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Ad%20Hoc%20Routing%20Metrics

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Title: Ad%20Hoc%20Routing%20Metrics


1
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
5
Understanding min-hop metricPerformance
6
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

7
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)

8
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

9
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.

10
Evaluation ETX performance
11
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

12
Comparison of Routing Metrics for Static
Multi-Hop Wireless Networks
  • Richard Draves, Jitendra Padhye and Brian Zill

13
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

14
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)

15
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

16
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
17
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

18
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

19
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

20
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

21
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

22
Wireless Testbed
23
LQSR Overhead Link Variability
24
Impact of TCP flows (one at a time)
  • ETX performs better by avoiding low-throughput
    paths
  • RTT suffers heavily from self-interference

25
Impact on Path Length
  • Path Length is generally higher under ETX

26
Throughput Vs Path Length
PktPair suffers from self-interference only on
multi-hop paths
27
Experimental results for mobile wireless networks
  • Shortest path routing is best in mobile scenarios
  • Why?

28
ExOR Opportunistic Multi-Hop Routing For
Wireless Networks
  • Sanjit Biswas and Robert Morris

29
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.

30
Why ExOR promises high throughput? - 1
31
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

32
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

33
Refresher
N7
N8
F
F
F
N1
N2
N5
S
F
N4
D
Batch
N3
N6
F
1st round
2nd round
3rd round
34
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.

35
Evaluation - 1
36
Evaluation - 2
37
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?

38
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

39
A Rate-Adaptive MAC Protocol for Multi-Hop
Wireless Networks
  • By Gavin Holland, Nitin Vaidya and Paramvir Bahl

40
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.

41
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)
42
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
43
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
44
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).

45
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.

46
Slow fading Channel
47
Fast Fading Channel
48
Variable Traffic Source
49
Multi-Hop Performance
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