Information Technology Reference
In-Depth Information
Figure19-3 LAPB Structure
User data
Packet level
header
Layer 3
X.25
Variable length
Variable length
Frame
Check
Sequence
Flag
0x7E
Address
field
Control
field
Information field
Flag
0x7E
Variable length
1 byte
1 byte
1 byte
1 byte
2 bytes
Note: Information and confirmation of field lengths and content were obtained from the following
publications: Internetworking by Mark A. Miller, P.E., published by M&T Books, 1991. The second topic is
Data and Computer Communications by William Stallings, published by Prentice Hall, 1994.
Layer 3
The Layer 3 X.25 header is made up of a general format identifier (GFI), a logical channel identifier
(LCI), and a packet type identifier (PTI). The GFI is a 4-bit field that indicates the general format of the
packet header. The LCI is a 12-bit field that identifies the virtual circuit. The LCI is locally significant
at the DTE/DCE interface. In other words, the PDN connects two logical channels, each with an
independent LCI, on two DTE/DCE interfaces to establish a virtual circuit. The PTI field identifies 1 of
X.25's 17 packet types.
Addressing fields in call setup packets provide source and destination DTE addresses. These are used to
establish the virtual circuits that constitute X.25 communication. ITU-T Recommendation X.121
specifies the source and destination address formats. X.121 addresses (also referred to as international
data numbers, or IDNs) vary in length and can be up to 14 decimal digits long. Byte four in the call setup
packet specifies the source DTE and destination DTE address lengths. The first four digits of an IDN are
called the
data network identification code
(DNIC). The DNIC is divided into two parts, with the first
three digits specifying the country and the last digit specifying the PSN itself. The remaining digits are
called the national terminal number (NTN) and are used to identify the specific DTE on the PSN. The
X.121 address format is shown in Figure 19-4.
