X.25

Introduction

X.25 is used in a Packet Switched Network and in 1964 was designed by Paul Baran of the RAND Corporation for use with the Public Data Network (PDN) and unreliable analogue telephone services. The idea was to connect a dumb terminal to a packet-switched network. In 1976 X.25 became a standard under the CCITT, now the International Telecommunications Union - Telecommunication Standardization Sector (ITU-T).

The Physical Layer commonly uses X.21/X.21bis provides the handshaking and electrical requirements at the Physical Layer. EIA/TIA-232/449/530 and G.703 can also be used. X.21bis is the equivalent of RS232C and RS449/442. Most routers support X.21, V.35, RS232C and RS449/442.

The Frame Layer (Layer 2) is Link Access Protocol Balanced (LAPB) and provides link setup, control, sequencing and error recovery. It also provides a windowing mechanism. LAPB is very robust and can be applied direct to serial links and is often found being used on satellite links. Private X.25 networks can use LAPB but when connecting to Public X.25 networks you must use the X.25 encapsulation only.

There is extensive error checking and recovery with buffering so that end stations can ensure that the network is error free. The X.25 Layer 3 is called the Packet Layer Protocol (PLP) and allows up to 4095 virtual circuits over one physical interface. Layer 3 (Network layer) protocols are tunnelled in X.25 layer 3 packets. Network layer addresses are mapped to the X.121 addresses. This is connection-oriented and forces sequencing. X.75 is used to link different X.25 networks from other countries.

Link Access Protocol Balanced (LAPB)

Although Link Access Protocol (LAP) can be used with X.25, most commonly LAPB (a variation on the HDLC frame format) is used to provide balanced relationships where either device can send commands and receive responses.

In the Address field, one side must be configured as a Layer 2 DTE and the other as a Layer 2 DCE (you must not confuse this with the more familiar Layer 1 DCE and DTE designations). The Control field indicates which of three types of LAPB frames there are:

Unnumbered: These establish and maintain communications and there are five types of U-frames; Set Asynchronous Balanced Mode (Extended) (SABM or SABME) which establish the DTE to DCE link; Unnumbered Acknowledgment (UA) which confirms acceptance of a frame; Disconnect (DISC) which terminates the link; Disconnect Mode (DM) which indicates a disconnected state and Frame Reject (FRMR) which reports an error condition.
Supervisory: These allow the control of the flow of data and there are three types of S-frames; Receiver Ready (RR) which acknowledges the reception of a frame and indicates that the device is ready to receive the next one in the sequence; Receiver Not Ready (RNR) which acknowledges a received frame but it indicates that it cannot receive any more I-frames because it is still busy; and Reject (REJ) which requests the retransmission of I-frames, the packet contains the error frame so that the DTE will retransmit all packets since the error frame.
Information: These contain data as well as Next Sent (NS) and Next Receive (NR) counts. The sequencing information of I-frames is piggybacked so streamlining the acknowledgment operation and a timer T1 defines how long a device waits for a response.

Packet Layer Protocol (PLP) (X.25) Packet

A PAD is a Packet Assembler/Disassembler whilst a CPAD is a Character Packet Assembler/Disassembler for use with a terminal. The PAD collects data from asynchronous terminals and periodically outputs the data in X.25 packets.

Because a router acts as an X.25 host, it must be configured as a DTE and the X.25 switch is the DCE. Layer 3 is concerned with DTE to DTE (router to router) connectivity.

This Layer 3 packet is encapsulated within the Information field of the LAPB layer 2 packet:

The header consists of the following:

General Format Identifier (GFI): This defines the sequencing requirements and contains; the Qualifier (Q) bit which identifies information destined for PADs; the Delivery (D) bit which calls for end to end acknowledgment and the Sequence Number (SN) which describes whether the number sequence used is modulo 8 or modulo 128.
Logical Channel Identifier (LCI): This identifies the virtual circuit (whether Switched or Permanent) between the two end stations and is made up of the 4-bit Logical Channel Group Number (LCGN) and the 8-bit Logical Channel Number (LCN). The virtual circuit is locally significant and can change from call setup to call setup.

Packet Type Identifier (PTI): This describes the packet type as tabled below:

Hex Code	DTE to DCE	DCE to DTE
0B	Call Request	Incoming Call
OF	Call Accept	Call Connected
13	Clear Request	Clear Indication
17	DTE Clear Confirmation	DCE Clear Confirmation
x1	DTE RR	DCE RR
x5	DTE RNR	DCE RNR
x9	DTE REJ	DCE REJ
1D	Reset Request	Reset Indication
1F	DTE Reset Confirmation	DCE Reset Confirmation
FD	Restart Request	Restart Indication
FF	DTE Restart Confirmation	DCE Restart Confirmation

Using the above PTI types, PLP operates in five stages:

Call Setup
Data Transfer
Idle - used once the SVC is up but no data transfer is occurring.
Call Clearing
Restarting

PVCs are permanently in Data Transfer mode.

Addressing X.121 and X.75

This International Data Number (IDN) is used for routing within the X.25 cloud and is located just after the PTI. It consists of 14 digits, the first four digits being the Data Network Identification Code (DNIC) used when a packet travels between PDNs, the first three describe the country and the fourth the PDN The UK uses the range 234-237). The remaining ten digits describe the National Terminal Number (NTN). An X.75 routing flag may be required by some PDNs, and this is a '0' or a '1' which sits just before the X.121 address to indicate the requirement for the use of X.75 routing tables.

The addresses are defined by the ITU-T and are detailed on their web site https://www.itu.int/en/Pages/default.aspx .

Several network layer protocol addresses can be mapped to a single X.121 address as described in RFC 1356.

Call Request

The PTI contains a '0B' to identify the packet as a Call Request, followed by two 4-bit fields which provide the lengths of the destination and source addresses. Next comes the 14 byte Call Address followed by the Facilities field which starts with the length of the requested facilities first (up to 109 bytes and is often used for things like fast select, credit card transactions, reverse charge etc.). The Protocol Identification (PID) describes the protocol which will cross the Switched Virtual Circuit (SVC) and this followed finally by the Call User Data (CUD) which is 15 bytes and is used by some manufacturers such as Nortel to pass the Connection ID parameter to the receiving router.

Call Setup is shown diagrammatically below:

The Call is accepted once the receiving DTE is sure that it can support the requested facilities.

During data transfer, the receiving device (which is the local switch) acknowledges the packet, not the ultimate destination, so that the sending device does not have to wait for the packet to travel the whole network before sending the next one.

The Clear Request terminates the connection.

Virtual Circuits

The X.25 provider will give you a range of VCs (out of the total range which is 1-4095) that you can use and these must be configured the same on the local switch as on the router. The following table details the type of circuit, the default values on Cisco routers and the relevant IOS commands:

Circuits	Default	Command
PVC		x25 pvc circuit
SVC Incoming Only	0	x25 lic circuit
	0	x25 hic circuit
SVC Two-way	1	x25 ltc circuit
	1024	x25 htc circuit
SVC Outgoing Only	0	x25 loc circuit
	0	x25 hoc circuit

VC number assignments have to follow the following order: 1 <= PVCs < lic <= hic < ltc <= htc < loc <= hoc <= 4095 where 'l' stands for 'low' and 'h' stands for 'high'.

An example of what a provider may give to a user is a range of VCs of say 1-15 where the incoming SVCs can be from 1 (lic) to 5 (hic), the two-way SVCs from 6 (ltc) to 10 (htc) and the outgoing SVCs from 11 (loc) to 15 (hoc).

There are two types of Virtual Circuits (Logical Circuits) between DTEs across a series of X.25 switches:

Permanent Virtual Circuit:

Like a leased line, being statically configured between DTEs.
No call setup is needed.
Node addressing is not required.
All packets follow a fixed path through the X.25 fabric.

Switched Virtual Circuit:

Like a Voice call, disconnects when data transfer has finished.
The path through the X.25 fabric may change during the call as links fail.
Path may be different each call.
For network protocols like IPX that provide higher layer resequencing, multiple SVCs provide an effectively larger window size.

X.25 Routing

X.25 can be configured as a Public Data Network (PDN), complying with OSI and allowing IP communication with other switches.

Local X.25 switching is where incoming calls on one interface are routed out of another local interface. Remote switching is where incoming calls are sent to other switches via tunnelling through TCP and the remote switch routes the call on to the destination. Remote Switching is commonly called X.25 Over TCP (XOT) and provides the facility to run X.25 over any medium and at much faster speeds.

X.25 can also be run over non-serial media using LLC2 using Connection Mode Network Service (CMNS).

RFC 1356 describes multiprotocols over X.25.

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