Презентации / 1-2 Wireless Intro
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Who Gets the Higher Transmission Priority?
1.Access Point
2.Wireless Client
3.They all get equal access
802.11 adopted CSMA/CA (Carrier Sense Multiple Access)/Collision Avoidance model where nodes (AP or clients) avoid collisions by transmitting only when the channel is sensed to be "idle”.
Other channel access models have been proposed. These models allow the AP to organize channel access in a structured way, but these have never been widely adopted
1.Legacy Point Coordination Function (PCF)
2.802.11e Hybrid Controlled Channel Access (HCCA)
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Basics of RF
Basic Understanding of Radio…
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AC Frequency 50 Hz or |
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Battery is DC |
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50 CPS – Cycles Per |
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Typical home is AC |
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Second |
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Direct Current |
Alternating Current |
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Waves travel back and forth so fast they actually leave the wire
How fast the AC current goes, is its “frequency” AC is very low frequency 50Hz (Cycles Per Second)
Radio waves are measured in kHz, MHz and GHz
The lower the frequency, the physically longer the radio wave –
Higher frequencies have much shorter waves, and as such, it
takes more power to move them greater distances. This is why 2.4 GHz goes further then 5 GHz (given same amount of RF
power).
Popular Radio Frequencies:
AM Radio 520-1610 kHz
Shortwave 3-30 MHz
FM Radio 88 to 108 MHz
Aviation 108-121 MHz
Weather Radio 162.40 MHz
GSM Phones 900 & 1800 MHz
DECT Phones 1900 MHz
Wi-Fi 802.11b/g/n 2.4 GHz
Wi-Fi 802.11a/n 5 GHz
Особенности
•Почему не МГц, не кГц? При ГГц радиоволны с нужными хар-ми
•Частота – Дл
•Чем выше частота, меньше область покрытия и выше DataRate
•Чем меньше частота, больше область покрытия и ниже DataRate.
•f -> бесконечность???
•Что происходит с сигналом при передаче в пространстве?
•Препятствия (преодоление + поглощение), отражение и т.д.
•Чем более F – хуже преодолевает препятствие.
•60 ГГц?
•Оптимально – 2,4/5 Ггц
•Баланс: Частота (DataRate)– Область покрытия (100 мВт) – Диапазон применяемых частот (нелиц)
Wi-Fi Radio Spectrum
2.4 GHz |
5 GHz |
Wi-Fi is an “unlicensed” service
It has beginnings in the ISM Industrial Scientific Medical band where it was not desirable or profitable to license such short range devices.
The first frequencies available for Wi-Fi use were in the 2.4 GHz range
As Wi-Fi popularity and usage increased, the regulatory bodies allocated additional spectrum in the
5 GHz band.
The spectrum we use today is also used
by Amateur (Ham Radio) and other services such as radio location (radar).
There is more bandwidth in 5 GHz with mechanisms in place to co-exist with licensed services such as (RADAR) RAdio Detection And Ranging using (DFS) Dynamic Frequency Selection (method of automatic channel selection)
Особенности
• Совпадение частоты?? Интерференция.
802.11 is the IEEE standard defining Wireless Access running in the Unlicensed Bands
q802.11 b/g (1997 / 1999 / 2003)
Ø11 Mb / 54 Mbps and runs in the 2.4 GHz frequency band
Ø3 non-overlapping 20 MHz channels (1, 6, 11) – or 4? (1, 5 , 9, 13)? – Hint: it is an odd number
Channel reuse scheme in the Network
Neighboring APs use different channels to reduce interference.
On 2.4 GHz, the “Reuse cluster” size is equal to 3
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Access
Point
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Особенности
• Принцип триангуляции
• Наличие этажей (учет вертикалей)
802.11 is the IEEE standard defining Wireless Access running in the Unlicensed Bands
q802.11 b/g (1997 / 1999 / 2003)
Ø11 Mb / 54 Mbps and runs in the 2.4 GHz frequency band
Ø3 non-overlapping 20 MHz channels (1, 6, 11) – or 4? (1, 5 , 9, 13)? – Hint: it is an odd number
q802.11 a |
(1999) |
Ø54 Mbps and runs in the 5 GHz frequency band (5.1 – 5.7 GHz)
Ø16 non-overlapping 20 MHz channels in ETSI or 3.464 Gbps with DFS
ØShorter range than 802.11 b/g
q802.11 n |
(2009) |
ØBackwards compatible to 802.11 a/b/g but adding the powerful MIMO technology
Ø144 Mbps runs in 2.4 GHz and 450Mbps in 5 GHz band
ØEither 20 MHz or 40 MHz wide channels in 5 GHz
ØMost useful running in the 5GHz band à Seven 40 MHz channels