Wireless in the real world

Gabriel WeiszOctober 8, 2010

Architecture and Evaluation of an Unplanned 802.11b Mesh Network (Roofnet)

• The idea: Share internet access via an unplanned mesh network, using omnidirectional roof mounted antennas and multi-hop routing to ease deployment

• Distributed devices for volunteers to install at home, consisting of a PC with an 802.11b card and an antenna to install on the roof– The 802.11b card used supports a non-standard “Pseudo-IBSS” ad-hoc mode to

avoid IBSS partitioning issues• Nodes attempt to auto configure

– If the user has an internet connection, the node shares – If the user doesn’t, the node is a DHCP server and provides internet access

• Nodes generate their own address for use on the Roofnet network, and perform NAT for their clients

• Users behind nodes cannot communicate with each other

Roofnet Routing• Tries to find highest throughput route between any

pair of nodes• Source based routing with link-state and DSR-style

on demand querying• Estimated transmission time metric• Bit rate adjusts automatically as packets sent,

based upon actual data transmissions• 627 kbit/second inter-node performance• 1395 kbit/second performance from internet

gateways, with average latency of 22 ms

Roofnet Questions and Issues• Why 802.11b? 802.11G was widely available by then.• Why PC’s? The Linksys WRT54G was widely available by

then, and the open source firmware had been available for several years

• Shouldn’t Roofnet network be isolated from user’s network for users sharing their internet connection?

• It seems like the different wiring configuration for clients providing internet connectivity could be confusing. Why not just connect the DSL or cable modem to the device? Would resolve this and the previous issue.

• Do modern devices support “Pseudo-IBSS?”• How does this scale to hundreds of nodes?

White Space Networking with Wi-Fi like Connectivity (WhiteFi)

• The idea: The FCC decided to allow unused UHF bandwidth to be used for unlicensed communication. Create devices for Wi-Fi like data transmission

• Lots of bandwidth exists here. Good spatial propagation characteristics may make this a good mechanism for city-wide deployments

• Issues are primarily due to the fact that white space devices must not interfere with incumbents (people who’ve paid for the spectrum). These users include TV stations and microphones– Spectrum is fragmented, and availability varies widely in space and time

• Solution: support variable channel widths, continuously probe for channel availability, and have a backup channel to communicate changes in availability

More on the WhiteFi Solution• Real Hardware:

– A PC with a standard wireless card– A UHF translator to downconvert the output of the wireless card to

UHF bands and limit the bandwidth– Universal Software Radio Peripheral (USRP) Software Defined Radio

(SDR) used to sample the UHF spectrum and detect other signals• Software:

– Channel selection by evaluating Multichannel Airtime Metric (Mcham)

– Signal Inspection before Fourier Transform (SIFT) used by clients to search spectrum in time domain rather than frequency domain in order to speed up AP discovery.

– Switch to backup channel if a primary user discovered, and send chirps about available whitespaces

WhiteFi Results• Sift has a high packet detection rate across

different channel sizes• Sift can detect AP’s with higher capture ratios

farther away• AP Discovery time goes down from 7.5 seconds

to 2.5 seconds in rural areas, and from 2.5 seconds to 1.8 seconds in urban areas. Would this actually matter to end users?

• Spatial variation mechanism makes good use of available bandwidth

WhiteFi Questions• How does the system behave with multiple

AP’s?• Do we need to avoid interfering with

microphones that may be working illegally?• Do chirps interfere with primary users (who

are legal)?• Should other MAC protocols (such as CDMA)

be considered?• How can the security issues be addressed?

Self-Management in Chaotic Wireless Deployments

• Many wireless deployments are unplanned (due to individuals or small groups implementing them) and unmanaged (due to management difficulty)

• A large number of access points were using channel 6 (40-odd %). Some were using 2 or 11, and almost nobody was using anything else– Other channels not really independent– Why not 1 instead of 2?

• Performance at endpoints can suffer significantly in dense environments

• Automated power control and rate adaptation algorithms can reduce interference and ensure robust end-client performance