HP 600 Series, Services And Applications

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Like LAPB (the link-access procedure used on X.25 networks) and LAPD
(the link-access procedure used in ISDN networks), Q.922 frames are
delimited by a flag character (7E hex), and for data integrity, a frame check
sequence (CCITT-CRC) is appended to the user data.
Table 2. CCITT Rec. Q.922 Two-Octet Address Field Format
8 7 6 5 4 3 2 1
Upper DLCI C/R EA
Lower DLCI FECN BECN DE EA
Q.922 Address
The format of the Q.922 address field is shown in table 2. In addition to the
two-octet (byte) format shown in table 2, there are three- and four-octet
formats. These formats can be used to provide additional addressing
capability and will be used in the future to provide switched virtual circuits.
For now we will consider only the two-octet format, since it is typically the
only addressing format supported on today’s public Frame Relay networks.
Data-Link Connection Identifier The six-bit upper DLCI and four-
bit lower DLCI are concatenated to form a ten-bit DLCI. This provides
for up to 1024 DLCIs per Frame Relay switching node (again, consider-
ing only the two-octet address format).
Control Field and Command/Response Bit Frame Relay frames
are sent with the control field (octet 4 in table 1) set to 03 hex. This
encoding indicates that the frame is a UI (unnumbered information)
type of frame (identical to the encoding of frames transmitted on most
LANs). The command/response bit (denoted by “C/R” in table 2) is
always encoded with a 0, which indicates that the frame is a command.
(UI frames are command frames.)
Address Extension Bits The EA bit in the Q.922 address field is the
address extension bit. The EA bit in the last octet of the Q.922 address
field is always set to 1, which indicates that it is the last octet of the
address field. All preceding octets in the address field have the EA bit
cleared.
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Linking Up with Frame Relay
Frame Relay Data-Link Interface
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