Page 26 - DCAP406_DCAP_207_Computer Networks

Unit 2: Network Software

 Inter-process dialog control: When two applications engage in the exchange of data, they Notes
establish a session between them. Consequently, a need arises to control the flow and the
direction of data flow between them for the duration of the session. Depending on the
nature of the involved applications, the dialog type may be full duplex, half-duplex, or
simplex mode of communication.
 Session Recovery: Another application-oriented concern is the capability to reliably recover
from failures at a minimum cost. This can be achieved by providing a check mechanism,
which enables the resumption of activities since the last checkpoint. Check pointing
circumvents this requirement by re-transmitting only the affected files, saving time and
bandwidth.
 Presentation Problems: Whenever two or more communicating applications run on
different platforms, another concern arises about the differences in the syntax of the data
they exchange. Resolving these differences requires an additional process. Good examples
of presentation problems are the existing incompatibilities between the ASCII and EBCDIC
standards of character encoding, terminal emulation incompatibilities, and
incompatibilities due to data encryption techniques.
Self Assessment

State whether the following statements are true or false:
1. The entities comprising the corresponding layers on different computers are called clients.
2. The International Organization for Standardization (ISO) took the initiative in setting up
OSI.
3. Data communication process allocates memory resources, commonly known as
communications buffers for the sake of transmission and reception of data.
4. The information exchanged between two computers is physically carried by means of
chemical signals assuming certain coding methods.

5. OSI reference model divides the required functions of the network architecture into five
layers and defines the function of each layer.

2.3 Interfaces and Services

Each layer provides services to the immediate layer above it. There are some associated terms,
which are used frequently:
 Entities: They are active elements. For example, processes, I/O chips, etc. in each layer.

 Peer entities: They are entities in the same layer on different computers.
 Service provider: This function of layer provides certain services.
 Service user: This function of layer uses certain services.
 SAP (Service Access Points): It is the point from where services can be accessed. Each SAP
has a unique address.
Connection-oriented and Connectionless Services

Connection-oriented Service is similar to the telephone system where a dedicated channel is
established between sender and receiver before transmission. They are suitable for

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