cse 461 university of washington1 where we are in the course finishing off the link layer! –...
TRANSCRIPT
![Page 1: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/1.jpg)
CSE 461 University of Washington 1
Where we are in the Course• Finishing off the Link Layer!– Builds on the physical layer to transfer
frames over connected links
PhysicalLink
Network
Transport
Application
![Page 2: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/2.jpg)
CSE 461 University of Washington 2
Topics1. Framing– Delimiting start/end of frames
2. Error detection/correction– Handling errors
Done
DSL
![Page 3: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/3.jpg)
CSE 461 University of Washington 3
Topics (2)3. Retransmissions– Handling loss
4. Multiple Access– Classic Ethernet, 802.11
5. Switching– Modern Ethernet
![Page 4: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/4.jpg)
CSE 461 University of Washington 4
Topic• Two strategies to handle errors:
1. Detect errors and retransmit frame (Automatic Repeat reQuest, ARQ)
2. Correct errors with an error correcting code
Done this
![Page 5: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/5.jpg)
CSE 461 University of Washington 5
Context on Reliability• Where in the stack should we
place reliability functions?
PhysicalLink
Network
Transport
Application
![Page 6: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/6.jpg)
CSE 461 University of Washington 6
Context on Reliability (2)• Everywhere! It is a key issue– Different layers contribute differently
PhysicalLink
Network
Transport
ApplicationRecover actions
(correctness)
Mask errors(performance optimization)
![Page 7: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/7.jpg)
CSE 461 University of Washington 7
ARQ• ARQ often used when errors are
common or must be corrected– E.g., WiFi, and TCP (later)
• Rules at sender and receiver:– Receiver automatically acknowledges
correct frames with an ACK– Sender automatically resends after a
timeout, until an ACK is received
![Page 8: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/8.jpg)
CSE 461 University of Washington 8
ARQ (2)• Normal operation (no loss)
Frame
ACKTimeout Time
Sender Receiver
![Page 9: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/9.jpg)
CSE 461 University of Washington 9
ARQ (3)• Loss and retransmission
Frame
Timeout Time
Sender Receiver
Frame
ACK
X
![Page 10: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/10.jpg)
CSE 461 University of Washington 10
So What’s Tricky About ARQ?• Two non-trivial issues:– How long to set the timeout? »– How to avoid accepting duplicate
frames as new frames »
• Want performance in the common case and correctness always
![Page 11: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/11.jpg)
CSE 461 University of Washington 11
Timeouts• Timeout should be:
– Not too big (link goes idle)– Not too small (spurious resend)
• Fairly easy on a LAN– Clear worst case, little variation
• Fairly difficult over the Internet– Much variation, no obvious bound– We’ll revisit this with TCP (later)
![Page 12: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/12.jpg)
CSE 461 University of Washington 12
Duplicates• What happens if an ACK is lost?
X
Frame
ACKTimeout
Sender Receiver
![Page 13: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/13.jpg)
CSE 461 University of Washington 13
Duplicates (2)• What happens if an ACK is lost?
Frame
ACK
X
Frame
ACKTimeout
Sender Receiver
New Frame??
![Page 14: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/14.jpg)
CSE 461 University of Washington 14
Duplicates (3)• Or the timeout is early?
ACK
Frame
Timeout
Sender Receiver
![Page 15: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/15.jpg)
CSE 461 University of Washington 15
Duplicates (4)• Or the timeout is early?
Frame
ACK
Frame
ACK
Timeout
Sender Receiver
New Frame??
![Page 16: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/16.jpg)
CSE 461 University of Washington 16
Sequence Numbers• Frames and ACKs must both carry
sequence numbers for correctness
• To distinguish the current frame from the next one, a single bit (two numbers) is sufficient– Called Stop-and-Wait
![Page 17: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/17.jpg)
CSE 461 University of Washington 17
Stop-and-Wait• In the normal case:
Time
Sender Receiver
![Page 18: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/18.jpg)
CSE 461 University of Washington 18
Stop-and-Wait (2)• In the normal case:
Frame 0
ACK 0Timeout Time
Sender Receiver
Frame 1
ACK 1
![Page 19: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/19.jpg)
CSE 461 University of Washington 19
Stop-and-Wait (3)• With ACK loss:
X
Frame 0
ACK 0Timeout
Sender Receiver
![Page 20: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/20.jpg)
CSE 461 University of Washington 20
Stop-and-Wait (4)• With ACK loss:
Frame 0
ACK 0
X
Frame 0
ACK 0Timeout
Sender Receiver
It’s a Resend!
![Page 21: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/21.jpg)
CSE 461 University of Washington 21
Stop-and-Wait (5)• With early timeout:
ACK 0
Frame 0
Timeout
Sender Receiver
![Page 22: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/22.jpg)
CSE 461 University of Washington 22
Stop-and-Wait (6)• With early timeout:
Frame 0
ACK 0
Frame 0
ACK 0
Timeout
Sender Receiver
It’s aResend
OK …
![Page 23: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/23.jpg)
CSE 461 University of Washington 23
Limitation of Stop-and-Wait• It allows only a single frame to be
outstanding from the sender:– Good for LAN, not efficient for high BD
• Ex: R=1 Mbps, D = 50 ms– How many frames/sec? If R=10 Mbps?
![Page 24: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/24.jpg)
CSE 461 University of Washington 24
Sliding Window• Generalization of stop-and-wait– Allows W frames to be outstanding– Can send W frames per RTT (=2D)
– Various options for numbering frames/ACKs and handling loss• Will look at along with TCP (later)
![Page 25: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/25.jpg)
CSE 461 University of Washington 25
Topic• Multiplexing is the network word
for the sharing of a resource
• Classic scenario is sharing a link among different users– Time Division Multiplexing (TDM) »– Frequency Division Multiplexing
(FDM) »
![Page 26: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/26.jpg)
CSE 461 University of Washington 26
Time Division Multiplexing (TDM)
• Users take turns on a fixed schedule
2 2 2 2
![Page 27: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/27.jpg)
CSE 461 University of Washington 27
Frequency Division Multiplexing (FDM)• Put different users on different frequency bands
Overall FDM channel
![Page 28: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/28.jpg)
CSE 461 University of Washington 28
TDM versus FDM• In TDM a user sends at a high rate a
fraction of the time; in FDM, a user sends at a low rate all the time
Rate
TimeFDM
TDM
![Page 29: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/29.jpg)
CSE 461 University of Washington 29
TDM versus FDM (2)• In TDM a user sends at a high rate a
fraction of the time; in FDM, a user sends at a low rate all the time
Rate
TimeFDM
TDM
![Page 30: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/30.jpg)
CSE 461 University of Washington 30
TDM/FDM Usage• Statically divide a resource– Suited for continuous traffic, fixed
number of users
• Widely used in telecommunications– TV and radio stations (FDM)– GSM (2G cellular) allocates calls using
TDM within FDM
![Page 31: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/31.jpg)
CSE 461 University of Washington 31
Multiplexing Network Traffic• Network traffic is bursty
– ON/OFF sources – Load varies greatly over time
Rate
TimeRate
Time
![Page 32: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/32.jpg)
CSE 461 University of Washington 32
Multiplexing Network Traffic (2)• Network traffic is bursty– Inefficient to always allocate user
their ON needs with TDM/FDM
Rate
TimeRate
Time
R
R
![Page 33: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/33.jpg)
CSE 461 University of Washington 33
Multiplexing Network Traffic (3)• Multiple access schemes multiplex users according to
their demands – for gains of statistical multiplexing
Rate
TimeRate
Time
Rate
Time
R
R
R’<2R
Two users, each need R Together they need R’ < 2R
![Page 34: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/34.jpg)
CSE 461 University of Washington 34
Multiple Access• We will look at two kinds of multiple
access protocols1. Randomized. Nodes randomize their
resource access attempts– Good for low load situations
2. Contention-free. Nodes order their resource access attempts– Good for high load or guaranteed
quality of service situations
![Page 35: CSE 461 University of Washington1 Where we are in the Course Finishing off the Link Layer! – Builds on the physical layer to transfer frames over connected](https://reader035.vdocuments.net/reader035/viewer/2022070403/56649f2f5503460f94c49a9a/html5/thumbnails/35.jpg)
CSE 461 University of Washington 35
END
© 2013 D. Wetherall
Slide material from: TANENBAUM, ANDREW S.; WETHERALL, DAVID J., COMPUTER NETWORKS, 5th Edition, © 2011. Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey