Physical paths

The physical path between two subarea nodes is an explicit route.

Explicit route (ER)

An explicit route is an ordered set of subarea nodes and transmission groups along a path between communicating subarea nodes, including:

The endpoint subareas
Any subareas between the endpoint subareas
The transmission group used to connect each subarea pair along the route

In connecting one endpoint subarea to another, an explicit route cannot include the same subarea more than once.

Forward explicit route

Explicit routes originating in the host are forward explicit routes and are numbered 0 through 15 (0 through 7 with releases before NCP Version 4 Release 3.1 or Version 5 Release 2.1 or if ERLIMIT is set to 8).

Reverse explicit route

Reverse explicit routes terminate in the host and must use the same set of subarea nodes and transmission groups as their corresponding forward explicit route. They are also numbered 0 through 15 (0 through 7 with releases before NCP Version 4 Release 3.1 or Version 5 Release 2.1 or if ERLIMIT is set to 8), but they do not have to have the same explicit route number as the corresponding forward explicit route.

Transmission group (TG)

A transmission group consists of a single SDLC link or data channel between two subarea nodes, or of a logical combination of up to eight parallel SDLC links between communication controller subarea nodes. Transmission groups are numbered 1 through 255.

Transmission groups appear as one link to the path control network, even if the transmission group contains parallel SDLC links. Data flows across a specific transmission group during a session, but not across a specific link in a transmission group. The specific link used within the transmission group is determined dynamically depending on which link is available at the time.

Transmission groups can also exist in parallel, creating multiple explicit routes between two subareas. For instance, parallel SDLC links between adjacent communication controller subarea nodes can be divided into multiple transmission groups. A maximum of 16 parallel transmission groups are allowed between adjacent subarea nodes.

Parallel links in one or more transmission groups can be active and carry data simultaneously. Each parallel link is controlled (activated and deactivated) independently of the others.

Routing tables

An entire explicit route is not defined to each subarea node along a path; instead, each subarea node has its own routing table that contains only the information that path control needs to forward data to the next subarea along the route (the adjacent subarea). Given the explicit route number being used and the destination subarea, the table provides the adjacent subarea to which the data is forwarded and the transmission group over which the data is forwarded.

Peripheral link

A peripheral link is the portion of the route between a subarea node and a peripheral node. A peripheral node uses local addresses for routing and requires boundary function assistance from an adjacent subarea node to communicate with a nonadjacent subarea node.

NCP/Token-Ring Interconnection (NTRI) Nondisruptive Route Switching (NDRS)

Token-Ring networks often contain bridges that allow multiple physical paths between two nodes. NCP Version 5 Release 3 and later provides the NDRS function that attempts to find an alternate route through the token ring network. NCP invokes this function automatically when timeout conditions occur and all retries have been exhausted for a connection.

VTAM® and NCP Version 7 Release 8 and later provide the MODIFY NCP operator command. This command can be used to force NCP to initiate NDRS for a 3745 token-ring attached subarea connection. This function is useful in order to switch back to the original route (when recovered) after NCP has automatically invoked NDRS to find a backup route. It can also be used at any time to search for a faster route through the token-ring network. NCP Version 7 Release 7 and later will generate a "Token-Ring Path Switch Notification" Generic Alert upon successful completion of NDRS. See z/OS Communications Server: SNA Operation for further information.

In the sample network configuration shown in Figure 1, HOSTA and peripheral node T1 can communicate over two explicit routes, one in each direction. Both explicit routes have been numbered 0 and include HOSTA, NCPA1, NCPA2, and transmission groups 1 and 14. A data channel assigned to TG1 connects HOSTA with NCPA1. An SDLC link assigned to TG14 connects NCPA1 and NCPA2. Each subarea node contains a routing table to route data between subareas along ER0.

Figure 1. One explicit route in each direction

In the sample network configuration in Figure 2, parallel SDLC links connect NCPA1 and NCPA2. Two different explicit routes between HOSTA and NCPA2 are now available in either direction. All routes take TG1 between HOSTA and NCPA1, but data flows between NCPA1 and NCPA2 over TG14 for ER0 or TG15 for ER1. The routing tables have been updated to reflect the additional explicit routes.

Figure 2. Two explicit routes in each direction