gtps2m0fACF/SNA Data Communications Reference

SNA Data Transfer

The 37x5 communications controller is a programmable control unit, a piece of hardware. It assumes many of the line handling and processing functions of the network. The network control program (NCP) regulates operation of the 37x5. Macro instructions, coded on input cards, define the network attached to the 37x5. The macro instructions are assembled and the resulting object deck is stored on file. This load module must then be loaded to the 37x5 from an MVS/VTAM system.

Common Characteristics of NCP and CTC Data Transfer

The standard SNA data unit is called a path information unit (PIU). To transmit data between TPF and the NCP, data must be in this format. The PIU contains routing information in addition to the data itself. PIUs follow in a certain order, based on their virtual route sequence number. PIUs with a virtual route sequence number that is higher orlower than the next expected virtual route sequence number are discarded. TPF builds a PIU for each message transferred to the NCP. TPF also executes the appropriate channel program to transfer the PIUs to the NCP.

NCP Data Transfer

The NCP is responsible for successful delivery of each message. The NCP presents an attention interrupt over the channel to TPF indicating that data is ready for transfer. TPF reads input data on a time-controlled basis. Therefore, it does not acknowledge every interrupt. Time-controlled reading of NCP input data prevents a single NCP from monopolizing the resources of the TPF system. Control of the network attached to an NCP occurs at two levels: TPF directs control of the network at channel speed, while the NCP controls line handling and data transfer.

Unique Characteristics of CTC Data Transfer

Data transfer on a SNA CTC connection consists of a matching set of CCWs (write/read on one side and read/write on the other side) with a number of 4K indirect data address word (IDAW) buffers. Either side can initiate a write by beginning the channel program with a write control (WCTL) CCW. The other side receives an attention interrupt from the CTC hardware (for example, 3088); in return, the other side issues a channel program beginning with a sense command byte (SCB) CCW. The read/write sequence of the channel program is fixed at exchance identification(XID) time.

As part of the data transfer, an 8-byte control field is passed by each side as the first 8 bytes of data. Control information, including byte counts and status (such as XID, error retry, and slowdown) is used by each side to determine how to process the data received.

TPF supports a maximum buffer size of 64K. Blocking is used to transmit or receive multiple PIUs on a single I/O operation. TPF initializes I/O operations on the CTC connection when any of the following conditions occur:

Note:
You should be aware of these conditions when specifying the value for the VTAM DELAY operand.