wireless macs reprise overlay mac n.
Download
Skip this Video
Download Presentation
Wireless MACs (reprise): Overlay MAC

Loading in 2 Seconds...

play fullscreen
1 / 27

Wireless MACs (reprise): Overlay MAC - PowerPoint PPT Presentation


  • 81 Views
  • Uploaded on

Wireless MACs (reprise): Overlay MAC. Brad Karp UCL Computer Science. CS 4C38 / Z25 24 th January, 2006. Many competing schemes for MACs, even slotted ones! This paper: measure underlying problem; build real implementation; evaluate it. Context: 802.11 MAC and Forwarding. MACAW (1994)

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Wireless MACs (reprise): Overlay MAC' - cleatus-angel


Download Now An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
wireless macs reprise overlay mac

Wireless MACs (reprise): Overlay MAC

Brad Karp

UCL Computer Science

CS 4C38 / Z25

24th January, 2006

context 802 11 mac and forwarding

Many competing schemes for MACs, even slotted ones!

This paper: measure underlying problem; build real implementation; evaluate it.

Context: 802.11 MAC and Forwarding
  • MACAW (1994)
    • Communication range = interference range
    • No carrier sense
    • RTS/CTS for hidden terminal problem
  • 802.11b standard (mid 90s)
    • Designed chiefly with base stations in mind
    • Carrier sense and RTS/CTS
    • Interference range > communication range
  • Roofnet (2005)
    • Multi-hop forwarding using 802.11b
    • RTS/CTS disabled (no help to performance)
    • Collisions between forwarders in a chain
    • Highly asymmetric packet loss rates on many links
  • Overlay MAC (2005)
    • Study pathologies when 802.11a applied to multi-hop forwarding
    • Propose time-slotted “overlay” for 802.11a to alleviate problems
motivation 802 11 s shortcomings
Motivation: 802.11’s Shortcomings
  • Asymmetric interaction between nodes
    • at senders
    • at receivers
  • Rigid allocation of bandwidth among flows
    • no application choice of bandwidth allocation
    • poor fairness among flows for some traffic workloads
802 11a testbed
802.11a Testbed
  • Indoor, chain topology
  • No other 802.11 traffic in band
  • UDP broadcast packets
  • TCP
motivation asymmetric carrier sense at senders
Motivation:Asymmetric Carrier Sense at Senders
  • All 15 node pairs: greedy broadcast UDP
  • Far apart nodes:
    • ca. 5.1 Mbps
    • senders send simultaneously; don’t sense one another’s carriers
  • Close nodes:
    • ca. 2.5 Mbps each
    • senders share fairly; sense one another’s carriers
  • Three cases:
    • one sender >= 4.5 Mbps, other <= 800 Kbps
    • no RTS/CTS; no ACKs; no transport protocol
    • only explanation: one sender can’t sense other’s carrier
    • doesn’t depend on receiver
motivation asymmetric symmetric interaction at receivers
Motivation: Asymmetric / Symmetric Interaction at Receivers
  • Sender pairs who can broadcast at full rate, each sends greedy UDP unicast
  • Example 1:
    • 1  2 3  4
    • 85% packet drops from 1 to 2
    • sending rate drops > 60% from 1 to 2
  • Example 2:
    • 1  2  3
    • 35% packet drops for both 1 and 2
    • channel utilization: drops by 55%
motivation rigid bandwidth allocation
Motivation: Rigid Bandwidth Allocation
  • How do you divide capacity when senders use auto bit-rate selection?
    • 802.11 answer: equal number of transmit opportunities for senders…
    • …but each packet may be at different bit-rate
  • Heterogeneous sending rates:
    • 1  AP  2
    • 1 sends at 54 Mbps
    • 2 sends at {6, 12, 18, 36, 54} Mbps
  • Fair, but total utilization drops as node 2 slows!
  • Unpredictable:
    • Node 1 alone: 24 Mbps
    • Node 2 joins at 6 Mbps: Node 1 gets 3.6 Mbps
motivation forwarding and fairness
Motivation: Forwarding and Fairness
  • 802.11 doesn’t consider forwarding in b/w allocation
  • Interference range twice transmission range
    • N2 can’t receive during xmits of {N4, N5, N6}
    • N3 can’t receive during xmits of N1
  • 802.11’s bandwidth allocation
    • N1 and N3: 1/3 each of N2’s bw
    • N4, N5, N6: 1/9 each (equal share of 1/3)
  • Fairer would be
    • N3: 3/7
    • N1, N4, N5, N6: 1/7

N1

N2

N3

N4

N6

N5

motivation more unfairness
Motivation: (More) Unfairness
  • Two flows: 1  2 and 3  4
  • One at a time:

each 4.6 Mbps

  • Simultaneously:

one > 4 Mbps, one < 100 Kbps

  • Rate limiting both to 2.3 Mbps:

one 2.3 Mbps,one 580 Kbps

assumptions
Assumptions
  • Unicast and broadcast transmission supported
  • Promiscuous mode listening
  • RTS/CTS configurable “off”
  • Limit transmit queue to 1-2 packets
    • Why?
oml design overview
OML: Design Overview
  • Divide time into slots
    • All nodes agree on slot boundaries
    • Need loosely synchronized clocks
  • Mutually interfering nodes contend for same set of slots
    • Which nodes mutually interfere?
  • Each slot in set owned by one sender
  • Senders may have weights; bandwidth divided proportionally to weights
oml clock synchronization
OML: Clock Synchronization
  • Real hardware clocks don’t tick at promised rate
    • oscillators in PCs are typically off by 1 – 100 μs per s
    • 1 – 2 μs change per degree C!
    • skew: difference in frequency between two clocks
  • Many proposed algorithms for sync’ing distributed clocks in many settings
  • OML solution:
    • single leader node broadcasts timestamps
    • estimate propagation delay to receivers
    • receivers estimate their own skew; apply correction
    • goal: error must be much smaller than slot length
oml slot length
OML: Slot Length
  • Constraints
    • longer than clock error
    • longer than packet transmission time
    • otherwise, as short as possible
  • Value in evaluation
    • 5 max-sized (1500-byte) packets
    • 10 ms @ 6 Mbps
oml algorithm 1 diameter one unit weights
OML Algorithm 1:Diameter One, Unit Weights
  • Pseudo-random hash function
    • Output uniformly random in (0, 1]
    • Hi = H(ni, t), for c nodes, 1 ≤ i ≤ c
      • ni = node ID of node i (integer, unique per node)
      • t = time slot ID (increasing integer)
    • Assume all nodes who contend know one another’s ni
    • Each node can locally compute Hi for all its neighbors
  • Biggest Hi wins; winner is r, where:
oml algorithm 2 diameter one arbitrary weights
OML Algorithm 2:Diameter One, Arbitrary Weights
  • Suppose node i wants weight wi
  • Redefine Hi() in terms of wi:
  • Nodes must know wi of all nodes they contend with (within interference zone)
  • Winner r of slot is still node with greatest Hi in that slot
  • Proven in tech report:
oml algorithm 3 diameter 1
OML Algorithm 3:Diameter > 1
  • Only nodes that can interfere with one another must compete for slots
  • What set of nodes interfere with one node?
    • Radio ranges highly variable
    • No very satisfying, scalable answer!
  • Solution in paper: assume a fixed, k-hop interference zone
    • nodes broadcast for k hops intent to contend
    • greater k  assume more nodes mutually interfere
    • greater k  utilization may decrease
oml algorithm diameter 1 cont d
OML Algorithm:Diameter > 1 (cont’d)
  • Overlapping interference regions reduce utilization
  • Suppose H1 < H2 < H3
  • H1 and H2 will both think they’ve “lost,” but H1 and H3 don’t interfere!

1

2

3

oml algorithm slot groups
OML Algorithm:Slot Groups
  • Each slot owner relinquishes slot with probability (1-p) in each group
  • Nodes know locally when slot relinquished; use another pseudo-random hash function in (0, 1]
  • After slot relinquished, others in zone compete for it
  • Reduces chance of race in previous slide
evaluation simulation
Evaluation: Simulation
  • QualNet simulator
  • 802.11a, 6 Mbps fixed rate
  • Two-ray ground reflection model (350 m range)
  • RTS/CTS disabled
  • 50 nodes / km2, randomly placed
  • Slot time: 10 ms (5 1500-byte pkts)
  • Group size: N = 20 slots
  • k = 2
  • AODV routing
  • 1 simulated minute
metric fairness index
Metric: Fairness Index
  • M flows
  • weights w1, …, wM
  • Throughputs x1, …, xM
  • Fairness index, F:
  • F = 1 when all flows’ throughputs proportional to weights
  • F = 1/M when one flow starves all others
simulation packet transmissions
Simulation: Packet Transmissions
  • Workload: 10 UDP flows, different sources, one sink
  • OML successfully avoids simultaneous contending transmissions
  • OML is too conservative; delivers fewer packets successfully than 802.11
simulation average throughput
Simulation: Average Throughput
  • 5- and 10-flow workloads
  • Throughput comparable for OML vs. 802.11
simulation fairness
Simulation: Fairness
  • Nodes set weights to number of unique source IPs in output queue; unit weight per flow
  • Per-source-IP queues; round-robin among queues
  • N.B. fairness of 1 impossible; not all flows contend with all others
  • OML more fair than 802.11
simulation throughput and path length
Simulation: Throughput and Path Length
  • Narrower span of throughputs for OML than for 802.11
  • Improved fairness across varying path lengths, but less total capacity
testbed heterogeneous data rates
Testbed: Heterogeneous Data Rates
  • Two senders: one fixed at 54 Mbps, one varying from 6 to 54 Mbps
  • Same weights at both senders; equal channel access time at each sender
  • Proportional sharing
  • Increased total throughput vs. 802.11
testbed chain topology
Testbed: Chain Topology
  • 5-hop chain testbed
  • Two one-hop flows on random links
  • One flow at a time
  • Simultaneous, no OML
  • Simultaneous, OML, k={1, 2}
  • Improved fairness at cost of reduced throughput
testbed chain topology throughput fairness trade off
Testbed: Chain Topology,Throughput-Fairness Trade Off
  • “Oracle”: global knowledge of interference; “perfect” scheduling
  • OML approaches optimal fairness with k=2, at some throughput cost
  • 802.11 appears to favor throughput over fairness