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What is network services terminology?
The real function of every layer in OSI is to provide services to the layer above it.
Network Services Terminology
The network services terminology is explained below −
Entities − In each layer there are so many active elements. An entity can be a software entity or a hardware entity. For example, a software entity is Process and a hardware entity is Intelligent I/O chip
Peer Entities − Entities in the same layer on different machines are called peer entities.
Service Provider and Service User − The entities in layer N-1 implement a service used by layer N. Layer N-1 is called the Service Provider and layer N is called the Service User.
Service Access Points (SAPs) − Services are available as Service Access Points. The layer N-1 SAPs are the places where layer N can access the services offered. Each Service Access Point has an unique address which identifies it.
Interface Data Unit (IDU) − If two layers want to exchange information then there has to be an agreed set of rules about the interface. At a typical interface, the layer N entity passes an IDU to the layer N-1 entity through the Service Access Points.
Service Data Unit − The Interface Data Unit consists of an SDU (Service Data Unit) and some other control information. The SDU is the information which is passed across the network to the peer entity and then up to layer N+1. The control information is needed to help the lower layer do its job, but is not part of the data itself.
Protocol Data Unit − In order to transfer the data to the SDU, the layer N entity may have to fragment it into several pieces, each of which is given a header and sent as a separate PDU (Protocol Data Unit) such as a packet. The PDU headers are used by the peer entities to carry out their peer protocol. They identify which PDUs contain data and which contain control information, provide sequence number counts, and so on.
Connection – Oriented Service − Modelled after the telephone system. If we want to use a connection-oriented network service, the service user first establishes a connection, uses the connection, and terminates the connection.
Connection-less Service − Modelled after the Postal System. Here each message carries the full destination address, and each one is routed through the system independent of all others.
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