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About Ad Hoc Networks

Published September 16th 2008

You may have recently received an email regarding Wireless Ad Hoc networks. This documents provides more background information about them. If you just want to get on with the job of removing and disabling them from your computer's configuration, go straight to Ad Hoc Networks - Disabling.

What is an Ad Hoc Network?

Ad Hoc networks are PC-to-PC wireless networks. This is different to the wireless connection such as provided by ANU-Access. ANU-Access used dedicated wireless access points (WAPs) to send and receive data over the wireless medium. This is called Infrastructure-mode wireless as network connections are made via the network infrastructure, i.e. the WAPs.

In Ad Hoc-mode wireless, there is no dedicated WAP. A PC or laptop acts as both a WAP and a client, and can send and receive wireless data from other Ad Hoc clients directly.

How do you tell an Ad Hoc network from a 'proper' Infrastructure network?

This screenshot shows the difference in Windows XP. The Ad Hoc network has an icon showing two PCs sharing wireless data, whereas the Infrastructure network shows an antenna.

How Do Ad Hoc Networks work?

Ad Hoc networks appear to work just the same as other wireless networks. Your computer detects the Ad Hoc network, you double-click on the icon for that network and you will be prompted for a login if that network requires authentication.

The difference is that you are really connecting to another person's computer.

In Windows the computer advertising the Ad Hoc network needs to have another active connection, such as a wired ethernet connection to the internet, as well as Internet Connection Sharing (ICS) enabled for you to be able to send and receive traffic over the Ad Hoc network. In most instances this is not the case and the Ad Hoc connection will not allow you to use the internet - even though it seems you have connected.

What is the issue with Ad Hoc Networks?

There are several issues; we'll give a brief overview of each.

Issue 1 - You don't know it's happening

Many people who have a Windows computer running an Ad Hoc network are unaware that it is doing so. This can cause a problem for both you, other clients, and other legitimate wireless networks in the area. Here is an example:

Say you have double-clicked on an Ad Hoc network icon at one time or another. Maybe your were in an airport and clicked on "Free Public WiFi" just to see if you could connect or maybe you thought you were connecting to ANU-Access but it had a different icon. The next time your computer is in range of that network, your computer will attempt to reconnect to the Ad Hoc network - even if it did not work properly the first time (your computer will only do this if there is not a more preferred wireless network available at the time).

Now, here is the problem: Say you now move to a different location with no wireless - your computer will now automatically start advertising itself as an available Ad Hoc network for "Free Public WiFi" - or whatever the name of the network you clicked on. Now you have other random clients trying to connect to you! As you move about, your computer advertises the Ad Hoc network whenever you are not connected to another wireless network.

Issue 2 - It spreads to others

Guess what? Now those clients who tried to connect to you roam somewhere else, and their PCs start advertising an Ad Hoc network, which is picked up by others, and so on and so on. In this way some Ad Hoc networks, such as the ubiquitous "Free Public WiFi" have spread and continue to spread around the world.

Issue 3 - It could be a trap

Waiting in an airport or some other place, it can be hard to resist the temptation for free internet access! But before you do connect to an unknown wireless network consider that you are offering that device (be it a Ad Hoc or an Infrastructure network) a view of everything you are doing. This could include snooping user-names and passwords without your consent. Even a mail client like Outlook Express may be storing your password, and sending it every time you connect so that it is visible to the device advertising the wireless network.

In the case of Ad Hoc networks, a person with malicious intent may have intentionally set up a free wireless service with this specific intention. Be very wary of what wireless networks you connect to, particularly those that are "unencrypted" or "unsecured" and do not require any authentication.

Issue 4 - It interferes with legitimate wireless services

Wireless networking uses a very narrow band in the radio spectrum around the 2.4GHz frequency. Many other types of devices besides those that support wireless networking also use this range, or cause interference at the same frequencies. These include cordless phones and microwave ovens. However the worst source of interference is other wireless networks. Ad Hoc networks are included in this category.

Even though the wireless spectrum is divided into 12 channels, these channels still create interference with each other. In the case of ANU we use all the available spectrum - on several different channels - to provide you the ANU-Access service. When there is interference present, the WAP automatically scales down the speed of it's connections to clients. Also interference will cause errors in the data being exchanged that requires that same data to be re-transmitted - causing even more traffic and thus more congestion and interference.

If there is large amount of interference the service can become basically unuseable. Ad Hoc networks can contribute a significant amount to this. In fact at the last count there were at least 500 computers on campus advertising an Ad Hoc network and causing interference with ANU-Access.

Who is Affected?

As far as we know, only Windows PCs will suffer from issues 1 & 2. However radio interference affects all users on campus by degrading the speed and reliability of the ANU Access service.

What Do I Do Now?

If you have a Windows PC, please follow the instructions in the next section, Ad Hoc Networks - Disabling, as a matter of urgency. We have made these instructions as easy to follow as possible.

If you have a non-Windows PC or Laptop you may still want to check your network connection profile anyway.