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232 Chapter 8: Establishing an X.25 Connection
Figure 8-4
X.121 Address Format
LAPB Layer
LAPB distinguishes between two types of hosts: DTE and DCE. At Level 2, LAPB enables
orderly and reliable exchange of data between a DTE and a DCE. As mentioned earlier, a router
using LAPB encapsulation can perform the role of a DTE or DCE device at the protocol level,
which is different from the physical DTE or DCE.
Using LAPB in noisy conditions can result in greater throughput than using HDLC encapsulation
in similar conditions. When LAPB detects a missing frame, the router retransmits the frame
instead of waiting for the higher layers to recover the lost information. This behavior is good
only if the host timers are relatively slow. In the case of quickly expiring host timers, however,
you will discover that LAPB is spending much of its time transmitting host retransmissions.
When using long delay satellite links, the modulo for the link should be increased to 128 from
8. This is discussed later in the chapter.
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 bidirectional conversation, both parties
keep send and receive sequence numbers. The poll bit is used to force a final bit message
in response, which 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.
·
Unnumbered (U) frames--These frames, as the name suggests, are not sequenced. They
are used for control purposes.
Destination DTE
address length
4 bits
Source DTE
address length
4 bits
Country
Code
PSN
National
terminal
number
Destination DTE
address length
4 bits
Source DTE
address length
4 bits
Country
Code
PSN
Destination DTE
address length
4 bits
Source DTE
address length
4 bits
PSN
National
terminal
number
International data number
4 digits DNIC
10-11 digits
NTN
Country
Code
Destination DTE
address length
4 bits
Source DTE
address length
4 bits
PSN
National
terminal
number