General Description of the X.25 Protocol

The X.25 user/network interface consists of three distinct layers of control procedures, consistent with the bottom three layers of the OSI model:

DTE DTE DTE Computer mainframe small business computer Terminal controller DCE _DCE DCE Switching node X.25 X.25 X.25 Figure 3.5.

circuit for sending data bits between the user machine (DTE) and the network (DCE). Its functions are to pass data, synchronisation and control signals between the DTE and the DCE and to handle failure detection and isolation procedures. The recommended form of this interface to digital circuits is described in CCITT Recommendation X.21.

Layer 2 The frame layer is essentially the HDLC layer of the control protocol described previously. In X.25 the control procedures for point-to-point balanced systems are referred to as balanced link access procedures (LAPB). This layer defines control procedures and the frame envelope, which is used to carry a frame of data over the physical link, and ensure that the frame is not lost or garbled.

Layer 3 The packet layer is the network layer or logical link control layer. This layer specifies the manner in which control information and user data are structured into packets. It describes the formats of packets that are used for setting up and clearing a virtual call, sending data over virtual circuits, controlling message flow, sequencing, interrupts and recovering from the various problems that might occur. It also allows a single physical circuit to support communications to numerous other DTEs concurrently.

Virtual circuit and Datagram service

The X.25 recommendation provides access to the following services that might be provided on public data networks:

3.2. The Network Layer in X.25 45 • permanent virtual circuits (PVCs) and

• datagrams.

A virtual circuit (VC) is a bidirectional, transparent, flow-controlled path between a pair of logical or physical parts. A switched virtual circuit is a temporary association between two DTEs. Like a telephone call there are three phases to a virtual call:

(i) call set-up,

(ii) data exchange and (iii) call clearing.

A permanent virtual circuit is a permanent association existing between two DTEs, which is analogous to a point-to-point private line. It requires no call set-up or call clearing action by the DTE.

A datagram is a self-contained entity of data containing sufficient information to be routed to the destination DTE without the need for a call to be set up (Figure 3.6).

The PAD Interface

Many terminals transmit characters rather than data packets. An interface machine is thus needed to assemble the terminal’s data into packets and disassemble them for X.25 operation. Most common carriers provide such an interface machine. A standard for this interface has been proposed as an extension to the CCITT Recommendation X.25 and is called the PAD (Packet Assembly/Disassembly) interface. CCITT Recommendations X.3, X.28 and X.29 define the CCITT PAD interface. X.3 defines the PAD parameters; X.28 defines the terminal-PAD interface; and X.29 defines the PAD-host computer (DTE) interface.

User machines access to the network might assume one of the set-ups in Figure 3.7. 3.2.2. X.25 End-to-end Virtual Circuit Service Characteristics

Establishment and clearing of a virtual circuit

A switched virtual circuit is established when the call request issued by the calling DTE is accepted by the called DTE. The call request identifies the called and calling addresses and facilities requested for the call, and may include user data.

If the call is refused by the called DTE, this DTE can signal the reason for call clearing to the calling DTE in a diagnostic code. If the call attempt fails for some reason, a call progress signal is transmitted across the network indicating one of the causes specified in X.25.

Once the call has entered the data transfer phase, either DTE can clear the call using the diagnostic code to signal to the remote DTE the reason for clearing. If the call is cleared, data may be discarded by the network since the clear is not sequenced with respect to user data.

Data transfer

D D D

The pointy part is the address. (c) Datagram conception.

(b) A virtual circuit conception (a) The communication subnet

C B A A B C C B A Figure 3.6.

passed transparently through the network. Virtual circuit flow control is applied to ensure that the transmitting DTE does not generate data at a rate that is faster than that which the receiving DTE can accept.

A considerable debate has taken place on whether the DTE or the network should determine the maximum number of data packets which may be in the network on a virtual circuit. It has been agreed that DTE-to-DTE acknowledgment of delivery be available as a standard characteristic

3.2. The Network Layer in X.25 47 X.25 X.25 DTE DTE DTE PAD interface Figure 3.7.

of X.25 virtual circuits. If a DTE wishes to receive end-to-end acknowledgment for data it is transmitting across the X.25 interface, it uses an indicator called the delivery confirmation, or D bit, contained in the header of DATA packets. Later sections will discuss this point further. There are thus independent mechanisms for transferring user control information between a pair of DTEs outside the normal flow of data or a virtual circuit. The first mechanism transfers user control information within the normal flow control and sequencing procedures in a virtual circuit except that the Q bit in the DATA packet header is set. The second mechanism bypasses the the normal DATA packet transmission sequence. This mechanism uses an INTERRUPT packet which may contain one byte of user data. This packet is transmitted as quickly as possible to its

destination, jumping the queues of normal DATA packets. Packet Format

X.25 describes the formats of packets that will be passed between a user machine and the DCE in order to set up and use virtual circuits. The packets have the general format shown in Figure 3.8.

The first four bits of a packet are a general format identifier. A virtual circuit is identified by the 12-bit Group and Channel number. When a user machine initiates a virtual call it selects a free logical channel from those available to it. This number is passed to the local DCE which then attempts to set up a virtual call using that logical channel.

There are two types of packets: DATA and CONTROL packets. The Control bit is set to 1 in all control packets and to 0 in all data packets.

Figure 3.9 illustrates a CALL REQUEST format, a DATA packet format and various CONTROL packet formats.