Week 3 – Data Link Layer COMP90007 Internet Technologies
Lecturer: Semester 2, 2021
© University of Melbourne 2021
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Flow Control
Principles to control when sender can send next frame
Feedback based flow control (usually used in Data Link layer)
Rate based flow control
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A Very Simple Protocol
Time
delay
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Acknowledged Transmission
Case: fast sender vs. slow receiver, the receiver’s buffer space constrained
Requires acknowledgement Ack
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Noisy Channel Protocol
Case: frames can be lost either entirely or partially
Requires timeout function to determine arrival or non-arrival of complete frames
Requires distinction between frames already sent/received and those being re-transmitted
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Stop and Wait Protocol
ARQ (Automatic Repeat reQuest) Ack and Timeout
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Link Utilisation in Stop and Wait Protocols
Link Utilisation (U) measures the efficiency of communication.
Tf = Transmission delay, time needed to transmit a frame of length L; Tp= Propagation delay;
Ta= Time for transmitting an Ack, and we can assume Ta = 0.
Tt= Tf + 2Tp
U = (Time of transmitting a frame)/(Total time for the transfer) = Tf / Tt Given bit rate B and Tf = L/B, we have
U=Tf /(Tf +2Tp)= (L/B)/(L/B+2Tp)=L/(L+2Tp B). Tf
Sender Receiver
Tp
Tf Ta
next
Time Axis
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Link Utilisation in Stop and Wait Protocols
For a link with B=1 Mbps, Tp=50ms and frame size 10Kb, what is the link utilisation?
U=L/(L+2Tp B)
= 10000/(10000+2*0.05*106)=1/11
Sender Receiver
Tt
next
Tf Ta
Tp
Time Axis
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Sliding Window Protocols
Sending window: Sender maintains a set of sequence numbers corresponding to frames allowed to send
Receiving window: Receiver maintains a set of sequence numbers corresponding to frames allowed to accept
What is the window size of Stop and Wait protocol?
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Sliding Window Protocols
Link Utilisation:
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Go-Back-N
Senders don’t need to wait for acknowledgement for each frame before sending next frame
sender
receiver
Long transmission time needs to be considered when programming timeouts e.g., low bandwidth or long distance
Receiver window size =1, Sender window size is N
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Selective Repeat
Receiver accepts frames anywhere in receive window
NAK (negative ack) triggers the retransmission of a missing frame
before a timeout
Cumulative ack indicates highest in-order frame
sender
receiver
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Go-Back-N vs Selective Repeat
Go-Back-N: receiver discards all subsequent frames from error point, sending no acknowledgement, until receiving the next frame in sequence
Selective Repeat: receiver buffers good frames after an error point, and relies on sender to resend oldest unacknowledged frames
Trade-off between efficient use of bandwidth and data link layer buffer space
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Examples of Data Link Protocols
Point-to-Point Protocol (PPP) Packet over SONET
PPP over ADSL
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Point-to-Point Protocol
PPP is a standard protocol for delivering packets
across links
Framing uses a flag (0x7E) and byte stuffing
Default is unnumbered mode: connectionless unacknowledged service
Errors are detected with a checksum
unnumbered mode
IP packet
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Packet over SONET
Packet over SONET: carry IP packets over SONET optical fibre links
Uses PPP (Point-to-Point Protocol) for framing
PPP frames may be Protocol stack split over SONET
payloads
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ADSL
Widely used for broadband Internet over local loops
ADSL runs from modem (customer) to DSLAM (ISP) IP packets are sent over PPP and AAL5, ATM
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ADSL
PPP data is sent in ATM cells over ADSL
ATM uses short, fixed-size cells (53 bytes); each cell has a virtual circuit identifier
1) PPP frame is converted to an AAL5 frame (PPPoA)
2) AAL5 frame is converted to ATM cells
Structure of AAL5 frame
It will be divided into 48-byte pieces, each of which goes into one ATM cell with 5-byte header
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