length in a bit Information passing across the interface at this level comprises oriented protocol is an arbitrary number of bits rather than a fixed three signal types: multiple of a selected character size. This makes life simpler by reducing the number of necessary framing and control characters. The CCITT adopted HDLC largely intact, producing the Link 1. Data signals transfer the higher level information between the Access Procedure Balanced (LAPB). two devices, a signal carrying data in one direction and another in the other direction. 2. Clock signals, in conjunction with data signals, reconstruct transmitted information at the destination. 54.4.2.1 The make-up of a frame 3. Where devices are connected directly via an X.25 port, soft- ware handles flow control (which monitors the amount of A basic X.25 level 2 or HDLC (Figure 54.9) frame opens and closes traffic on the circuit) automatically as part of the protocol with a flag, a specific pattern of bits transmitted on the link to act as functions. Asynchronous traffic, however, makes use of control reference points for frame synchronisation. When there are no Frame Address Control Information field check 01111110 field field 01111110 sequence 8 bits 8 bits 8 bits N bits 16 bits 8 bits Figure 54.9 An X.25 level 2 frame: HDLC structure Packet switching and the OSI model 54/9 frames to transmit on a circuit, flags are sent continuously to fill in frame from another, each information frame is assigned a pair of time and maintain synchronisation between the two end points. sequence numbers, carried for X.25 level 2 in the control field, When a recipient detects a flag, it examines the data stream for an operating in both the send and receive directions. eight bit address field or area, used to identify to which direction of The initial frame carries a sequence number in a three bit field. transfer the frame belongs. A control field specifies what the frame The receiver, maintaining a count of error free frames received, contains and carries frame sequence numb