IEEE 802.11
IEEE 802.11 neboli Wi-Fi. Wi-Fi je zkratka pro termin Wireless
Fidelity, kterym se oznacuje jeden z technologickych standardu
bezdratove komunikace (prakticky jsou jako Wi-Fi kompatibilni
oznacena bezdratova zarizeni ruznych vyrobcu, ktere spolu navzajem
bez problemu spolupracuji). Technologie Wi-Fi vyuziva bezlicencni
frekvencni pasmo 2,4GHz. Technicky jde o standard IEEE (Institute
of Electrical and Electronics Engineers) 802.11b pro
vysokorychlostni bezdratove prenosy, datove kompatibilni s
metalickou (dratovou) siti ethernet. Wi-Fi umoznuje uzivatelum
snadne pripojeni k siti v maximalni prenosove rychlosti 125Mbit/s
(Afterburner technology) na vzdalenost nekolika stovek metru ci
kilometru. (Vzdalenost, na kterou se lze pripojit vzdy zalezi na
typu pouzite anteny a konkretnich mistnich podminkach).
Aktualizovano: Nejnovejsi Wi-Fi zarizeni umeji komunikovat
rychlosti az 270Mbit/s. Toto je umozneno novou technologii MIMO
(Multiple In/Multiple Out), ktera umoznuje vysilat i prijimat
nekolik datovych toku najednou v ramci jednoho kanalu.
Zajimavost:
Vzdalenost 200 kilometru preklenuli prostrednictvim Wi-Fi
vysokoskolaci z Cincinnati. Komunikace probihala plnou rychlosti
standardu 802.11b – 11Mbit/s, bez zesileni signalu. Pro vytvoreni
tohoto noveho svetoveho rekordu pouzili vlastnorucne upravenou
satelitni antenu o prumeru 3,66 metru.
Historie IEEE 802.11
V roce 1991 si firmy vyrabejici tato bezdratova zarizeni
uvedomily, ze pokud chteji dosahnout co nejvetsiho rozsireni
bezdratovych LAN siti a jejich vseobecneho prijeti, musi byt
standardizovany. Zacaly proto smerovat sve konani vzhledem k tomuto
dulezitemu kroku.
Nekdy v roce 1992 zacali vyrobci bezdratovych zarizeni vyvijet
produkty pracujici v bezlicencnim pasmu 2.4GHz. S timto krokem
prislo rozsireni bezdratovych siti do dalsich dvou, velmi
lukrativnich trhu. Prvnim bylo zdravotnictvi, ktere zacalo pouzivat
mobilni prenosna zrizeni pro pristup k lekarskym zaznamum pacientu.
Pocitace si take nasly cestu do skol a trid, skoly a vyukova
zarizeni zacala instalovat bezdratove site aby se vyhnula pokladani
kabelu klasickych lokalnich siti.
V roce 1997 vydala standardizacni organizace IEEE (Institute of
Electrical and Electronic Engineers) standard 802.11 upravujici
pravidla a technicke parametry pro lokalni bezdratove site.
Standard obsahuje definici pro prenos v infracervenem pasmu a dve
metody prenosu v pasmu 2.4GHz. Maximalni rychlost prenosu dle
tohoto standardu cini 2 Mbit/s.
V roce 1998 byl schvalen standard 802.11c, ktery pouze resi praci
komunikacnich mostu (bridges) v ramci podvrstvy MAC. Je pouzivan
vyrobci mostu a pristupovych bodu.
V roce 1999 byl schvalen novy standard 802.11b, poskytujici
prenosovou kapacitu az 11Mbit/s na fyzicke vrstve v pasmu 2.4GHz,
pouziva technologii CCK (Complementary Code Keying).
V roce 1999 byl dale schvalen standard 802.11a, poskytujici
prenosovou kapacitu az 54Mbit/s na fyzicke vrstve, pracuje vsak v
licencovanem pasmu 5GHz. Prace na tomto standardu zapocaly jeste
pred zapocetim praci na 802.11b (jak pismenko napovida), ale
implementace si diky vyssi slozitosti vyzadala vice casu. Pouziva
OFDM modulaci (Orthogonal Frequency Division Multiplexing).
V roce 2001 byla schvalena norma 802.11d, ktera upravuje 802.11b
pro jine kmitocty, nez je pasmo 2.4GHz. (Hlavne pro pasmo
5GHz)
V roce 2003 byla schvalena norma 802.11g, ktera nabizi prenosovou
rychlost az 54Mbit/s v bezlicencnim pasmu 2.4GHz. Je zpetne
kompatibilni s normou 802.11b (pomoci CCK a RTS/CTS). Pouziti
technologii CCK a OFDM je povinne.
Velmi pekny clanek o Wi-Fi:
(visit link)
CACHE:
Specialni vybaveni:
1) Zarizeni podporujici standard IEEE 802.11b nebo IEEE 802.11g
Na souradnicich N 50° 02.772 E 014° 26.004 najdete e-mailovou
adresu. Spocitejte vsechny znaky pred zavinacem @ a dostanete cislo
X, pote spocitejte vsechny znaky za zavinacem @ a dostanete cislo
Y. Dale zde naleznete cislo ve tvaru KLMNO. Nyni budete potrebovat
zarizeni podporujici standard IEEE 802.11b Zapnete jej a dejte
vyhledat vsechny dosptupne site. Zjistete cislo ve tvaru PQRST.
Nyni vse dosadte do vzorcu:
A = K+N+R-X-O
B = T+Y+L-M-Q
C = P+S+L-X-N
D = R-T-S-K
E = Y+M-P-N
F = L+R+O-X-P-T
G = Q+T-K-N-S
H = P+Q+R-Y-O
I = X+O+Y-L-R
J = M+R-T-S-N-K-Q
Cache je na souradnicich:
N 50° AB.CDE
E 014° FG.HIJ
Cache je na fregventovanem miste, je zde spousta mudlu, budte
opatrni. Nejlepsi je odlov nekdy rano, nebo taky pozde vecer.
POZOR! Pro uznani nalezu je nutne, aby jste v logu na gc.com
uvedli, jake zarizeni jste pouzili pro odloveni teto cache.
(znacka, model, typ) napr.: PCMCIA ASUS WL-100G Deluxe
IEEE 802.11
IEEE 802.11, the Wi-Fi standard, denotes a set of Wireless
LAN/WLAN standards developed by working group 11 of the IEEE
LAN/MAN Standards Committee (IEEE 802). The term 802.11x is also
used to denote this set of standards and is not to be mistaken for
any one of its elements. There is no single 802.11x standard. The
term IEEE 802.11 is also used to refer to the original 802.11,
which is now sometimes called "802.11legacy." For the application
of these standards see Wi-Fi.
The 802.11 family currently includes six over-the-air modulation
techniques that all use the same protocol. The most popular (and
prolific) techniques are those defined by the b, a, and g
amendments to the original standard; security was originally
included and was later enhanced via the 802.11i amendment. Other
standards in the family (c–f, h–j, n) are service enhancements and
extensions or corrections to previous specifications. 802.11b was
the first widely accepted wireless networking standard, followed
(somewhat counterintuitively) by 802.11a and 802.11g.
802.11b and 802.11g standards use the 2.4 gigahertz (GHz) band,
operating (in the USA) under Part 15 of the FCC Rules and
Regulations. Because of this choice of frequency band, 802.11b and
802.11g equipment can incur interference from microwave ovens,
cordless telephones, Bluetooth devices, and other appliances using
this same band. The 802.11a standard uses the 5 GHz band, and is
therefore not affected by products operating on the 2.4 GHz
band.
Which part of the radio frequency spectrum may be used varies
between countries, with the strictest limitations in the USA. While
it is true that in the USA 802.11a and g devices may be legally
operated without a license, it is not true that 802.11a and g
operate in an unlicensed portion of the radio frequency spectrum.
Unlicensed (legal) operation of 802.11 a & g is covered under
Part 15 of the FCC Rules and Regulations. Frequencies used by
channels one (1) through six (6) (802.11b) fall within the range of
the 2.4 gigahertz Amateur Radio band. Licensed amateur radio
operators may operate 802.11b devices under Part 97 of the FCC
Rules and Regulations that apply.
CACHE:
Special equipment:
1) Device with IEEE 802.11b or IEEE 802.11g standards
At the coordinates N 50° 02.772 E 014° 26.004 you find e-mail
address. Count all letters before @ and you will get number X. Now
count all characters after @ and you will get number Y. You find
here namber KLMNO too. Now you will need device with IEEE 802.11b
standard. Switch on device and find available wireless networks.
Write down the number PQRST. Now fill all it into the formula:
A = K+N+R-X-O
B = T+Y+L-M-Q
C = P+S+L-X-N
D = R-T-S-K
E = Y+M-P-N
F = L+R+O-X-P-T
G = Q+T-K-N-S
H = P+Q+R-Y-O
I = X+O+Y-L-R
J = M+R-T-S-N-K-Q
The cache is placed at the coordinates:
N 50° AB.CDE
E 014° FG.HIJ
NOTICE! For approval „Find it“ you must write in log, what IEEE
802.11 device you use. (producer, type) For example: PCMCIA ASUS
WL-100G Deluxe
