Data Link Laye Services-Services Provided to Network Layer
Services Provided to the Network LayerThe function of the data link layer is to provide service to the network layer.
• The principal service is transferring data from the network layer on the source machine to the network layer on the destination machine.
• The network layer hands some bits to the data link layer for transmission to the destination, the job of the data link layer is to transmit
the bits to the destination machine, so they can be handed over to the network layer on the destination machine.
The data link layer can be designed to offer various services.
Three possibilities that are commonly provided are:
1. Unacknowledged connectionless service.
2. Acknowledged connectionless service.
3. Acknowledged connection-oriented service.
• Unacknowledged connectionless service consists of
having the source machine send independent frames to the destination
machine without having the destination machine acknowledge them. No
connection is established beforehand or released afterward. Good channels
with low eror rates, for real-time traffic, such as speech.
• Acknowledged connectionless service. When this service
is offered, there are still no connections used, but each frame sent is ndividually acknowledged. This way, the sender knows whether or not a
frame has arrived safely. Good for unreliable channels, such as wireless.
• Connection-oriented service. With this service, the source
and destination machines establish a connection before any data are
transferred. Each frame sent over the connection is numbered, and the
data link layer guarantees that each frame sent is received. Furthermore, it
guarantees that each frame is received exactly once and that all frames are
received in the right order.
• When connection-oriented service is used, transfers have three
distinct phases.
1. In the first phase the connection is established by having
both sides initialize variable and counter need to keep track of which
frames have been received and which ones have not.
2. In the second phase, one or more frames are actually
transmitted.
3. In the third phase, the connection is released, freeing up
the variables, buffers, and other resources used to maintain the
connection.
