Data Link Layer

1
Data Link Layer
2
Data link layer

• The communication between two adjacent machines.
• Basic property
• If bit A is sent before bit B, bit A will be
  received earlier than bit B
• Fundamental constraints
• Errors
• Computers have different capabilities
• Delay is not zero

3
Data link layer

• What the data link layer does is to encapsulate
  the packets from the network layer into frames
  and send out these frames.
• So we will discuss how to deliver these frames.

4
Framing

• How does the receiver know where is the start and
  where is the end of a frame?
• Counter. Tell the receiver how many bytes there
  are in this frame. Problems?
• The counter part could be corrupted and you are
  done.

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Flag Bytes

• Add special bytes to the beginning and the end of
  the frame.
• Problems?
• What if the data contains the flag bytes?
• Add ESC byte to each flag or ESC in the data.
  Stuffing.
• Any time you see an ESC, the next byte is either
  ESC or FLAG naturally occurred in the data.

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Using Bits

• More flexibility gained by sending bits. Using
  01111110 as the flag bits.
• If the data contains this pattern, add a 0 after
  the fifth 1.

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Error detection

• So the frames are sent. At the receiver side, how
  do you know that you received the frame
  correctly?
• Using the error control code discussed last time.
  
• Calculate the syndrome, if it is 0, then assume
  no error, if it is not zero, then some error.
• Could this scheme give you the wrong judgment
  sometime? Can it be 100 accurate?

8
Data link layer protocols

• Assume that the physical layer, the data link
  layer, and the network layer are three processes
  and communicate with each other.
• Assume that the sender always has enough data to
  send.
• Assume that the machines dont crash.
• Assume that the data flows only one direction
  (simplex).

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Protocol 1.

• Your protocol really depends on the world you are
  living in.
• If (1) the link never screws up your bits, and
  (2) the receivers network layer process reads
  the bits infinitely fast, what should be your
  protocol?

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Protocol 1

• Sender.
• Grab data from the network layer.
• Send to physical layer. Goto 1.
• Receiver
• Get data from the physical layer
• Give it to the network layer. Goto 1.

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Protocol 2

• Now remove assumption (2). The receivers
  network layer process is not infinitely fast. How
  to design the protocol?

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Protocol 2

• The receiver must provide some kind of
  acknowledgements. If the receiver never tells the
  sender anything, the sender has no way of knowing
  whether this speed is too high or too low.
• The simplest form of ack is the receiver sends
  back an ack every time he receives a frame.
• The sender waits for this ack to start sending
  the next frame.
• Called stop and wait.

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Protocol 2

• Sender.
• Grab data from the network layer.
• Send to physical layer
• Wait for ack. Goto 1.
• Receiver
• Get data from the physical layer
• Give it to the network layer
• Send ack. Goto 1.

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Protocol 3

• Now, lets remove assumption (1). There is now
  noise in the channel and frames could be
  corrupted. Both the data frame and the ack
  frame.
• How to design your protocol? What are the
  considerations?

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Protocol 3

• So we can add a timer. If the sender does not
  receive the ack before the timer expires, he
  knows something is wrong and resend this frame.
• How long should the timer be set?
• Problems?

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Protocol 3

• A sent B frame F.
• B received the frame. Sent ack to A. Ack got
  lost.
• A time out. Resend F.
• B received F. AGAIN!

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Sequence Number

• Every frame has a sequence number.
• Sender says I am sending the frame with sequence
  number m.
• Receiver acks saying that I have successfully
  received the frame with sequence number m, please
  send me frame with sequence number m1.
• This will solve the duplication problem. Why?

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Protocol 3

• Sender.
• Grab data from the network layer.
• Send to physical layer with the current sequence
  number and start timer.
• Wait for ack(Expc). If got ack before time out
  for the frame with the current sequence number,
  increment the sequence number, repeat 1. Else,
  timeout, repeat 2.
• Receiver
• Get data from the physical layer
• If it is with the expected sequence number, give
  it to the network layer, increment the sequence
  number.
• send ack (Expc). Goto 1.

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Protocol 3

• Question 1. After sender sends frame m, will he
  receive Expc for lower than m?

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Protocol 3

• Answer. No. Because when sender sent frame m, he
  must have received Expc from the receiver of m.
  Receiver keeps the sequence number and wont
  decrease it.

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Protocol 3

• Question 2. After sending out an Expc of m1,
  what are the possible frames the receiver can
  receive?

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Protocol 3

• Answer. The only possible frames he could
  receive is m or m1.
• In what case he receives m? In what case he
  receives m1? Why cant he receive m-1 (Because
  when sender sends m, he knows that m-1 is
  received correctly.)? Why cant he receive m2
  (because sender wont send m2 before the
  receiver sends Expc m2)?

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Protocol 3

• Question 3. How many bits should the sequence
  number have?

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Protocol 3

• Answer. 1 bit.

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Sliding window

• Think the frames the sender has to send as a
  continuous stream. The window contains the
  current frame being sent and is not acked yet.
  Every ack (Expc) moves the window forward one
  frame.

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Correctness Part is Done!

• So our Protocol 3 will work without causing
  errors. ARQ. (automatic repeat request)
• So the next step is the optimization.
• Any one sees the problem?

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Large link delays

• 500ms delay. 50kbps. Each frame is 1000 bits.
• What is the link efficiency?

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Solution

• So, increase the window size from 1 to n.
• Would want to do this whenever the bandwidth
  delay product is large.
• Go-back N
• Selective-repeat