Before you start
Objectives: Learn about different wireless standards and their specifics, including 802.11n.
Prerequisites: no prerequisites.
Key terms: 802.11n, channels, network, standards, wireless, frequency, range, devices, GHz
The wireless standards that we are going to talk about here are 802.11a, 802.11b, 802.11g, and 802.11n. Different standards work on different frequency ranges.
The reason why the frequency in wireless networks is important is because of the interference and compatibility with other devices. For example, in the 2.4 GHz range we may have other devices operating. Such devices might be cordless phones and microwave ovens. All those devices use the same frequency range so we might have interference between all those devices and our wireless network.
The frequency is also important when it comes to compatibility. Standards which work on the same frequency range are compatible (can work together).
The 802.11a supports speeds of up to 54 Mbps. It works on a 5.75 GHz range. It has less interference, but it is not compatible with other standards. The maximum distance that it can transmit is about 150 feet (about 45 meters).
The 802.11b standard supports speeds of up to 11 Mbps. It has a good signal range (up to 300 feet or 90 meters), but also a higher chance of interference. It works on a 2.4 GHz range. It is not compatible with the 802.11a standard.
The 802.11g also gives us up to 54 Mbps, but it works on the 2.4 GHz frequency which is the same as 802.11b (it is compatible with 802.11b). It has a good signal range (up to 300 feet or 90 meters).
802.11n is a new standard which is backwards compatible with 802.11b/g or with 802.11a. The compatibility depends on the frequency used. The 802.11n standard can use 2.4 GHz frequency or 5.75 GHz frequency. It can provide speeds of up to 600 Mbps (it depends on how it is implemented), with less interference and longer range (up to 1200 feet or 365 meters).
In wireless networks, a channel is a portion of frequency range. By dividing the frequency range into channels, we can have multiple devices in separate wireless networks transmitting at the same time.
The 2.4 GHz range is divided into 11 channels. When we form a wireless network with an access point and clients, all devices on that network will use the same channel. Because of this we can have a separate network with the same standard in the same location, but which works on different channel. Although we have 11 channels, we can’t actually create 11 networks which work on different channels in the same location. This is because some channels actually overlap. When creating networks on the same location, we have to choose channels which don’t overlap. With 802.11b/g standard, we have three channels that don’t overlap. Those are 1, 6, and 11. So, when implementing a wireless network with multiple APs in the same location, we have to be sure to choose non-overlapping channels for neighboring access points.
Networks on Non-overlapping Channels
In 5.75 GHz range we have 23 channels, of which we have 12 non-overlapping channels. This is great if we need to create big wireless networks since we have more channels to work with.
Have in mind that wireless networks are considered as broadcast networks. This means that all computers that connect to the Access Point are sharing the signal of the network. The speed of the wireless network depends on the distance and obstructions such as walls. The speed decreases as the distance between the transmitter and receiver increases. When we connect a legacy device to the wireless network, all devices on the network will operate at the legacy speed. For example, connecting an 802.11b device to an 802.11g Access Point slows down the network to 802.11b speeds. The same thing is if we connect a 802.11g device to the 802.11n AP.
With 802.11n we can deal with that by using a dual band access point which can use one radio to transmit at one frequency, and a different radio to transmit at a different frequency. For example, we can our AP to use one antenna to communicate at 5.75 GHz with 802.11a devices, and all other antennas to use 2.4GHz to communicate with 802.11n devices. In some configurations we will see the term “mixed mode” to indicate that our network has both 802.11n and non-802.11n clients. In this configuration one antenna is used for legacy clients and others are used for 802.11n clients. When a mix of clients using different standards are connected, the AP must disable some 802.11n features in order to be compatible with non-802.11n clients. Some newer 802.11a and 802.11g devices can provide up to 108 Mbps by using technologies from 802.11n (MIMO and channel bonding).
When it comes to wireless network cards that we use on our clients, different cards will be compatible with different standards. If we use a 802.11g standard on our network we should use (buy) a network card that is compatible with 802.11g standard or with both 802.11g/b standards. Some network cards are compatible with 802.11a/b/g standards. Such cards have multiple transmit radios built in. We can also buy cards that are only compatible with 802.11n standard, 802.11n/g/b standards, or all standards (this is rare).
New Technologies in 802.11n
802.11n uses new technology called Multiple Input Multiple Output (MIMO), which simply adds multiple antennas to transmit and receive signals. This improves the overall performance of the wireless network (stronger signal and increased speed), since separate antennas can be used to simultaneously send the same or even different data. The MIMO system is usually described by the number of sending and receiving antennas. For example, the 4×3 designation indicates 4 transmit and 3 receive radios. The 802.11n specification allows up to 4 sending and 4 receiving antennas. For highest speed we configure all radios to transmit the same data. This also increases the distance. With MIMO we can also configure some antennas to transmit by using one standard (for example 802.11n), and other antennas to transmit by using other standards (for example 802.11a). In this case, client devices which support 802.11n standard will connect to the 802.11n radios, while 802.11a devices will connect to the 802.11a radios. That way, the speed for 802.11n devices won’t be decreased to 802.11a speed. Similar thing is for 802.11b/g devices on 2.4 GHz frequency.
Another technology used in 802.11n standard is Channel bonding. Channel bonding combines two non-overlapping channels in order to double the transmission speed. The 2.4 GHz range has a total of 11 channels, and 3 of them are non-overlapping (1,6, and 11). With channel bonding we combine two of those channels into one logical unit. So, if we have 54 Mbps by using one channel on 2.4 GHz network, we can get 108 Mbps by combining two channels. The problem in the 2.4 GHz range is that we can have a maximum of 1 bonded channel, and one non-bonded channel. The 5.75 GHz range has a total of 23 channels, and 12 of those are non-overlapping. This allows a maximum of 6 non-overlapping bonded channels. Because of that, channel bonding is more often used with 5.75 GHz range.