Unnumbered (U) frames – mainly used for control purposes, can be used to initialize secondary stations.S-frames can request and suspend transmission, report on status and acknowledge receipt of I-frames. Supervisory (S) frames – are providing control information.The secondary station uses the P/F bit to tell the primary one if the current frame is the last in its current response. A P/F bit is used by the primary station to tell the secondary weather it requires an immediate response. I-frames also contain the poll final (P/F) bit. I-frames are sending and receiving sequence numbers, used to refer to the number of the frame to be sent or received next. Information (I) frames – used to carry upper layer information and some control information.Control – uses three different formats, depending on the type of HDLC frame used:.The primary address is defined as either the source or the destination address, but since WAN connections are point-to-point, there’s no need to specify that. Address – contains the HDLC address of the secondary station, which can be a specific address, a group address or a broadcast address.The flag field is used to initiate and terminate error checking. In consecutive frames, the end flag of the first frame is used as the start flag for the next frame. HDLC inserts a 0 bit every five 1s in the data field.
Flag – the flag field’s bit pattern is 01111110.The HDLC frame structure contains the following fields, in order of precedence: Cisco uses a proprietary version of the HDLC protocol, Cisco HDLC (cHDLC), but allowed other vendors to implement it in their network equipments.
0 kommentar(er)
