Understanding Ad Hoc Mode - Page 2

By Jim Geier

August 23, 2002

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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