Using a Unix computer as a 802.11 wireless base station

If you have an Internet connection, and wish to access it using 802.11 wireless LAN cards, then one way to do this is to purchase a specialized 802.11 "Access Point" - e.g., Apple's "AirPort" - and connect this to your Internet connection. Alternatively, you can use a Unix computer as a base station. Using an open system for your base station gives you more flexibility in how you manage the network - for instance, you can also use the Unix system for specialized access control, statistics gathering, DNS, web caching, etc. etc. (Besides, if you already have a Unix system, then it's cheaper to use this, rather than buying a separate, dedicated Access Point.)

This page summarizes my experience in getting a general-purpose FreeBSD and Linux computer to act as a base station for a 802.11 wireless LAN. This should also be possible for other Unix systems (e.g., OpenBSD, Solaris). (It might also be possible with non-Unix systems such as Windows NT and Windows 2000, as they, too, can act as a router.)

(Another, slightly different approach is to take special-purpose 802.11 Access Point hardware, and port a open operating system to it. One recent example of this is "OpenAP", which is a port of Linux to access points based on the Eumitcom WL11000SA-N board.)

I assume here that Proxim (formerly Agere (formerly Lucent)) "Orinoco" (aka. "WaveLAN") PCMCIA cards (Silver or Gold) are used for the 802.11 interface. It's probably possible to use other brands of 802.11 interface instead, although the instructions for these (most notably, the Unix kernel device names) will be slightly different. In any case, I recommend that, whenever possible, wireless clients should use the same type of 802.11 interface as the base station. Although different brands of 802.11 interface are supposed to be interoperable, this is often not the case in practice.

The basics

There are four possible ways to connect the wireless LAN card to your computer:

1. Directly, using a PCMCIA slot
2. Using a ISA<->PCMCIA adaptor, with the card plugged into this
3. Using a PCI<->PCMCIA adaptor, with the card plugged into this
4. (only as a last resort) Using an Orinoco Ethernet converter, with the card plugged into this

If, however, your system doesn't have a PCMCIA interface, you can instead use an Orinoco ISA or PCI Adaptor (option 2 or 3). These are ISA or PCI cards - each currently costing about $US67 - with an integrated PCMCIA slot into which you insert your wireless LAN card. The kernel will then treat the wireless LAN card as if it were in a directly-attached PCMCIA interface. If your system has both a free ISA slot and a free PCI slot, then I strongly recommend that you use the ISA adaptor, rather than the PCI adaptor. Most people have had better results using the ISA adaptor.

• If you're running FreeBSD, you should be running at least version 4.4-STABLE to use the PCI adaptor. You may also have to add special commands to your "/boot/loader.conf" file; see the FreeBSD release notes for details. (To use the ISA interface, you should be running at least version 3.5-RELEASE, although I recommend at least 4.1-STABLE.)
• Alberto Escudero reports that the ISA adaptor works OK with Linux. Eberhard Wildermuth reports that the PCI adaptor also works with Linux, but that a patch must be applied to make it work.

We may not be able to support "BSS mode"

There's no reason in principle why a Unix base station cannot support BSS mode. The problem, though, is that this requires having (read and write) access to the low-level 802.11 data-link layer packet headers, and the magic incantation needed to do this appears to be a closely guarded secret - at least, for the Orinoco card.

Notes

An alternative: "IBSS mode"

The main disadvantage of IBSS mode is that many of 802.11's "power-saving" features cannot be used. (In BSS mode, the access point can keep track of when clients are asleep in low-power mode, and buffer packets for later delivery.)

• Another mode, similar to IBSS, is "ad hoc demo" mode. This, however, is non-standard, is not supported by recent versions of drivers for Windows clients, and may not be supported by future versions of drivers for Apple AirPort clients. Therefore, it should not be used. (The terminology here can be rather confusing, because some 802.11 vendors, e.g., Aironet, refer to IBSS mode as "ad hoc".)

Configuring the base station to run in IBSS mode

• If you also plan to use encryption (WEP), your base station and clients' firmware version should be at least 6.16; earlier versions did not implement encryption correctly in IBSS mode.
• Christophe Kalt reports that he needed to upgrade to firmware version 7.28 in order to get NetBSD to work. (Firmware version 6.06 did not work reliably for him.)

If you need to upgrade your cards' firmware, you can find software to do this on Proxim's web site. (This software will probably need to be run on a Windows computer.)

The base station will also need to be configured with:

1. A network name (just as with BSS mode)
2. A special "allow IBSS" bit that needs to be turned on in the network interface

Instructions for FreeBSD

/usr/sbin/wicontrol -i wi0 -n <network_name>
/usr/sbin/wicontrol -i wi0 -p 1
/usr/sbin/wicontrol -i wi0 -c 1
/usr/sbin/wicontrol -i wi0 -s <station_name>

Instructions for Linux

1. Download and unpack the latest beta version of "pcmcia-cs version 3.1.22" from here.
2. Run "./Configure; make; make install"

Next, if you don't already have the "iwconfig" application, you'll need to download and install it. (This is one of Jean Tourrilhes' "Wireless Tools".)

Finally, configure your wireless network interface with the appropriate parameters for this network:

    iwconfig wvlan0 essid <network_name> mode Ad-Hoc nick <station_name>
    /etc/pcmcia/network start wvlan0

    case "$ADDRESS" in
    # Lucent Wavelan IEEE, wvlan_cs driver
    # Settings for an IBSS network:
    *,*,*,00:60:1D:*|*,*,*,00:02:2D:*)
        ESSID="<network_name>"
        MODE="Ad-Hoc"
        ;;
    esac

Configuring wireless clients to access the network

To configure a FreeBSD or Linux client to access the network, follow the same instructions given above for configuring a base station. (If you're running Linux on a PowerPC Macintosh, see Paul Lucas's page for additional information.)

To configure a Windows client to access the network, follow the usual instructions (in particular, entering the correct network name). Depending on which client configuration/driver software you're using, you may also need to select "peer-to-peer networking" mode. (Do not select "ad hoc demo" mode.)

To configure a MacOS client to access the network, use Apple's "AirPort" driver, available from http://www.apple.com/airport/. (Make sure you're using the latest version.) Note that you should use the AirPort software even if you're using an Orinoco card plugged into a PCMCIA slot. (The AirPort card is really just an Orinoco card without an antenna.) To connect to the network, you may also need to select "computer-to-computer" mode.

Acknowledgements