Giuseppe Bianchi Layer 2 Framing HDLC (High-level Data Linl Control)

Giuseppe Bianchi HDLC  Typical Layer 2 protocol  Services:  Framing  Error detection and frame retransmission  Flow control  Most of current line-based L2 protocols are derived from HDLC  PPP (dial-up, ADSL, …)  LAP-D (ISDN)  LAP-B (X.25)  …

Giuseppe Bianchi Why framing? Framing received frames transmitted frames PHY: - bit oriented byte oriented FRAMES = “structured” information & control fields PHY = bit/byte streams

Giuseppe Bianchi HDLC Frame Format Flag 1 byte Address 1-n bytes Control 1-2 bytes Information (variable) FCS 2-4 bytes Flag 1 byte  Each frame starts & ends with reserved flag:  = 0x7e  Just one flag required between consecutive frames  Two consecutive flags = empty frame  Just ignore flagHDLC frameflagHDLC frame flagHDLC frameflagHDLC frameflag

Giuseppe Bianchi The need for “stuffing” Flag FRAME Flag Flag FRAME Flag FRAME Flag

Giuseppe Bianchi Bit stuffing  HDLC Frame delineated by flag character   Six consecutive 1s  Bit stuffing: prevent occurrence of more than five 1s!  Transmitter inserts extra 0 after each consecutive five 1s inside the frame  Receiver checks for five consecutive 1s  if next bit = 0, it is removed  if next two bits are 10, then flag is detected  If next two bits are 11, then frame has errors

Giuseppe Bianchi Bit stuffing example Data to be sent: After stuffing and framing * * Data received After destuffing and deframing EXAMPLE: send EXAMPLE: receive

Giuseppe Bianchi Byte stuffing  Byte-oriented channels?  Same stiffing idea, but stuffing via control escape octes:  Send (“reserved” byte) as (control escape octet) + (reserved byte)XOR( )  Extra XOR (bit 5 complemented) for improved robustness  Control escape octet  = 0x7d – of course NOW this is a reserved byte, too! 113F7E027D10 113F7D027D105E5D Receiver detects control escape And removes it restoring original byte as part of the frame (e.g. not as flag) See RFC 1662 for bit-stuffing over bit synchronous links (versus byte-oriented links)

Giuseppe Bianchi HDLC Frame Content Flag 1 byte Address 1-n bytes Control 1-2 bytes Information (variable) FCS 2-4 bytes Flag 1 byte  Address:  Receiving (secondary) station  Or Transmitting (secondary) station in case of response  = broadcast address  Control  Frame type and transmission control information  FCS  CRC16 or CRC32 for error detection  Size of fields decided during link set-up/initialization procedure

Giuseppe Bianchi Control field  Three types of frames  I = Information frame  S = Supervisory frame  U = Unnumbered frame

Giuseppe Bianchi Information Frame  N(S)  # of currently transmitted frame  N(R)  # of next frame expected to be received  i.e., ACK of all frames until N(R)-1  P/F bit  For polling operation 0N(S) P/F 0 8 bit case 16 bit case N(R) P/F N(S)N(R)

Giuseppe Bianchi Supervisory Frame  Control frame used to manage ACK and flow control  S=RR (Receive Ready)  Carries cumulative ACK up to N(R)-1  S=RNR (Receive Not Ready)  Blocks sender (flow control), carries cumulative ACK as well  S=REJ (Reject)  Negative ACK for frame # N(R), for Go-Back-N case  S=SREJ (Selective Reject)  Negative ACK for frame # N(R), for Sel. Repeat case  Extension: OTHER frame # to be retransmitted may be included in the frame information field 1 P/F 8 bit case 16 bit case N(R) P/F N(R) 0S 10S0000

Giuseppe Bianchi Unnumbered Frame  Frames used for datalink management  Initialization, Setup/release, Reset, Management and notification of malfunctions and issues  Several control messages  See details and examples in book  Different datalink scenarios  Asynchronous Balanced Mode (ABM)  LAP-B  Two full-duplex stations  Normal Response Mode (NRM)  Unbalanced (half-duplex) operation: primary station can only access link, secondary station is slave  Asynchronous Response Mode (ARM)  Like NRM, but secondary station may further initiate an unsolicited transmission 1 P/F M1M