%deffont "standard" tfont "times.ttf" %deffont "thick" tfont "thick.ttf" %deffont "typewriter" tfont "courier.ttf" %default 1 leftfill, size 2, fore "black", font "thick", bimage "slide-bgwide.jpg" %default 2 size 7, vgap 40, fore "black", prefix " " %default 3 size 2, bar "gray70" 10 12 88, vgap 10, fore "black" %default 4 size 5, fore "white", font "standard", fore "black", prefix " " %tab 1 size 5, vgap 40, prefix " ", icon box "green" 50 %tab 2 size 4, vgap 40, prefix " ", icon arc "yellow" 50 %tab 3 size 3, vgap 40, prefix " ", icon delta3 "black" 40 %page %center Wireless networking with Linux and IEEE 802.11b %vgap 50 David Gibson OzLabs, IBM Linux Technology Center %font "typewriter" %size 4 %prefix " " david@gibson.dropbear.id.au dgibson@au1.ibm.com %pcache 1 1 0 0 %page Overview Wireless LANs IEEE 802.11 features IEEE 802.11 architecture Current 802.11 devices Current Linux support Future work %page Wireless LANs "Ethernet-like" LANs without physical cabling Use radio or infrared signals Many use the 2.4GHz "Industrial-Scientific-Medical" (ISM) radio band Maximally absorbed by water Short range Noisy - used by microwave ovens Limited commercial value No license required (at sufficiently low power) %page Issues in wireless networking Unreliable medium Random errors orders of magnitude more frequent than with wired networks Hidden transmitters Stations might not be able to receive transmissions from all other stations Walls or other obstructions complicate the situation Mobile stations Stations can access the LAN while moving from place to place Environment changes can cause radio effects indistinguishable from moving stations No fixed extent Range of radio is difficult to determine and varies with environment Can't restrict access by physical connectivity %page Hidden transmitter %newimage -zoom 95 hiddentx.eps Station B can see (i.e. receive from and transmit to) both other stations But A can't see C and C can't see A %page IEEE 802.11 Wireless NICs have been available for a while e.g. "WaveLAN" ISA cards Not very common Different brands weren't interoperable 802.11 is a standard for wireless LANs 802.11b is a revision allowing higher bitrates (to 11Mb/s) It has helped wireless LANs become a lot more popular Other wireless standards HomeRF - similar in concept, but hasn't caught on as much Bluetooth - aimed mostly at shorter range, lower power applications HiperLAN 2 - 5GHz band, more telephony oriented (e.g. ATM-like MAC) %page How standard is a standard? Multiple (non-interoperable) PHYs Frequency Hopping (FH) in the 2.4GHz band (1,2Mb/s) Direct Sequence Spread Spectrum (DSSS) in the 2.4GHz band (1,2Mb/s, 5.5,11Mb/s with 802.11b) Infrared (1,2Mb/s) OFDM in the 5GHz band (802.11a) 2.4GHz DSSS at 11Mb/s is the most prominent currently Optional protocol features Enough is specified for interoperability But a device might not support a particular feature Distribution System isn't specified %page IEEE 802.11 features Part of the IEEE 802 family of LAN standards So lots of similarities to 802.3/Ethernet But also quite a few differences to handle vagaries of wireless MAC level ACKs and retransmits and fragmentation (unreliable medium) Frames are acknowledged and can be retransmitted if there is an error Frames can be broken into smaller fragments More likely to be transmitted without error Fragments can be individually retransmitted Allows the medium to appear reliable at the data link layer CSMA/CA (hidden transmitter) Can't reliably detect collisions as on Ethernet (CSMA/CD) Use back-offs at strategic points to reduce the chance of collisions %page IEEE 802.11 features (cont.) RTS/CTS - virtual carrier sense (hidden transmitter) Stations can exchange short Ready-To-Send/Clear-To-Send before sending data Reserves the wireless medium - other stations will act as if they could physically sense a carrier A station only needs to see the RTS or the CTS, not both, to reserve the medium Access points / infrastructure networking (hidden transmitter & mobility) Stations can relay transmissions via one or more Access Points (APs) MAC level encryption (no fixed extent) RC4 encryption of each frame Known as "Wired Equivalent Privacy" (WEP), supposed to provide similar security to a wired network Algorithm is flawed and has been broken more esoteric features: PCF, etc... %page IEEE 802.11 (cont.) 802.11 MAC protocol introduces a lot of overhead 11Mb/s 802.11 is significantly slower in practice than 10Mb/s Ethernet The 802.11 MAC header is over twice as long as 802.3 %newimage -zoom 75 frame-format.eps %page Architecture of an 802.11 network Several different possible configurations allowed Ad-hoc networks Just a set of stations, nothing else required Infrastructure networks One or more Wireless Access Points (APs) Co-ordinate and facilitate %page Ad-hoc network (IBSS) Stations communicate directly with no central co-ordination Problems are likely if there are hidden transmitters %cont %newimage -zoom 85 ibss.eps %page Simple infrastructure network All transmissions relayed via Access Point (AP) AP and associated stations are called a Basic Service Set (BSS) %cont %newimage -zoom 95 bss.eps %page Extended infrastructure network Multiple APs connected by a Distribution System (DS) %cont %newimage -zoom 55 ess.eps %page Infrastructure networks (cont.) Collection of all BSSes connected by the DS called the Extended Service Set (ESS) BSSes can by physically separate To extend coverage of the network Or they can overlap To provide redundancy if an AP fails Stations can move freely between the BSSes No loss of connectivity No change of address Standard doesn't specify the DS Can be wired or wireless Most often runs over an Ethernet wired network %page Current 802.11b devices Nearly all are 16-bit PCMCIA cards Many, many vendors, but most are just rebadges "PCI" NICs are usually PCMCIA cards bundled with an adaptor Wireless AP appliances also common - usually a PCMCIA card plus controller and firmware Some include a modem and can act as a NATing firewall and DHCP server etc. Dominated by Intersil's Prism 2 chipset (and related) Several firmwares: Intersil, Lucent/Agere/ORiNOCO, Symbol, Aironet/Cisco %page Linux Support Most devices are supported Nearly cards are Prism 2, but different firmwares make supporting them all non-trivial Support is not entirely mature and consolidated Many drivers, which overlap in coverage a lot Some features (AP mode, monitor mode) not supported by all drivers or on all hardware No 802.11 support at anything higher than driver level (each driver encapsulates packets so they appear as Ethernet to the kernel) Development slow because hardware and firmware specs are not easily available Intersil does give out specs under NDA (which allows open source drivers) Lucent/Agere release very little information %page Driver coverage %cont %newimage driver-support.eps %page orinoco driver Supports most Intersil, Agere and Symbol based cards Supports PCMCIA cards, PLX based PCI cards and Apple Airport %center %newimage "driver-structure.eps" %page Further work Consolidation of drivers %cont %font "typewriter" wvlan_cs %cont %font "standard" now deprecated Plans to merge %cont %font "typewriter" orinoco %cont %font "standard" and %cont %font "typewriter" prism2 %cont %font "standard" drivers. Protocol support - %cont %font "typewriter" net/80211 %font "standard" A standard set of functions for use in 802.11 drivers (as there are for Ethernet) Easier to write drivers for new hardware Proper support for software AP mode, 802 bridging etc. Common configuration Because the 802.11 protocol has so many features there are many configuration options Many drivers can't set them all, and they don't set them all the same way Jean Tourrilhes' Wireless Extensions are a partial solution Don't always map perfectly to the 802.11 model Uses %cont %font "typewriter" ioctl() %cont %font "standard" s which are always nasty %page The End %center %size 4 %prefix " " %font "standard" IEEE 802 standards %font "typewriter" http://standards.ieee.org/getieee802/ %font "standard" Wireless HOWTO %font "typewriter" http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux %font "standard" orinoco driver %font "typewriter" http://www.ozlabs.org/people/dgibson/dldwd %font "standard" linux-wlan project %font "typewriter" http://www.linux-wlan.net