Fast Handoff in 802'11a WiFi Networks

1
Fast Handoff in 802.11a WiFi Networks

• Honours Proposal
• David Murray
• Supervisors Mike Dixon and Terry Koziniec

2
Handoff in WiFi Networks

• WiFi networks are short range 50 100m
• Large deployments need many Access Points (APs)
• WiFi Networks have a cell based topology just
  like regular mobile phones
• Handoff is the connectivity transfer that occurs
  when moving between cells

50 - 100m
3
Why do we need fast handoff

• WiFi Networks were designed for nomadic
  operation.
• Wireless networks no longer solely service
  laptops and computers
• VoWiFi phones
• This research in fast handoff is a subset of
  VoWiFi
• VoIP (Voice over IP)
• WiFi (Wireless Networks)

4
VoIP (Voice over IP)

• VoIP is the use of the IP (data) network for
  voice traffic
• Cost savings arise from only requiring one
  network.

5
WiFi (802.11)

• Wireless networks allow mobile connectivity
• Tremendous growth
• WLANs are now found in homes, cafes, businesses
  and universities

6
VoWiFi

• VoWiFi is the transmission of voice over a WiFi
  network
• Enterprise market
• Telstra
• Home dual band cell/WiFi market
• Future

7
VoWiFi Issues Handoff

• Handoff occurs when a client moves between
  wireless cells
• Difficult and time consuming process and results
  in loss of connectivity
• The longer the handoff process, bigger the
  disruption to the conversation
• Research has shown that handoff is possible in
  802.11b networks
• It is believed that handoff in 802.11a networks
  will be 8/3 or 2.66 times longer

8
802.11 Physical Layer Standards

• 802.11b
• DSSS Modulation 11Mbps data rate
• 2.4 GHz band 3 non-overlapping channels
• 802.11g
• OFDM modulation 54mbps data rate
• 2.4 GHz band 3 non-overlapping channels
• Backwards compatible with 802.11b
• 802.11a
• OFDM modulation 54mbps data rate
• 5 GHz band 8 non-overlapping channels
• Not compatible with 802.11b/g

9
802.11a A better medium for voice

• Interference causes packet loss (a disaster for
  voice calls)
• 2.4 GHz spectrum current and future problems
• Bluetooth
• Cordless phones
• Microwaves
• 802.11n Channel bonding
• Power consumption
• OFDM reduces power
• 802.11a devices typically use less power
• 802.11a 40mw
• 802.11b 100mw
• If 802.11a can support low latency (sub 50 ms)
  handoffs it is a prime candidate for VoWiFi

10
Handoff Explained

• Detection
• Scanning
• Decision

11
Handoff Explained Detection

• The detection phase is about as advanced as your
  car radio
• Physical limitations of wireless (radio) networks
• A client card can only be on one channel at any
  one time.
• Clients are unaware of surrounding APs until
  they begin scanning
• The client will wait until the signal drops below
  a predefined threshold before scanning for better
  APs

12
Handoff Explained Scanning

• When a client has detected the need to start
  scanning they will scan every channel.
• Remember 802.11b/g has 3 non-overlapping
  channels. 802.11a has 8 non-overlapping channels
• This has obvious implications on scanning latency
  in 802.11a and 802.11b/g networks
• The IEEE define two scanning mechanisms
• Passive scanning
• Active Scanning
• Depending on the mechanisms used the scanning
  phase can take up to 1.1 seconds

13
Handoff Explained Decision

• The decision phase compares the strongest signal
  found in the scanning phase with the signal
  received from the old AP
• Signal strengths are not static
• To prevent clients from ping ponging between
  APs the new AP must offer a considerably better
  signal strength

14
Experiment 1 Empirically Test Scanning in
802.11a Networks

• This experiment involves moving a 802.11a client
  between two APs and sniffing the traffic
• This is harder than it looks!
• The difficulty is in sniffing management frames,
  which requires specific hardware software and
  Linux knowledge
• By sniffing the management frames, it will be
  possible to determine total handoff time and the
  intricacies (if any) of 802.11a scanning

15
Passive Scanning

• Passive scanning is the slowest scanning
  mechanism
• Passive scanning relies on beacons which are
  broadcasted by default every 100 ms
• Every wireless device using passive scanning must
  passively scan for 100ms on every channel.
• Passive scanning in 802.11b/g 300 ms
• Passive scanning in 802.11a 800 ms
• Why would anyone use passive scanning when there
  are faster mechanisms available? Answer power
  consumption

16
Improvements to Passive Scanning

• By lowering the beacon interval it is possible to
  reduce passive scanning delays
• A beacon interval of 50 ms results in
• Passive scanning in 802.11b/g 150ms
• Passive scanning in 802.11a 400 ms
• While this is a good way to reduce scanning
  delays, it suffers from scalability issues.
• Halving the beacon interval has the effect of
  halving the scanning delay but doubling the
  bandwidth used by

17
Experiment 2 Passive Scanning in 802.11a networks

• Scaling the beacon interval will be more
  efficient in 802.11a of OFDM networks
• DSSS (802.11b) management frames _at_ 1 2 mbps
• OFDM (802.11a/g) management frames _at_ 6 24 mbps
• We should see efficiencies in the order of 12x
• Despite the fact that we have 2.66 more channels
  to scan, at what cost can we support fast (sub
  50ms) handoffs

18
Active Scanning

• Active scanning involves aggressively probing
  channels for APs
• There are two timers that dictate how long a
  client will scan each channel
• Min Channel Time
• Max Channel Time
• Lowering these timers will lower the time spend
  scanning each channel
• Lowering these timers is a double edged sword. If
  timers are set to low clients will miss APs

19
Experiment 3 Improving Active Scanning

• By optimising min and max channel times it is
  possible to lower active scanning times
• The changing of these timers is beyond what can
  be done in the labs
• I will use OPNET modeller to model the impact of
  changing scanning times

20
Conclusion

• I propose to research the scanning phase of
  802.11a handoff to support voice traffic
• Three experiments
• An empirical study of 802.11a handoff
• Scaling beacon intervals to reduce passive
  scanning times
• Optimising min and max channel timers to improve
  active scanning

21
Questions?