Introduction to Tandem X.25 Capabilities

X.25 Layers
What is X.25?
065307 Tandem Computers Incorporated 1–11
reach individual employees by dialing directly to the telephone number of an
individual office.)
The actual number of subaddresses supported depends upon the particular network
and computer system.
X.25 Layers The X.25 standard provides rules for communicating on the first three functional
layers of a data communications model (see Figure 1-8). The following subsections
describe the components of each of these layers, starting with the layer that is closest
to the user, the packet layer.
Physical Layer — Layer 1 defines the electrical and physical characteristics of the
interface used to connect a DTE to a DCE via a leased or switched line.
Link (Frame) Layer — Layer 2 defines the link-control procedures to be used
between a DTE and a DCE. Two types of link access procedures were defined by
the CCITT: Link Access Procedure (LAP) and Link Access Procedure Balanced
(LAPB). The most widely used access procedure is LAPB. It provides a
full-duplex, point-to-point, asynchronous balanced mode link. LAP is no longer
included in the 1988 CCITT Recommendation, and it is not supported by Tandem
X25AM software release C30.08 and later versions.
The Link Layer is responsible for link initialization, error detection, error recovery,
sequence checking, flow control, and link disconnection. The Link Layer uses the
frame format of High Layer Data Link Control (HDLC), a protocol specified by the
International Organization for Standardization (ISO) for bit-synchronous data
communications.
Packet (Network) Layer — Layer 3 defines how information is to be switched and
routed, and establishes the logical connection of remote DTEs.