System Network Architecture

Systems Network Architecture (SNA) is a data communication architecture established by IBM to specify common conventions for communication among the wide array of IBM hardware and software data communication products and other platforms. Among the platforms that implement SNA in addition to mainframes are IBM’s Communications Server on Windows, AIX, and Linux, Microsoft’s Host Integration Server (HIS) for Windows, and many more.

Layers of SNA

• Physical control Similar to the OSI physical layer, this layer is concerned with electrical, mechanical, and procedural characteristics of the transmission media and the methods used for interfacing. No specific protocols are defined for this layer. This layer can be implemented using anyone of the international standards.
• Data link control This layer is similar to the data link layer of the OSI. SNA defines SDLC protocol for message transfer across a communication link. It also supports X.25 and Token ring protocols. It allows primary stations to communicate with secondary and token ring networks communicate with the peer network, using this protocol.
• Path control Path control layer of the SNA includes many functions of the OSI network layer. It performs packet formation, path selection, routing, packet reassembling, controlling virtual routes and a few functions of OSI data link layer also.
• Transmission control The functions of this layer are similar to OSI transport layer. This layer also performs few of the functions performed by the OSI presentation layer such as data encryption and decryption.
• Data flow control The role of this layer is to arrange sessions between the source and destination stations. It also assigns data flow sequence number, receives chains of requests and responses from calling and called stations and forms brackets by grouping related chains. It roughly matches the functions of OSI session layer.
• Presentation services The primary role of this layer is to run data transmission algorithms in accordance with a well-defined conversation (communication) protocol, by using conversation verbs. Coordinating the resource sharing and synchronization are the other functions of this layer. This layer has resemblance with OSI presentation layer.
• Transaction services This layer is on the top of SNA architecture. It performs distributed processing and management. The following are the functions of this layer.

• End user services

Transaction services layer supports SNADS (SNA Distributed Services) provides asynchronous transmission between SNA applications.

• Network Configuration services

It is used to activate and deactivate links during transactions. It also assigns network address during dynamic network reconfigurations. At each domain in the network, it performs the functions of loading and maintaining the domain software.

• Session services

Translation of network names into valid network addresses, before activating LU-LU connections, it verifies the authenticity of the end user.

• Management services

The important network management functions performed by the transaction layer are managing the network problems, improving the performance of network, collecting the accounting information, managing the network configuration, etc.

Components of SNA

Hosts or type 5 A host is a mainframe or a midrange system.. The host controls a collection of physical and logical units, communication links, and other network resources called a domain. A host may control portions of one or more SNA networks called sub areas. Single SNA network may include several host nodes. A terminal in one host domain can access an application on another host domain.

Communication controllers or type 4 These nodes are in charge of managing the network’s data flow. Direct high-speed lines connect hosts and communication controllers. A data link is a type of communication link that connects two communication controllers.

Peripheral nodes or type 2 Peripheral nodes are other peripheral devices on a hierarchical network, such as cluster controllers, printers, and terminals. End users can use this node to access the network and end user services. This node supports the following logical unit types: LU2, LU3, LU6.2, and LUI (for non-SNA interconnections). The SNA network divides the network entities into two types of addressable units. They are physical units (PU) and logical units (LU). It uses control points to monitor the data, control data flow, and perform network management.

Physical Units (PU) Physical units are combination of entities in the form of hardware, software, and firmware that manages the resources of a node: The following are some of the node types.