Spread Spectrum Technologies Overview
One of the many things that must be known in order to be a successful CCNA wireless candidate is an understanding of the basics of spread spectrum technologies. This is important because many of the most commonly used wireless technologies in use today take advantage of spread spectrum techniques. This article takes a high-level look at how spread spectrum technologies work and where they are in use in today’s modern networks.
Simply put, Spread Spectrum is the use of a technology that spreads a signal over a frequency spectrum. For example, 802.11b uses the 2.4 GHz band (2.4000–2.4835 GHz) and utilizes channels that are 22 MHz wide with a defined center frequency. The signal is able to be spread across that entire 22 MHz area.
Spread Spectrum Technologies
When dealing with modern wireless networks there are a number of different technologies that are in use. This section takes a look at the different technologies in use on modern wireless LAN networks, specifically 802.11b, 802.11a, 802.11g and 802.11n. In other words, let’s take a look at the different encoding and modulation techniques that are used by these four and how they are used to achieve greater bandwidths.
At the low end of the bandwidth spectrum are the 1 and 2 Mbps options that are available in the 802.11b and 802.11g standards (Originally standardized with 802.11 prime). The encoding method in use at these bandwidths is the Direct-Sequence Spread Spectrum (DSSS) technique using Barker 11 code. The modulation method is Differential Binary Phase-Shift Keying (DBPSK) for 1 Mbps and Differential Quadrature Phase-Shift Keying (DQPSK) for 2 Mbps.
The next step-up in bandwidth was initially defined in the 802.11b standard and is also used with 802.11g, these options include 5.5 and 11 Mbps. To achieve these rates the same two modulation methods (DBPSK and DQPSK) are used but are paired with a different encoding method. To achieve the 5.5 and 11 Mbps rates, the Direct-Sequence Spread Spectrum (DSSS) technique is used with Complementary Code Keying (CCK).
The 802.11a, 802.11g and 802.11n standards utilize the Orthogonal Frequency-Division Multiplexing (OFDM) technique offering a variety of different bandwidth options depending on the modulation technique. 802.11a and 802.11g both utilize OFDM to provide bandwidths from 6 Mbps through 54 Mbps. 6 and 9 Mbps are provided by using Binary Phase-Shift Keying (BPSK), 12 and 18 Mbps are provided by using Quadrature Phase-Shift Keying (QPSK), 24 and 36 Mbps are provided by using 16-Quadrature Amplitude Modulation (16-QAM), and 48 and 54 Mbps are provided using 64-QAM. The difference between the 802.11a and 802.11g implementations is the frequency band being used; the 802.11a standard utilizes the 5 GHz band while the 802.11g standard uses the 2.4 GHz band.
802.11n introduces a number of different features that allow it to not only utilize some of the features of all the other standards, but also reach very high bandwidth potentials. One feature is the ability to utilize 40 MHz channels instead of the 20 MHz channels used by the 802.11a, b and g standards. Unfortunately this can be a blessing and a curse, as the 802.11n standard supports the 2.4 and 5 GHz frequency bands. When using 40 MHz channels, two 20-MHz channels are combined and their combined frequency space is used to provide a single channel. If implementing 40 MHz channels using the 2.4 GHz band, the amount of space and interference can quickly become a large issue, making expansion almost impossible. The other major feature that was introduced in the 802.11n standard is multiple-input multiple-output (MIMO); MIMO provides the ability to utilize multiple spatial streams that can each provide bandwidth numbers equivalent to previous standards. The table below shows the average speeds that are available using a single spatial stream:
Table 1 – 802.11n Speeds and Modulations
|Modulation||20 MHz (Mbps)||40 MHz (Mbps)|
There are a number of different 802.11n devices out there that support 2, 3 and 4 spatial streams offering a total theoretical bandwidth of ~600 Mbps with 4 spatial streams.
There are a number of techniques that can be used to offer various amounts of bandwidth depending on the application; this article simply takes a look at the ones that are used on modern Wireless LAN networks. As the different wireless technologies evolve other new techniques will be developed to squeeze more information into a wireless signal. Hopefully this article gives a good look at what technologies are involved, and offers a starting point for further researching each of these different technologies.