Wireless Computing

Introduction to Wireless

Wireless Technologies


Network Security



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Inter-LAN Interference

Basics - Spread Spectrum - Error Correction - Inter-LAN

Unfortunately, background interference protection and error checking will not help a bit if different nodes on the same system start transmitting at the same time. In order to prevent this, most modern wireless protocols include some sort of carrier sense multiple access collision avoidance (CSMA/CA). Any system with data to send first senses the medium to see whether any other transmissions are currently in process. If not, it waits one interframe space (IFS - see below) and then sends. If the medium is busy, it waits until the current transmission is over, waits one IFS, and then resamples, repeating the process.

Interframe Space

The concept of an interframe space allows wireless systems to implement a simple priority system. Systems with high priority data to send have shorter IFS times, so whenever the medium is clear they will be the first to send (and thus muscle out all competing senders).

IFS is also important because it incorporates the concept of exponential backoff. If a unit keeps finding the medium busy when it samples, its wait times begin to increase with each failure. Exponential backoff allows for the gradual reduction of traffic on overloaded airwaves back to a manageable level.

Preventing the Hidden Node Problem

Assume that there is a simple network consisting only of two computers and a wireless hub in between them. If all three are in range of each other, then CSMA/CA as described above works fine: they will all be able to detect when the airwaves are busy and wait to transmit. But what if the two computers are both within range of the hub, but out of range of each other? One computer, when sampling the airwaves, will have no idea whether the other computer is sending or not. If it decides to go ahead and send there is the chance that the second computer will also be sending, in which case both transmissions collide and interfere.

The solution to this quandary, known as the hidden node problem, is a four-step sending process requiring verification from the receiving unit. After determining that things are clear within its own range, a unit with data to send transmits a request to send (RTS) signal to its destination. The receiver senses the medium for itself, and if clear responds with a clear to send (CTS) signal. The sender transmits the data, and the receiver sends a acknowledgement (ACK) response. If any of the signals in the process are not received as expected, the data is assumed dropped and the entire sequence resets and begins again.

Diagram of CSMA/CA in action
Diagram of CSMA/CA in action

802.11b and Bluetooth

In addition to inter-LAN interference, problems can also occur with other nearby wireless networks. Both 802.11b and Bluetooth, for example, use the same 2.4 Ghz bandwidth. At close range, the two systems can cause each other significant interference. As the number of Bluetooth devices, in particular, continues to explode, this has the potential to be a major problem. Several different solutions promise to help prevent conflicts in the future:

Collaborative solutions rely on communication between 802.11 and Bluetooth devices. One proposal is to divide every given time period into two distinct components, one for WLAN devices to send and the other for WPAN devices. Another idea is to have a central controlling device with which every wireless unit communicates; the controller would keep track of all transmission requests and signal each unit when it was clear to send. It has the advantage of allowing for complicated priority filtering, but requires extra hardware and mechanisms not currently built in to most wireless devices.

Noncollaborative solutions mostly involve the Bluetooth end. The most promising include adaptive packet selection and adaptive frequency hopping. Both rely on a Bluetooth unit's ability to scan the range of its 79 channels and classify every channel as 'good' (clear) or 'bad' (interference). Adaptive packet selection means that the unit will choose to transmit larger packets on good channels. Adaptive frequency hopping allows the Bluetooth unit to alter its frequency-hopping sequence to avoid the bad channels altogether. This solution is currently awaiting a FCC ruling before being widely implemented; current FCC rules mandate a minimum range of frequency hops that is much higher than would be used by adaptive hopping in most cases.

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