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Q.1 Explain all design issues for several layers in Computer. What is connection – oriented and connectionless service?


Answer:
The various key design issues are present in several layers in computer networks. The important design issues are:

1. Addressing: Mechanism for identifying senders and receivers, on the network need some form of addressing. There are multiple processes running on one machine. Some means is
needed for a process on one machine to specify with whom it wants to communicate.

2. Error Control: There may be erroneous transmission due to several problems during communication. These are due to problem in communication circuits, physical medium, due to thermal noise and interference. Many error detecting and error correcting codes are known, but both ends of the connection must agree on which one being used. In addition, the receiver must have some mechanism of telling the sender which messages have been received correctly and which has not.

3. Flow control: If there is a fast sender at one end sending data to a slow receiver, then there must be flow control mechanism to control the loss of data by slow receivers. There are several mechanisms used for flow control such as increasing buffer size at receivers, slow down the fast sender, and so on. Some process will not be in position to accept arbitrarily long messages. Then, there must be some mechanism to   disassembling, transmitting and then reassembling messages.

4. Multiplexing / demultiplexing: If the data has to be transmitted on transmission  media separately, it is inconvenient or expensive to setup separate connection for each pair of communicating processes. So, multiplexing is needed in the physical layer at sender end and demultiplexing is need at the receiver end.

5. Routing: When data has to be transmitted from source to destination, there may be multiple paths between them. An optimized (shortest) route must be chosen. This decision is made on the basis of several routing algorithms, which chooses optimized route to the destination.

Connection Oriented and Connectionless Services:
Layers can offer two types of services namely connection oriented service and connectionless service.

Connection oriented service:
The service user first establishes a connection, uses the connection and then releases the connection. Once the connection is established between source and destination, the path is fixed. The data transmission takes place through this path established. The order of the messages sent will be same at the receiver end. Services are reliable and there is no loss of data. Most of the time, reliable service provides acknowledgement is an overhead and adds delay.

Connectionless Services:
In this type of services, no connection is established between source and destination. Here there is no fixed path.
Therefore, the messages must carry full destination address and each one of these messages are sent independent of each other. Messages sent will not be delivered at the destination in the same order. Thus, grouping and ordering is required at the receiver end, and the services are not reliable.

There is no acknowledgement confirmation from the receiver. Unreliable connectionless service is often called datagram service, which does not return an acknowledgement to the sender. In some cases, establishing a connection to send one short messages is needed. But reliability is required, and then acknowledgement datagram service can be used for these applications.
Another service is the request-reply service. In this type of service, the sender transmits a single datagram containing a request from the client side. Then at the other end, server reply will contain the answer. Requestreply is commonly used to implement communication in the client-server model.

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