Data Link Layer

Right now, we are going to talk about Data Link Layer.
Data link layer had a function to make sure that message sent to correct device and translate message from network layer to bits form to transmitted by Physical layer.
In this layer, message is in form data frame with additional header which contain source address and destination hardware.
In device layer 3 (router), device worked to choose the best way to reach network destination, but the device didn’t care where the host is located. In Device layer 2 had responsibility to identified every device in network.

When host send package to another host, data link layer will frame the package with information data link control include hardware address, but if this package go through a router, this information will be left, router just receive the original package.

In Data link layer had 2 sublayers which is MAC and LLC.

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Media Access Control (MAC) also know as 802.3.
this define how the package been replace by media. Physical address and logical topology being defined in here.
Logical topology is signal path which go trough a physical topology.

MAC may refer to the sublayer that determines who is allowed to access the media at any one time (usually CSMA/CD). Other times it refers to a frame structure with MAC addresses inside.

CSMA/CD (Carrier Sense Multiple Access with Collision Detection) is a access method that trace every possibility collision will happen. If a computer want to send data and detect that data which will be send is already in network, it will wait for a few times to try again send the data, so the collision can be avoid.
If in the checking step found another data transmission and being collision with the data which being send, the node must send again the request in a few minutes in random time.

There are generally two forms of media access control: distributed and centralized.
To make you more easy to understand,
for example in a conversation, If two people speak at the same time, they will back off and begin a long and elaborate game of saying “no, you first”.
The MAC determines where one frame of data ends and the next one starts – frame synchronization
There four means of frame synchronization.
- time based
- character counting
- byte stuffing
- bit stuffing.

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The second of is:
Logical link control (LLC) also known as 802.2.
This sublayer had responsible to identify network layer protokol and then encapsulate it. Header LLC told to data link layer what should be done for package when frame is received.
For example:
a host that received frame and will saw header LLC to understand the packet destination in IP protocol on network layer. LLC is also give flow control and sequence the control bit.

A example device that work in Data link Layer is Switch and Bridges