计算机代考程序代写 Week 3 – Data Link Layer COMP90007 Internet Technologies – cscodehelp代写

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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