The X.25 Protocol

The X.25 protocol really comprises many protocols, including LAPB and X.25 itself, which
is a layer 3 protocol. X.25 also uses various standards, including X.121, X.75, and X.3,
among others.
We have included information regarding X.25 in this text; however, it is very
likely that this information will not be included in the exam as it is modified in
the future. Readers can expect to see questions relating to the technology, and
given its historical significance, learning about X.25 is not unwarranted.
LAPB operates at layer 2 of the Open System Interconnect (OSI) model and is responsible
for providing reliability. Specifically, LAPB provides windowing functions and detects
missed frames.
Readers who wish to review the OSI model should refer to CCNA: Cisco
Certified Network Associate Study Guide, 4th Edition, by Todd Lammle
(Sybex, 2004).
X.25 (which can be described as also belonging to layers 1 through 3) was designed to catch
errors, because it was developed to operate on poor-quality telecommunications systems. At
layer 3, X.25 describes the formation of data packets and the methods to be used for connectivity,
in addition to addressing.
Some consider the X.25 standards to be recommendations from the International Telecommunications
Union–Telecommunication (ITU-T) Standardization Sector. In practice, this
can be accurate because private X.25 networks are free to operate over any methodology that
works. However, the standards can simplify matters and become very important in public
X.25 networking.
The X.25 addressing standard is X.121. X.121 addresses are composed of a Data Network
Identification Code (DNIC) and a Network Terminal Number (NTN). These numbers work
similarly to the way area codes and phone numbers work—the DNIC is akin to an area code
that is defined on a country basis, and the NTN is a specific node identifier.