6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
069225 Tandem Computers Incorporated 1–13
Extended Control. The basic control field of 1 byte allows a maximum window size of 8
frames. In extended control, the control field is 2 bytes long, allowing a maximum
window of 128 frames. The poll/final bit for extended control is in bit position 9 for
I-frames and S-frames. For U-frames, it is set in bit positions 5 and 9.
IBM Extended Control (IEC). The IBM Systems Network Architecture (SNA)/SDLC
implementation of extended control differs from the basic ADCCP specification. With
IEC, only numbered frames have a 2-byte field. Unnumbered frames have a 1-byte
field. The following types of ADCCP command/response frames use a 2-byte field in
extended control:
Information
Receive Ready (RR)
Receive Not Ready (RNR)
Reject (REJ)
Selective Reject (SREJ)
All other frame types in IEC have a 1-byte field.
Information-Frame Control Field. An I-frame transfers data between two stations. The
control field of an I-frame contains the following three parameters, in addition to the
low-order bit that identifies the frame as an I-frame:
Ns parameter The link-level sequence number of the I-frame. It places this frame in
relation to others being sent to the same station. For successive
frames, this number increments from 0 to 1 to 2, and so forth, until it
reaches the window size for the line control mode, which is 7 for
standard mode and 127 for an extended mode. The link sequence
number wraps to 0 once it reaches the limit of the window size.
Nr parameter The link-level sequence number of the next expected I-frame. Thus
the Nr parameter acknowledges receipt of all I-frames with sequence
numbers less than Nr; the next I-frame receive from the other station
should have a sequence number of Nr.
Poll/final bit The bit that the primary station uses to poll the secondary station. In
NRM, the primary station is requesting a response or a series of
responses from secondary station when the primary station polls a
secondary station. If a secondary station uses this bit, it indicates that
this frame containing the poll/final bit is the last frame in the
sequence of frames sent to the primary station.
In ABM, the primary station uses the poll/final to synchronize the
send and receive count between itself and the secondary station. The
primary station sends an RR frame with the poll bit set to the
secondary station. The secondary station responds with an RR frame
with a final bit set and a count of the frames received from the
primary station.