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.

ARPANET

The ARPANET (American Research Project agency NET work), now called DARPA, owned by US Defense, doest not follow the OSI model at all. The IMP-IMP protocol really corresponds to mixture of the layer 2 and layer 3 protocols. Layer 3 also contains an elaborate routing mechanism. In addition, there is a mechanism that explicitly verifies the correct reception at the destination IMP of each and every of protocol sent by the source IMP.

The ARPANET does have protocols that roughly cover the same territory as the OSI network and transport protocols. The network protocols, called IP (Internet Protocol) is connectionless and was designed to handle the interconnection of the vast number of WAN and LAN. The ARPANET transport protocol is a connection-oriented protocol called TCP (Transmission Control Protocol). There is no session or presentation layer protocol in ARPANET.
The ARPANET services include files transfer, email and remote login. These services are supported by well-known protocols FTP, SMTP (Simple Mail Transfer protocols) and TELNET (remote login)

Public Networks

The networks which is owned neither by Government or by private organization but by an individual network operator, provides communication services for the customers' hosts and terminals. Such a system is called public networks.

All of them use OSI model and the standard CCITT (Consultative Committee International for Telephony and Telegraphy, a French standard organization) or OSI protocols for all the layers. For the lowest three layers, CCITT has issued recommendations that have been universally adopted by public networks worldwide. These layers are always known collectively as X.25 standards.

They physical layer protocol, called X.25, specifies the physical, electrical and procedural interface between the host and the network. The data link layer standard has a number of variations. They all are designed to deal with transmission errors on the telephone line between the user's equipment and the network. The network layer protocol deals with addressing, flow control, delivery confirmation, interrupts and related issues.

ISO has developed standards for a connection-oriented transport layer service definition and a connection-oriented transport layer protocol. Also it has adopted standards for the connection-oriented session service, protocol and presentation service and protocol.

The Application layer contains following protocols:

1. FTAM (File transfer, access and management) protocol, which provides a way to transfer, access and generally manipulate remote files in a uniform way.
2. MOTIS (Message-Oriented Text Interchange Systems) protocol is used for electronic mail.
3. VTP (Virtual Terminal Protocol) provides a terminal-independent way for programs to access remote terminals.
4. JTM (Job Transfer and Manipulation) protocol is used for submitting jobs to remote mainframe computers for batch processing.

Example Networks

Numerous networks are currently operating around the world. Some of them owned by governments, some for research work, some are owned by private organizations and so on.