SNA Network Design Fundamentals

There are three objectives which must be satisfied in any network design:
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  • Service
  • Availability
  • Efficiency
The relative importance of each of these factors determines (or at least should determine) the design of the network.
Service is usually measured by response time. Response time may be loosely defined as the interval between an end user dispatching a request and the arrival of the corresponding response from the session partner. Minor differences in this definition revolve around the interpretation of when the response has actually arrived.
Availability is the state of a given network facility being functional at a given time. Availability is generally measured as the ratio of the actual functional amount of time to the committed functional amount of time, expressed as a percentage. Availability is the most difficult network attribute to measure because the availability of a given network element is dependent on the availability of other network elements in the path to it. Thus, the availability of a given network element may vary with the point of access to the network.
Efficiency is the quality of meeting service and availability objectives for the minimum cost. Because there are so many differences in the resource requirements, efficiency is generally measured on an application-by-application basis and frequently by user group too.

Network Topology

Network topology is the general name given to the physical layout of network nodes and links most often represented as "ball-and-stick" diagrams on network managers' office walls. As complex as these diagrams often appear at first glance, virtually all networks use one of three basic designs or topologies:
  • Bus
  • Ring
  • Star
The majority of network traffic is routed to or from applications that reside in the network's mainframe processors (host subarea nodes). The communications processor subarea nodes which are channel-attached to these processors and the links which interconnect them are subject to the highest traffic loads and are of the highest concern in network design. These nodes and links together are referred to as the network backbone. Defining the structure of the network backbone in terms of the three basic architectures clarify much of any network structure being examined.

Bus Topology

Bus topologies are the simplest form of network design. Bus structures result when there are only two nodes in the network backbone. In practice, this usually results when an organization has two operational centers (one or both of which may have mainframe processors) which are fairly widely separated geographically. The following diagram, Bus Topology, is a schematic representation of a network employing bus topology.
Bus Topology Diagram
+--------------+ | HOST | | PROCESSOR | | B | | | +--------------+ || +-------------------------------------------------||--------+ | || | | +--------------+ +--------------+ | | | |--------------- | | | | | CCU |------------- / | CCU | | | | NODE A | / ------------| NODE B | | | | | --------------| | | | +--------------+ TRANSMISSION GROUP +--------------+ | | || | +-------||--------------------------------------------------+ || BUS BACKBONE +--------------+ | | | HOST | | PROCESSOR | | A | | | +--------------+

Ring Topology

Ring topologies result when each node in the backbone is connected to two adjacent backbone nodes. This is typically done to reduce the dependence on the availability of an intermediate or "through" node by providing an alternate path. Ring structures are common in organizations with decentralized operational centers. The following Ring Topology diagram is a schematic representation of a network employing ring topology.
Ring Topology Diagram
| PROCESSOR | | A | +--------------------+ || RING BACKBONE +--------------------------||-----------------------------+ | || | | +---------+ | | | CCU | | | | | | | | NODE A | | | +---------+ | | / \ | | TG AB / \ TG AC | | / \ | | /\ / \ /\ | | / \ / \ / \ | | / \/ \/ \ | | / \ | | / \ | | / \ | | +---------+ +---------+ | | | CCU |--------------- | CCU | | | | | / | | | | | NODE B | ----------------| NODE C | | | +---------+ TG BC +---------+ | | || || | +------||---------------------------------------||--------+ || || +---------------+ +---------------+ | PROCESSOR | | PROCESSOR | | B | | C |

Star Topology

Star topologies are used when the operational centers of an organization are geographically separated but the mainframe processing capabilities are centralized. The star design reduces communications line costs by multiplexing multiple sessions across the transmission group(s) linking the remote node to the central node. The following Star Topology diagram is a schematic representation of a network employing star topology.
Star Topology Diagram
STAR BACKBONE +---------------------------------------------------------+ | | | +---------+ | | | CCU | | | | | | | | NODE A | | | +---------+ | | | | | | /| | | +-----------------+ |/ | TG DA | | | | | | | | | +---------+ | | | | | CCU | | | | PROCESSOR D |==| | | | | | | NODE D | | | | | +---------+ | | | | / \ | | +-----------------+ / \ | | TG DB / \ TG DC | | / \ | | /\ / \ /\ | | / \ / \ / \ | | / \/ \/ \ | | / \ | | / \ | | +---------+ +---------+ | | | CCU | | CCU | | | | | | | | | | NODE B | | NODE C | | | +---------+ +---------+ | | | +---------------------------------------------------------+
While the network backbone is not an SNA concept, it is nonetheless a functional entity. Its utility is twofold: it makes it easier to visualize the network structure and it identifies the nodes and links that are most vital to network performance and availability. Once the backbone structure has been designed, it must be defined using the SNA routing techniques, which are briefly outlined below.

Network Link Configuration and Data Flow

SNA network links are organized into several different logical and physical configurations based on the complexity of the network and the type of sessions supported.
Adjacent communications controllers can be connected by multiple links operating concurrently. These are called parallel links. Each parallel link must be assigned to a logical entity called a transmission group (TG). A transmission group appears as a single link to the path control network. Software within the CCU acts to evenly distribute the load across the links which make up a TG.
To allow data to flow in the network, one or more paths must be defined between each pair of network addressable units (NAUs). Paths are defined by specifying an explicit route (ER) and, if necessary, a peripheral link. The explicit route is the portion of the path between two subarea nodes. The peripheral link is the portion of the path between a subarea node and a peripheral node.
Path definitions are distributed among all the nodes along the path and stored in routing tables. A routing table entry exists for each path and consists of the destination node, explicit route number, next subarea node, and applicable transmission group number. Thus each node retains only enough information to route the message to the next node.
Session traffic is assigned to a logical connection called a virtual route. Virtual routes are then mapped to an explicit route. Virtual routes can have up to three different transmission priorities. Messages are queued and selected for transmission on a TG based on these transmission priorities.
Virtual routes are assigned to a session when it is initiated. A session-initiation request contains a class of service. The SSCP looks up this class of service in its class of service table, which contains pairs of virtual routes and transmission priorities, to assign a particular virtual route to the session.

Flow Control

Because of the relatively autonomous and asynchronous nature of session traffic throughout the network, the amount of data to be sent to a given node at any time may exceed that node's ability to accept data. Since any network element's ability to buffer data is limited, some method of preventing overload is required; this technique is called flow control. SNA flow control is implemented through pacing. Pacing allows a receiving node to limit the rate at which traffic is sent to it. SNA defines two types of pacing:
  • Session-level pacing
  • Virtual-route pacing
Both session-level and virtual-route pacing are based on a pacing window whose size is the number of message units that can be outstanding between origin and destination. When this limit is reached, the sender stops sending until the receiver acknowledges readiness by sending a pacing response.
Specifying explicit routes defines the network topology in an SNA environment. The specification of virtual routes, transmission priorities, and pacing parameters controls service levels and throughput for network services.