Is It Quiz Time

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Is It Quiz Time??

• What benefit do you get from replicating the root
  node in a Distributed Hash Table (DHT)?
• Answers due by 539PM

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CS176C Spring 2006Wireless MAC Protocols

• Administrivia / reminders
• Next Tues no class
• Project proposals due next Tues (5/9) midnight
• Email to krishnap_at_cs (cc me)
• HW 2 due next Sunday (5/14) midnight
• Email to krishnap_at_cs
• Today
• Media Access Control (MAC) protocols
• Tues no class
• Thur Proactive Wireless Routing (DSDV)

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MAC Protocols

• Media Access Control (MAC) Protocols
• Wireless networks use a shared medium air
• Protocol to determine who speaks, and when
• How is this handled in wired networks?
• Ethernet
• Multiple Access ProtocolCSMA/CD
• The acronyms
• CS carrier sense (listen before you talk)
• MA multiple access (multiple speakers)
• CD collision detection (detect instead of avoid)

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Ethernet

• A senses channel, if idle
• Transmit and monitor channel
• If detect another transmission (hear ! sent)
• Abort and send jam signal
• Update of collisions
• Delay according to exponential backoff algorithm
• Start again at beginning (channel sense)
• Else done with frame, set collisions to 0
• Elsewait until transmission over, then repeat

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CSMA/CD Details

• Jam signal
• Make sure all transmitters aware of collision
  (48bits)
• Exponential backoff
• Adapt retransmission attempts to current load
• Heavier load ? longer, random wait
• First collision choose K from 0,1delay is K
  x 512 bit transmission times
• Second collision choose K from 0,1,2,3
• After jth collision choose K from 0, ..., 2j
  1
• After 10 or more collisions, choose K
  from0,1,2,3,4,, 1023

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Why Not Collision Detection for Wireless?

• Most radios are half-duplex
• Send or receive, not both simultaneously
• Near-far problem
• Transmission so loud at sender it drowns out
  others
• Fundamental issues
• Sender cannot hear the same thing as the listener
• Hidden Terminal problem
• If medium is free near the sender, it may not be
  free near the receiver
• Exposed Terminal problem
• If medium is busy near sender, it may be free
  near the receiver

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Hidden Terminal Problem
Senders A and C separated by obstacle. Each
thinks the medium is free.
Senders A and C out of range of each other.
Each thinks medium is free.
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Exposed Terminal Problem
Range of B
Range of C
sending
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Types of MAC Protocols

• MAC protocol design specific to application
• Fixed assignment approaches
• Allocate medium to a fixed number of transceivers
• Central server (AP) makes allocation assignment
• Great for cellular networks
• Examples TDMA, FDMA, CDMA
• Random assignment approaches
• Dynamic number of transceivers vie for medium
• Distributed (peer-to-peer) algorithms for
  contention
• Great for dynamic / unplanned or distributed
  networks
• Examples Aloha, CSMA, MACA

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Fixed Assignment MACs

• Given a static number of users
• Can schedule each user evenly across available
  spectrum
• Advantage
• Efficiently assign spectrum evenly to users
• Fair
• guaranteed slice of spectrum
• Minimal interference and conflicts
• Disadvantages
• Requires single (central) entity to perform
  assignment
• Does not adapt well to changing network
• Assumes equal use of B/W by each transceiver
• No contention
• Scales to large of users (hence ideal for
  cellular)

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FDMA and TDMA

• FDMA Frequency division multiple access
• Divide frequency into channels
• Each user assigned to specific channel
• Guard band between channels to avoid interference
• Not scalable to large of users (very old
  cellular systems)
• TDMA Time division multiple access
• All users on same channel or frequency
• Divide time into many small slots
• Each user assigned a sequence of timeslots (or
  fight for slots)
• Requirement time synchronization between
  user??AP
• Basis for GSM cellular networks (Cingular,
  T-Mobile etc)

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Cellular Networks

• Downlink (provider ? phone) is easy
• AP sends, appropriate phone listens
• Uplink (phone ? network)is harder
• Need to coordinatebetween users
• Large of users
• Contention ? bad
• Assignment ? good
• Roaming
• Overprovision for expectedroaming traffic

Wireless Cell
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Problem with Fixed Assignment

• Assignment for given set of users
• Can be recomputed over time by central entity
• Assumes uniform traffic model across users
• Every user has same slice of bandwidth
• What about data networks such as Wireless LAN?
• of users changes quickly over time
• Each users bandwidth requirement varies over
  time
• No central entity to compute assignment
• Need distributed approach to media access

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Basic Random Assignment (Aloha)

• Most of these assume single frequency, time
  division
• Designed by Abramson in 1970 for linking various
  Hawaiian Islands
• Protocol
• Transmit whenever you want
• Receiver acknowledges message
• If no ack, resend after random wait
• Low throughput peak performance is only 18
• Variant slotted Aloha
• Senders synchronized in time
• Time divided into slots
• Can only transmit at beginning of time slot
• Maximum throughput about 36

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Improving Throughput

• CSMA carrier sense multiple access
• Listen to channel first, transmit if not busy
• Improves over Aloha
• Problem in wireless networks
• Sender cannot hear what receiver hears
• Hidden terminals
• Collisions despite CS
• Exposed terminals
• Reduced throughput

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Fixing Carrier Sense w/ MACA

• Multiple Access with Collision Avoidance
• Replaces carrier sense with collision avoidance
• CSMA MA/CA ? MACA
• Eliminating carrier sense removes previous
  problems
• Hidden terminal, exposed terminal
• Add collision avoidance RTS/CTS signaling
  between transmitting a packet
• Small control packets announce your intention to
  send
• Neighbors hear, reduces collisions

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Collision Avoidance

• Replaces carrier sense with explicit signaling
• For unicast transmissions only
• Request to send (RTS) packet
• Is it ok to send to receiver?
• Clear to send (CTS) packet
• Receiver says OK to send
• RTS/CTS include length of upcoming packet
• Neighbors know how long to get out of the way
• Carrier sense ? sender-driven
• Collision avoidance ? receiver-driven

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Hidden Terminal Avoidance

• B sends RTS to C
• Neighbors hear RTS
• C responds with CTS
• Neighbors hear and back off

Range of C
Range of B
RTS
CTS
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CA as Virtual Carrier Sense

• B and C use RTS/CTS to reserve medium
• A overhears RTS, estimates backoff time
• D overhears CTS, estimates backoff time

A overhears RTS, knows to stay away
A
RTS
Data Frame
B
CTS
C
D
D overhears CTS, stays away
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Collisions Can Still Occur

• No carrier sense, so RTS messages can collide
• RTS messages sent random time after last
  transmission
• If collision, each sender performs random binary
  exponential backoff
• Any collisions are on short RTS messages, cheaper
  than full message collision

Range of C
Range of B
RTS
RTS