1 Topic How do nodes share a single link Who sends when eg in WiFI Explore with a simple model Assume noone is in charge this is a distributed system CSE 461 University of Washington ID: 798755
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CSE 461 University of Washington
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Topic
How do nodes share a single link? Who sends when, e.g., in WiFI?Explore with a simple modelAssume no-one is in charge; this is a distributed system
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Topic (2)
We will explore random multiple access control (MAC) protocolsThis is the basis for classic EthernetRemember: data traffic is bursty
Zzzz..
Busy!
Ho hum
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ALOHA Network
Seminal computer network connecting the Hawaiian islands in the late 1960sWhen 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 resendThat’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 loadAnalysis shows at most 18% efficiencyImprovement: 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 1973Nodes share 10 Mbps coaxial cableHugely 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 wirelessSo 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 timeAgain, we can do this with wires
X
X
X
X
X
X
X
X
Jam!
Jam!
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CSMA/CD Complications
Want everyone who collides to know that it happenedTime 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 secondsSo node can’t finish before collisionEthernet 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 collideMore load, more of a problem
Now!
Now!
Uh oh
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CSMA “Persistence” (3)
Intuition for a better solutionIf there are N queued senders, we want each to send next with probability 1/N
Send p=½
Whew
Send p=½
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Binary Exponential Backoff (BEB)
Cleverly estimates the probability1st collision, wait 0 or 1 frame times2nd collision, wait from 0 to 3 times3rd collision, wait from 0 to 7 times …BEB doubles interval for each successive collision
Quickly gets large enough to workVery efficient in practice
Slide18Classic Ethernet, or IEEE 802.3
Most popular LAN of the 1980s, 1990s
10 Mbps over shared coaxial cable, with baseband signalsMultiple access with “1-persistent CSMA/CD with BEB”
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Modern Ethernet
Based on switches, not multiple access, but still called EthernetWe’ll get to it in a later segment
Switch
Twisted pair
Switch 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!)Nodes may have different areas of coverage – doesn’t fit Carrier Sense »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
Slide23Hidden Terminals
Nodes A and C are hidden terminals
when sending to BCan’t hear each other (to coordinate) yet collide at B
We want to avoid the inefficiency of collisionsCSE 461 University of Washington23
Slide24Exposed Terminals
B and C are exposed terminals
when sending to A and DCan hear each other yet don’t collide at receivers A and D
We want to send concurrently to increase performanceCSE 461 University of Washington24
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Nodes Can’t Hear While Sending
With wires, detecting collisions (and aborting) lowers their costMore wasted time with wireless
Time
XXXXXXXXX
XXXXXXXXX
Wireless
Collision
Resend
X
X
Wired
Collision
Resend
Slide26Possible Solution: MACA
MACA uses a short handshake instead of CSMA (Karn
, 1990)802.11 uses a refinement of MACA (later)
Protocol rules:A sender node transmits a RTS (Request-To-Send, with frame length)The receiver replies with a CTS (Clear-To-Send, with frame length)Sender transmits the frame while nodes hearing the CTS stay silentCollisions on the RTS/CTS are still possible, but less likely
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MACA – Hidden Terminals
AB with hidden terminal CA sends RTS, to B
D
C
B
A
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MACA – Hidden Terminals (2)
AB with hidden terminal CB sends CTS, to A, and C too
D
C
B
A
RTS
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MACA – Hidden Terminals (3)
AB with hidden terminal CB sends CTS, to A, and C too
D
C
B
A
RTS
C
TS
C
TS
Alert!
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MACA – Hidden Terminals (4)
AB with hidden terminal CA sends frame while C defers
Frame
Quiet...
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MACA – Exposed Terminals
BA, CD as exposed terminalsB and C send RTS to A and D
D
C
B
A
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MACA – Exposed Terminals (2)
BA, CD as exposed terminalsA and D send CTS to B and C
D
C
B
A
RTS
RTS
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MACA – Exposed Terminals (3)
BA, CD as exposed terminalsA and D send CTS to B and C
D
C
B
A
RTS
RTS
C
TS
C
TS
All OK
All OK
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MACA – Exposed Terminals (4)
BA, CD as exposed terminalsA and D send CTS to B and C
D
C
B
A
Frame
Frame
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802.11, or WiFi
Very popular wireless LAN started in the 1990sClients get connectivity from a (wired) AP (Access Point)It’s a multi-access problem Various flavors have been developed over time
Faster, more features
Access
PointClient
To Network
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802.11 Physical Layer
Uses 20/40 MHz channels on ISM bands802.11b/g/n on 2.4 GHz802.11 a/n on 5 GHzOFDM modulation (except legacy 802.11b)Different amplitudes/phases for varying SNRs
Rates from 6 to 54 Mbps plus error correction802.11n uses multiple antennas; see “802.11 with Multiple Antennas for Dummies”
Slide37802.11 Link Layer
Multiple access uses CSMA/CA (next); RTS/CTS optional
Frames are
ACKed and retransmitted with ARQFunky addressing (three addresses!) due to APErrors are detected with a 32-bit CRCMany, many features (e.g., encryption, power save)
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Packet from Network layer (IP)
Slide38802.11 CSMA/CA for Multiple Access
Sender avoids collisions by inserting small random gapsE.g., when both B and C send, C picks a smaller gap, goes first
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Time
Send?
Send?
Slide39The Future of 802.11 (Guess)
Likely ubiquitous for Internet connectivityGreater diversity, from low- to high-end devices
Innovation in physical layer drives speedAnd power-efficient operation too
More seamless integration of connectivityToo manual now, and limited (e.g., device-to-device)CSE 461 University of Washington
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Topic
A new approach to multiple accessBased on turns, not randomization
1
3
2
4
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Issues with Random Multiple Access
CSMA is good under low load:Grants immediate accessLittle overhead (few collisions)But not so good under high load:High overhead (expect collisions)Access time varies (lucky/unlucky)
We want to do better under load!
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Turn-Taking Multiple Access Protocols
They define an order in which nodes get a chance to sendOr pass, if no traffic at presentWe just need some ordering …E.g., Token Ring
»E.g., node addresses
Slide43Token Ring
Arrange nodes in a ring; token rotates “permission to send” to each node in turn
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Node
Direction of
transmissionToken
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Turn-Taking Advantages
Fixed overhead with no collisionsMore efficient under loadRegular chance to send with no unlucky nodesPredictable service, easily extended to guaranteed quality of service
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Turn-Taking Disadvantages
ComplexityMore things that can go wrong than random access protocols!E.g., what if the token is lost?Higher overhead at low load
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Turn-Taking in Practice
Regularly tried as an improvement offering better serviceE.g., qualities of serviceBut random multiple access is hard to beatSimple, and usually good enough
Scales from few to many nodes