1. Field of the Invention
This invention is for use with wireless local area networks (LANs). Such networks include a plurality of mobile devices such as portable computers or other information processing devices, which communicate in a wireless fashion. The mobile devices normally communicate with a wired infrastructure via access points. Each access point represents a base station for a microcell which operates in accordance with a frequency hopping scheme in a frequency band, for example the frequency band from 2.4 to 2.4835 GHz. The mobile devices must establish communication links with new access points when they move out of range of old access points. Also, for proper load handling, it may be preferable to switch a mobile device to another access point to more uniformly distribute network capacity, even when the mobile device is still within range of its previous access point.
2. Description of the Related Art
One of the frequency bands on which wireless local area networks often communicate is the frequency band between 2.4 and 2.4835 GHz. This frequency band is relatively unregulated due to its poor characteristics and resulting low desirability. The poor nature of this frequency band is due to the fact that it is also the vibratory frequency range of a water molecule. Microwave ovens, which heat objects by vibrating the water molecules within them, therefore operate in this frequency range. Thus, a microwave oven can be viewed as a jammer or noise source which, when in operation, jams at least a portion of the 2.4 to 2.4835 GHz frequency spectrum.
As a result of the poor quality, wireless local area networks in this band typically employ a frequency hopping scheme with rapid switching from one channel within this frequency band to another channel within this frequency band. The band being 83.5 MHz wide, conventional channel distribution as set by current FCC regulations establishes 83 individual channels, each with a 1 MHz width. Thus, the first channel is from 2.405 to 2.4015 GHz, the second channel is from 2.4015 to 2.4025 GHz, and so on. Conventionally, 82 individual channels are used, leaving a guardband on either side of the frequency range.
A typical system will hop from one channel to another at a uniform hopping rate of, for example 10 hops per second. In order to maintain communication, each of the communicating devices must know when and where to hop. Normally, each access point continuously operates in accordance with a predetermined hopping scheme. Thus, once a mobile device knows the hopping scheme of the access point with which it is communicating, the mobile device can hop frequencies coincident with the access point.
About 65 relatively uncorrelated frequency hopping schemes have been established which provide effective orthogonal hopping, wherein for any given hopping scheme there will be few if any instances where two access points are communicating on the same frequency. Thus, each access point operates in accordance with one of these 65 hopping schemes.
In a typical system, a time mark frame is transmitted by each access point at the beginning of each frequency hop. The time mark frame includes identification of the channel number (i.e. the particular frequency) upon which the access point is broadcasting and an identification of the hopping sequence employed by the access point.
A problem arises when a mobile device transitions from communicating with one access point to communicating with another access point. In order to continue to communicate, the mobile device must determine which hopping scheme is employed by the new access point. A conventional method of establishing a communication link with the new access point has the mobile device operate in a "hunt" mode wherein it simply selects one of the 82 channels in use and waits until the new access point hops to this channel. Upon receipt of the time mark frame, the mobile device will then determine the hopping sequence being employed by the new access point and thereafter hop along with the new access point according to its hopping scheme.
Waiting for the access point to hop to the frequency selected by the mobile unit can result in a long transition time, causing a low data transmission rate. A further problem can occur if a nearby radiating source, such as a microwave oven or other source, happens to be operating at the selected "hunt" frequency. The operation of the radiating source at the frequency of interest could cause jamming for a long period of time, such that the communication link is not reestablished or is reestablished after an unacceptably long transition time.
Alternatively, the mobile device could transition by randomly hopping, eventually arriving at a frequency coincidentally used by the new access point. This strategy is as inefficient as the first, although it may reduce the effects of a nearby noise source and thus increase the statistical probability of establishing a communication link.
Alternatively, when the network is established and the access points are initially put in place, the manager of the local area network could set the hopping scheme for each access point and establish a system by which each access point could inform the mobile devices with which it communicates of the hopping schemes of the neighboring access points. Such a strategy, however, requires an extensive amount of skill and effort on the part of the local area network manager. Also, a significant amount of work would be required when changes in the local area network topology occur, such as incorporating additional access points to handle increased capacity on a busy local area network. It is therefore beneficial to have a local area network topology wherein the access points and mobile devices can, over time, determine the relationships therebetween and continuously update the relationships such that when a mobile device transitions from communicating with one access point to communicating with another access point, the mobile device has a good idea which frequency hopping scheme is likely to be employed when it completes the transition.