Page 213 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes 1’s indicates that the true distance is 2 – 1 plus the value of the following symbols. Consider the
K
following example:
000100001010011000000000000000000000001000000011 (48 bits) comprises of runs of length 3, 4,
1, 2, 0, 23, 7 and 0. Using 4-bit symbols, it is encoded as:
0011 0100 0001 0010 0000 1111 0100 0111, for 32 bits and a savings of 16/48 = 33%.
Using 3-bit symbols, it would be encoded as: 011 100 001 010 000 111 111 111 010 111 000 for 33
bits.

Self Asessment

Fil in the blanks:
7. Encryption/decryption is taken care of at ……………….. layer.

8. The ……………….. layer is responsible for establishing, maintaining, synchronizing and
terminating dialogs.
9. The ……………….. layer disconnects a session abruptly, while the ……………….. layer
provides for graceful closure.
10. ……………….. points recovers data that have been delivered but not yet used.
11. ……………….. is the major function of the presentation layer.

13.5 Summary

The session layer is located above the transport layer and intends to provide value added
services to the underlying transport layer services. The session layer creates, manages and
releases communication sessions between presentation layer entities at source and
destination machines.

The session layer implements the mechanism for managing the dialogue between end-
user application processes. It facilitates in creating either duplex or half-duplex operation
to implement check-pointing, adjournment, termination, and restart procedures.

The synchronization points which are introduced as reference points in the data control
flow and error in the session layer.

Major synchronization points divide a message into a series of dialogs. Each major
synchronization points are acknowledged before the session is continued. When an error
happens, data is recovered only up to the last major point.

Minor synchronization points are just markers and inserted into the middle of dialogs.
They, depending on the applications, may or may not require acknowledgement. They
are security zones to recover the data from one or more minor synchronization points
within a dialog when an error happens.

The presentation layer deals with the translation, encryption/decryption, authentication
and compression.
Encryption is used to scramble the data so that only authorized persons can unscramble
the data of a conversation. Decryption reverses the encryption process to translate the
message back into its original form.

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