Information Technology Reference
In-Depth Information
Figure19-4 The X.121 Address Format
DNIC
4 digits
NTN up to
10 digits
4 bits
4 bits
Called DTE
address
length
Calling DTE
address
length
National
terminal
number
Country
PSN
International data number
The addressing fields that make up the X.121 address are necessary only when an SVC is used, and then
only during call setup. After the call is established, the PSN uses the LCI field of the data packet header
to specify the particular virtual circuit to the remote DTE.
Layer 3 X.25 uses three virtual circuit operational procedures: call setup, data transfer, and call clearing.
Execution of these procedures depends on the virtual circuit type being used. For a PVC, Layer 3 X.25
is always in data transfer mode because the circuit has been permanently established. If an SVC is used,
all three procedures are used.
Packets are used to transfer data. Layer 3 X.25 segments and reassembles user messages if they are too
long for the maximum packet size of the circuit. Each data packet is given a sequence number, so error
and flow control can occur across the DTE/DCE interface.
Layer 2
Layer 2 X.25 is implemented by LAPB, which allows each side (the DTE and the DCE) to initiate
communication with the other. During information transfer, LAPB checks that the frames arrive at the
receiver in the correct sequence and free of errors.
As with similar link layer protocols, LAPB uses three frame format types:
•
Information (I) frames
—These frames carry upper-layer information and some control
information (necessary for full-duplex operations). Send and receive sequence numbers and the poll
final (P/F) bit perform flow control and error recovery. The send sequence number refers to the
number of the current frame. The receive sequence number records the number of the frame to be
received next. In full-duplex conversation, both the sender and the receiver keep send and receive
sequence numbers. The poll bit is used to force a final bit message in response; this is used for error
detection and recovery.
•
Supervisory (S) frames
—These frames provide control information. They request and suspend
transmission, report on status, and acknowledge the receipt of I frames. They do not have an
information field.
•
Unnumbered (U) frames
—These frames, as the name suggests, are not sequenced. They are used
for control purposes. For example, they can initiate a connection using standard or extended
windowing (module 8 versus 128), disconnect the link, report a protocol error, or carry out similar
functions.
The LAPB frame is shown in Figure 19-5.
