Ad%20Hoc%20Routing%20Metrics

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Ad Hoc Routing Metrics

• 15-849 E -- Wireless Networks
• 02/27/2006
• Kaushik Sheth
• Jatin Shah

2
A High-Throughput Path Metric for Multi-Hop
Wireless Routing(ETX)

• Douglas S. J. De Couto, Daniel Aguayo, John
  Bicket, Robert Morris

3
Minimum Hop Count

• Assumes links either work or dont work
• Minimize hop count -gt Maximize the distance
  traveled by each hop
• Minimizes signal strength -gt Maximizes the loss
  ratio
• Uses a higher Tx power -gt Interference
• Arbitrarily chooses among same length paths

4
Understanding min-hop metricTestbed
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Understanding min-hop metricPerformance
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Is there a better metric?

• Cut-off threshold
• Disconnected network
• Product of link delivery ratio along path
• Does not account for inter-hop interference
• Bottleneck link (highest-loss-ratio link)
• Same as above
• End-to-end delay
• Depends on interface queue lengths

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ETX metricDesign goals

• Find high throughput paths
• Account for lossy links
• Account for asymmetric links
• Account for inter-link interference
• Independent of network load (dont incorporate
  congestion)

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ETX metricDefinition

• ETX predicted of data tx required to
  successfully send a packet over link/path
  including retransmissions
• ETX (link) 1 / df x dr
• ETX (path) ? ETX(link)
• ETX (link) measured by broadcasting periodic
  probe packets
• Reverse-delivery ratio piggybacked in forward
  probe packets

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ETX caveats

• ETX estimates are based on measurements of a
  single link probe size (134 bytes) i.e. Probe
  size ? Data/Ack size
• Under-estimates data loss ratios, over-estimates
  ACK loss ratios
• ETX assumes all links run at one bit-rate
• Broadcast has lower priority.
• ETX assumes that radios have a fixed transmit
  power level.

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Evaluation ETX performance
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Take aways

• Pros
• ETX performs better or comparable to Hop Count
  Metric
• Accounts for bi-directional loss rates
• Can easily be incorporated into routing protocols
  as detailed experiments on a real test bed show
  it
• Cons
• May not be best metric for all networks
• Mobility, Power-limited, Adaptive Rate
  (multi-rate)
• Predications of loss ratios not always accurate
  as seen in experiments sometimes.
• Experiments (30 sec transfer of small packets)
  may not complement real-world scenarios

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Comparison of Routing Metrics for Static
Multi-Hop Wireless Networks

• Richard Draves, Jitendra Padhye and Brian Zill

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Routing in Multi-hop Wireless Networks

• Mobile Networks
• Minimum-hop routing (shortest path)
• DSR, AODV, TORA (covered previously)
• Static Networks
• HOP based routing chooses short but lossy
  wireless links thereby reducing throughput
• Taking more hops on better quality links can
  improve throughput

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Contribution of the paper

• Design and Implementation of a routing protocol
  based on notion of link quality
• LQSR (Link Quality Source Routing)
• Experimental comparison of three link quality
  metrics
• Per-hop Round Trip Time (RTT)
• Per-hop Packet Pair Delay (PktPair)
• Expected Transmission (ETX)

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Summary of Results

• ETX Provides best performance for static wireless
  network
• Performance of RTT and PktPair suffer due to
  self-interference
• HOP performs well over ETX in mobile wireless
  networks

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LQSR Architecture

• Implemented in a shim layer between Layer 2 and
  3.
• The shim layer acts as a virtual Ethernet adapter
• Virtual Ethernet addresses
• Multiplexes heterogeneous physical links
• Advantages
• Supports multiple link technologies
• Supports IPv4, IPv6 etc unmodified
• Preserves the link abstraction
• Can support any routing protocol

• Architecture
• Header Format

Ethernet
MCL
Payload TCP/IP, ARP, IPv6
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LQSR

• Source Routed, link state protocol
• Derived from DSR
• Each node measures quality of its link to its
  neighbor
• The info regarding link quality propagates
  through the mesh
• Source selects route with best cumulative metric
• Packets are source-routed using this route

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Link Quality Metrics

• Per-hop Round Trip Time (RTT)
• Routing based on minimizing total RTT
• Per-hop Packet Pair Delay (PktPair)
• Routing based on minimizing PktPair
• Expected Transmission (ETX)
• Routing based on maximizing ETX
• Minimum hop routing (HOP)
• Routing based on minimizing HOP

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Metric 1 Per-hop RTT

• Advantages
• Easy to implement
• Accounts for link load and bandwidth
• Also accounts for link loss rate
• 802.11 retransmits lost packets up to 7 times
• Lossy links will have higher RTT
• Disadvantages
• Expensive
• Self-interference due to queuing

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Metric 2 Per-hop Packet-Pair

• Advantages
• Self-interference due to queuing is not a problem
• Implicitly takes load, bandwidth and loss rate
  into account
• Disadvantages
• More expensive than RTT

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Metric 3 Expected Transmissions

• Advantages
• Low overhead
• Explicitly takes loss rate into account
• Disadvantages
• Loss rate of broadcast probe packets is not the
  same as loss rate of data packets
• Probe packets are smaller than data packets
• Broadcast packets are sent at lower data rate
• Does not take data rate or link load into account

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Wireless Testbed
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LQSR Overhead Link Variability
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Impact of TCP flows (one at a time)

• ETX performs better by avoiding low-throughput
  paths
• RTT suffers heavily from self-interference

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Impact on Path Length

• Path Length is generally higher under ETX

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Throughput Vs Path Length
PktPair suffers from self-interference only on
multi-hop paths
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Experimental results for mobile wireless networks

• Shortest path routing is best in mobile scenarios
• Why?

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ExOR Opportunistic Multi-Hop Routing For
Wireless Networks

• Sanjit Biswas and Robert Morris

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Contributions

• This paper contributes the first complete design
  and implementation of a link/network-layer
  diversity routing technique that uses standard
  radio hardware.
• It demonstrates a substantial throughput
  improvement and provides insight into the sources
  of that improvement.

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Why ExOR promises high throughput? - 1
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Why ExOR promises high throughput? - 2
N5
N1
N3
N7
N6
N2
N4
N8
S
D
Traditional Path

• Gradual falloff of probability with distance
  (80, 40, 20..)
• Lucky longer path can reduce transmission count
• Shorter path ensures some forward progress

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Design Challenges

• The nodes must agree on which subset of them
  received each packet Protocol ?
• A metric to measure the probable cost of moving
  packet from any node to destination
• Choosing most useful participants
• Avoid simultaneous transmission to minimize
  collisions

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Refresher
N7
N8
F
F
F
N1
N2
N5
S
F
N4
D
Batch
N3
N6
F
1st round
2nd round
3rd round
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Evaluation Setup

• 65 node pairs from a physical layout of 38
  Roofnet nodes participated
• No ExOR Traditional routing, hence the ExOR run
  was asked to transfer 10 more.
• One hop at a time for fair comparison in
  traditional routing.

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Evaluation - 1
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Evaluation - 2
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Take aways

• Pros
• ExOR achieves 2x to 4x throughput improvement for
  more distant pairs
• ExOR implemented on Roofnet and evaluated in
  detail
• Exploits radio properties, instead of hiding them
• Does not require changes in the MAC layer
• Cons
• Not scalable to large network as traditional
  routing
• Overhead in packet header (batch info)
• Batches affect the TCP performance
• What if not enough packets to make the batch?

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Extra related work

• Opportunistic Channel Protocols
• Use channel reservation to avoid collisions
• Cons require channel stability, use signal
  strength to predict reception, does not use
  intermediate nodes to relay
• Opportunistic Forwarding
• Select forwarding nodes based on channel
  conditions
• Cons use channel measurements or distance to
  predict the delivery success rate
• Multiple Path Routing
• Maintain multiple routes to use as alternative
  routes or split the traffic among them
• Cons Ensure the paths are disjoint, need to
  identify specific paths in advance
• Cooperative Diversity Routing
• Exploit nearby nodes which overhear the
  transmission
• Cons duplicate transmissions

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A Rate-Adaptive MAC Protocol for Multi-Hop
Wireless Networks

• By Gavin Holland, Nitin Vaidya and Paramvir Bahl

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Introduction

• Rate Adaption
• Rate adaption is the process of dynamically
  switching data rates to match the channel
  conditions. There are two aspects to rate
  adaption
• Channel quality estimation
• By Sender
• By receiver-gt RBAR(Receiver Based Auto rate)
• Rate Selection
• By Sender -gtARF(Auto rate Fallback)
• By Receiver -gt RBAR(Receiver Based Auto rate)
• Why receiver based rate adaption
• The goal of rate adaption is to provide optimum
  throughput.
• Rate selection can be improved by proving more
  timely and more complete channel quality.
• Channel quality information is best acquired at
  the receiver.

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RBAR modified DCF Protocol

• DCF To coordinate the transfer of data packet.
• NAV To announce the duration of packet.

DRSH Final reservation Time
DCTS Reservation time
DRTS Reservation time (IEEE 802.11)
DRTS Tentative reservation time (RBAR)
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RBAR EVENT FLOW

• S choose a data rate r1, using some heuristic,
  and sends r1 and the size of the data packet n in
  the RTS to R.
• A, overhearing the RTS, uses r1 and n to
  calculate the duration of the reservation,
  marking it as tentative.
• R, having received the RTS, uses some channel
  quality estimation and rate selection technique
  to select the best rate r2 for the channel
  conditions, and sends r2 and n in the CTS to S.
• B, overhearing the CTS, calculates the
  reservation using r2 and n.
• S responds to the CTS by placing r2 into the
  header of the data packet and transmitting the
  packet at the selected rate. If r1?r2, S uses a
  unique header signaling the rate change.
• A, overhearing the data packet, looks for the
  unique header. If it exists, it recalculates the
  reservation to replace the tentative reservation
  it calculated earlier.

S
R
B
A
r1, n
r1, n
r2, n
r2, n
r2, n
r2, n
ACK
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RBAR MAC Header

Framl control
Duration
Dest. Address
Source Address
BSSID
Sequnce control
Body
FCS
IEEE 802.11 MAC Header
Framl control
Duration
Dest. Address
Source Address
BSSID
Sequnce control
HCS
Body
FCS
RBAR Reservation SubHeader
RBAR MAC Header
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RBAR RTS/CTS Implementation
Frame control
Duration
Dest. Address
Source Address
FCS
Rate Length
IEEE 802.11 RTS
RBAR RTS
Frame control
Duration
Dest. Address
FCS
Rate Length
IEEE 802.11 CTS
RBAR CTS

• In RBAR, instead of carrying the duration of the
  reservation , the packets carry the modulation
  rate and the size of the data packet.
• If there is rate mismatch between sender and
  receiver DRTS refer to as tentative reservation.
• Final reservations are confirmed by the presence
  or absence of Reservation SubHeader (RSH).

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RBAR PLCP Header
Sync
SFD
Signal
Service
Length
CRC
Data Rate
RSH Rate
RBAR PLCP header
802.11 PLCP header

• In standard 802.11, the PLCP header contains an
  8 bit signal field.
• In RBAR, the PLCP header has been divided into
  two 4 bit rate subfields.
• Thus, the PLCP transmission protocol is modified
  as follows when the MAC passes a packet down to
  the physical layer, it specifies two rates, one
  for the subheader and one for the remainder of
  the packet.

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Slow fading Channel
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Fast Fading Channel
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Variable Traffic Source
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Multi-Hop Performance