Chapter 9: Data Link Control Business Data Communications, 4e

Flow Control Necessary when data is being sent faster than it can be processed by receiver Computer to printer is typical setting Can also be from computer to computer, when a processing program is limited in capacity

Stop-and-Wait Flow Control Simplest form Source may not send new frame until receiver acknowledges the frame already sent Very inefficient, especially when a single message is broken into separate frames, or when the data link is long enough for significant delays to be introduced

Sliding-Window Flow Control Allows multiple frames to be in transit Receiver sends acknowledgement with sequence number of anticipated frame Sender maintains list of sequence numbers it can send, receiver maintains list of sequence numbers it can receive ACK (acknowledgement) supplemented with RNR (receiver not ready)

Error Control Process All transmission media have potential for introduction of errors All data link layer protocols must provide method for controlling errors Error control process has two components Error detection Error correction

Error Detection: Parity Bits Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) Good for detecting single-bit errors only High overhead (one extra bit per 7-bit character=12.5%)

Error Detection: Cyclic Redundancy Check (CRC) Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder For a CRC of length N, errors undetected are 2-N Overhead is low (1-3%)

Error Correction Two types of errors Automatic Repeat reQuest (ARQ) Lost frame Damaged frame Automatic Repeat reQuest (ARQ) Error detection Positive acknowledgment Retransmission after time-out Negative acknowledgment and retransmission

Stop-and-Wait ARQ One frame received and handled at a time If frame is damaged, receiver discards it and sends no acknowledgment Sender uses timer to determine whether or not to retransmit Sender must keep a copy of transmitted frame until acknowledgment is received If acknowledgment is damaged, sender will know it because of numbering

Go-Back-N ARQ Uses sliding-window flow control When receiver detects error, it sends negative acknowledgment (REJ) Sender must begin transmitting again from rejected frame Transmitter must keep a copy of all transmitted frames

Data Link Control Specified flow and error control for synchronous communication Data link module arranges data into frames, supplemented by control bits Receiver checks control bits, if data is intact, it strips them

High-Level Data Link Control On transmitting side, HDLC receives data from an application, and delivers it to the receiver on the other side of the link On the receiving side, HDLC accepts the data and delivers it to the higher level application layer Both modules exchange control information, encoded into a frame

HDLC Frame Structure Flag: 01111110, at start and end Address: secondary station (for multidrop configurations) Information: the data to be transmitted Frame check sequence: 16- or 32-bit CRC Control: purpose or function of frame Information frames: contain user data Supervisory frames: flow/error control (ACK/ARQ) Unnumbered frames: variety of control functions (see p.131)

HDLC Operation Initialization: S-frames specify mode and sequence numbers, U-frames acknowledge Data Transfer: I-frames exchange user data, S-frames acknowledge and provide flow/error control Disconnect: U-frames initiate and acknowledge