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

2. Flow Control 8Necessary when data is being sent faster than it can be processed by receiver 8Computer to printer is typical setting 8Can also be from computer to computer, when a processing program is limited in capacity

3. Stop-and-Wait Flow Control 8Simplest form 8Source may not send new frame until receiver acknowledges the frame already sent 8Very 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

4. Sliding-Window Flow Control 8Allows multiple frames to be in transit 8Receiver sends acknowledgement with sequence number of anticipated frame 8Sender maintains list of sequence numbers it can send, receiver maintains list of sequence numbers it can receive 8ACK (acknowledgement) supplemented with RNR (receiver not ready)

5. Error Control Process 8All transmission media have potential for introduction of errors 8All data link layer protocols must provide method for controlling errors 8Error control process has two components 8Error detection 8Error correction

6. Error Detection: Parity Bits 8Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) 8Good for detecting single-bit errors only 8High overhead (one extra bit per 7-bit character=12.5%)

7. Error Detection: Cyclic Redundancy Check (CRC) 8Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame 817-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder 8For a CRC of length N, errors undetected are 2 -N 8Overhead is low (1-3%)

8. Error Correction 8Two types of errors 8Lost frame 8Damaged frame 8Automatic Repeat reQuest (ARQ) 8Error detection 8Positive acknowledgment 8Retransmission after time-out 8Negative acknowledgment and retransmission

9. Stop-and-Wait ARQ 8One frame received and handled at a time 8If frame is damaged, receiver discards it and sends no acknowledgment 8Sender uses timer to determine whether or not to retransmit 8Sender must keep a copy of transmitted frame until acknowledgment is received 8If acknowledgment is damaged, sender will know it because of numbering

10. Go-Back-N ARQ 8Uses sliding-window flow control 8When receiver detects error, it sends negative acknowledgment (REJ) 8Sender must begin transmitting again from rejected frame 8Transmitter must keep a copy of all transmitted frames

11. Data Link Control 8Specified flow and error control for synchronous communication 8Data link module arranges data into frames, supplemented by control bits 8Receiver checks control bits, if data is intact, it strips them

12. High-Level Data Link Control 8On transmitting side, HDLC receives data from an application, and delivers it to the receiver on the other side of the link 8On the receiving side, HDLC accepts the data and delivers it to the higher level application layer 8Both modules exchange control information, encoded into a frame

13. HDLC Frame Structure 8Flag: 01111110, at start and end 8Address: secondary station (for multidrop configurations) 8Information: the data to be transmitted 8Frame check sequence: 16- or 32-bit CRC 8Control: purpose or function of frame 8Information frames: contain user data 8Supervisory frames: flow/error control (ACK/ARQ) 8Unnumbered frames: variety of control functions (see p.131)

14. HDLC Operation 8Initialization: S-frames specify mode and sequence numbers, U-frames acknowledge 8Data Transfer: I-frames exchange user data, S-frames acknowledge and provide flow/error control 8Disconnect: U-frames initiate and acknowledge