Introduction

X.25 is an ITU (International Telecommunication Union) protocol standard for WAN communications. X.25 protocol standard defines interface between Data Terminal Equipment (DTE) and Data Circuit-terminating Equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit. Hillstone Software's HS X.25 C Source Library fully implements ITU-T X.25 protocol.

X.25 Network Topology

X.25 network consists of DTEs (data terminal equipment: terminals, personal computers, networked hosts, bank ATM machines, payment terminals) communicating across X.25 network, via X.25 DCEs (data circuit-terminating equipment: modems, PADs (packet assembler disassembler) packet switches). DCEs are connected to or integrated within PSE (packet switching exchange typically based on carrier premises. X.25 switches transfer (switch) packets between DTEs in the core of X.25 PSN (public switching network). HS X25 implementation can be used in any part of X.25 network component.

X.25 PAD (Packet Assember Disassember)

Some of X.25 DCEs are either connected to or are a one device with X.25 PAD - Packet Assembler Disassembler. An X.25 PAD is used to connect an Asynchronous character mode terminal to X.25 network. There are several types of PADs: Triple X PAD which operate according to X.28, X.29 and X.3 ITU-T protocol standards; there is also a TPAD - Transactional Packet Assembler Disassember which is often used to connect payment terminals to X.25 network.

X.25 Virtual Circuits

X.25 protocol supports 2 types of virtual circuits (connections): switched virtual circuits (SVC) where calls are established on demand for a desired period of time (similar to telephone (PSTN) calls) and permanent virtual circuits (PVC), where a circuit is permanently connected by agreement between X.25 network operator and X.25 subscriber. HS X.25 supports SVC and PVC operation.

X.25 Protocol Layers

With reference to OSI protocol layer model, X.25 physical layer fits in OSI physical layer, X.25 data link layer fits in OSI data link layer and X.25 packet layer fits in OSI network layer.

• Physical layer: This layer specifies the physical, electrical, functional and procedural characteristics to control the physical link between a DTE and a DCE. Common implementations use X.21, EIA-232, EIA-449 or other serial protocols. X.25 protocol can be run over ISDN (integrated services digital network) D channel or B Channel. In this case physical layer is an ISDN channel driver. X.25 can also be run over XOT (X.25 over TCP /IP). In this case physical layer is LAN (Ethernet) packet driver or serial packet driver such as PPP or SLIP. HS X25 includes full implementation of XOT protocol stack for X.25 operating over TCP IP.
• Data link layer: The data link layer consists of the link access procedure for data interchange on the link between a DTE and a DCE. In its implementation, the Link Access Procedure, Balanced (LAPB) is a data link protocol that manages a communication session and controls the packet framing. It is a bit-oriented protocol that provides error correction and orderly delivery. A variation on LAPB is LAPD which is used on ISDN as opposed to serial synchronous links.
• Packet layer: This layer defined a packet-layer protocol for exchanging control and user data packets to form a packet-switching network based on virtual circuits.

X.25 Packet Layer

Channels and Addressing

VC may be established using X.121. The X.121 address consists of a three-digit Data Country Code (DCC) plus a network digit, together forming the four-digit Data Network Identification Code (DNIC), followed by the National Terminal Number (NTN) of at most ten digits. Note the use of a single network digit, seemingly allowing for only 10 network carriers per country, but some countries are assigned more than one DCC to avoid this limitation. Networks often used fewer than the full NTN digits for routing, and made the spare digits available to the subscriber (sometimes called the sub-address) where they could be used to identify applications or for further routing on the subscribers networks.

NSAP addressing facility is supported from 1984 X.25 revision and this enabled X.25 to better meet the requirements of OSI Connection Oriented Network Service (CONS). Later revisions of X.25 allow multiple addresses in addition to X.121 addresses to be carried on the same DTE-DCE interface: Telex addressing (F.69), PSTN addressing (E.163), ISDN addressing (E.164), Internet Protocol addresses (IANA ICP), and local IEEE 802.2 MAC addresses.

PVCs are permanently established in the network and therefore do not require the use of addresses for call setup. PVCs are identified at the subscriber interface by their logical channel identifier.

One DTE-DCE interface to an X.25 network has a maximum of 4095 logical channels on which it is allowed to establish virtual calls and permanent virtual circuits, although networks are not expected to support a full 4095 virtual circuits. For identifying the channel to which a packet is associated, each packet contains a 12 bit logical channel identifier made up of an 8-bit Logical Channel Number and a 4-bit Logical Channel Group Number. Logical channel identifiers remain assigned to a virtual circuit for the duration of the connection. Logical channel identifiers identify a specific logical channel between the DTE (subscriber appliance) and the DCE (network), and only has local significance on the link between the subscriber and the network. The other end of the connection at the remote DTE is likely to have assigned a different logical channel identifier. The range of possible logical channels is split into 4 groups: channels assigned to permanent virtual circuits, assigned to incoming virtual calls, two-way (incoming or outgoing) virtual calls, and outgoing virtual calls. (Directions refer to the direction of virtual call initiation as viewed by the DTE - they all carry data in both directions.) The ranges allowed a subscriber to be configured to handle significantly differing numbers of calls in each direction while reserving some channels for calls in one direction. All International networks are required to implement support for permanent virtual circuits, two-way logical channels and one-way logical channels outgoing; one-way logical channels incoming is an additional optional facility. DTE-DCE interfaces are not required to support more than one logical channel. Logical channel identifier zero will not be assigned to a permanent virtual circuit or virtual call. The logical channel identifier of zero is used for packets which don't relate to a specific virtual circuit (e.g. packet layer restart, registration, and diagnostic packets).

Packet Layer Initialization

X.25 packet layer is initialized or re-initialized using X.25 restart procedure which simultaneously clears all virtual calls or permanent virtual circuits

Connection Setup for Switched Virtual Circuits (SVC)

A virtual call is established by transferring call request packet over DTE DCE interface and receiving call confirmation packet.

Call request and optionally call confirmation packet contains address block which carries calling and called X.25 DTE addresses.

Connection Clearing for Switched Virtual Circuits (SVC)

A virtual call is disconnected by transferring clear request packet over DTE DCE interface and receiving clear confirmation packet.

Permanent Virtual Circuits

Permanent virtual circuits require no call setup. X.25 reset and reset confirmation packets are transferred across DTE DCE interface to transition PVC endpoints into data state.

Data Transfer and Flow Control

Data is transferred across DTE DCE interface in DATA packets.

For the purpose of flow-control, a sliding window protocol is used with the default window size of 2. The acknowledgements may have either local or end to end significance. A D bit (Data Delivery bit) in each data packet indicates if the sender requires end to end acknowledgement. When D=1, it means that the acknowledgement has end to end significance and must take place only after the remote DTE has acknowledged receipt of the data. When D=0, the network is permitted (but not required) to acknowledge before the remote DTE has acknowledged or even received the data.

DATA packets are sequentially numbered modulo 8 or modulo 128. The sequence number is carried in Ps field of DATA packets. The sequence number cycles from 0 to 8 or 128. Each DATA packet is acknowledged by an RR, RNR or DATA packet in the reverse direction. DATA packets carry both Ps and Pr fields, meaning Send and Receive sequence numbers. RR and RNR packets carry Pr (receive sequence) number only. Pr - receive sequence number acknowledges all DATA packets numbered received up to Pr -1. Maximum number of packets so acknowledged is the window size which is either pre-agreed or negotiated during call setup. For modulo 8 the maximum window size is 7. For example in a windows size is 7 is configured, a DTE or DCE is allowed to send a maximum of 7 packets without waiting for acknowledgement which comes with RR, RNR or DATA packet from remote DTE or DCE.

RR packet is used to acknowledge reception of data packets and to open or re-open remote transmit window. RNR packet is used to acknowledge reception of data packets and to tell the remote side to stop transmitting more data. REJ packet is used to request retransmission of DATA packets which are detected to be lost by analyzing sequence numbers and using timeouts. HS X.25 currently supports modulo 8 only operation.

Some X.25 networks support data packet re-transmissions via special facility to which user may subscribe.

Data Segmentation and Re-assembly

An X.25 protocol is responsible for segmenting the data submitted by application layer for transmission into data packets according to agreed maximum packet size and transmitting fragmented data sequence. All DATA packets except last in sequence has the M (more) bit set. The remote X.25 re-assembles the DATA packets into complete data sequence before passing it up to application layer.

Error Correction

Error recovery procedures at the packet layer assume that the data link layer is responsible for retransmitting data received in error. Packet layer error handling focuses on resynchronizing the information flow in calls, as well as clearing calls that have gone into unrecoverable states.

Basic Packet Format

All X.25 packets have the following fields:

• GFI: general format identifier
• LCN+LCGN: logical channel group number and logical channel number
• Packet type identifier

Further depending on packet type, call setup and clearing packets contain or may contain an address block, clearing cause and diagnostic, facilities and call user data

X.25 Packet Types:

Packet Type	DCE -> DTE	DTE -> DCE	Service	VC	PVC
Call setup and Clearing	Incoming Call	Call Request		X
	Call Connected	Call Accepted		X
	Clear Indication	Clear Request		X
	Clear Confirmation	Clear Confirmation		X
Data and Interrupt	Data	Data		X	X
	Interrupt	Interrupt		X	X
	Interrupt Confirmation	Interrupt Confirmation		X	X
Flow Control and Reset	RR	RR		X	X
	RNR	RNR		X	X
	REJ	REJ		X	X
	Reset Indication	Reset Request		X	X
	Reset Confirmation	Reset Confirmation		X	X
Restart	Restart Indication	Restart Request	X
	Restart Confirmation	Restart Confirmation	X
Diagnostic	Diagnostic		X
Registration	Registration Confirmation	Registration Request	X

X.25 Facilities

X.25 allows users to subscribe to facilities defined and described in ITU-T Recommendation X.2. The categories of X.2 facilities are:

• essential facilities
• additional facilities
• conditional facilities
• mandatory facilities
• optional facilities

X.25 also provides X.25 and ITU-T DTE optional user facilities defined in Recommendation X.7. The X.7 optional user facilities fall into four categories of user facilities that require:

• subscription only
• subscription followed by dynamic invocation
• subscription or dynamic invocation and dynamic invocation only.

X.25 Diagnostic error codes

Description	Decimal code
No additional information	0
Invalid P(S)	1
Invalid P(R)	2
Packet type invalid	16
For state r1	17
For state r2	18
For state r3	19
For state p1	20
For state p2	21
For state p3	22
For state p4	23
For state p5	24
For state p6	25
For state p7	26
For state d1	27
For state d2	28
For state d3	29
Packet not allowed	32
Unidentifiable packet	33
Call on one-way logical channel	34
Invalid packet type on a permanent virtual circuit	35
Packet on unassigned logical channel	36
Reject not subscribed to	37
Packet too short	38
Packet too long	39
Invalid general format identifier	40
Restart packet with non-zero in bits 1 to 4 of octet 1, or bits 1 to 8 of octet 2	41
Packet type not compatible with facility	42
Unauthorized interrupt confirmation	43
Unauthorized interrupt	44
Unauthorized reject	45
TOA/NPI address subscription facility not subscribed to	46
Time expired	48
For incoming call	49
For clear indication	50
For reset indication	51
For restart indication	52
For call deflection	53
Call set-up or call clearing problem	64
Facility code not allowed	65
Facility parameter not allowed	66
Invalid called DTE address	67
Invalid calling DTE address	68
Invalid facility length	69
Incoming call barred	70
No logical channel available	71
Call collision	72
Duplicate facility requested	73
Non-zero address length	74
Non-zero facility length	75
Facility not provided when expected	76
Invalid ITU-T specified DTE facility	77
Maximum number of call redirections or call deflections exceeded	78
Miscellaneous	80
Improper cause code from DTE	81
Not aligned octet	82
Inconsistent Q-bit setting	83
NUI problem	84
ICRD problem	85
Not assigned	96
International problem	112
Remote network problem	113
International protocol problem	114
International link out of order	115
International link busy	116
Transit network facility problem	117
Remote network facility problem	118
International routing problem	119
Temporary routing problem	120
Unknown called DNIC	121
Maintenance action	122
Reserved for network specific diagnostic information 128 – 255

X.25 Data link layer - LAPB

LAPB is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that 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.