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Understanding Ad Hoc Mode
By Jim Geier

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.

Ad hoc operates differently

Much of the 802.11 standard defines a common operation whether you're using ad hoc or infrastructure mode. The use of ad hoc mode only affects the protocols, so there is no impact on the Physical Layers (i.e., 802.11a and 802.11b). Within the MAC Layer, all of the carrier sensing and most of the frame types and corresponding usage are the same regardless of which mode you choose. The absence of an access point, however, means that an ad hoc wireless LAN must take on more of the MAC Layer responsibilities.

The first ad hoc station (radio NIC) active establishes an IBSS and starts sending beacons, which are needed to maintain synchronization among the stations. (With infrastructure mode, only the access point sends beacons.) Other ad hoc stations can join the network after receiving a beacon and accepting the IBSS parameters (e.g., beacon interval) found in the beacon frame.

All stations that join the ad hoc network must send a beacon periodically if it doesn't hear a beacon from another station within a very short random delay period after the beacon is supposed to be sent. The random delay minimizes the transmission of beacons from multiple stations by effectively reducing the number of stations that will send a beacon. If a station doesn't hear a beacon within the random delay period, then the station assumes that no other stations are active and a beacon needs to be sent.

After receiving a beacon, each station updates their local internal clock with the timestamp found in the beacon frame, assuming the timestamp value is greater than the local clock. This ensures that the all stations are able to perform operations, such as beacon transmissions and power management functions, at the same time.

As with infrastructure networks, an ad hoc sleeping station (i.e., power management "on") indicates that they're entering sleep state by setting the power management bit in the control field of any frame. All other stations learn of this by monitoring the frame control fields of all frames. Stations will then hold off transmitting to the sleeping station and buffer the corresponding packets locally.

Regularly, all sleeping stations wake up at the same time during the announcement traffic indication map (ATIM) window, which corresponds with each beacon transmission. If a station is holding packets for a sleeping destination, the station will send an ATIM frame to the sleeping station indicating that packets are awaiting transmission. The station that had been asleep then knows to stay awake through the next beacon interval, which is hopefully long enough for the station buffering the packet to send it successfully. After receiving and acknowledging reception of the packet, the station can go back to sleep.

Summary

As you can see, ad hoc mode offers enough advantages to consider when deploying wireless LANs. The thought of saving the cost on access points is certainly a compelling reason to strongly consider this configuration. Unless you implement routing among the wireless users, however, you'll find that ad hoc mode mostly applies to smaller, spontaneous networks when there isn't a strong need for interfacing with a wired network.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs (SAMs, 2001), and regularly instructs workshops on wireless LANs. Join Jim for discussions as he answers questions in the 802.11 Planet Forums.

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