AT&T 7400 series manual OSI Model, Physical Layer, Data-Link Layer, Network Layer, Transport Layer

D-2COMMUNICATIONS PROTOCOLS

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OSI MODEL

To provide a model through which all protocols could be classified and studied, the International Organization for Standardization (ISO) created the Open Systems Interconnection (OSI) architecture.

The OSI model has seven layers (see Figure D-1).

Physical Layer

The physical layer, or layer 1, covers the physical interface between devices and the rules by which bits are passed. The physical layer concerns itself with the mechanical and electrical aspects of communication. Among the physical level protocols are: RS-232C, RS-449, and X.21.

Data-Link Layer

The data-link layer, or layer 2, sends and receives blocks of data with the necessary codes for synchronization, error control, or flow control. Using these codes, the data-link layer checks the physical link reliability, corrects any transmission errors, and provides a means to activate, deactivate, and maintain the link. The principle service provided by the link layer is error detection and correction. Examples of layer 2 standards are: HDLC, ADCCP, and LAPB.

Network Layer

The network layer, or layer 3, provides for the transparent transfer of data between networks. The network layer is responsible for establishing, maintaining, and terminating connections across the intervening communications facility. Among the services performed by layer 3 are network addressing, error notification, and segmenting, blocking, and multiplexing messages. The best known example of layer 3 is the X.25 layer 3 standard.

Transport Layer

The transport layer, or layer 4, provides a mechanism for the exchange of information across systems. This layer ensures that data units are delivered error-free, in sequence, with no losses or duplications. The size and complexity of a transport protocol depends on the type of service layer 3 provides. For a reliable layer 3 with a virtual circuit capability, a minimal layer 4 is required. The ISO has defined five classes of transport protocols, each oriented toward a different underlying service.