Envoy ACP/XF Application Programming Manual

Normal Response Mode (NRM) Protocols
EnvoyACP/XF Application Programming Manual–132179
3-4
Extended Control Fields
2: 3-octet address field
3: 4-octet address field
The address-field size that you select when you call SETMODE 19 applies only for the
duration of the current line opening. For multipoint supervisor application processes,
you must issue the SETMODE 19 call before building the station address list.
Extended Control Fields
By using the SETMODE and CONTROL procedures, you can configure either a 1-octet
or a 2-octet control field. The difference between the basic and extended control fields is
as follows:
Using SCF, you can specify a 2-octet control field by specifying the modifier
EXTENDEDCONTROL. Set a 1-octet or 2-octet control field by issuing a SETMODE
19 call before issuing any CONTROL, READ, WRITE, or WRITEREAD calls. Bit 3 of
parameter 2 in the SETMODE 19 call specifies the desired control-field length, as
follows:
CALL SETMODE (fnum,19,parameter1,parameter2)
parameter2.<3> = 0: 1-octet control field
= 1: 2-octet control field
When you use extended control fields, you must make sure that you have specified an
appropriate window size. The window size actually determines the number of I-frames
you can send to a remote station before requiring acknowledgment. For basic control
modes, the window-size value must be in the range 1 through 7. For extended control
modes, however, the window-size value may be in the range 1 through 127. If you do
not specify another value, the window-size defaults to 7, no matter what the length of the
control field.
Although you normally set window size during system configuration, you also can set it
dynamically by using a SETMODE 18 call; put the value in parameter 2.
IBM Extended Control Fields
The basic ADCCP specification of extended control fields indicates that all control
fields are two bytes long (that is, for both numbered frames and unnumbered frames).
However, in the IBM implementation of extended control, only numbered frames
Control
Field
Frame
Sequence
Numbers Modulo Description
1-octet 3 bits modulo-8 Can transmit up to 7 I-frames in succession without
receiving an acknowledgment from the remote
station.
2-octet 7 bits modulo-128 Can transmit up to 127 I-frames in succession
without receiving an acknowledgment from the
remote station.
Note. The window size specified by SETMODE 18 cannot exceed the value specified in the
PROFILE.