System information

X.25 Frame Format
Book Title
19-412
Figure 19-3 An X.25 Frame
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, 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.
Figure 19-4 The X.121 Address Format
Packet-level
header
Flag Data
Frame
Bit stream
Frame-level
control and
address
FlagFCS
User
data
Layer 3
X.25
Layer 2
X.25
Layer 1
X.25
Packet
Called DTE
address
length
Calling DTE
address
length
National
terminal
number
Country
International data number
PSN
4 bits 4 bits
DNIC
4 digits
NTN up to
10 digits