; Flow Control
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Flow Control


  • pg 1
									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?
   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
       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 An exchange using the OSI model

   Byte oriented
       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
   What if flag was part of the data (non-text files)?
   Bit oriented
       More typical of streaming, binary files, graphics,
       Frame interpreted as a bit stream
       Start and end of frame marked with a
   Again, what if flag is part of the data?
   Stuff a bogus 0 after 5 consecutive 1s.
Flow control

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 Taxonomy of protocols discussed in this chapter

   ARQ – Automatic Repeat reQuest
   ACK – acknowledgment
   NAK – negative acknowledgment (indicates a
    problem with a frame – damaged or never arrived)
Figure 11.6 The design of the simplest protocol with no flow or error control

       Has similarities to a streaming protocol
Algorithm 11.1 Sender-site algorithm for the simplest protocol

Algorithm 11.2 Receiver-site algorithm for the simplest protocol

Figure 11.7 Flow diagram for Example 11.1

   No mechanism for error control or acknowledgments
Figure 11.8 Design of Stop-and-Wait Protocol

Algorithm 11.3 Sender-site algorithm for Stop-and-Wait Protocol

Algorithm 11.4 Receiver-site algorithm for Stop-and-Wait Protocol

Figure 11.9 Flow diagram for Example 11.2

   Assumes no errors in frames
   Assumes frames are not lost
   Assumes Acks are not lost
   Each frame has a sequence number
   Sequence nos range from 0 to 2m-1, where m is the
    number of bits used to represent the sequence
   If m=3, sequence nos are as follows
                0,….7, 0,….7, 0,….7, etc
   ARQ: adds simple error control
   Distinguish Ack frames from NAK frames
   Implement a timer if neither of the above does not
    arrive in timely fashion
Figure 11.10 Design of the Stop-and-Wait ARQ Protocol

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

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

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

Algorithm 11.6 Receiver-site algorithm for Stop-and-Wait ARQ Protocol

        Option: Do an error check and send either an Ack or
         NAK frame
Figure 11.11 Flow diagram for Example 11.3

   What if an ack is not lost but just delayed past when
    the timer expires?

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