Bussiness data communications 4e chapter 9

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 broke

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: 16-

or 32-bit CRC

✘ Control: purpose or function

of frame

✘ I nformation frames: contain user data

✘ S upervisory frames:

flow/error control (ACK/ARQ)

✘ U nnumbered 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