Page 24 - DCAP406_DCAP_207_Computer Networks

Unit 2: Network Software

The English person passes his message in English to his translator, who translates it into Notes
French or other language, depending on the layer 2 protocol.

Figure 2.1: Functions of Layered Architecture

The translator then passes the message to secretary at layer 1 to transmit the message by
telephone, email, or some other means, depending on layer 1 protocol.

When the message reaches the destination, the peer secretary passes the message to the
peer translator, who translates it into French and passed across the 2/3 interface to French
speaking person.

Thus an effective conversation takes place between two persons, not understanding each
other’s language. Similarly, two computers on different networks communicate with each
other.
2.2.1 Design Issues for the Layers

In information exchange between computers, communication processes need to have the
following to accomplish these aspects of exchange process:

Physical Data Encoding: The information exchanged between two computers is physically
carried by means of electrical signals assuming certain coding methods. For two computers
to reliably exchange data, they must have a compatible implementation of encoding and
interpreting data carrying electrical signals.
Multiplexing: This uses the same connection for multiple, unrelated conversations. For
example, a few physical circuits are used for all virtual connections.
Transmission Media: This concern deals with the type of media used (fiber, copper, wireless,
and so on), which is dictated by the desirable bandwidth, immunity to noise, and
attenuation properties. These factors affect the maximum-allowable media length while
still achieving a desirable level of guaranteed data transmission.

Flow Control: Data communication process allocates memory resources, commonly known
as communications buffers for the sake of transmission and reception of data. It keeps a fast
sender from swamping a slow receiver with data. Some kind of feedback from receiver is
needed. A proper data flow control technique desires that the receiving process in
transmission of data should send a “stop sending” signal to the sending computer whenever
it does not have resources to cope with the rate at which data is being transmitted. On the
other hand, when receiving device has sufficient resources available, it should send a “resume
sending” signal. The resources available at receiving end to cope up with the sending
computer are buffers availability. Figure 2.2 shows the mechanism of data flow control.

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