As we all know, Data Link Layer is the second layer in the OSI model. When the Data Link Layer is combined with physical layer of the OSI model with some interconnecting medium, it provides a connection for data transfer. This connection is reliable. The main processes which are been followed in the data Link Layer are error control, flow control, link management etc.
FUNCTION OF DATA LINK LAYER:
The Data Link Layer receives the data, i.e. to be transmitted to other device, from the higher layer of the OSI model. The data comes in the form of block, this layer adds some control bits to the blocks of data which is in the form of data bits. Data blocks with the combination of control blocks are called as FRAME.
DATA LINK PROTOCOL:
Protocols can be defined as these are called as set of rules. In the Data Link Layer some sets of rules and procedures are made to carry the control functions in the data link layer. These are known as data link control.
Some of the data link controls used in data link layer are :
- High level data link control (HDLC)
- Synchronous data link control (SDLC)
- Advance data communication control procedure (ADCCP)
- Binary synchronous data link control (BSC, BISYCNC)
SERVICES PROVIDED TO THE NETWORK LAYER :
The main work of the data link layer is to provide services to the network layer. The main service of the data link layer is to transfer data of network layer from the source machine to the network layer of the destination machine.
Following are the services provided by the data link layer to the network layer :
- Acknowledged connectionless service- in this type of service of data link layer an acknowledged connectionless service is established between the source and the destination. That means here the acknowledgement bit is sent between the source and destination but no connection is established.
- Unacknowledged connectionless service- As the name suggest, when the data is transferred from the source machine to the destination machine it is not known to the destination machine i.e. no acknowledgement is sent either from the source side nor from the destination side. No connection is established or released between them. And if the data is lost in between the transmission then it cannot be recovered.
- Acknowledged connection-oriented service- In this type of service properly an acknowledgement bit is sent from both the sides and a proper connection is established between the source and the destination. Proper connection is established and released.
To provide services to the network layer, data link layer uses the services provided to it by the physical layer. Physical layer sends the data in the form of bits in streams. Data link layer break the bit stream into the discrete frames and calculates the sum which is called the checksum. This checksum at the source machine should be equal to the checksum at the destination machine. The breaking of bit stream into discrete frames by adding time gaps or inserting spaces is called as framing.
ERROR CONTROL :
To ensure proper services and safe delivery of the packets data link layer uses a mechanism which is called as error control. In this the receiver sends back an acknowledgment bit which shows that the data is safely delivered to the destination machine.For this the receiver when receives the data or does not receives it sends back the either the positive or negative acknowledgement. If the data is properly received it sends back the positive acknowledgement and if it does not receive the correct data or incomplete data it sends back the negative acknowledgement. This method is known as error control.
FLOW CONTROL :
It is very important that the transmission speed of both the machines should be same. because if this is not the case then there occur many problems like when sender sends the data at a very fast speed or at very slow speed and the receiver receives the data at very slow speed or at very fast speed, in either of the case the receiver will start loosing the data. To prevent this flow of data flow control mechanism is incorporated in them. like Automatic Repeat Request (ARQ).
Sample Questions :
- WHAT ARE THE DIFFERENT DESIGN ISSUES OF DATA LINK LAYER?
- WHAT IS DATA LINK PROTOCOL?
- MENTION SOME OF THE IMPORTANT FUNCTIONS OF DATA LINK LAYER?
- EXPLAIN THE CONCEPT OF DATA LINK LAYER?
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