Packet-switched networks (PSNs) allow customers to share network facilities, reducing costs. PSNs use virtual circuits to allow remote sites to maintain multiple connections over a single interface. X.25 was an early packet switching technology that defined the connection between terminals and PSNs. It provided reliability through error checking at the data link and network layers. X.25 addresses remote devices using X.121 addresses and establishes permanent or switched virtual circuits to transmit data between them.

1. X-25, Benefits of Packet
Switched Networks
 PSNs, packet-switched networks, provide
remote offices with either permanent or
switched connections that feature high-levels
of throughput (typically up to DS1).
 An important advantage of PSNs is that they
offer customers a way to share facilities with
other customers, thereby reducing the cost of
WAN service.

2. X-25, Benefits of Packet
Switched Networks
 Paths through the PSN are called
virtual circuits (VCs). A virtual circuit is a
logical path, not a physical one.
 Virtual circuits make it possible for a
remote site to maintain connections to
multiple sites over the same physical
interface.

3. X-25, Benefits of Packet
Switched Networks
 A site can send data directly to several
other remote sites via different virtual
circuits in the carrier network. This
requires that the customer mark, or
tag, each unit of data in some way so
the provider's WAN switch can
determine which destination to route the
traffic through the cloud (refer to the
figure).

5. X-25, Benefits of Packet
Switched Networks
 In frame relay networks, the VC
information is called a data link control
identifier (DLCI) and is included in the
frame header. In X.25 networks, the VC
information is called the logical
channel identifier (LCI) and is included
in the packet header.

6. X-25, Benefits of Packet
Switched Networks
 PSNs allow providers to charge their
customers on the basis of the number of
packets transmitted. Because the
customer can "pay as they go," PSNs
can provide optimal cost-effectiveness.

7. X-25, Benefits of Packet
Switched Networks
 X.25 was one of the earliest packet switched
technologies and the first to be deployed
worldwide. In fact, since X.25 is still frequently
used in developing countries and for legacy
equipment, X.25 continues to be the world's
most common packet-switched technology,
and can be found in virtually every region that
supports data communications.

9. X.25
 X.25 is a standard that defines the
connection between a terminal and a
PSN. In other words, X.25 is an
interface specification. It does not
specify the characteristics of the PSN
itself. Despite this, the networking
industry commonly uses the term X.25
to refer to the entire suite of X.25
protocols.

10. X.25
 Developed in the early 1970s, X.25 was
designed to transmit and receive data
between alphanumeric "dumb" terminals
through analog telephone lines. X.25 enabled
these terminals to remotely access
applications on mainframes or
minicomputers. Later, X.25's capability was
expanded to support a variety of networking
protocols, including TCP/IP, Novell IPX, and
AppleTalk.

11. X.25
 The X.25 suite of protocols includes
Packet Layer Protocol (PLP), Link
Access Procedure, Balanced (LAPB),
and various physical-layer serial
interfaces (e.g., X.21bis, EIA/TIA-232,
EIA/TIA-449, EIA-530, and G.703). The
figure maps the key X.25 protocols to
the layers of the OSI reference model.

13. X.25
 Both PLP and LAPB include mechanisms for
error checking, flow control, and reliability. By
including these mechanisms at both Layer 2
and Layer 3, X.25 provides a high level of
reliability.
 If a network is built on unreliable circuits,
error checking at the hardware level (the Data
link layer) can handle transmission errors
more efficiently than processes in software
(the Network layer and above).

14. X.25
 X.25 is "over-engineered" when
implemented over modern WAN links.
Newer technologies, such as Frame
Relay, take advantage of lower error
rates by providing a stripped-down,
unreliable data link. Such technologies
rely on error detection and correction in
the upper layers (typically the Transport
layer).

15. X.25 Network Devices
 X.25 network devices fall into three
general categories:
 Data terminal equipment (DTE).
 Data circuit-terminating equipment
(DCE).
 Packet switching exchange (PSE).

16. X.25 Network Devices
 DTE devices are end systems that
communicate across the X.25 network. They
are usually terminals, routers, or network
hosts, and are located on the premises of
individual subscribers.
 DCE devices are communications devices
such as modems and packet switches. They
provide the interface between DTE devices
and a PSE. X.25 DCEs are typically located
in the carrier's facilities.

17. X.25 Network Devices
 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
illustrates the relationships between the
three types of X.25 network devices.

19. Packet
Assembler/disassembler (PAD
 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:

20. Packet
Assembler/disassembler (PAD
 buffering.
 packet assembly.
 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 operation
includes adding an X.25 header.

21. Packet
Assembler/disassembler (PAD
 Finally, the PAD disassembles incoming
packets before forwarding the data to
the DTE. This includes removing the
X.25 header.

22. Packet
Assembler/disassembler (PAD
 Some ITU-T recommendations defining the
PAD are as follows:
 X.3 - Specifies the parameters for terminal-
handling functions (e.g., baud rate, flow
control, character echoing, and other
functions) for a connection to an X.25 host.
The X.3 parameters are similar in function to
Telnet options or attention (AT) command set
for modems.

23. Packet
Assembler/disassembler (PAD
 X.28 - Specifies the user interface for
locally controlling a PAD. X.28 identifies
the keystrokes that you would enter at a
terminal to set up the PAD, similar to the
AT command set for modems.

24. Packet
Assembler/disassembler (PAD
 X.29 - Specifies a protocol for setting the X.3
parameters via a network connection. When
a connection is established, the destination
host can request that the PAD or terminal
change its parameters by using the X.29
protocol. A PAD cannot tell the destination
host to change its X.3 parameters, but it can
communicate that its own parameters were
changed.

25. Packet
Assembler/disassembler (PAD
 X.75 - Specifies the gateway between the
clouds. It defines the signaling system
between two PDNs. X.75 is essentially a
Network-to-Network Interface (NNI).
 When discussing X.25, the term virtual circuit
(VC) is used interchangeably with the
following terms: logical channel identifier
(LCI), virtual circuit number (VCN), logical
channel number (LCN), and virtual
channel identifier (VCI).

26. Virtual Circuits
 X.25 connection can be a permanent virtual
circuit (PVC) or, more commonly, a switched
virtual circuit (SVC).
 A PVC is similar to a leased line. Both the
network provider and the attached X.25
subscriber must provision the VC. PVCs use
no call setup or call clear that is apparent to
the subscriber. Any provisioned PVCs are
always present, even when no data traffic is
being transferred.

27. Switched Circuits
 An SVC exists only for the duration of
the session. Three phases are
associated with X.25 SVCs:
 Call setup.
 Information transfer.
 Call clear.

28. Switched Circuits
 The X.25 protocol offers simultaneous
service to many hosts. An X.25 network
can support configurations of multiple
SVCs and PVCs over the same physical
circuit attached to the X.25 interface.
 Cisco routers provide numbering for up
to 4095 VCs per X.25 interface. VCs are
identified using the LCI.

29. X.25 Encapsulation
 Delivery of Network layer data through
the internetwork usually involves
encapsulation of Layer 3 packets inside
Layer 2 frames.
 In an X.25 environment, an LAPB frame
is used.

30. X.25 Encapsulation
 In an X.25 WAN, the Layer 3 packet must
include X.25 Packet Layer Protocol (PLP).
The Layer 3 PLP header provides reliability
through sequencing, and manages packet
exchanges between DTE devices across
virtual circuits.
 Layer 3 encapsulation occurs twice in an
X.25 TCP/IP packet: once for the IP datagram
and once for X.25 PLP.

31. X.25 Encapsulation
 When configuring X.25 on a Cisco
router's interface, you can choose
between Cisco's encapsulation type and
the Internet Engineering Task Force
(IETF) type. The Cisco encapsulation
method is the default.
 The Layer 3 X.25 header is made up of
the following components:

32. X.25 Encapsulation
 A general format identifier (GFI) - The GFI
is 4-bit field that indicates the general format
of the packet header.
 A logical channel identifier (LCI) - The LCI
is a 12-bit field that identifies the virtual
circuit. The LCI is locally significant at the
DTE/DCE interface.
 A packet type identifier (PTI) - The PTI field
identifies one of X.25's 17 packet types.

33. X.25 Encapsulation
 Thus, in an X.25 environment, the
virtual circuit information (the LCI) is
carried in the Layer 3 header. An end-
to-end virtual circuit is established in the
PSN via two logical channels, each with
an independent LCI on two DTE/DCE
interfaces.

34. X.121, the X.25 Addressing
Standard
 Addressing fields in PLP call setup
packets provide source and destination
DTE addresses. These are used to
establish the virtual circuits that
constitute X.25 communication.

35. X.121, the X.25 Addressing
Standard
 ITU-T recommendation X.121 specifies
the source and destination address
formats. X.121 addresses (also referred
to as international data numbers, or
IDNs) vary in length and can be up to
15 decimal digits long.

36. X.121, the X.25 Addressing
Standard
 The first four digits of an IDN are called the
data network identification code (DNIC).
The DNIC is divided into two parts, the first
three digits specifying the country and the last
digit specifying the PSN itself.
 The remaining digits are called the national
terminal number (NTN) and are used to
identify the specific DTE on the PSN.

37. X.121, the X.25 Addressing
Standard
 For your specific DNIC code, consult
your service provider. For a listing of
ITU-T country code assignments, refer
to the ITU-T recommendation X.121.

38. X.121, the X.25 Addressing
Standard
 For different network protocols to connect
across X.25, mapping statements are entered
on the router. These statements map the
next-hop Network layer address to an X.121
address (refer to Figure ). For example, an IP
network layer address is mapped to an X.121
address to identify the next-hop host on the
other side of the X.25 network.

41. X.25 Configuration
 When you select X.25 as a WAN
protocol, you must set appropriate
interface parameters. The interface
configuration tasks include:
 Define the X.25 encapsulation (DTE is
the default). .

42. X.25 Configuration
 Assign the X.121 address (usually
supplied by the PDN service provider).
 Define map statements to associate
X.121 addresses with higher-level
protocol addresses.

43. X.25 Configuration
 Other configuration tasks can be
performed to control data throughput
and to ensure compatibility with the
X.25 network service provider.
Commonly used parameters include the
number of VCs allowed and packet size
negotiation.

44. X.25 Configuration
 X.25 is a flow-controlled protocol. The default
flow-control parameters must match on both
sides of a link. Mismatches because of
inconsistent configurations can cause severe
internetworking problems.
 Before configuring X.25 parameters, you
should enter interface configuration mode
and assign a higher-layer address, such as
an IP address to the interface.

45. X.25 Configuration
 The following sections describe X.25
SVC configuration, X.25 PVC
configuration, and optional
configurations including:
 VC ranges.
 Packet sizes.
 Window parameters.

47. Configuring X.25 SVC’s
 To activate X.25 on an interface, you must
enter the encapsulation x25 command to
specify the encapsulation type to be used:
 Router(config-if)#encapsulation
x25 [dte | dce] [ddn | bfe] |
[ietf].
 The router can be an X.25 DTE device, which
is typically used when the X.25 PSN is used
to transport various protocols.

48. Configuring X.25 SVC’s
 The router can also be configured as an
X.25 DCE device, which is typically
used when the router acts as an X.25
switch.
 You can choose between two
encapsulation methods; Cisco and
IETF. The default is Cisco and is not
specified by a keyword.

49. Configuring X.25 SVC’s
 The x25 address command defines
the local router's X.121 address. Only
one address per interface can be
defined (refer to Figure ). The value
specified must match the address
designated by the X.25 PDN:
 Router(config-if)#x25 address
x.121-address

50. Configuring X.25 SVC’s
 The x25 map command provides a
static map of higher-level addresses to
X.25 addresses. The command maps
the network layer addresses of the
remote host to the X.121 address of the
remote host:
 Router(config-if)#x25 map
protocol address x.121-
address [options]

51. Configuring X.25 PVC’s
 When configuring a PVC, you must configure
the interface using the encapsulation x25
command. You must also assign an X.121
address using the x25 address command.
These tasks are the same, whether you are
configuring an SVC or a PVC.
 However, instead of using the x25 map
command to establish a PVC, you use the
x25 pvc command.

52. Configuring X.25 PVC’s
 PVCs are the X.25 equivalent of leased lines;
they are never disconnected. You do not
need to configure an address map before
defining a PVC because the x25 command
does the mapping for you, as follows:
 Router(config-if)#x25 pvc circuit
protocol address [protocol2
address2 [...[protocol9
address9]]] x121-address
[options].

57. Configuring X.25 PVC’s
 Multiple protocols can be routed on the
same PVC. Multiple circuits can also be
established on an interface by creating
another PVC.