The data link layer is layer 2 in the seven-layer OSI model of computer networking. DLL protocol layer transfers data between nodes on the same local area network segment or between adjacent network nodes in a wide area network. Data-link protocols specify how devices detect and recover from such collisions, and then provide mechanisms to reduce or prevent them.
Here in this article we will be discussing about the DLL Protocols. DLL protocols can be divided into two. They are :
- Elementary Protocols
- Sliding Window
1. ELEMENTARY PROTOCOLS :-
At first we will discuss about the elementary Protocols. Elementary DLL Protocols are further divided into three protocols. They are :
- An Unrestricted simplex protocol
- A simplex stop and wait protocol
- A simplex protocol for a noisy channel
I. AN UNRESTRICTED SIMPLEX PROTOCOL:
Before learning the protocol, we must assume some facts like : Data is transmitted in only one direction. Both the network layers are always ready for transmitting and receiving. we will ignore the processing time and most of all the communication channel between the data link layer will never damage or loose the frames.
This protocol consists of two procedures : sender and receiver. The sender sends the data to the receiver and runs on the data link layer of the sender’s machine and receiver receives the data from the sender and it runs on the data link layer of the receiver’s machine. And here a frame arrival is used which sends the information that an undamaged frame has arrived.
This is a very simple protocol in which the sender uses a while loop and it continuously sends the data to the receiver. The initial work of the receiver is to wait for the frame. as soon as it starts receiving the frame it set the event to frame_arrival or else event is set to wait for frame.
II. A SIMPLEX STOP AND WAIT PROTOCOL:
Here we have to deal with the problem of flooding in which the sender sends the data continuously to the receiver and a condition like flood is created at the receiver’s end.
To prevent this we use stop and wait protocol. According to this the sender will send the data to the receiver. Now receiver will receive the frame and transmit it to the network layer of receiver’s machine and after doing this it sends back the dummy frame to the sender that the frame is received. This is also called acknowledgement. So in more general terms it can be explained as : the sender sends the frame and then wait until the acknowledgement is received by the receiver. This is called stop and wait protocol.
III. A SIMPLEX PROTOCOL FOR A NOISY CHANNEL:
According to this protocol, it deals with the noisy channel. Here the main task is of acknowledgement which is received from the receiver. The receiver sends back a positive and a negative acknowledgement and after receiving the acknowledgement the sender starts again. The working of this protocol is the sender sends the frame to the receiver, the receiver receives the frame if it found it correct it sends back a positive acknowledgement and after receiving positive ack the sender sends the next frame but if the receiver the frame and the frame is damaged due to the channel nose then it sends back an negative ack and after receiving the negative ack the sender resends the frame until the receiver sends back a positive ack. This is called as simplex protocol for noisy channel.
Now we will discuss about the sliding window protocol in Data link layer.
2. SLIDING WINDOW PROTOCOL :-
In stop and wait protocol, as we all know only one frame can be transmitted at a time. This results in serious inefficiencies when the bit length is increased. To increase its efficiency we must try to transmit more than one frame at a time that means multiple frames need to be transmitted so that the efficiency can be greatly increased.
For this we will assume that there are two stations connected through a full duplex link. Station B allocates buffer space so it can take W frames so A can transmit W frames without waiting for the acknowledgement. To keep track of the frames a sequence number is labeled with each. By sending ack means B acknowledges a frame which includes the sequence number of the next frame to be accepted. This implicitly announces that B is ready to receive next W frames. This method can also be used for acknowledging multiple frames. A maintains the sequence numbers of those frames which is acknowledged to be send and B maintains the sequence number which is acknowledged to be received. Every list can be thought as a window frame. Therefore this complete operation is called as sliding window protocol.
Some important Questions on DLL Protocol:
- EXPLAIN DLL PROTOCOLS.
- DEFINE DIFFERENT TYPES OF DLL PROTOCOLS.
- WHAT IS SLIDING WINDOW PROTOCOL?
- WHAT IS STOP AND WAIT PROTOCOL?
- EXPLAIN ELEMENTARY AND SLIDING WINDOW DLL PROTOCOL.
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