SNA (System Network Architecture)

It is IBM's network architecture. It was developed mainly to interconnect dissimilar user machines. An SNA network consists of machines called nodes of which there are four types:

• Type 1 - Terminals.
• Type 2 - Controllers, machines that supervise the behaviors of the terminals and other peripherals.
• Type 3 - Front End Processor (FEP), relieve the Host of the work and interrupt handling associated with data communication.
• Type 4 - Hosts.

Each nodes contains one or more NAUs (Network Addressable Unit) which is a piece of software that allows a process to use the network. There are three kinds of NAUs. An LU (Logical Unit) is the usual variety to which user processes can be attached. A PU (Physical Unit) is used to being a node on-line, take it off-line, test it and perform similar network management functions. The third kind of NAU is SSCP (System Services Control Point) has complete knowledge of and control over all the front ends, controllers and terminals attached to the host.

Protocol Hierarchy in SNA

End User

NAU services

Data Flow Control

Transmission Control

Path Link Control

Data Link Control

Physical Link Control

Layer1 (Physical Link Layer) take care of physically transporting bits from one machine to another. The Layer2 (data link layer) constructs frames from the raw bit stream, detecting and recovering from transmissions errors.

Layer3 in SNA, called Path Control, is concerned with establishing a logical path from source NAU to destination NAU. Path control consists of three sublayers. The highest sublayer doest the global routing, deciding which sequence of subareas should be used to get from the source subarea to the destination subarea. The sequence is called virtual route. Two subareas may be connected by several kinds of communication lines so that the next sublayer chooses the specific lines to use, giving and explicit route. The lowest sublayer splits traffic among several parallel communication links of the same type to achieve greater bandwidth and reliability.

It is the job of Layer4 (Transmission Control Layer) to create, manage and delete transport connections (sessions). The Data Link Layer keeping track of which end of session is supposed to talk next. This layer is also doing error recovery functions. The NAU services layer provides two classes of services to the user processes. First there are presentation services such as text compression. Second there are session services for setting up connections. In addition there are network services which maintain the operation of the network.