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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
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Context on Reliability• Where in the stack should we
place reliability functions?
PhysicalLink
NetworkTransportApplication
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Context on Reliability (2)• Everywhere! It is a key issue– Different layers contribute differently
PhysicalLink
NetworkTransportApplication
Recover actions(correctness)
Mask errors(performance optimization)
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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
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ARQ (2)• Normal operation (no loss)
Frame
ACKTimeout Time
Sender Receiver
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ARQ (3)• Loss and retransmission
Frame
Timeout Time
Sender Receiver
Frame
ACK
X
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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
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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)
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Duplicates• What happens if an ACK is lost?
X
Frame
ACKTimeout
Sender Receiver
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Duplicates (2)• What happens if an ACK is lost?
Frame
ACK
X
Frame
ACKTimeout
Sender Receiver
New Frame??
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Duplicates (3)• Or the timeout is early?
ACK
Frame
Timeout
Sender Receiver
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Duplicates (4)• Or the timeout is early?
Frame
ACK
Frame
ACK
Timeout
Sender Receiver
New Frame??
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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
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Stop-and-Wait• In the normal case:
Time
Sender Receiver
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Stop-and-Wait (2)• In the normal case:
Frame 0
ACK 0Timeout Time
Sender Receiver
Frame 1
ACK 1
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Stop-and-Wait (3)• With ACK loss:
X
Frame 0
ACK 0Timeout
Sender Receiver
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Stop-and-Wait (4)• With ACK loss:
Frame 0
ACK 0
X
Frame 0
ACK 0Timeout
Sender Receiver
It’s a Resend!
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Stop-and-Wait (5)• With early timeout:
ACK 0
Frame 0
Timeout
Sender Receiver
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Stop-and-Wait (6)• With early timeout:
Frame 0
ACK 0
Frame 0
ACK 0
Timeout
Sender Receiver
It’s aResend
OK …
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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?
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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)
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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) »
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Time Division Multiplexing (TDM)
• Users take turns on a fixed schedule
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2 2 2 2
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Frequency Division Multiplexing (FDM)• Put different users on different frequency bands
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Overall FDM channel
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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
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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
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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
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Multiplexing Network Traffic• Network traffic is bursty
– ON/OFF sources – Load varies greatly over time
Rate
TimeRate
Time
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Multiplexing Network Traffic (2)• Network traffic is bursty– Inefficient to always allocate user
their ON needs with TDM/FDM
Rate
TimeRate
Time
R
R
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Multiplexing Network Traffic (3)• Multiple access schemes multiplex users according to
their demands – for gains of statistical multiplexing
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Rate
TimeRate
Time
Rate
Time
R
R
R’<2R
Two users, each need R Together they need R’ < 2R
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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
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Topic• How do nodes share a single link?
Who sends when, e.g., in WiFI?– Explore with a simple model
• Assume no-one is in charge; this is a distributed system
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Topic (2)• We will explore random multiple
access control (MAC) protocols– This is the basis for classic Ethernet– Remember: data traffic is bursty
Zzzz..Busy! Ho hum
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ALOHA Network• Seminal computer network
connecting the Hawaiian islands in the late 1960s– When should nodes send?– A new protocol was devised
by Norm Abramson …
Hawaii
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ALOHA Protocol• Simple idea:– Node just sends when it has traffic. – If there was a collision (no ACK
received) then wait a random time and resend
• That’s it!
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ALOHA Protocol (2)• Some frames will
be lost, but many may get through…
• Good idea?
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ALOHA Protocol (3)• Simple, decentralized protocol that
works well under low load!
• Not efficient under high load– Analysis shows at most 18% efficiency– Improvement: divide time into slots
and efficiency goes up to 36%
• We’ll look at other improvements
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Classic Ethernet • ALOHA inspired Bob Metcalfe to
invent Ethernet for LANs in 1973– Nodes share 10 Mbps coaxial cable– Hugely popular in 1980s, 1990s
: © 2009 IEEE
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CSMA (Carrier Sense Multiple Access)
• Improve ALOHA by listening for activity before we send (Doh!)– Can do easily with wires, not wireless
• So does this eliminate collisions?– Why or why not?
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CSMA (2)• Still possible to listen and hear
nothing when another node is sending because of delay
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CSMA (3)• CSMA is a good defense against
collisions only when BD is small
X
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CSMA/CD (with Collision Detection)• Can reduce the cost of collisions by
detecting them and aborting (Jam) the rest of the frame time– Again, we can do this with wires
X X X X X X X XJam! Jam!
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CSMA/CD Complications• Want everyone who collides to
know that it happened– Time window in which a node may
hear of a collision is 2D seconds
X
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CSMA/CD Complications (2)• Impose a minimum frame size that
lasts for 2D seconds– So node can’t finish before collision– Ethernet minimum frame is 64 bytes
X
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CSMA “Persistence”• What should a node do if another
node is sending?
• Idea: Wait until it is done, and send
What now?
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CSMA “Persistence” (2)• Problem is that multiple waiting
nodes will queue up then collide– More load, more of a problem
Now! Now!Uh oh
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CSMA “Persistence” (3)• Intuition for a better solution– If there are N queued senders, we
want each to send next with probability 1/N
Send p=½WhewSend p=½
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Binary Exponential Backoff (BEB)• Cleverly estimates the probability
– 1st collision, wait 0 or 1 frame times– 2nd collision, wait from 0 to 3 times– 3rd collision, wait from 0 to 7 times …
• BEB doubles interval for each successive collision– Quickly gets large enough to work– Very efficient in practice
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Classic Ethernet, or IEEE 802.3• Most popular LAN of the 1980s, 1990s– 10 Mbps over shared coaxial cable, with baseband signals– Multiple access with “1-persistent CSMA/CD with BEB”
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Ethernet Frame Format• Has addresses to identify the sender and receiver• CRC-32 for error detection; no ACKs or retransmission• Start of frame identified with physical layer preamble
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Packet from Network layer (IP)
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Modern Ethernet• Based on switches, not multiple
access, but still called Ethernet– We’ll get to it in a later segment
Switch
Twisted pairSwitch ports
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Topic• How do wireless nodes share a
single link? (Yes, this is WiFi!)– Build on our simple, wired model
Send? Send?
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Wireless Complications• Wireless is more complicated than
the wired case (Surprise!)1. Nodes may have different areas of
coverage – doesn’t fit Carrier Sense »2. Nodes can’t hear while sending –
can’t Collision Detect »
≠ CSMA/CD
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Different Coverage Areas• Wireless signal is broadcast and
received nearby, where there is sufficient SNR
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Hidden Terminals• Nodes A and C are hidden terminals when sending to B– Can’t hear each other (to coordinate) yet collide at B– We want to avoid the inefficiency of collisions
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Exposed Terminals• B and C are exposed terminals when sending to A and D– Can hear each other yet don’t collide at receivers A and D– We want to send concurrently to increase performance
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Nodes Can’t Hear While Sending• With wires, detecting collisions
(and aborting) lowers their cost• More wasted time with wireless
Time XXXXXXXXX
XXXXXXXXX
WirelessCollision
ResendX
X
WiredCollision
Resend
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Possible Solution: MACA• MACA uses a short handshake instead of CSMA (Karn, 1990)
– 802.11 uses a refinement of MACA (later)
• Protocol rules:1. A sender node transmits a RTS (Request-To-Send, with frame length)2. The receiver replies with a CTS (Clear-To-Send, with frame length)3. Sender transmits the frame while nodes hearing the CTS stay silent– Collisions on the RTS/CTS are still possible, but less likely
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MACA – Hidden Terminals
• AàB with hidden terminal C1. A sends RTS, to B
DCBA
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MACA – Hidden Terminals (2)
• AàB with hidden terminal C2. B sends CTS, to A, and C too
DCBARTS
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MACA – Hidden Terminals (3)
• AàB with hidden terminal C2. B sends CTS, to A, and C too
DCBARTS
CTSCTS
Alert!
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MACA – Hidden Terminals (4)
• AàB with hidden terminal C3. A sends frame while C defers
FrameQuiet...
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MACA – Exposed Terminals• BàA, CàD as exposed terminals– B and C send RTS to A and D
DCBA
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MACA – Exposed Terminals (2)• BàA, CàD as exposed terminals– A and D send CTS to B and C
DCBARTSRTS
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MACA – Exposed Terminals (3)• BàA, CàD as exposed terminals– A and D send CTS to B and C
DCBARTSRTS
CTSCTS
All OKAll OK
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MACA – Exposed Terminals (4)• BàA, CàD as exposed terminals– A and D send CTS to B and C
DCBAFrameFrame
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802.11, or WiFi• Very popular wireless LAN
started in the 1990s• Clients get connectivity from a
(wired) AP (Access Point)• It’s a multi-access problem J• Various flavors have been
developed over time– Faster, more features
AccessPoint
Client
To Network
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802.11 Physical Layer• Uses 20/40 MHz channels on ISM bands
– 802.11b/g/n on 2.4 GHz– 802.11 a/n on 5 GHz
• OFDM modulation (except legacy 802.11b)– Different amplitudes/phases for varying SNRs– Rates from 6 to 54 Mbps plus error correction– 802.11n uses multiple antennas; see “802.11
with Multiple Antennas for Dummies”
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802.11 Link Layer• Multiple access uses CSMA/CA (next); RTS/CTS optional • Frames are ACKed and retransmitted with ARQ• Funky addressing (three addresses!) due to AP• Errors are detected with a 32-bit CRC• Many, many features (e.g., encryption, power save)
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Packet from Network layer (IP)
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802.11 CSMA/CA for Multiple Access• Sender avoids collisions by inserting small random gaps– E.g., when both B and C send, C picks a smaller gap, goes first
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Time
Send?
Send?