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X.25 333
Figure 10-2
X.25 Addressing
NOTE
With LAPB encapsulation, when an error occurs, the packet is retransmitted by the source. The
extra error-recovery overhead in LAPB decreases performance, so you should use it only on
lines that are prone to errors.
X.25 provides
·
Cost-effective access
·
Fairly inexpensive but low-speed transport
·
International acceptance
You might end up working with companies in remote locations such as Singapore, Hong Kong,
and South America. But distant locations aren't the only reason to use X.25. If you have noisy
analog circuits, you should be thinking of using X.25 because of its built-in error-checking
capability, which Frame Relay doesn't have. That noise factor makes X.25 an ideal candidate
for satellite links. Airlines use X.25 extensively because airports exist internationally.
X.25 is most commonly configured as a transport for datagrams across an X.25 network. Layer
3 datagrams are reliably carried inside X.25 frames, as shown in Figure 10-3.
Data network ID code
"DNIC"
4 Decimal digits
Up to 10 or 11 decimal
digits
Network terminal
number
(Country code)
The first 4 digits denote country code and provider,
this address is refferred to as the ТDNICУ
87200333.book Page 333 Wednesday, August 22, 2001 2:53 PM