Packet Loss Characterization in WiFibased Long Distance Networks

1
Packet Loss Characterization in WiFi-based Long
Distance Networks

• Authors Anmol Sheth, Sergiu Nedevschi,
• Rabin Patra, Lakshminarayanan Subramanian
  
• INFOCOM 2007
• Reporter ???

2
Motivation

• Do some measurements for understanding how WiLD
  (WiFi-based Long Distance) networks perform in
  practice
• Analyze the loss variability across time
• Explore the solution and propose some methods to
  mitigate loss

3
Outline

• Methodology
• Loss Variability analysis
• Remedies
• Conclusion
• Novelty Strength
• Weakness

4
Methodology

• Measurements on a WiLD network testbed comprising
  of links in both rural and urban environment.
• Use wireless channel emulator (Spirent 5500) to
  study each source of packet loss in isolation.

5
Methodology (cont.)

• Testbed setup
• 802.11 a/b/g
• CBR UDP traffic streams
• Turn off MAC-layer ACKs and set the maximum
  retries limit to zero
• Modify Atheros madwifi driver to pass up
  frames with CRC and PHY errors

6
Methodology (cont.)

• Channel losses
• External WiFi interference
• External non-WiFi interference
• Multipath interference
• 802.11 protocol-induced losses
• Timeouts due to propagation delay
• Breakdown of CSMA over long distances

7
Channel losses

• External WiFi interference
• External non-WiFi interference
• Multipath interference

8
External WiFi interference

• Any WiFi traffic that is not a part of the
  primary WiLD link is categorized as external WiFi
  interference

experiment
emulation
9
External WiFi interference

• Effect of hidden terminals in WiLD networks

10
External WiFi interference

• Effect of relative power and rate of external
  interference

Emulation result
11
Channel losses

• External WiFi interference
• External non-WiFi interference
• Multipath interference

12
External non-WiFi interference

• Other devices that share the 2.4 GHz band.
• microwave
• cordless phone
• Wide-band noise
• Result no significant correlation between noise
  and loss rate.

13
Channel losses

• External WiFi interference
• External non-WiFi interference
• Multipath interference

14
Multipath interference

• Comparing to mesh network deployment, there are
  two factors contributing to lower delay spreads
  in the WiLD networks.
• long distance between two end hosts
• line-of-sight deployment of the nodes

Delays between a primary and secondary
reflection at midway and quarter-way point.
15
802.11 protocol-induced losses

• Link layer recovery mechanism
• Breakdown of CSMA

16
Link layer recovery mechanism

• The 802.11 MAC uses a simple stop-and-wait
  protocol, when MAC ACKs are enabled, the sender
  has to wait for an ACK after each transmission,
  and this leads to decreasing channel utilization
  with increasing link distance.

Emulation result
17
Breakdown of CSMA

• On longer distance links, it is possible that the
  two nodes will begin transmission within the
  window defined by the propagation delay.
• The throughput of the WiLD link degrades as the
  distance is increased.

18
Main Result
19
Loss Variability

• Burst-Residual Separation
• Burst
• time-periods with sharp spikes in the loss rate
• Residual
• the losses that constantly occur in the
  underlying channel over time.

0
1
P1
P2
Loss Variability distribution
Burst
Residual
20
Burst characteristic

• Short burst
• majority of the bursts to be short bursts that
  last for less than 0.3s
• Long burst
• a single long burst is followed by a string of
  other long bursts separated by short
  time- periods (in the order of a few seconds).

21
Residual characteristic

• The residual loss distribution is stationary over
  hourly time scales
• The residual loss rate on any link remains
  roughly constant over a few minutes even in the
  presence in short bursts during such periods.

22
Remedies

• Frequency Channel Adaption
• Rate Adaptation
• Adaptive Forward Error Correction

23
Frequency Channel Adaption

• The advantages of channel switching could be
  significant in presence of long or high-loss
  bursts

Simulation result
24
Rate Adaptation

• The increased transmission time of the frame
  increases the probability of a collision with the
  external traffic.

Simulation result
25
Adaptive Forward Error Correction

• At the end of each time slot the receiver informs
  the transmitter of the loss observed in the
  previous slot. Based on this link information,
  the transmitter adjusts the redundancy for the
  next round.

Simulation result
26
Conclusion

• Most of the losses arise due to external WiFi
  interference on same and adjacent channels.
• The loss due to external WiFi interference
  depends on the relative power level between the
  primary and external traffic, their channel
  separation, and the rate of external
  interference.

27
Novelty Strength

• The loss rate is strongly related to the amount
  of external traffic received on the same and
  adjacent channels in contrast to the
  omni-directional antennas used in the mesh
  networks deployment.
• From the emulation traces they observed that
  almost 100 of the lost frames contained CRC
  errors.

28
Weakness

• Switching the frequency channel could mitigate
  interference however, it is not always possible
  to switch a frequency channel in a large scale
  deployment.
• Most of the 802.11 radios have built in
  rate-adaptation algorithms which selects a lower
  rate with feasible encoding on experiencing high
  loss.

29
Thank you!