Computer Network Architecture: The OSI Model

Name the seven names of OSI reference model.

The OSI reference model has seven layers as follows:

1. Physical layers
2. Data link layers
3. Network layers
4. Transport layers
5. Session layers
6. Presentation layers
7. Application layers

what is service access points (SAPS)?

Each entity communicates with entities in the layers above and below it across an interface. The interface is realized as one or more service access points (SAPs).

The OSI model is communication-oriented. Two (N) entities communicate, using a protocol, by means of an (n-1) connection. This logical connection is provided by(N-1) entities between (N-1) SAPs. 

Primitives and parameters

Each layer is defined in two parts: the protocol between peer entities in different systems and the service provided be one layer to the next higher layer in the same system. Protocols are defined in terms of the formats of protocol data units (PDUs) that are exchanged and rules governing the use of those PDUs. The services between adjacent layers are expressed in terms of primitives and parameters. A primitive specifies the function to be performed and the parameter are used to pass data and control information. 

 Discuss the OSI reference model with neat diagram.

The OSI reference model has seven layers as follows:

1. Physical layers
2. Data link layers
3. Network layers
4. Transport layers
5. Session layers
6. Presentation layers
7. Application layers

It takes two to communicate, so the same set of layered functions must exist in two systems. Communication is achieved by having corresponding (“peer”) entities in the same layer in two different systems communicate via a protocol.

Figure (below) illustrates that the OSI model. Each system contains the seven layers. Communication is between application in the systems, leveled application X and application Y in the figure. If application X wishes to send a message to application Y, it invokes the application layer (layer 7). Layer 7 establishes a peer relationship with layer 7 of the target machine, using a layer 7 protocol. This protocol requires services from layer 6. So the two layer 6 entities use a protocol of their own and so on down to the physical layer, which actually passes the bits through a transmission.

There is no direct communication between layer accept at the physical layer. This is, above the physical layer, each protocol entity sends data down to the next lower layer in order to get the data across to its peer entity.

To assure the effective communication between two heterogeneous systems, standards are needed. Standards must define the functions and services to be provided by the layer (but not how it is to be done- they may differ from system to system). Standard must also define the protocols between peer layers). The OSI model, by defining seven-layer architectures, provides a framework for defining these standards.

Discuss four kinds of standards primitive used to define the interaction between adjacent layers.

 Four types of primitives are used in standards to define the interaction between adjacent layer in the architecture:

1. Request: A primitive issued by a service user to invoke some service and to pass the parameters needed to fully specify the requested service.
2. Indication: A primitive issued by a service to either (i) indicate that a procedure has been invoked by the service user on the connection and to provide the associated parameters or (ii) notify the service user of a provider – imitated action.
3. Response: A primitive issued by a service user to acknowledge or complete some procedure previously invoked by an indication to that user.
4. Confirm: A primitive issued by the service provider to acknowledge or complete some procedure previously invoked by a request by the service user.

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