Computer Network Unit 1 q what are the topologies in computer n/w ?

Q 3. What is session ?
Ans: a session is a series of interactions between two communication end points that occur during the span of a single connection. Typically, one end point requests a connection with another specified end point and if that end point replies agreeing to the connection, the end points take turns exchanging commands and data ("talking to each other"). The session begins when the connection is established at both ends and terminates when the connection is ended.


Q 4. What do you mean by synchronization in session layer ?
Ans:
Synchronization: Move the two session entities into a known state. The transport layer handles only communication errors, synchronization deals with upper layer errors. In a file transfer, for instance, the transport layer might deliver data correctly, but the application layer might be unable to write the file because the file system is full. Users can split the data stream into pages, inserting synchronization points between each page. When an error occurs, the receiver can resynchronize the state of the session to a previous synchronization point. This requires that the sender hold data as long as may be needed. Synchronization is achieved through the use of sequence numbers. The ISO protocols provide both major and minor synchronization points. When resynchronizing, one can only go back as far as the previous major synchronization point. In addition, major synchronization points are acknowledged through explicit messages (making their use expensive). In contrast, minor synchronization points are just markers.
Q 5. Data compression and cryptography changes data format, give conceptual difference.
Ans: Data compression is known for reducing storage and communication costs. It involves transforming data of a given format, called source message, to data of a smaller sized format, called codeword. Data encryption is known for protecting information from eavesdropping. It transforms data of a given format, called plaintext, to another format, called cipher text, using an encryption key. The major problem existing with the current compression and encryption methods is the large amount of processing time required by the computer to perform the tasks. To lessen the problem, I combine the two processes into
one.