The data link layer is made up of two sublayers:
- MAC (Media Access Control) Layer
- LLC (Logical Link Control) Layer
Both of these two sublayers are responsible for different functions for the data link layer. Today, we are going to talk about the MAC layer in details.
MAC layer is also known as Media Access Layer. As it is evident from the name itself that for the Data link layer, the MAC layer serves the purpose of managing the media access to different devices.
Let’s say there is a bus topology in which many computers are connected in a series. However, there is only one common media cable on which all the data has to be transferred.
We know the data is carried over on a cable in terms of electric signals. So, if more than one computer sends an electric signal at the same time, there will be a signal collision which will turn into the data packets loss on the wire. And that’s where MAC layer comes into the picture.
Channel access control Mechanisms applied by Mac layer:
Different kind of channel access control mechanisms are applied by the MAC layer for media access and here are some of the popular channel access control mechanisms:
- Token passing
- CSMA/ CD
- CSMA/ CA
- Slotting mechanism etc.
A token is passed among the data competitors for a limited amount of time to release the data on the common channel. There are many variants of this mechanisms. Here are some:
- For the quality of service, the token is passed for longer times to a particular node
- A node with more data gets the token for the longer time.
CSMA/ CD stand for Carrier sense multiple access with collision detection (CSMA/CD). It’s a media access control method used most notably in local area networking using early Ethernet technology.
When two nodes send data packet at the same time, then a collision happens. When a collision happens, a jam signal is sent along the wire which tells all the nodes that there has been a collision in the wire. All the nodes stop sending data for a while, for a random time interval before resuming the sending process again.
CSMA/ CA stand for Carrier sense multiple access with collision Avoidance (CSMA/CA).
CSMA/ CA came into the picture because the problem with CSMA/ CD was that it detected collision after it happened. That means the damage was already done and the mechanism was looking for the recovery after the damage.
However, in the case of CSMA/ CA, the data packet will only be sent if the media is free from packets. When a node senses that there is some transmission going on the media channel then it will wait for a random time before sensing the media channel once again.
Slotting mechanism mostly came from the Aloha system. However, the Aloha system has also seen a lot of modifications.
Like in pure aloha, all the nodes sent packets at any time but this mechanism was improved in slotted aloha where all the nodes could send packets only at the beginning, at one slot.
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