–Fresnel Zone
Think about football shaped zone from piont A to piont B
Then Think about a larger football shaped zone from A to piont B
Then Think about a larger football shaped zone from A to piont B
The first one is zone 1, the next is zone 2 and then zone 3
you need have 60% of zone 1 visable for the signal to work
it is reccomended you have 80% of zone 1 visable for the signal to be optimal
—RSSI (Relative signal strength)
Is measured in DBm, the closer you get to 0 the better the strength.
dbm is measured in negative DBm, -30 is better then -90 DBm
This is an indication out of a maxium of 255 values (can be less) how
strong the signal strength is
—SNR (Signal to noise ratio)
The difference between the RSSI and the amount of interference is known as the
signal to noise ratio.
Noise can be defined as the amount of signal that is lost due to other devices
broadcasting on the same frequency
–Link budget
This is the amount of signal that must be broadcast for the reciever to get
the signal and be able to communicate
–Line of sight (LOS)
referrs more to radio line of sight which is not always visual,
It is important to note that in outdoor areas, where it is humid then the signal
will bend towards the earth. Where the air is thinner it will bend away from the
earth.
—Service sets
iBSS = term to define ad-hoc basic service set network segment
BSS = from an access piont, broadcasting to clients in a network segment, inlcudes the switch
eSS = wired network segment up multiple wireless acess pionts through the multilayer switch
BSSID = mac address associated to the SSID
—SSID
32 characters in length defining a basic service set broadcast segment
Remember with 54mb 802.11g it is operating at half duplex so you only have a
theoretical maximium of 27mbps.
—Polarization
There are 3 types of polarization, vertical, horizontal and circluar
Vertical polarization means that the electro field moves in a sine wave up and then down
The magnetic field is perpendicular to the electric field so it moves in a sine
wave left to right perpendicular to the electric field. cisco uses vertical
polarization for its devices.
Horizontal polarization means that the electric field moves left and right
in a sine wave towards a destination, the magnetic field moves vertically up
and down perpendicular to the electric field
Circular polarization means that the electric field moves in the spiral towards
the destination, magnetic field is perpendicular.
In a piont to piont antenna, the polarization must be the same or the signal
will be weak.
—diveristy
Diverisity is when the access piont has 2 antenna’s, it does not transmit from both antenna’s
at the same time, the client sends a pre-amble to the access piont and then
the AP chooses the best antenna to use.
Diversity is a solution to multipath, the 2 antenna’s are one cycle away from each other.
—Antenna types
omnidirecitonal – refers to the sphere transmission
directional – buble shape cone outward
YAGI antenna – cone shape outbound
Patch Antenna’s – Oval sphere outbound, good covering only 1 room.
dipole 2.2dbi antenna – Emmits in a isostropic donut sphere.
cisco air-ant1728 – Emmits more on the horiztonal plane flat than the elevation plane (up and down).
cisco air-ant24120 – Very poor elevation plan transmission(up down), great hozizontal plane transmittions (distance)
cisco air-ANT2485P-R Directional antenna that I installed at bendigo.
cisco 2410/1949 – Yagi Antenna
ciso Air-Ant3338 – Piont to Piont senerio’s
—RP-TNC
Connector used for Cisco Antenna’s.
—Attenuator
If you need to reduce the signal because you are getting bleed-over in the cabling
then you place an attenuator between the radio and the antenna.
A DC injector sits between the attenuator and the radio.
—Amplification
If you are need additional strength signal in the cabling you can add an amplifier
—Phase
Phase is height of the signal as it arrives at the destination.
In phase is when the signal is arriving synronised at the same peak and valley
180 degree’s out of phase means that the 2 arriving waves one is upside down
90 degress is 25% off, 270 degree’s is 75% off.
—lightning arrestor
This is a connector that stops lightning from hitting a dish and frying your
whole local area network
—splitter
2 directional antenna’s back to back can use a splitter rather than
having another wireless transmitter with the antenna’s,
incurrs a -4dbB loss. (half distance covered)
—DSSS
Uses all the frequency in the channels to send data, does not sub split the channels
into sub frequencies
—Barker 11
Used in 802.11b to provide a reliable transport. Operates over a 22mhz channel to
send data, encodes the data into frames where its ok for 9 bits of data to be
lost in tranmission and the bit is still recieved and can ensure it can calculate
the bit value is some information is lost
Called symbols or chips, this is a set of characters to represent a 1 or a 0
—OFDM
Used in 802.11a/g/n/ac to provide the same reliable tramission mechanism as DSSS however
6 bits delivered per control packet which allows faster data transmissions.
Uses sub-carriers, by splitting the channels into 315.5khz sub channels.
48 are used for different streams of data, 4 are controll and protection streams
to ensure the quality of the waves.
—QAM
Comes in different flavours like 16-QAM,64-QAM, 128-QAM, 256-QAM(AC),
uses different phase and amplitude to deliver different streams of
data, IE if send one signal, then another at 90 degree’s that is 2 different
waves that can be understood by the client, adjusting the amplitude, ie the
height of the wave, that is another stream that can be understood.
—Clean channels
In the US and Australia there are 3 clean channels that are 22Mhz apart.
Using multiple access points on the same channel can cause overlap.
Co-channel interference is much better because you are on the same channel
it can be detected, if someone is overlapping 2 clean channels there will be
alot of noise to both clean channels and it is less likely to be detected
and result is alot more re-transmits.
—Duty Cycle
This is a percentage of time a device is transmitting and causing interference,
IE a baby monitor has a 100% duty cycle always sending data and causing interference
—DCF vs PCF
DCF is when the clients are responsible for the control of sending and recieving data,
IE all the laptops transmit a RTS and CTS messages. PCF is when the AP controlls who
can send and who can recieve, this has not been implemented by any vendors.
—Null Data Frame
This is an empty frame that is sent from the client to let the AP know that it is going
into power saving mode and the amount of traffic happening will be minimal to save battery,
The AP sends a TIM message to let the client know to wake up.
—802.11 Packets
Can contain up to 4 mac address fields, depending on the direction of the packet (IE)
From the AP or TO the AP, the mac address information is in different positions within
the packet. There is at least 3 MAC addresses used, source,destination and BSSID.
The order of operations for a Client to send is
0-31 wait timer -> Listen -> Announce slot time -> Wait SIF/DCF + ACK -> Transmit
If when someone is transmitting when you listen the wait timer is doubled and the
sequence starts from the begining.
–ETSI
Standard body for GSM mobile signals in Europe
—802.11b
Supports up to 3 Clean channels and different data rates up to 11mbps.
As you start to move away in distance from the access piont it negoaites a lower
speed on the fly. The modulation is changing to allow for more resilancy in packets
getting dropped for example barker 11 + DBPSK can allow for more dropped bits
than CCK-DQPSK
—802.11g
3 Clean channels, can allow 802.11b clients to connect, be wary that if a single
802.11b client connects the speed of everyone will drop to .11b speeds because
the modulation will have to be all DSSS not the higher data rate OFDM.
—802.11n
Sends data over 2 channels at the same time, uses relection and refraction to
its advantage, uses MIMO and multipath to its advantage to send multiple signals
at the same time, it has uses a 20mhz and 40mhz channel, has more sub channels.
Still uses OFDM, can have up to 4 radio’s
—802.11ac
support for 8 different radios, uses OFDM, and 256-QAM
—Frequency
Is the number of cycles (up and down) waves per second
1hertz – 1 cycle per second
1megahurtz – 1 million cycles per second
1ghertz – 1 billion cycles per second
—Amplitude
Amplitude is the strength of the signal which is the measure of the wave from the top (crest) of the cycle
to the pit or bottom of the cycle. The term used to describe a loss in signal is called “attenuation”
—Free path loss model
without any obsticals what is the degredation of the signal between the transmitter and the reciever.
—multipath
When there are multiple signals comming from the sender due to possible reflection, it is possible that both
signals to be received at the same time, this will degrade both signals being received.
—RF Mathematics
1 Watt = 1 Joule of power
1 Watt = 1v x 1A
Access pionts are measured in milliWatts (1000th of a Watt). usually 100millewatts.
//Decibels.
Used to express a strength, can be dBHz, 1 Decibel is 1 Tenth of a Bell
dBm is often used to express an transmitter power in relation to 1 milliwatt of power
dBi is often used to express an istropic antenna power, which goes in a sphere direction
dBd is another type of antenna called a “bipole” which goes in 1 direction.
The scale of dB doubles every 3 db
For example
-9dBm = (1/8) x 1milliwatt
-6dBm = (1/4) x 1milliwatt
-3dbm = (1/2) x 1milliwatt
0dbm = 1 x 1milliwatt
3dBm = 2 x 1milliwatt
6dBm = 4 x 1milliwatt
9dBm = 8 x 1milliwatt
If you want to convert from a dBi to a dbd measurement add + 2.14
If you want to convert from a dBd to a dBi measurement minus -2.14
Find the dBM if you have the mW
dbm = mw log10 x 10
dbm = 15 log10 x 10
dbm = 1.17 x10
dbm = 11.7
Find the mW if you have the dBm
mW = 10 ^(power) (dBm /10)
mW = 10 ^ (20/2)
mW = 10 ^ 10
mW = 100
—EIRP (effective Isotropic Radiated Power)
This is the amount of signal that is being sent from your transmitter,
counteries specify the amount of maximum EIRP and maximum dBm that can be sent.
EIRP can be calculated with the following equation
EIRP = dbm (transmit power) + dbi (antenna gain) – cable loss (db)
—LWAPP (Lightweight wireless access access piont)
Dump WAP access pionts which pull there configuration from the controller.
LWAPP is the protocol between the two.
—SA (standalone)
No Wireless lan controller sharing.
—WLC
16 SSID, 512 VLANs + support for trunks
changes the channels, transmit power and monitoring.
—Monday morning syndrome
When everyone gathers in 1 room and connects to the same AP. The WLC can deny some connections so that clients
connect to another AP automatically.
—Wism (Wireless Internal Service Module)
Blade which goes into a 6500 series Switch, can be a wireless controller
—WLCM-e
Expansion module which goes into ISR 2800 series router. wireless controller module
—Wireless Beacon
The AP sends a wireless beacon once every 100 ms, that has information about the wireless network.
Channel, speed and capabilities such as security
—Probe Request
Request information about about an AP (aka Active scanning)
—Beacon Scanning
Listen for information about an AP (aka Passive scanning)
—WLC change IP address
config
interface address management
—Product Hierachy
APS managed by WLC -> WLCs mananaged by WCS Prime -> WCS Prime Mananaged by Cisco Navigator.