The Systems Network Architecture (SNA) from IBM uses a 7 layer architecture similar to the OSI model.
SNA is designed to provide networking facilities for IBM systems only. Because of this, it is used by only a limited set of users. SNA, as a proprietary networking architecture, describes the general characteristics of computer hardware and software required for interconnection. The OSI reference model was developed a decade after the development of SNA. It has used SNA as a model. SNA was first released during 1974. The main idea was to connect many different hardware and software via links. A link consists Of a link connection and one or more link stations. The transmission media can be telephone cables, microwave links, optical fibers, and coaxial cables, etc. SNA supports distributed processing, internetworking, network management, and many advanced features.
Layers of SNA
The SNA layers are briefly discussed below.
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. The main functions are verification of sequence number when the packet is received, managing the rate at which requests are sent and received between logical units. 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
The architecture of SNA is based on entities called nodes. SNA describes three types of nodes addresses discussed below.
Hosts or type 5 A main frame or a mid range system is known as a host. 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 used for controlling the flow of data in the network. Direct high speed lines are used for connecting hosts to communication controller. Data links are used for connecting a communication controller to another communication controller.
Peripheral nodes or type 2 Other peripheral devices on the hierarchical network such as cluster controllers, printers, and terminals are known as peripheral nodes. 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.
Type 1.0: Terminal nodes
Type 2.0: Terminals printers cluster controllers and other nodes that can only communicate with the mainframes.
Type 2.1: Minis, gateways, work stations, and cluster controllers that can communicate with a mainframe or other Type 2.1 node.
Type 4.0: Communication controllers that link main frame hosts and cluster controllers.
Type 5.0: Host computers
Logical Units (LU) Logical units are network entities that allow applications to access the network.
They provide connectivity to other logical units as well. VTAM (Virtual Terminal Access Method) provides the functions for users and applications to access the network. LU types are listed below.
Type 0: General purpose LU used in program to program connections.
Type 1: Used for application program that communicates with single or multiple device.
Type 2: Terminals through which application programs communicate with workstations.
Type 4: Used for application programs that communicate with single or multiple devices or logical units in peripheral devices that communicates with each other.
Type 6.1: Used for two application sub-systems to communicate with each other.
Type 6.2: Supports communication between two applications. Advanced program-to-program communication (APPC) is a protocol that uses LU 6.2.
Type 7: Supports session between host applications and midrange system.