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Page updated Thursday
14th 2003f August 2003
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  Wide area wireless networking - Air Stream

Date:August 8, 2003
Time:7:00 pm.
Place:WEA, 223 Angas Street, ADELAIDE
Presenter:Troy Vodopivec & James Day

Air-Stream Wireless Incorporated (a not-for-profit association), represented by Troy Vodopivec and James Day, presented a very informative meeting regarding the development of an innovative metropolitan wide community wireless computer network. This is achieved using the current 2.4GHz Wireless LAN (WLAN) technology.

One of their goals is to encourage development of smaller sub-networks between close neighbouring nodes, ultimately interconnected to build a meshed network with redundant links. Each node comprises 5-6 home users arranged within a 7.5 km radius around a central Access Point, each with their antenna (e.g a modified ex Galaxy TV antenna) aimed toward the centrally located omni-directional antenna (looks like a 50 mm PVC tube approx 600 mm long). Each user requires Coax cable (up to a maximum of about 10 metres) from their antenna to their "radio" card. Either a PMCIA card with an external aerial socket (or it is modified to provide one) or a similar PCI card version is used (replace the attached removable antenna with your cable and external antenna). Support is available within the group to adapt other suitable parabolic microwave antennas to suit this application.

A single hop range of up to 14 kms has so far been achieved, with a total distance of 50-60 kms by linking Access Points together.

An Access Point (AP) comprises an omni-directional antenna coupled to a Unix box to provide a central hub to its local users as well as providing 1 or 2 directional links to other Access Points. There is a maximum limit of 11 MBits/Sec total traffic at an AP. Only one action can occur at a time at an AP - i.e. receive a data packet before retransmitting that packet to its local destination (or next AP). Up to 5 or 6 Nodes per AP are the limit, providing a typical data rate of 600 k/sec.

Spread spectrum radio technology is used (max of 14 channels) with typically channels 1 + 6 + 10 being utilised without interference as the channels actually overlap. No license is required (2.4 to 2.48 GHz). The PCMCIA card produces typically 30 mWatts and costs about $50. With a suitable external antenna providing 12 dB of gain this equates to about 400 mW EIRP (equivalent isotropically radiated power). With a high gain antenna, such as a parabolic, you can achieve outputs up the the maximum permissable of 4 W EIRP.

A LinkSys device was used as the first AP in Adelaide, but it was mostly for experimentation and is no longer being used. Once the site's performance had stabilized and the group had discovered what hardware was needed to create the desired network, the LinkSys AP was replaced by a *nix box permanently. The LinkSys Wireless unit produces 100 mWatts of power at the base unit (this is the same unit that is normally applied in a wireless office situation to support several computer terminals). Where possible it is HIGHLY recommend to use a *nix PC to control functions for an AP or client, as the hardware devices are less configurable and more prone to failure than a PC interface card.

The IP Address range administered within the group uses the South Australian IP space of 10.96.0.0/11. This IP space under the current allocation scheme equates to:

  • 250 Wireless Access Points
  • 30,000 Wireless Nodes
  • 1,800,000 Fully routed LAN IPs
  • 250 Backbone links

Increasing the distance from your PC to the antenna (unless you want to mount your PC in the roof space) may be achieved by the use of a USB (limited) or CAT5 cable and appropriate wireless adapter at the external antenna.

Security can be improved by the use of Wired Equivalent Privacy (WEP) encryption (up to 128 bit) or for serious security, by implementing a Virtual Private Network (VPN) eg IPSec over the WLAN.

Interest in this project attracted 80 people to their open day 12 months ago with another open day expected soon. The group relies on sponsors and member funding to provide hardware for the backbone links between network segments. Membership is not compulsory but any person connected to the Air-Stream Network must become a member of the association. Many others volunteer to become members before they are connected to provide input into the expansion of the network. Community support is required to provide Access Points on suitable high locations such as a friends house on the hills face.

Airstream have meetings on the third Wednesday of the month, all are welcome to attend.

Visit their website at www.air-stream.org for additional information on this groups activities.

Thanks very much to Colin Huckel for the write-up on the presentation and to Rick Matthews for the photographs.

Andrew Braund