Hello Friends, In this blog post(design issues of the data link layer) I am going to let you know about the design issues of the data link layer.
Inside this blog post(design issues of the data link layer), we will explore What are the design issues of the data link layer? What is the responsibility of the data link layer? What is the data link layer of the OSI model? Datalink layer protocols.
(Design issues of the data link layer)…Data Link incorporates certain processes that carry out error control, flow control, and the associated link management functions. It receives the data to be sent to the destination from the next higher layer and adds some control bits to block data bits. This data block along with the control bits is called a frame.
Some important functions of the data link layer include a well-defined service interface to the network layer, framing flow control, error detection and error control, frame formatting, and sequencing. The design issues for the data link layer are discussed below.
design issues of the data link layer:
Service protocol to the network layer:
The primary responsibility of the data link layer is to provide the service to the network layer. The principle is transferring data from the network layer on the source machine to the network layer on the destination machine. The two data link layer communicates with each other by the data link control protocol.
Following are the important services provided by the data link layer to the data link layer:
Unacknowledged connectionless service.
Acknowledged connectionless service.
Acknowledged connection-oriented service.
Unacknowledged connectionless service:
As the name suggests it is an unacknowledged form of transmission. Here the source machine sends the data to the destination machine without any acknowledgment.
For this, no connection either is established or released. If the data is lost due to noise or interface, the lost data is not even recovered by the layer.
Acknowledged connectionless service:
In acknowledged connectionless service each data frame is acknowledged by the destination machine. If any data frame is lost or not arrived in time, the same can be transmitted again. In this service, no connections are used.
Acknowledged connection-oriented service:
Acknowledged connection-oriented service establishes a connection prior to data transmission. Each frame is numbered before transmission is carried out in distinct phases.
This service provided to be more reliable than other services.
To service the network layer, the data link layer uses the service provided to it by the physical layer accepts the raw bitstream and delivers it to the destination.
This bitstream may contain an error that is the number of bits received may not be equal to the number of bits transmitted. The data link layer breaks the streams into discrete frames and computes the checksum for each frame.
At the destination, the checksum is recomputed. The breaking of bitstream by inserting spaces and time gaps is called framing.
Since it is difficult and risky to count on timing and mark the start and end of each frame, a various simple method used for framing are:
Starting and ending character with character stuffing.
Starting and ending flags, with bit stuffing.
To ensure the proper sequencing and safe delivery of frames at the destination, an acknowledgment should be sent by the destination network. The send back special control frames bearing positive or negative acknowledgment about the incoming frames.
If the sender receives a positive acknowledgment it means the frame has arrived safely. A negative acknowledgment means that something has gone wrong and the frame is to be retransmitted.
A timer at the receiver’s and sender’s end is introduced. Also, sequence number to the outgoing frames are maintained so that the receiver can distinguish a retransmission from originals. This is one of the most important parts of the data link layer duties.
When the sender is running on a fast machine or lightly loaded machine and receiver is on the slow or heavily loaded machine. Then the transmitter will transmit frames faster than the receiver can accept them.
Even if the transmission is error-free at a certain point the receiver will simply not be able to handle the frames as they arrive and will start to lose some.
To prevent this flow control, the mechanism is incorporated which includes a feedback mechanism requesting transmitter retransmission of incorrect message block. The most common retransmission is known as automatic repeat request (ARQ).
Error control in the data link layer (DDL) is based on automatic repeat request (ARQ) that is a retransmission of data in three cases:
If the frames are to be distributed to different systems on the network, the data link layer adds a header to the frame to define the physical address of the sender and/or receiver of the frame.
In this blog post(design issues of the data link layer), we have seen all the possible issues with the data link layer. We have also explored What are the design issues of the data link layer? What is the responsibility of the data link layer? What is the data link layer of the OSI model? Datalink layer protocols.
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