FLAG-8 bit sequence with a bit pattern (7E) ADDRESS-address of secondary station that is either the originator or destination of the frame (or the station acting as the secondary in the case of combined stations) CONTROL-1 or 2 byte segment used for flow and error control. Information-Users data from network layer or network management information. Length can vary from network to network, but is always fixed within each network FCS-Frame Check Sequence contains either a 2 or 4 byte ITU-T CRC.

HDLC FRAME TYPES

Information Frame If the first bit of the control field is a zero, then the frame is an information Frame N(S)- These three bits define the sequence number of the frame in travel P/F-When the bit is set, this means either poll or final. It means poll when the frame is being sent by the primary station to a secondary station. N(R)- These three bits correspond to the value of the ACK when piggybacking is used

Supervisory Frame If the first two bits of the control field are 10, this means this is an S-frame The second 2 bits of the control field is a code that defines four types of S-frames 00-RR S-Frame (receive ready) Acknowledges a safe and sound frame or group of frames 01-RNR S-Frame (Receive Not Ready) Acknowledges the receipt of a frame or group of frames, and announces that the receiver is busy and cannot receive more frames. 10-REJ S-Frame (Reject) This is a NAK frame. NAK as used in Go Back NARQ 11- SREJ S-Frame(sSelective Reject) NAK frame used in Selective repeat ARQ

Unnumbered Frame

Bit-Oriented protocol(cont’d) U-Frame control command and response Command/ response Meaning SNRM SNRME SARM SARME SABM SABME UP UI UA RD DISC DM RIM SIM RSET XID FRMR Set normal response mode Set normal response mode(extended) Set asynchronous response mode Set asynchronous response mode(extended) Set asynchronous balanced mode Set asynchronous balanced mode(extended) Unnumbered poll Unnumbered information Unnumbered acknowledgement Request disconnect Disconnect Disconnect mode Request information mode Set initialization mode Reset Exchange ID Frame reject

Bit-Oriented protocol(cont’d) U-Frame ~ can be divided into five basic functional category –Mode setting –Unnumbered-Exchange –Disconnection –Initialization Mode –Miscellaneous

Bit-Oriented protocol(cont’d) Mode Setting Command ~ are sent by the primary station, or by a combined station wishing to control an exchange, to establish the mode of the session Unnumbered-Exchange ~ are used to send or solicit specific pieces of data link information between device Disconnection Initialization Mode Miscellaneous

Bit-Oriented protocol(cont’d) Example 1 : Poll/Response

Bit-Oriented protocol(cont’d) Example 2 : Select/Response

Bit-Oriented protocol(cont’d) Example 3 : Peer Devices(1)

Bit-Oriented protocol(cont’d) Example 3 : Peer Devices(2)