation delay) from station A, if station B requests to send data and senses the medium it will also find it idle and will also send data. This will result in collision of data from station A and B.