Implementing Cisco Unified Wireless Networking Essentials
Europe USA Japan Frequency
–2.4835 GHz 2.4 GHz 2.4 GHz ISM 2.0
2.4 GHz 2.0–2.495 GHz
CEPT A UNII-1 5.15–5.25 GHz 5.15–5.25 GHz
CEPT B UNII-2 5.25–5.35 GHz
CEPT C UNII-2 Extended 5.47–5.7253 GHz
ISM 5.725–5.850 GHz
5.0 GHz 5.038–5.091 GHz
4.9 GHz 4.9–5.0 GHz
In Europe,the2.4-GHz range and the 5.0-GHz range are used.The 5.0-GHz frequency ranges that are used in Europe are called the Conference of European Post and Telecommunication(CEPT)A, CEPTB, and CEPTC bands.
In the United States, the900-MHz, 2.4-GHz ISM, and 5.0-GHz Unlicensed National Information Infrastructure(UNII) bands are used.
Japan has its own ranges in the 2.4- and 5.0-GHz range.
The United States has 11 channels, and each channel is 22-MHz wide The 5-GHz range is also subdivided into channels, each being 20-MHz wide. A total of 23 nonoverlapping channels exist in the 5-GHz range
A modulated wave form consists of three parts:
Amplitude: The strength of the signal
Phase: The timing of the signal between peaks
Frequency: How often the signal repeats in one second
Wireless networks use a few different modulation techniques, including these: DSSS
DSSS is the modulation technique that 802.11b devices use to send data. In DSSS, the transmitted signal is spread across the entire frequency spectrum that is being used.
A chip and a bit are essentially the same thing, but a bit represents the data, and a chip is used for the carrier encoding.
Encoding is the process of transforming information from one format to another.
The chipping code process takes each data bit and then expands it into a string of bits. To achieve rates of 1 Mbps and 2 Mbps, 802.11 uses a Barker code. This code defines the
use of 11 chips when encoding the data.
When you are using DSSS, the Barker code works well for lower data rates such as 1-Mbps and 2-Mbps. DSSS uses a different method for higher data rate, which allows the 802.11 standard to achieve rates of 5.5 and 11Mbps. Complementary code keying(CCK) uses a series of codes called complementary sequences. There are 64 unique code words. Up to 6 bits can be represented by a code word, as opposed to the 1 bit represented by a Barker code.
?Encoding is how the changes in RF signal translate to the 1s and 0s.
?Modulation is the characteristic of the RF signal that is manipulated.
Data Rate Encoding Modulation
1 11 chip Barker coding Differential Binary Phase Shift Keying
2 11 chip Barker coding Differential Quadrature Phase Shift Keying
5.5 8 chip CCK encoding (4 chipping sequences) DQPSK 11 8 chip CCK encoding (64 chipping sequences) DQPSK
DBPSK, two phases are used that are separated by 180 degrees. DBPSK can modulate 1 bit per symbol. To simplify this, a phase shift of 180 degrees is a 1, and a phase shift of 0 degrees is a 0.
In DBPSK,1 bit per symbol is encoded. This is okay for lower data rates. DQPSK has the
Capability to encode 2 bits per symbol. This doubles the data rates available in DBPSK while staying within the same bandwidth. At the 2-Mbps data rate, DQPSK is used with Barker encoding. At the 5.5-Mbps data rate, DQPSK is also used, but the encoding is CCK-
4. At the 11-Mbps data rate, DQPSK is also used, but the encoding is CCK-8.
OFDM is not considered a spread spectrum technology, but it is used for modulation in wireless networks. Using OFDM, you can achieve the highest data rates with the maxi- mum resistance to corruption of the data caused by interference. OFDM defines a num- ber of channels in a frequency range. These channels are further divided into a larger number of small-bandwidth subcarriers. The channels are 20MHz, and the subcarriers are
802.11g and 802.11a
both use OFDM. The way they are implemented is a little different because 802.11g is designed to operate in the 2.4-MHz range along with 802.11b devices.
With the RTS/CTS method, the sending device uses an RTS (request to send) packet, and the access point uses a CTS (clear to send) packet addressed to the sender of the RTS frame. This alerts other devices that they should not send for a period.
The Federal Communications Commission (FCC) is an independent agency in the United States that regulates communication methods. It is held directly responsible by Congress.
It is the FCC in the United States that governs the frequency ranges that can be used without a license, the transmit power of devices, the types of devices that can be used indoors as well as outdoors, and how the various types of hardware can be used.
Cisco uses a connector known as the Reverse-Polarity-Threaded Neil-Concelman (RP-TNC) connector,
Effective Isotropic Radiated Power (EIRP) is a way to measure the amount of energy radiated from an antenna.
Similar to the FCC, the ETSI has 2.4-GHz EIRP output rate standards that you should be familiar with. The ETSI’s rules, however, are different from the FCC’s rules. ETSI defines 20-dBm EIRP on point-to-multipoint and on point-to-point with 17-dBm maximum transmit power with 3-dBi gain. In a way, this is easier to remember, because these numbers are the same value for both point-to-point and point-to-multipoint connections. Of course, a professional installer can increase the gain as long as he or she lowers the transmit power
below 17 dBm at a ratio of 1:1. Therefore, a professional installer could drop the transmit power by 1 dBm and increase the gain by 1 dBm and still stay within the guidelines.
What makes Wi-Fi Alliance different from the ETSI, FCC, and IEEE
is that it gives its seal of approval to devices that plan in interoperability.
To calculate EIRP, use the following formula:
EIRP = transmitter output power – cable loss + antenna gain
Free Path Loss
An effect of absorption is heat. When something absorbs a wave, it creates heat in whatever absorbed the wave.
Walls, bodies, and carpet can absorb signals.
Reflection happens when a signal bounces off of something and travels in a different direction.
One characteristic of multipath is that a receiver might get the same signal several times over. This is dependent on the wavelength and the position of the receiver.
Anothercharacteristicofmultipathisthatitcancausethecopies of the signaltobecome outofphase.
The issue of wirelesssignals scattering happens when the signal is sent in many different directions.
Refractionis the change in direction of, or the bending of, a waveform as it passes through something that has a different density causing the wave to change direction.
Signal-to-noiseratio(SNR) is the term used to describe how much stronger the signal is Compared to the surrounding noise that corrupts the signal.
Link budgetis a value that accounts for all the gains and losses between sender and receiver, including attenuation, antenna gain, and other miscellaneous losses that might occur. This can be useful in determining how much power is needed to transmit a signal that the receiving end can understand.
The following is a simple equation to factor link budget:
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) – Losses (dB)
RSSI and SNR
We’ve logged a lot of ink discussing signals and signal strength, but so far, I haven’t told you how these are measured. There are two terms used to discuss signal strength: received signal strength indicator (RSSI) and signal-to-noise ratio (SNR). RSSI is designed to describe the strength of the signal received, and SNR refers to the ratio of the signal to the surrounding RF noise that is always present in the environment.
First, let’s talk about RSSI, which is a measure of the amount of signal strength that actually arrives at the receiving device. It’s a grade value ranging from 0 to 255. For each grade value, an equivalent dBm (decibels relative to a milliwatt) value is displayed. For example, 0 in the scale may equal –95dBm and 100 might be –15dBm. So 0 would equal a much greater loss of
signal than 100 would.
I’ll get into dBm in more detail soon, but for now understand that dBm is not an absolute measure; it’s a relative one. What I mean by relative is that it’s a value referenced against
another value, in this case, milliwatts. Decibels are used to measure an increase or decrease in power as opposed to an absolute value, meaning that decibel values come through as positive (gain) and negative (loss). RSSI values are negative and represent the level of signal loss that can be experienced en route with the card still able to receive the signal correctly. Most manufacturers will have a table listing the RSSI that’s required at each frequency.
RSSI values can’t be compared from one card vendor to another because each company typically uses a different scale. For example, Company A might be using a scale of 0 to 100, while Company B is using a scale from 0 to 60. Since the scales are different, the resulting RSSI values can’t be compared, right?
SNR is a critical comparison of the amount of signal as compared to the surrounding noise. If the level of noise is too close to the level of signal, the signal can’t be picked out from the noise and understood. Think of this as someone whispering in a really loud room. A higher value is good for SNR.
Now, let’s have some fun doing RF math! Let me show you how easy this can be
dB A decibel (dB) is a general term that describes either positive or negative change. The difference in the types I’ll go over next relate to the dB as the value that’s being referenced against. Basically, think of this concept as the starting point.
dBi A dBi is referenced against an isotropic antenna — a theoretical antenna that radiates
in all directions equally that doesn’t really exist. This theoretical antenna is important because it allows us to compare one antenna to another, since the antenna will be labeled in reference to the same point of comparison.
what dBi measures is essentially the effective gain of an antenna compared to an isotropic antenna, and it tells us that the greater the dBi value, the higher the gain, and the higher the gain, the more acute the angle of coverage.
dBd Okay—here’s something real and not theoretical—a dBd is referenced against a dipole
antenna, which does actually exist. The radiation patterns of a dipole antenna are shown in Figure 2.15; the shape is probably familiar to you.
So how does dBi compare to dBd? Converting one to the other is simply a matter of adding
or taking away 2.14:
dBi = dBd + 2.14ÛN
Therefore, the dBd measures the effective gain of an antenna as compared to a dipole antenna.
dBm A dBm is referenced against a milliwatt. The arbitrator reference point is 1 milliwatt, so 1 milliwatt equals 0dbm, or no change from the reference point.
Rule of 3s and 10s
Examine the graph closely for a minute—you know, like you would those illusory dot matrix
pictures you stare at and, suddenly, you see the picture? If you checked it out, you probably discerned the following relationships:
Increase of 3dB = double transmit (Tx) powerÛN
Decrease of 3dB = half the powerÛN
Increase of 10dB = 10× powerÛN
Decrease of 10dB = 1/10 powerÛN
You can use this ―rule of 3s and 10s‖ to perform RF math calculations. Here’s what I mean if a radio transmitter emits a signal at 100mW and an amplifer introduces a 3dB gain into the signal, the resulting signal will be double to 200mW. Taking that another step, if the antenna introduces 10dB of gain, then the signal leaving the antenna will now be 2000mW.
First, you’ve got to nail down the net gain:
Start with the transmitter power +20dB Equals 100mW
Add the antenna gain +6dB Each 3dB doubles
Total power with antenna gain +26dB 100mW × 2 × 2 = 400mW
Add the power from the amp +10dB 10dB equals 10× power
Total power with amp +36dB 400mW × 10 = 4000mW
The same as 4W +36dB 4000mW = 4W
Now let’s take a look at our loss:
Start with the total power gain +36dB Equals 4W
Subtract loss from 50 feet of cable: -3.35dB Round to –3dB,
which divides by 2, Total power minus cable +33dB 4W/2 = 2W
Subtract loss from attenuator -3dB Again divides by 2
Total power minus attenuator +30dB 2W/2 = 1W
A WPAN has the following characteristics:
? The range is short—about 20 feet.
? Eight active devices
? Unlicensed 2.4-GHz spectrum
? Called a piconet ( Bluetooth communicates with a shared hopping sequence in a local area is what makes it a piconet)
WLANs have the following characteristics:
? 2.4-GHz or 5-GHz spectrum.
? A larger range than a WPAN—close to 100 meters from AP to client.
? To achieve further distance, more power output is required.
? It’s not personal; rather, more clients are expected.
?WLANs are very flexible, so more than eight active devices/clients are expected, un-like a
A wireless metropolitan-areanetwork (WMAN) covers a large geographic area and has the following characteristics:
? Speeds decrease as the distance increases.
? Close to broadband speeds versus Ethernet speeds.
? Used as a backbone, point-to-point, or point-to-multipoint.
? Most well-known is WiMax.
A wireless wide-area network (WWAN) covers a large geographic area. WWANs have the following characteristics:
? Low data rates
? High cost of deployment
Ad hoc networks don’t require a central device to allow them to communicate. Rather, one device sets an SSID or network name and a channel as radio parameters. This is called a Basic Service Set (BSS), which defines the area in which a device is reachable. Because the two machines don’t need a central device to speak to each other, it is called an Independent Basic Service Set (IBSS). This type of adhoc network exists as soon as two devices see each
Infrastructure Mode : An AP is very much a hub or a bridge from wired networking, except of course that it uses radio waves instead of cabled connections. Here’s why:
? There is one radio, which cannot send and receive at the same time. This is where the
AP is likened to a hub. It’s a half-duplex operation.
? APs have some intelligence that is similar to that of a bridge. Like the bridge, the AP
maintains a MAC address forwarding database, helping it know which interface a frame has to be forwarded through.
What is different on an AP versus a bridge is that wireless frames are more complex. Standard Ethernet frames have a source MAC address and a destination MAC address. Wireless frames can have three or four MAC addresses. Two of them are the source and destination MAC addresses, like is found in the Ethernet frame, and one is AP’s MAC. The
fourth could be NEXT-HOP address when you are using workgroup bridge.
If we are using a repeater or mesh-based network or a workgroup bridge,
then we will also use a transmitter address, to indicate the address of a repeater or mesh AP or workgroup bridge that is forwarding traffic on behalf of the original client.
the coverage area of the AP is called a Basic Service Area (BSA), which is also sometimes known as a wireless cell.
When more than one AP is connected to a common distribution system, as shown in Figure 4-4, the coverage area is called an Extended Service Area (ESA).
On the AP, the network is associated with a MAC address. This network or workgroup that your clients connect to is called a Service Set Identifier (SSID). So on an AP, the SSID is a combination of MAC address and network name. This MAC address can be that of the wireless radio or another MAC address generated on the AP. When an AP offers service for only one network, it is called a Basic Service Set Identifier (BSSID). APs offer the ability to use more than one SSID; they can support up to 16 SSIDs. This would let you offer a Guest Network and a Corporate Network and still use the same AP. When the AP has more than one network, it is called a Multiple Basic Service Set Identifier (MB-SSID)