8 Communication Networks
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and checks for transmission errors by requiring the receiving end to send back acknowledgment
frames. Data link protocols are concerned with the following
issues:
•
How to divide the data into frames.
•
How to delimit frames by adding special bit patterns to the beginning and end of each frame. This allows the receiving end to detect where each frame begins and
where it ends.
•
Error detection. Some form of error check is included in the frame header. This is constructed by the transmitting end based on the contents of the frame, and
checked for integrity by the receiving end. A change in the frame bits can be detected in this way.
•
Error correction. When a frame arrives corrupted or is for any reason lost in the network, it is retransmitted. Lost acknowledgment frames may result in duplicate
frames, which need to be detected and corrected as well.
•
Flow control. In general, not all communication devices in a network operate at the same speed. Flow control provides a means of avoiding a slow receiver from
being swamped by data from a fast transmitter.
The data link layer hides the above details from the higher layers. To the network
layer, it appears as a reliable communication channel which can send and receive data packets as frames.
1.3.3. The Network Layer
The network layer is concerned with the routing of data across the network from one end to another. To do this, the network layer converts the data into packets and
ensures that the packets are delivered to their final destination, where they can be converted back into the original data. Network layer protocols are concerned with
the following issues:
•
The interface between a host and the network.
•
The interface between two hosts across the network.
•
Routing of packets across the network, including the allocation of a route and handling of congestion.
•
Correct ordering of packets to reflect the original order of data.
•
Collection of statistical information e.g., number of transmitted packets for performance measurement and accounting purposes.
•
Internetworking: communication between two or more networks.
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The network layer hides the above details from the higher layers. To the
transport layer, it appears as a uniform data transfer service, regardless of the location of the communicating devices and how they are connected.
1.3.4. The Transport Layer
The aim of the transport layer is to isolate the upper three layers from the network, so that any changes to the network equipment technology will be confined to the
lower three layers i.e., at the node level. Transport layer protocols are concerned with the following issues:
•
Establishment and termination of host-to-host connections.
•
Efficient and cost-effective delivery of data across the network from one host to another.
•
Multiplexing of data, if necessary, to improve use of network bandwidth, and demultiplexing at the other end.
•
Splitting of data across multiple network connections, if necessary, to improve throughput, and recombining at the other end.
•
Flow control between hosts.
•
Addressing of messages to their corresponding connections. The address information appears as a part of the message header.
•
Type of service to be provided to the session layer e.g., error-free versus error- prone connections, whether messages should be delivered in the order received
or not.
The transport layer hides the above details from the higher layers. To the session
layer, it appears as a customized data transfer service between two hosts, isolating the underlying network technology from it.
1.3.5. The Session Layer
