2022-01-30
Flow Control Error Control
2022-01-30
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Flow Control/1
Definition:
f low control is a technique for assuring that a transmitting station does not overwhelm a receiving station with data
Flow Control/2
two f low control mechanisms stop‐and‐wait
also referred as “alternating bit” or “send and wait” sliding window
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Flow Control: Stop‐and‐Wait
Anything Negative?
Stop and Wait
source transmits frame
destination receives frame and replies with
acknowledgement (ACK)
source waits for ACK before sending next
destination can stop f low by not send ACK
works well for a few large frames
Stop and wait becomes inadequate if large block of data is split into small frames
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Flow Control: Sliding Window
allows multiple numbered frames to be in transit
receiver has buffer W long
transmitter sends up to W frames without ACK
ACK includes number of next frame expected
sequence number is bounded by size of field (k) frames are numbered modulo 2k
giving max window size of up to 2k ‐ 1
receiver can ack frames without permitting further transmission (Receive Not Ready)
must send a normal acknowledge to resume
if have full‐duplex link, can piggyback ACks
Sender’ s Sliding Window progress
Window size=7 packets
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Sliding Window Diagram
Sliding Window: Example
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Line Utilization as a Function of Window Size
Error Control
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Error Control
Error control techniques use some or all of these mechanisms
Automatic repeat request (ARQ)
error detection
positive acknowledgment
retransmission after timeout
negative acknowledgment and retransmission
SomeversionsofARQ Stop‐and‐Wait ARQ
Go‐back‐N ARQ
Selective‐rejectARQ
Stop and Wait
Source transmits single frame
Wait for ACK
If received frame damaged, discard it
Transmitter has timed‐out
If no ACK within timeout, retransmit
If ACK damaged, transmitter will not recognize it
Transmitter will retransmit
Receiver gets two copies of frame
use alternate numbering and ACK0 / ACK1
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Stop and Wait: Damaged frame
Stop and Wait: Lost frame
, 2/4/2021
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Stop and Wait: Lost ACK
Stop and Wait – Example
see example with both types of errors
pros and cons simple
inefficient
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Stop and Wait ‐ Pros and Cons
Inefficient use of resources (low utilization, especially for systems with long propagation delays)
Based on sliding window
If no error, ACK as usual, indicating next frame
Use window to control number of outstanding frames
If error, reply with rejection
discard that frame and all future frames until error
frame received correctly
transmitter must go back and retransmit that frame and all subsequent frames
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Sliding Window: Damaged frame
Go Back N ‐ Damaged Frame
Receiver detects error in frame i Receiver sends rejection‐i (RRi) Transmitter gets rejection‐i
Transmitter retransmits frame i and all subsequent frames
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Sliding Window: Lost frame
Go Back N ‐ Lost Frame (1)
Framei lost
Transmitter sends i+1
Receiver gets frame i+1 out of sequence Receiver send rejection i (RRi)
Transmitter goes back to frame i and retransmits frame i and all subsequent frames
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Go Back N ‐ Example
Go Back N ‐ Lost Frame (2)
Framei lostandnoadditionalframesent
Receiver gets nothing and returns neither
acknowledgement nor rejection
Transmitter times out and sends acknowledgement frame with P bit set to 1
Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )
Transmitter then retransmits frame i
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Sliding Window: Lost ACK
Go Back N –
Damaged or Lost Acknowledgement
Receiver gets frame i and sends acknowledgement (i+1) which is lost
Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame i
If transmitter times out, it sends acknowledgement with P bit set as before
This can be repeated a number of times before a reset procedure is initiated
NOTE: either damaged or lost, for sender is the same since it can’t reconstruct the Acknowledgement frame to be able to “read” it.
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Go Back N ‐ Damaged Rejection
As for lost frame
Go Back N ‐ Diagram
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Selective Reject
also called selective retransmission
only rejected frames are retransmitted
subsequent frames are accepted by the receiver and buffered
minimizes retransmission
receiver must maintain large enough buffer
more complex logic in transmitter
hence less widely used saves bandwidth
useful for satellite links with long propagation delays
Selective Reject: Example
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Go Back N vs Selective Reject
High Level Data Link Control
ISO 33009, ISO 4335
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HDLC Station Types
Primary station
Controls operation of link
Frames issued are called commands
Maintains separate logical link to each secondary station
Secondary station
Under control of primary station Frames issued called responses
Combined station
May issue commands and responses
HDLC Link Configurations
Unbalanced
One primary and one or more secondary stations Supports full duplex and half duplex
Balanced
Two combined stations
Supports full duplex and half duplex
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HDLC Transfer Modes (1)
Normal Response Mode (NRM)
Unbalanced configuration
Primary initiates transfer to secondary
Secondary may only transmit data in response to command from primary
Used on multi‐drop lines
Host computer as primary
Terminals as secondary
HDLC Transfer Modes (2)
Asynchronous Balanced Mode (ABM)
Balanced configuration
Either station may initiate transmission without receiving permission
Most widely used
No polling overhead
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HDLC Transfer Modes (3)
Asynchronous Response Mode (ARM)
Unbalanced configuration
Secondary may initiate transmission without permission form primary
Primary responsible for line
rarely used
Frame Structure
Synchronous transmission
All transmissions in frames
Single frame format for all data and control exchanges
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Frame Structure Diagram
Flag Fields
Delimit frame at both ends
01111110
May close one frame and open another
Receiver hunts for f lag sequence to synchronize
Bit stuffing used to avoid confusion with data containing 01111110
0 inserted after every sequence of five 1s
If receiver detects five 1s it checks next bit
If 0, it is deleted
If 1 and seventh bit is 0, accept as flag
If sixth and seventh bits 1, sender is indicating abort
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Bit Stuffing
Example with possible errors
Address Field
Identifies secondary station that sent or will receive frame Usually 8 bits long
May be extended to multiples of 7 bits
LSB of each octet indicates that it is the last octet (1) or not (0) All ones (11111111) is broadcast
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Control Field
Different for different frame type
Information ‐ data to be transmitted to user (next layer up)
Flow and error control piggybacked on information frames
Supervisory ‐ ARQ when piggyback not used
Unnumbered ‐ supplementary link control
First one or two bits of control filed identify frame
Remaining bits explained later
Control Field Diagram
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Poll/Final Bit
Use depends on context Command frame
1 to solicit (poll) response from peer Response frame
1 indicates response to soliciting command
Information Field
Only in information and some unnumbered frames Must contain integral number of octets
Variable length
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Frame Check Sequence Field
Error detection
16 bit CRC
Optional 32 bit CRC
HDLC Operation
Exchange of information, supervisory and unnumbered frames
Three phases Initialization Data transfer Disconnect
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Examples of Operation (1)
Examples of Operation (2)
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Other DLC Protocols (LAPB,LAPD)
Link Access Procedure, Balanced (LAPB)
Part of X.25 (ITU‐T)
Subset of HDLC ‐ ABM
Point to point link between system and packet switching network node
Link Access Procedure, D‐Channel ISDN (ITU‐D)
Always 7‐bit sequence numbers (no 3‐bit)
16 bit address field contains two sub‐addresses One for device and one for user (next layer up)
Other DLC Protocols (LLC)
Logical Link Control (LLC)
IEEE 802
Different frame format
Link control split between medium access layer (MAC)
and LLC (on top of MAC)
No primary and secondary ‐ all stations are peers
Two addresses needed
Sender and receiver
Error detection at MAC layer
32 bit CRC
Destination and source access points (DSAP, SSAP)
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Other DLC Protocols (Frame Relay) (1)
Streamlined capability over high speed packet witched networks
Used in place of X.25
Uses Link Access Procedure for Frame‐Mode Bearer
Services (LAPF)
Two protocols
Control ‐ similar to HDLC Core ‐ subset of control
Other DLC Protocols (Frame Relay) (2)
7‐bit sequence numbers
16 bit CRC
2, 3 or 4 octet address field
Data link connection identifier (DLCI)
Identifies logical connection More on frame relay later
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Other DLC Protocols (ATM)
Asynchronous Transfer Mode
Streamlined capability across high speed networks Not HDLC based
Frame format called “cell”
Fixed 53 octet (424 bit)
Details later
• synchronous transmission Vs asynchronous transmission
Error detection
• parity checks; LRC; CRC
Flow control
• stop-and-wait; sliding-window
Error control
• stop-and-wait; go-back-N; selective reject
• synchronous transmission; CRC; go-back-N; selective reject
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