DESIGN ISSUES OF LAYERS

Design issues for the Layers





· In the designing the layer, data transfer mode is considered for transmission.

The type of modes are –
A) Simplex communication
B) Half duplex communication
C) Full duplex communication.

· In simplex communication, data travel only in the direction (unidirectional).

· In half duplex mode, each node can both transmit and receive, but not at the same time.

· In full duplex mode, both nodes can transmit and receive simultaneously.

· The protocol also checks the logical channel for connection and the priority of the each logical channel.

· Many networks provide at least two logical channels per connections, one for normal data and one for urgent data.

· Actual data transfer takes place through physical channel. So error control is an important issue for error free data transfer. Many error detecting and error correcting codes are available, by both sides of the connection must agree correction codes are available, but both sides of the connection must agree on which one is being used.

· To receive the data in the same sequence, protocol must make provision for the receiver to allow the pieces to be put back together properly. For this, assign the number to each and every piece.

· While designing the layer, we also consider the loss of data because of slow receiver and fast sender.

· Various solutions are possible on this condition. One of the solutions is that sender agreed upon data transmission rate with receiver.

· If there is multiple path or channel in between sender and receiver, a route must be chosen.


Connection Oriented and Connectionless Services

· Connection oriented and connectionless are the two types of services that are offered by the layer.
· In connection oriented, direct path is established between source and destination. The telephone system is the example of the connection oriented service. This type of service provides a substantial amount of care for the user data.
· The connectionless (also called datagram) service goes directly from an idle condition into a data transfer mode, followed directly by the idle condition.
· The connectionless service is comparable to mailing a letter. Each message carries the full destination address, and each one is routed through the system independent of all the others.
· Each service can be characterized by a quality of service (QOS). Some service is reliable in the sense that they never lose data.
· Usually, a reliable service is implemented by having the receiver acknowledge the receipt of each message, so the sender is sure that it arrived. The acknowledgement process introduces overhead and delays, which are often worth it but are sometimes undesirable.

Service Primitives

· A service is formally specified by a set of primitives available to a user or other entity to access the service.

· These all primitives are implemented as software procedure calls.

· When user application invokes a service provider function by sending a request to the next lower layer, service provider confirm for request.

· If the service is going to provide a function for another user, the service provider must send identification to Y, after which Y is required to provide response.

· User X and Y are connected together by layer service access points (SAPs). If you consider service provider as a layer, the SAP contains the address of the specific function.

· Services can be either confirmed or unconfirmed.

· The request, an identification, a response, and a confirm are all in confirmed services.

· Connect and disconnect are the examples for confirmed and unconfirmed services respectively.

Relationship of Services to Protocols

· Service interface provides an entry point that users use to access the functionality exposed by the application.

· Service interface is usually network addressable.

· Service interface provides a much more coarse-grained interface while preserving the semantics and finer granularity of the application logic. It also provides a barrier that enables the application logic to change without affecting the users of the interface.

· The service interface should encapsulate all aspects of the network protocol used for communication between the user and service. For example, suppose that a service is exposed to consumers through HTTP over a TCP/IP network. User can implement the service interface as an ASP.NET component published to a well-known URL.