Data Link Layer

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Data Link Layer
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Data link layer

• The communication between two machines that can
  directly communicate with each other.
• 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

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Data Link Layer Framing
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Data link layer framing

• 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.

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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.
• 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. Or, a
  FLAG is a real FLAG if not proceeded by ESC.

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Using Bits

• More flexibility gained by sending bits. Using
  01111110 as the flag bits.
• If the data contains 011111, add a 0 after the
  fifth 1. When the receiver sees in the data part
  0111110, it removes the last 0.
• What if the original data contains 0111111?
• It will be stuffed as 01111101, and wont be
  regarded as the flag. The receiver will destuff
  it back into 0111111.
• What if the original data contains 0111110?
• It will be stuffed as 01111100. The receiver
  will destuff it back into 0111110.

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Data Link Layer Protocols
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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
  live 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?
• Note that the receiver may not be able to process
  a frame if it is still working on a frame
  received earlier.

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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 and send to
  physical layer.
• Wait (blocked here) for ack.
• After getting ACK, goto 1.
• Receiver
• Wait (blocked here) for data.
• After getting the data, give it to the network
  layer and 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?
• When a frame is lost, how can the sender know?

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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.

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Protocol 3

• Sender (maintains m as the sequence number up
  to which has been ACKed)
• Grab data from the network layer.
• Send to physical layer with the current sequence
  number , i.e., m1 and start timer.
• Wait for ACK.
• If got an ACKm1, mm1, repeat 1.
• Else, timeout, repeat 2.
• Receiver (maintains m as the sequence number up
  to which has been received)
• Wait for data.
• Get data from the physical layer. If it is with
  the expected sequence number, i.e., m1, give it
  to the network layer, mm1.
• send ACKm. Goto 1.

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Protocol 3

• Question 1. After sender sends frame m, will he
  receive ACK lower than m-1? Will it receive ACK
  higher than m?

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Protocol 3

• Answer. No. Because when sender sent frame m, he
  must have received ACK from the receiver of m-1.
  Receiver keeps the sequence number and wont
  decrease it. And it will not receive ACK higher
  than m, because m1 has not been sent yet.

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Protocol 3

• Question 2. After sending out an ACKm, 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? (ACKm gets lost) In
  what case he receives m1 (when everything goes
  fine)? 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
  ACKm1)?

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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. At any time, there are only two
  possible frames both for the sender and the
  receiver.