6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
069225 Tandem Computers Incorporated 1–17
USER3 This a user-defined frame type as defined in the ANSI ADCCP standard.
I-fields are allowed in this frames.
XID A primary station sends this command to the secondary station, asking it to
identify itself. The secondary responds with another XID frame. This
command is used most frequently in switched networks so that the called
station can find out the identity of the calling station. An XID frame has an
information field containing the necessary data. Specific applications and
devices define what information is required.
Information Field
The information field, if present, contains the application data to be delivered to the
other station. The ADCCP protocol (including the contents of the other fields) exists to
ensure the orderly transmission of the data in the information field. The contents of
the information field is transparent to ADCCP, which regards the data as only a
stream of bits.
The information field can be subject to character code translation. You can specify the
part of the field that you want translated, if any, at system generation time, or through
an application request that makes a change to the configuration block (see the
description of the (1) SET CONFIGURATION request and response in Section 6,
“Requests and Responses”). An application can replace the standard ASCII-EBCDIC
translation tables, supplying its own tables for incoming and outgoing data (see the
description of the (64) TRANSLATE TABLE request and response in Section 6,
“Requests and Responses”).
Sometimes the information field contains control information as well as data. For
example, it might support protocol layers higher than those provided by ADCCP. The
interpretation of the control field determines whether an information field is present
and, if so, what it contains.
Frame Check Sequence Field
The frame check sequence (FCS) field contains the cyclic redundancy check (CRC)
value for the frame. The sending station’s hardware computes the CRC value by
using the address field, control field, and information field (if present) as a continuous
bit stream. An end flag follows the frame check sequence field.
Abort Sequence
An abort sequence occurs when the transmission of a frame must stop abruptly and
tells the receiving station to disregard the frame in which it appears. The ADCCP
protocol module can insert the sequence at any point in the frame. An abort is
distinguished from a flag by the number of consecutive one bits it contains. An abort
sequence consists of at least 7 but no more than 15 consecutive one bits.
A transmission of 15 or more one bits is not an abort sequence. It is one of the options
available for time fill between frames.
A sequence of six or more consecutive one bits cannot occur randomly within the
frame because of zero bit insertion. Zero bit insertion is an error technique in which