8.2 Spread Spectrum Technology

   

As mentioned previously, the most common form of security provided for fixed wireless Internet access is SST. SST was originally developed for the military so that wireless military transmissions could be broadcast without fear of an enemy deciphering the communication or jamming the signal.

SST is a wideband RF technology. It converts the narrowband fixed wireless signal and outputs a wideband signal. The WMTS receives the wideband signal and converts it back to a narrowband and pieces the information back together. When the narrowband signal is distributed over a wideband signal, it scrambles the data, making it difficult for any device that does not have the conversion information to piece it back together.

Because of the way SST divides the signal, a random observer would see a modem-to-WMTS transmission as noise. In other words, it makes it difficult for someone to randomly pick up on transmissions. Instead, an attacker has to make a conscious effort to track down and decode your signal.

To get an idea of how SST works, imagine a 20-page report. Each page of that report is a data packet. Now, run the 20-page report through a wood chipper. [2] The wood chipper provides the same function as SST; it cuts up the document and spreads it out over the yard. On the other side of your yard, you have a reverse wood chipper; it catches the debris from your report and restores it to its original shape. This function is performed by the WMTS in a wireless transaction.

[2] Warning: Do not actually do this unless you have a backup of the report.

SST does a great job of scrambling the data, and makes it very difficult for an attacker to sniff data through the air, because the attacker has to know how the wireless modem distributed the data, in order to start piecing it back together. Understand, that with enough equipment, and some information about the make and model of your wireless networking equipment, it is very possible for an attacker to do just that. Once the SST has been decoded it is possible for the attacker to intercept all unencrypted transmissions, without the knowledge of the network administrators.

Three types of SST are commonly deployed:

  1. Frequency- hopping spread spectrum (FHSS)

  2. Direct-sequence spread spectrum (DSSS)

  3. Code-Division Multiple Access (CDMA)

When FHSS is deployed, a signal will hop from frequency to frequency over a period of time. The current standard is that the signal will switch frequencies at no longer than 400 millisecond intervals. Current standards also dictate that a signal must hop between at least 75 different frequencies. The WMTS must know the intervals at which the origination modem will be hopping frequencies, and the set of frequencies that will be used, so data can be pieced back together properly.

DSSS accomplishes the same thing as FHSS, but in a different manner. With DHSS the signal from the originating modem is joined with a higher rate bit sequence, sometimes called a chirping code. This chirping code is then spread across the spectrum in a manner similar to FHSS. The chirping code provides redundant information about the data, making the transmission less susceptible to atmospheric interference and providing an extra level of redundancy. DSSS technology is generally used for high-speed fixed wireless connections, while FHSS technology is used for lower speed connections.

CDMA is most often associated with the cellular telephone industry. In fact, CDMA is so common within the cellular industry that it is often thought of as synonymous with SST. CDMA converts data in the same manner that FHSS and DSSS do, but it spreads the signal through far fewer frequencies than FHSS and DSSS. To compensate for the smaller signal distribution CDMA generates fake noise. The signal is still distributed along the narrower band, but extra noise is added to the band , making the signal more difficult to detect than the signals from FHSS and DSSS devices. In addition to the enhanced security CDMA also digitizes the data, allowing for a mostly digital transmission from the modem to the WMTS. These two enhancements allow modem manufacturers to squeeze more bandwidth from a CDMA-enabled modem.

The different types of SST are useful for different types of fixed wireless connections. While CDMA is increasingly becoming more popular, it is not a fully accepted standard. If your company is deploying a fixed wireless Internet solution, find out from your provider what type of equipment is in use, and what type of SST is supported.

   


The Practice of Network Security. Deployment Strategies for Production Environments
The Practice of Network Security: Deployment Strategies for Production Environments
ISBN: 0130462233
EAN: 2147483647
Year: 2002
Pages: 131
Authors: Allan Liska

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