Lecture 4: IEEE 802.11
description
Transcript of Lecture 4: IEEE 802.11
Lecture 4: IEEE 802.11
ECE 591
IEEE 802.11 Requirements• Wi-Fi often used by the public as a synonym for IEEE 802.11-wireless
LAN (WLAN).
• Design for small coverage (e.g. office, home)
• Low/no mobility
• High data-rate applications
• Ability to integrate real time applications and non-real-time applications
• Use un-licensed spectrum
802.11: Infrastructure Mode
• Architecture similar to cellular– networks station (STA)
• terminal with access mechanisms to the wireless medium and radio contact to the access point
– access point (AP)• station integrated into the wireless LAN
and the distribution system– basic service set (BSS)
• group of stations using the same AP– portal
• bridge to other (wired) networks– distribution system
• interconnection network to form one logical network (EES: Extended Service Set) based on several BSS
Distribution System
Portal
802.x LAN
Access Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access Point
STA1
STA2 STA3
ESS
802.11 - Architecture of an ad-hoc network
• Direct communication within a limited range– Station (STA):
terminal with access mechanisms to the wireless medium
– Independent Basic Service Set (IBSS):group of stations using the same radio frequency
802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3
IEEE standard 802.11mobile terminal
access point
fixedterminal
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLC
infrastructurenetwork
LLC LLC
IEEE 802.11 Physical Layer• Family of IEEE 802.11 standards:
– unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz
and 802.11b/g 802.11a
300 MHz
5.15-5.35 GHz 5.725-5.825 GHz
The IEEE 802.11 Family
Protocol
Release Data
Freq. Rate (typical)
Rate (max)
Range (indoor)
Legacy 1997 2.4 GHz 1 Mbps 2Mbps ?802.11a
1999 5 GHz 25 Mbps 54 Mbps
~30 m
802.11b
1999 2.4 GHz 6.5 Mbps 11 Mbps
~30 m
802.11g
2003 2.4 GHz 25 Mbps 54 Mbps
~30 m
802.11n
2008 2.4/5 GHz
200 Mbps
540 Mbps
~50 m
802.11a Modulation• Use OFDM to divide each physical channel (20
MHz) into 52 subcarriers (312.5 KHz each)– 48 data, 4 pilot
• Adaptive modulation– BPSK: 6, 9 Mbps– QPSK: 12, 18 Mbps– 16-QAM: 24, 36 Mbps– 64-QAM: 48, 54 Mbps
802.11 MAC Layer: Access Methods
• DFWMAC-DCF CSMA/CA (mandatory)– collision avoidance via randomized “back-off“ – ACK packet for acknowledgements
• DFWMAC-DCF w/ RTS/CTS (optional)– additional virtual “carrier sensing: to avoid hidden terminal
problem
• DFWMAC- PCF (optional)– access point polls terminals according to a list
Hidden and Exposed Terminals• Hidden terminals
– A sends to B, C cannot receive A – C wants to send to B, C senses a “free” medium (CS fails)– collision at B, A cannot receive the collision (CD fails)– A is “hidden” for C
• Exposed terminals– B sends to A, C wants to send to another terminal (not A or B)– C has to wait, CS signals a medium in use– but A is outside the radio range of C, therefore waiting is not necessary– C is “exposed” to B
BA C
Near and Far Terminals• Terminals A and B send, C receives
– signal strength decreases proportional to the square of the distance– the signal of terminal B therefore drowns out A’s signal– C cannot receive A
• If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer
• Also severe problem for CDMA-networks - precise power control needed!
A B C
MACA - collision avoidance• MACA (Multiple Access with Collision Avoidance) uses short signaling
packets for collision avoidance– RTS (request to send): a sender request the right to send from a receiver
with a short RTS packet before it sends a data packet– CTS (clear to send): the receiver grants the right to send as soon as it is
ready to receive• Signaling packets contain
– sender address– receiver address– packet size
• Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)
MACA examples• MACA avoids the problem of hidden terminals
– A and C want to send to B
– A sends RTS first– C waits after receiving
CTS from B
• MACA avoids the problem of exposed terminals– B wants to send to A, C
to another terminal– now C does not have
to wait for it cannot receive CTS from A
A B C
RTS
CTSCTS
A B C
RTS
CTS
RTS
MACA variant: DFWMAC in IEEE802.11
idle
wait for the right to send
wait for ACK
sender receiver
packet ready to send; RTS
time-out; RTS
CTS; data
ACK
RxBusy
idle
wait fordata
RTS; RxBusy
RTS; CTS
data; ACK
time-out data; NAK
ACK: positive acknowledgementNAK: negative acknowledgement
RxBusy: receiver busy
time-out NAK;RTS
802.11 CSMA/CA
• CSMA: Listen before transmit• Collision avoidance
– when transmitting a packet, choose a backoff interval in the range [0, CW]
• CW is contention window• Count down the backoff interval when medium
is idle– count-down is suspended if medium becomes busy
• Transmit when backoff interval reaches 0
Congestion Avoidance: Example
data
waitB1 = 5
B2 = 15
B1 = 25
B2 = 20
data
wait
B1 and B2 are backoff intervalsat nodes 1 and 2
B2 = 10busy
busy
802.11 – RTS/CTS + ACK• Sender sends RTS with NAV (Network allocation Vector, i.e. reservation
parameter that determines amount of time the data packet needs the medium)
• Receiver acknowledges via CTS (if ready to receive)CTS reserves channel for sender, notifying possibly hidden stations
• Sender can now send data at once, acknowledgement via ACKOther stations store NAV distributed via RTS and CTS
t
SIFS
DIFS
data
ACK
defer access
otherstations
receiver
sender data
DIFS
new contention
RTS
CTSSIFS SIFS
NAV (RTS)NAV (CTS)
Not END