Bussiness data communications 4e chapter 9

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 ✘ 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: 16or 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, Sframes acknowledge and provide flow/error control ✘ Disconnect: U-frames initiate and acknowledge ... 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... 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... configurations) ✘ Information: the data to be transmitted ✘ Frame check sequence: 16or 32-bit CRC ✘ Control: purpose or function of frame ✘ Information frames: contain user data ✘ Supervisory frames: