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

2. 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

3. 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

4. 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)

5. 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

6. 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%)

7. 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%)

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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)

14. 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