Chapter 11 flow control can occur at layer 2 data link and at layer 4 transport
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Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport). You saw the need in the most recent assignment. Data packets can be damaged, but it’s not only data that can be changed. If the sequence number is changed how do you know what packet was damaged?

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Chapter 11 flow control can occur at layer 2 data link and at layer 4 transport
Chapter 11: Flow Control – can occur at layer 2 (data link) and at layer 4 (transport)

  • You saw the need in the most recent assignment.

  • Data packets can be damaged, but it’s not only data that can be changed.

  • If the sequence number is changed how do you know what packet was damaged?

  • What if the acknowledgment is damaged?


  • What if a data packet is lost? link) and at layer 4 (transport)

  • What if an acknowledgment is lost?

  • How many data packets can we acknowledge with one acknowledgment packet?

  • Book does some calculation of bit rates; you can skip that stuff. We’ll focus on the protocols.

  • Main thing is that the download speed is a function of not only the raw bit rate, but the flow control protocol used.


  • Recall the layering of protocols link) and at layer 4 (transport)

    • Frame – layer 2 unit of transmission

    • Packet – layer 3 unit of transmission

    • Text uses layer 2 context for flow control, but it does occur at layer 4 (TCP) as well


Figure 2.4 link) and at layer 4 (transport)An exchange using the OSI model


  • Byte oriented link) and at layer 4 (transport)

    • Frame interpreted as a sequence of bytes

    • Each byte means something

    • Old protocol typical of transferring text files

    • Flags (e.g. 01111110) delimit start and end of frame



  • Bit oriented link) and at layer 4 (transport)

    • More typical of streaming, binary files, graphics, etc

    • Frame interpreted as a bit stream

    • Start and end of frame marked with a flag=01111110



Flow control
Flow control link) and at layer 4 (transport)

Flow control refers to a set of procedures used to restrict the amount of data

that the sender can send before

waiting for acknowledgment.


Figure 11.5 link) and at layer 4 (transport)Taxonomy of protocols discussed in this chapter


Acronyms
Acronyms link) and at layer 4 (transport)

  • ARQ – Automatic Repeat reQuest

  • ACK – acknowledgment

  • NAK – negative acknowledgment (indicates a problem with a frame – damaged or never arrived)


Figure 11.6 link) and at layer 4 (transport)The design of the simplest protocol with no flow or error control

  • Has similarities to a streaming protocol


Algorithm 11.1 link) and at layer 4 (transport)Sender-site algorithm for the simplest protocol


Algorithm 11.2 link) and at layer 4 (transport)Receiver-site algorithm for the simplest protocol


Figure 11.7 link) and at layer 4 (transport)Flow diagram for Example 11.1



Figure 11.8 link) and at layer 4 (transport)Design of Stop-and-Wait Protocol


Algorithm 11.3 link) and at layer 4 (transport)Sender-site algorithm for Stop-and-Wait Protocol


Algorithm 11.4 link) and at layer 4 (transport)Receiver-site algorithm for Stop-and-Wait Protocol


Figure 11.9 link) and at layer 4 (transport)Flow diagram for Example 11.2



  • Each frame has a sequence number link) and at layer 4 (transport)

  • Sequence nos range from 0 to 2m-1, where m is the number of bits used to represent the sequence number

  • If m=3, sequence nos are as follows

    0,….7, 0,….7, 0,….7, etc


  • ARQ: adds simple error control link) and at layer 4 (transport)

  • Distinguish Ack frames from NAK frames

  • Implement a timer if neither of the above does not arrive in timely fashion


Figure 11.10 link) and at layer 4 (transport)Design of the Stop-and-Wait ARQ Protocol


Algorithm 11.5 link) and at layer 4 (transport)Sender-site algorithm for Stop-and-Wait ARQ

(continued)


(continued) link) and at layer 4 (transport)

Algorithm 11.5Sender-site algorithm for Stop-and-Wait ARQ

  • Option: If a NAK frame arrives, proceed as in the timeout


Algorithm 11.6 link) and at layer 4 (transport)Receiver-site algorithm for Stop-and-Wait ARQ Protocol

  • Option: Doan error check and send either an Ack or NAK frame


Figure 11.11 link) and at layer 4 (transport)Flow diagram for Example 11.3


  • What if an link) and at layer 4 (transport)ack is not lost but just delayed past when the timer expires?


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