CS代写 ISO 33009, ISO 4335

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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