Understanding Ad Hoc Mode

By Jim Geier

August 23, 2002

Don't let the bounds of an access point limit your WLAN applications. Consider 802.11's ad hoc mode to enable greater levels of mobility and portability with a truly wireless solution.

Most installed wireless LANs today utilize "infrastructure" mode that requires the use of one or more access points. With this configuration, the access point provides an interface to a distribution system (e.g., Ethernet), which enables wireless users to utilize corporate servers and Internet applications.

As an optional feature, however, the 802.11 standard specifies "ad hoc" mode, which allows the radio network interface card (NIC) to operate in what the standard refers to as an independent basic service set (IBSS) network configuration. With an IBSS, there are no access points. User devices communicate directly with each other in a peer-to-peer manner.

Ad hoc applications provide truly wireless solutions

Ad hoc mode allows users to spontaneously form a wireless LAN. For example, a group of people with 802.11-equipped laptops may gather for a business meeting at their corporate headquarters. In order to share documents such as presentation charts and spreadsheets, they could easily switch their NICs to ad hoc mode to form a small wireless LAN within their meeting room. Another example is when you and your associates are waiting for a flight at the airport, and you need to share a relatively large PDF file. Through ad hoc mode, you can easily transfer the file from one laptop to another. With any of these applications, there's no need to install an access point and run cables.

The ad hoc form of communications is especially useful in public-safety and search-and-rescue applications. Medical teams require fast, effective communications when they rush to a disaster to treat victims. They can't afford the time to run cabling and install networking hardware. The medical team can utilize 802.11 radio NICs in their laptops and PDAs and enable broadband wireless data communications as soon as they arrive on the scene.

Some product vendors are beginning to base their solutions on ad hoc mode. As an example, MeshNetworks offers a wireless broadband network system based on 802.11 ad hoc mode and a patented peer-to-peer routing technology. This results in a wireless mesh topology where mobile devices provide the routing mechanisms in order to extend the range of the system. For example, a user on one side of the building can send a packet destined to another user on the far side of the facility, well beyond the point-to-point range of 802.11, by having the signal hope from client device to client device until it gets to its destination. This can extend the range of the wireless LAN from hundreds of feet to miles, depending on the concentration of wireless users.

Pros and cons to consider

Before making the decision to use ad hoc mode, you should consider the following:

  • Cost savings. Without the need to purchase or install access points, you'll save a considerable amount of money when deploying ad hoc wireless LANs. Of course this makes the bean counters happy, but be sure you think about all of the pros and cons before making a final decision on which way to go.
  • Rapid setup time. Ad hoc mode only requires the installation of radio NICs in the user devices. As a result, the time to setup the wireless LAN is much less than installing an infrastructure wireless LAN. Obviously this time savings only applies if the facility you plan to support wireless LAN connectivity doesn't already have a wireless LAN installed.
  • Better performance possible. The question of performance with ad hoc mode is certainly debatable. For example, performance can be higher with ad hoc mode because of no need for packets to travel through an access point. This assumes a relatively small number of users, however. If you have lots of users, then you'll likely have better performance by using multiple access points to separate users onto non-overlapping channels to reduce medium access contention and collisions. Also because of a need for sleeping stations to wake up during each beacon interval, performance can be lower with ad hoc mode due to additional packet transmissions if you implement power management.
  • Limited network access. Because there is no distribution system with ad hoc wireless LANs, users don't have effective access to the Internet and other wired network services. Of course you could setup a PC with a radio NIC and configure the PC with a shared connection to the Internet. This won't satisfy a larger group of users very well, though. As a result, ad hoc is not a good way to go for larger enterprise wireless LANs where there's a strong need to access applications and servers on a wired network.
  • Difficult network management. Network management becomes a headache with ad hoc networks because of the fluidity of the network topology and lack of a centralized device. Without an access point, network managers can't easily monitor performance, perform security audits, etc. Effective network management with ad hoc wireless LANs requires network management at the user device level, which requires a significant amount of overhead packet transmission over the wireless LAN. This again leans ad hoc mode away from larger, enterprise wireless LAN applications.
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