3745 Dual CCU Modes of Operation

The IBM 3745-61A is a dual CCU model, allowing for the operation of two separate copies of NCP (Network Control Program) while paying to license only a single copy from IBM. Employing a console procedure, your Mid-Atlantic Research field service technician or your network operator can set up your dual CCU 3745-61A to operate in one of three modes of operation.

Twin Backup Mode - Two NCPs operate simultaneously. Each NCP controls its own network, but each NCP also is generated to backup the other NCP. Hardware resources are divided during normal operation between the two NCPs. In the event of a CCU failure, hardware resources are switched to the surviving NCP.

Twin Standby Mode - A single NCP operates. The second CCU is used as a hot standby CCU in the event of a failure of the primary CCU.

Twin Dual Mode - Two NCPs operate, with each assigned its own hardware resources. Operation is similar to having two separate 3745-31As. If one CCU fails, the second CCU and its associated NCP will not be affected.

When Twin Dual mode is employed, 3745 base frame hardware resources are split between the NCPs. Each NCP has its own control storage. Half of the busses are associated with the first NCP and half are associated with the second NCP. This means that each NCP may control four base frame channel adapters and four low speed scanners. If the 3746-A11 and A12 are installed, each NCP may control up to eight base frame channel adapters and up to sixteen low speed scanners.

The 3746-900 allows a greater level of resource sharing when the FEP (front end processor) is operated in Twin Dual mode.

A single ESCON coupler may be shared by the two operating NCPs, using the same fiber channel simultaneously.
For token ring, a single TIC may be activated by only one NCP at a time. However, TICs that are physically controlled by a single TRP (token ring processor) may be split between NCPs. For example, the first NCP may activate TIC 2176 while the second NCP activates TIC 2208, even though both TICs are controlled by the token ring processor at 2176. If the second NCP attempts to activate TIC 2176, the attempt is rejected unless the first NCP deactivates the TIC.