X.25 is an International
Telecommunication Union-Telecommunication Standardization Sector (ITU-T)
protocol standard for WAN communications that defines
how connections between user devices and network devices are established
and maintained. X.25 is designed to operate effectively regardless of
the type of systems connected to the network. It is typically used in
the packet-switched
networks (PSNs) of common carriers, such as the telephone companies.
Subscribers are charged based on their use of the network. The
development of the X.25 standard was initiated by the common carriers in
the 1970s. At that time, there was a need for WAN protocols capable of
providing connectivity across public data networks (PDNs). X.25 is now
administered as an international standard by the ITU-T. This chapter
covers the basic functions and components of X.25.
X.25 network devices fall into three general
categories: data terminal equipment (DTE), data
circuit-terminating equipment (DCE), and packet
switching exchange (PSE). Data terminal equipment
devices are end systems that communicate across the X.25 network. They
are usually terminals, personal computers, or network hosts, and are
located on the premises of individual subscribers. DCE devices are
communications devices, such as modems and packet switches, that provide
the interface between DTE devices and a PSE and are generally located in
the carrier's facilities. PSEs are switches that compose the bulk of the
carrier's network. They transfer data from one DTE device to another
through the X.25 PSN. Figure 17-1 illustrates the relationships
between the three
types of X.25 network devices.
Figure 17-1: DTEs,
DCEs, and PSEs make up an X.25 network.
The packet assembler/disassembler
(PAD) is a device commonly found in X.25 networks. PADs are used when a
DTE device, such as a character-mode terminal, is too simple to
implement the full X.25 functionality. The PAD is located between a DTE
device and a DCE device, and it performs three primary functions:
buffering, packet assembly, and packet disassembly. The PAD buffers data
sent to or from the DTE device. It also assembles outgoing data into
packets and forwards them to the DCE device. (This includes adding an
X.25 header.) Finally, the PAD disassembles incoming packets before
forwarding the data to the DTE. (This includes removing the X.25
header.) Figure 17-2 illustrates the basic operation
of the PAD when receiving packets from the X.25 WAN.
Figure 17-2: The
PAD buffers, assembles, and disassembles data packets.
X.25 sessions are
established when one DTE device contacts another to request a
communication session. The DTE device that receives the request can
either accept or refuse the connection. If the request is accepted, the
two systems begin full-duplex information transfer. Either DTE device
can terminate the connection. After the session is terminated, any
further communication requires the establishment of
a new session.
A virtual circuit is a logical
connection created to ensure reliable communication between two network
devices. A virtual circuit denotes the existence of a logical,
bidirectional path from one DTE device to another across an X.25
network. Physically, the connection can pass through any number of
intermediate nodes, such as DCE devices and PSEs. Multiple virtual
circuits (logical connections) can be multiplexed onto a single physical
circuit (a physical connection). Virtual circuits are demultiplexed at
the remote end, and data is sent to the appropriate destinations. Figure
17-3 illustrates four separate virtual circuits being multiplexed onto a
single physical circuit.
Figure 17-3: Virtual circuits
can be multiplexed onto a single physical circuit.
Two
types of X.25 virtual circuits exist: switched and permanent. Switched
virtual circuits (SVCs) are temporary connections used for sporadic data
transfers. They require that two DTE devices establish, maintain, and
terminate a session each time the devices need to communicate. Permanent
virtual circuits (PVCs) are
permanently established connections used for frequent and consistent
data transfers. PVCs do not require that sessions be established and
terminated. Therefore, DTEs can begin transferring data whenever
necessary, because the session is always active.
The basic operation of an X.25 virtual
circuit begins when the source DTE device specifies the virtual circuit
to be used (in the packet headers) and then sends the packets to a
locally connected DCE device. At this point, the local DCE device
examines the packet headers to determine which virtual circuit to use
and then sends the packets to the closest PSE in the path of that
virtual circuit. PSEs (switches) pass the traffic to the next
intermediate node in the path, which may be another switch or the remote
DCE device.
When the traffic arrives at the remote
DCE device, the packet headers are examined and the destination address
is determined. The packets are then sent to the destination DTE device.
If communication occurs over an SVC and neither device
has additional data to transfer, the virtual circuit is terminated.
The X.25 protocol suite maps to the
lowest three layers of the OSI reference model. The following protocols
are typically used in X.25 implementations: Packet-Layer Protocol (PLP),
Link Access Procedure, Balanced (LAPB), and those among other
physical-layer serial interfaces (such as EIA/TIA-232, EIA/TIA-449,
EIA-530, and G.703). Figure 17-4 maps the key X.25 protocols to the
layers of the OSI reference model.
Figure 17-4: Key X.25
protocols map to the three lower layers of the OSI reference model.
Packet-Layer
Protocol (PLP)
PLP is the X.25 network-layer protocol.
PLP manages packet exchanges between DTE devices across virtual
circuits. PLPs also can run over Logical-Link Control 2 (LLC2)
implementations on LANs and over Integrated Services Digital Network
(ISDN) interfaces running Link Access Procedure on the D channel (LAPD).
The PLP operates in five distinct modes: call
setup, data transfer, idle, call clearing,
and restarting.
Call setup mode is used
to establish SVCs between DTE devices. A PLP uses the X.121 addressing
scheme to set up the virtual circuit. The call setup mode is executed on
a per-virtual circuit basis, which means that one virtual circuit can be
in call-setup mode while another is in data-transfer mode. This mode is
used only with SVCs, not with PVCs.
Data-transfer
mode is used for transferring data between two
DTE devices across a virtual circuit.
In this mode, PLP handles segmentation and reassembly, bit padding, and
error and flow control. This mode is executed on a per-virtual circuit
basis and is used with both PVCs and SVCs.
Idle mode is used when a virtual
circuit is established but data transfer is not occurring. It is
executed on a per-virtual circuit basis and is used only with SVCs.
Call-clearing mode is used to
end communication sessions between DTE devices and to terminate SVCs.
This mode is executed on a per-virtual circuit basis and is used only
with SVCs.
Restarting mode is used to
synchronize transmission between a DTE device and a locally connected
DCE device. This mode is not executed on a per-virtual circuit basis. It
affects all the DTE device's established virtual circuits.
Four types of PLP packet fields exist:
LAPB is a data link-layer protocol that
manages communication and packet framing between DTE and DCE devices.
LAPB is a bit-oriented protocol which ensures that frames are correctly
ordered and error free.
Three types of LAPB frames exist: information,
supervisory, and unnumbered. The information
frame
(I-frame) carries upper-layer information and some control information.
I-frame functions include sequencing, flow control, and error detection
and recovery. I-frames carry send and receive sequence numbers. The supervisory
frame (S-frame) carries control information. S-frame functions
include requesting and suspending transmissions, reporting on status,
and acknowledging the receipt of I-frames. S-frames carry only receive
sequence numbers. The unnumbered frame (U-frame) carries
control information. U-frame functions include link setup and
disconnection, as well as error reporting. U-frames carry no sequence
numbers.
X.21bis is a physical-layer
protocol used in X.25 that defines the electrical and mechanical
procedures for using the physical medium. X.21bis handles the activation
and deactivation of the physical medium connecting DTE and DCE devices.
It supports point-to-point connections, speeds up to 19.2 kbps, and
synchronous, full-duplex transmission over four-wire media. Figure
17-5 shows the format of the PLP packet and its relationship to the LAPB
frame and the X.21bis frame.
Figure 17-5: The PLP packet
is encapsulated within the LAPB frame and the X.21bis
frame.
LAPB frames include a header, encapsulated
data, and a trailer. Figure 17-6 illustrates the format of the LAPB
frame and its relationship to the PLP packet and the X.21bis frame.
The following descriptions summarize the
fields illustrated in Figure 17-6:
Figure 17-6: An
LAPB frame includes a header, a trailer, and encapsulated data.
X.121 addresses are used by the
X.25 PLP in call-setup mode to establish SVCs. Figure 17-7
illustrates the format of an X.121 address.
Figure 17-7: The X.121
address includes an IDN field.
The X.121 Address field includes the International
Data Number (IDN), which consists of two fields: the Data Network
Identification Code (DNIC) and the National Terminal Number
(NTN).
DNIC is an optional
field that identifies the exact PSN in which the destination DTE device
is located. This field is sometimes omitted in calls within the same PSN.
The DNIC has two subfields: Country and PSN. The Country subfield
specifies the country in which the destination PSN is located. The PSN
field specifies the exact PSN in which the destination DTE device is
located.
The NTN identifies the exact DTE device
in the PSN
for which a packet is destined. This field varies in length.
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