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Method for Recognizing a Protocol Mismatch when using Integrated Services Digital Network Q.922

IP.com Disclosure Number: IPCOM000108196D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 2 page(s) / 86K

Publishing Venue

IBM

Related People

Fisher, ME: AUTHOR [+3]

Abstract

Devices attached to an Integrated Services Digital Network (ISDN) may communicate using data link protocols, such as Synchronous Data Link Control (SDLC), Link Access Protocol Balanced (LAPB) of CCITT Recommendation X.25, or CCITT Recommendation Q.922. It is possible to establish a connection to a device not employing the CCITT Q.922 protocol when only Q.922 is expected. Such a mismatch could occur when the user enters a wrong phone number on the calling device. Thus, a device expecting Q.922 protocol may find itself connected to a device using X.25 LAPB or SDLC protocol. Since all of these protocols use the same rules for frame delineation ('7E'X flags), bit transparency (zero-bit insertion and deletion), and Frame Check Sequence (FCS) generation and checking, an FCS error does not occur in the event of a mismatch.

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This is the abbreviated version, containing approximately 52% of the total text.

Method for Recognizing a Protocol Mismatch when using Integrated Services Digital Network Q.922

       Devices attached to an Integrated Services Digital
Network (ISDN) may communicate using data link protocols, such as
Synchronous Data Link Control (SDLC), Link Access Protocol Balanced
(LAPB) of CCITT Recommendation X.25, or CCITT Recommendation Q.922.
It is possible to establish a connection to a device not employing
the CCITT Q.922 protocol when only Q.922 is expected.  Such a
mismatch could occur when the user enters a wrong phone number on the
calling device.  Thus, a device expecting Q.922 protocol may find
itself connected to a device using X.25 LAPB or SDLC protocol.  Since
all of these protocols use the same rules for frame delineation
('7E'X flags), bit transparency (zero-bit insertion and deletion),
and Frame Check Sequence (FCS) generation and checking, an FCS error
does not occur in the event of a mismatch.  This article describes a
method for a device expecting only Q.922 frames to report a protocol
mismatch.

      Q.922, X.25 LAPB, and SDLC protocol data units (PDUs) start
with an address field.  (In this article, PDU refers to the part of a
frame immediately following the leading flag.)  The format and use of
the address field is different for each protocol:
o    In valid Q.922 PDUs, the address field is always 2 bytes long.
The low-order bit of the first byte is always off, and the low- order
bit of the second byte is always on.  Q.922 allows multiple
point-to-point links to share a channel.  Ten bits in the address
field are used to distinguish links within a channel.
o    In X.25 LAPB PDUs, the address field is always 1 byte long.  The
low-order bit of this first byte is always on.
o    In SDLC PDUs, the address field is 1 byte long.  The setting of
the low-order bit of this first byte varies with the station address.
The second byte of the PDU is the SDLC...