Ch3-2-Data Link

Sliding Window Protocol
ACK received Current Sequence Number
ACK sent
Window for Expected Frame
(c) After the first frame has been received. (d) After the first acknowledgement has been received.
One-Bit Sliding Window Protocol
Abnormal Case
A
B
Expect ACK 0, get ACK 1 Send B0 again!
Expect ACK 0, get ACK 1 Send A0 again! ACK 0 Disregard B0 (received before) Send A1 (A0 is acknowledged) Send A1 again Disregard A0 Send B1
7
Stop-and-Wait ARQ Software Coding
s.kind = data;
What if the event = “time_out” ?
8
Stop-and-Wait ARQ: The Receiver
1. At start, sequence number is 0 2. The receiver waits until event = frame_arrival 3. Receive the frame from the physical layer
Timer
4
Stop-and-Wait ARQ: Lost ACK
S is the current frame being sent
R is the current frame to receive
Importance of numbering
5
Delayed ACK & Lost Frame
S is the current frame being sent
15
One-Bit Sliding Window Protocol
A
B
One-bit = 2 sequence numbers (0 and 1) Window size is one.
Piggybacking but not pipelining (It is stop & wait) The notation is (seq, ack, info) An asterisk indicates where a network layer accepts a packet
Send B1 again
Every frame is sent twice!
Sliding Window Protocol
Use of Sliding Window 1. Sender: to hold the frames that have been sent, but are not acknowledged. Unacknowledged frames are caused by pipelining and piggybacking (in this case, if no data is sent from the receiver, then no ACK is sent as well) 2. Receiver: to hold unordered frames so that they can be reassembled in order and therefore no resend is necessary
Chapter 3
The Data Link Layer
(Part 2)
CSCI-345/EENG 480 Dr. Sunan Han, NYIT
1
Protocol 3: Stop-and-Wait ARQ
(Automatic Repeat reQuest)
A flow and error control mechanism for noisy channel The sending device keeps a copy of the last frame transmitted until it receives an acknowledgement (ACK)
2
Stop-and-Wait ARQ: Normal Operation
S is the current frame being sent
R is the current frame to receive
The sender will not send the next piece of data until it is sure that the current one is correctly received Sequence number necessary to check for duplicated packets
18
Sliding Window Protocol: The Sender
Window Size = 8 Frames from network layer
3 4 5 6 7 0 1
– Retransmits lost/damaged frame or lost/delayed ACK
Timer is introduced: expired response = frame loss ACK sent for frames received correctly Frame sequence numbers are 0 and 1 only
11
Protocol 4: Sliding Window Protocol
The sequence number of a frame is stored in the header in binary: if the number of bits in the header is m then the sequence number goes from 0 to 2m – 1 Sliding Window: Acknowledged (ACKed) frames are removed from the window and new frames are added into the window to maintain the window size
a) If sequence number (seq) is correct, send info to the network layer. Then send an ack frame to the sender, and invert sequence number (0 to 1 or 1 to 0) b) If seq is not correct, send the correct sequence number to the sender in an ack frame c) Go to step 2
a) If event = frame_arrivel and ack# is correct, then invert the sequence number (0 to 1 or 1 to 0) and go to step 2 b) If event = time_out or ack# is not correct, go to step 5
If it is the expected frame, frame_expected = unchanged! Meaning sending an ACK of the expected frame
10
Protocol 4: Sliding Window Protocol
Sequence number: each frame is assigned a sequence number, 0, 1, 2, 3, ... Window: set of sequence numbers for frames that are allowed to be sent without acknowledgment
3
Stop-and-Wait ARQ: Lost Frame
A timer is set each time a frame is sent
When ACK not received for the time set in the timer, the same frame will be resent
பைடு நூலகம்
R is the current frame to receive
I
I
R=0 S=0
Importance of ACK numbering
6
Stop-and-Wait ARQ: The Sender
1. The start sequence number is 0 2. The sender obtain the packet from network layer and put the packet into the info field of a frame 3. Assign the sequence number in the seq field 4. Assign “Data” in the kind field 5. Send the current frame to the physical layer 6. Start the timer: it starts to tick, … 7. Wait for “event”
12
Sliding Window Protocol
Current Sequence Number Current Frame on Hold