IBM 3708 Description page 105

Chaining Protocol Definition:
A
data chain
is a complete unit of data that originates
at a single LU. Data RU chaining provides a method of logically defining a
complete unit of data regardless of whether the data is transmitted as a single RU or
as a series of consecutive RUs. Each RU is associated with only one chain. An
individual RU may be the beginning, middle, ending, or only (both beginning and
ending) RU in the chain. The chaining indicators, Begin Chain (BC) and End Chain
(EC), are contained in the request header. The following are definitions of each type
of RU in a chain:
First in Chain (FIC)
Middle in Chain (MIC)
Last in Chain (LIC)
Only in Chain (OIC)
Identifies an RU that begins a chained
transmission (RH = BC -, EC).
Is transmitted with all RUs following the BC
transmission, with the exception of the last RU
in that chain (RH=-,BC -,EC).
Identifies the RU that completes a chained
transmission (RH = EC -,BC).
Indicates a transmission consists of a single RU.
Both the BC and EC indicators are included.
The single RU is termed a single-element chain
(RH= BCEC).
A chain is in the correct order if the RUs consist of the following:
• FIC, LIC; or
• FIC, MIC, ... , LIC; or
• OIC.
Any other sequence of chaining indicators causes a chaining error.
Chaining Operations:
When the 3708 receives a chain with chaining indicators in
an incorrect sequence (for example, FIC, MIC, FIC), a negative response with sense
data indicating a chaining error (sense code X' 2002 '), is returned to the host
program. The 3708 purges the chain, ignoring subsequent elements of that chain
until a data RU with the LIC or a Cancel command is received. Receipt of an OIC
data RU terminates the purging of a chain. The 3708 responds to the OIC message
with a negative sense code of X '2002'. Sending RUs with chaining indicators in the
sequence FIC, MIC, OIC is a violation of chaining protocol. In this case, when the
3708 receives the OIC transmission, the chaining error is detected, the OIC
transmission is purged, purging of chain elements is stopped, and a negative response
is sent for the OIC transmission. The 3708 is now ready to process the next chain
normally.
The 3708's chain processing for protocol enveloping sessions differs from the 3708's
chain processing for protocol conversion sessions. For protocol envelopingjNTO,
each RU of a chain is processed and data may be transmitted to the device as soon
as each RU is processed. For protocol conversion, each RU (chain element) is
processed as it is received, but the entire chain must be received before transmitting
data to the downstream device. Waiting for the entire chain is a requirement for LU
type 2 and LU type 3 protocol conversion sessions. It does not apply to LU type 1
SCS protocol conversion session.
3-36
IBM 3708 Description