Piggybacking • A method to combine a data frame with ACK.

• Station A and B both have data to send.

• Instead of sending separately, station A sends a data frame that includes an ACK.

• Station B does the same thing.

• Piggybacking saves bandwidth.

HDLCHigh-level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It implements the ARQ mechanisms we discussed in this chapter.

HDLC Station TypesPrimary station

Controls operation of link Issues commands (frames)Maintains separate logical link to each secondary station

Secondary stationUnder control of primary station Issues responses (frames)

Combined stationMay issue commands and responsesCombines the features of primary and secondary

stations

HDLC Link ConfigurationsUnbalanced

One primary and one or more secondary stations

Supports full duplex and half duplexBalanced

Two combined stationsSupports full duplex and half duplex

HDLC Transfer Modes (1)Normal Response Mode (NRM)

Unbalanced configurationPrimary can only initiate transmission Secondary may only transmit data in response

to command (poll) from primaryHost computer as primaryTerminals as secondary

HDLC Transfer Modes (2)Asynchronous Balanced Mode (ABM)

Balanced configurationEither station may initiate transmission

without receiving permissionMost widely used

HDLC Transfer Modes (3)Asynchronous Response Mode (ARM)

Unbalanced configurationSecondary may initiate transmission without

permission form primaryPrimary is responsible for connect, disconnect,

error recovery, and initializationrarely used

Frame Structure

01111110 01111110

Defines 3 types of frames (I,S,U frames)

Flag Fieldsdefine frame at both ends01111110Receiver hunts for flag sequence to

synchronizeBit stuffing used to avoid confusion with data

containing 01111110The transmitter inserts 0 bit after every

sequence of five 1s with the exception of flag fields

Address FieldIdentifies secondary station that sent or will

receive frameUsually 8 bits long

Control FieldDifferent for different frame type

I-frame (information frame) data to be transmitted to user (next layer up) Flow and error control piggybacked on information

framesS-frame (Supervisory frame)

Used for flow and error controlU-frame (Unnumbered frame)

supplementary link control

First one or two bits of control filed identify frame type

Control Field Diagram

I-frameContains the sequence number of transmitted

frames and a piggybacked ACK

1 2 3 4 5 6 7 8

0 N(S) P/F N(R)

S-frameUsed for flow and error control

1 2 3 4 5 6 7 8

1 S P/F N(R)

• RR --- receive ready

• RNR --- receive not ready

• REJ --- reject on frame N(R)

• SREJ --- selective reject on N(R)

0

U-frameMode setting, recovery, connect/diconnect

1 2 3 4 5 6 7 8

1 M P/F M1

Unnumbered function bits

Unnumberred framesSet normal response mode (SNRM)Set asynchronous response mode (SARM)Set asynchronous balanced mode (SABM)Disconnect (DISC)Unnumberred acknowledgement (UA)Disconnect mode (DM)Request disconnect (RD)Unnumberred poll (UP)Reset (RSET)Exchange identification (XID)Test (TEST)Frame reject (FRMR)

Information FieldOnly in information and some unnumbered

framesMust contain integral number of octetsVariable length

Frame Check Sequence FieldFCSError detection16 bit CRCOptional 32 bit CRC