I thought it'd be cool to share some of my revision notes :cool:
Chap 1: Introduction.
Wireless started out in the mid eighties (1986) for warehouses & retail business to ease stock control. It was from this technology that Enterprise and HomeUsers saw that they could be freed from the wires on their desks. Wireless data networks span a large area of technologies from infrared to satellite, fortunately cisco's Fundamentals course only covers LANS (Local Area Networks), Cisco is also part of the body which standardises wireless communications -Wireless Fidelity (WI-FI), This is apparently important to remember because they provide a stamp of approval, which means different vendors will interact. Here a table about wireless tech's which is shown on the presentation:
PAN\ LAN\ MAN\ WAN
Personal Area Network Local Area Network Metropolitan Area Network Wide Area Network
Standards Bluetooth 802.11a, 802.11b, 802.11g, HiperLAN2 802.11 MMDS, LMDS GSM, GPRS, CDMA, 2.5 - 3G
Range Short Medium Medium to Long Long
Application Peer2Peer or Device to Device Enterprise Networks Fixed, Last Mile Access Mobile Phones, PDAs.
Chap 2: Radio Freq Spread Spectrum.
This chapter talks about radio transmission, mainly frequency modulation type stuff, points I've picked up....
Radio Frequencies are licensed, i.e. Radio & TV stations need licences to broadcast; the powers that be put aside a portion of the frequency spectrum for unlicensed use. The part that Cisco use is the ISM (Inductrial, Scientific & Medical) Frequencies. In the spectrum of frequencies, from low to high, where audio is at the low end, and light, x-rays etc. are at the higher, these frequencies are available to use:
- 902 - 928 MHz & 26MHz
- 2.4 - 2.4835 GHz & 83.5MHz
IEEE 802.11b & 802.11g
HyperLan, HyperLan2 & 802.11a
IEEE 802.11 is the industry standard for wireless lans, and it comes in 3 flavours:
802.11b 802.11a 802.11g Freq Band 2.4GHz 5GHz 2.4GHz Availability World USA & Asia + Percific Wold Max Data Rate 11Mbs 54Mbs 54Mbs Sources of Interference Cordless Phone, Microwaves , Wireless Video or Bluetooth HyperLan Devices Same as .11b
Notice, that 802.11a isn't available worldwide, so Cisco don't use it so much in their products, 802.11g is the way forward ;)
A Note about the Laws of Radio Dynamics:
- The Higher the Data Rate, the Shorter the Range (Distance)
- The Higher the Power Output, The Longer the Range, the lower the Battery Life (Obviously)
- The Higher the Radio Frequency, The Higher the Data Rate, but the Shorter the Range
RF Technologies is DATA sent over the air waves, with a primary goal of sending as much as quickley as possible.It is a half duplex technology, using the same frequency to send and recieve - devices take in turns, and because they use the ISM frequencies no Licenses are needed.
Different Spread Spectrum RF Technologies are:
- FHSS (Frequency Hopping Spread Spectrum)
- DSSS (Direct Sequence Spread Spectrum)
- OFDM (Orthogonal Frequency Division Modulation)
Noise or Interference is caused by other transmissions; The further away you move from a transmission source, the more the noise interfers. Other factors that affect the transmission is the complexity of the modulation; The more complex the modulation, the shorter the distance away from the transmitter the noise begins to interfere... The more noise there is on a signal, the slower the data rate.
Signal Modulation is produced by the input of data and a carrier signal, the picture show's (roughly/badly) what happens. Aironet devices have 3 type of modulations, and what is used depends on the data rate:
- BPSK (Binary Phase Shift Keyed) for 1Mbs
- QPSK (Quardrature Phase Shift Keying) for 2Mbs
- CCK (Complementary Code Keying) or 5.5Mbs & 11Mbs
Direct Sequence Modulation in DSSS:
The data is broken into chunks, which can be send simultaneously down different channels. The chunks are combined with a chipping code - this is the chipping sequence, this special code allows one of the chunks to do missing (due to interference) and it's contents can be calculated from the other chunks - a little like RAID5 I guess.
The DSSS 2.4Ghz can be boken into 11Channels of 22Mhz, but these overlap - there is a set of 3 Channels which don't overlap, so different devices can use the same frequency with different channels so they don't interfere with each other.
Note: The more data you want to send, the more bandwidth you need i.e. the more of the frequency spectrum you will need to use. This combined with the LAWS of RF , i.e. the further away from a signal you are the less bandwidth you have available causes a problem with Data Comms.
Cisco implement Rate shifting, this is where as your signal drops then so does your connection speed. In the case of 802.11b DSSS the stages are 11Mbs -> 5.5Mbs -> 2 Mbs -> 1Mbs.
From the above two paragraphs you can work out the maxium available bandwidth for 802.11b, assuming you were close enough to the AP, you could have 3 devices on different channels running at 11Mbs which is a total bandwith of 33Mbs :)
802.11b uses OFDM as it's RF Technology, the main thing about this is it converts the frequency into sub carriers (or sub channels) if you use different modulation on these channels you get different speeds:
- BPSK = 6 & 9 Mbs
- QPSK = 12 & 18 Mbs
- 16QAM = 24 & 36 Mbs
- 64QAM = 48 & 54 Mbs
The 5GHz Range can be broken into 52 of the avilable 64 sub carriers, and 48 of those are used to transmit data (12 are used as zero side guards & 4 are used for sync + tracking) The 5GHz are broken into UNII Bands, what's important is UNII 1 & UNII 2 are for inside use and UNII 3 is for out-doors !
Rate shifing in 802.11a is the same as in .11b other than there are more layers of bandwidth degredation.
802.11a has 8 channels avilable in it's UNII bands. It also has different scaleability , 8 x 54 is 432Mbs of bandwidth but 802.11a doesn't have as far range as 802.11b on the 2.5GHz frequency.