IEEE 802.11b Wireless LANs

 
WiFi Networks:
IEEE 802.11b Wireless LANs
 
Carey Williamson
Department of Computer Science
University of Calgary
 
Background (1 of 2)
 
In many respects, the IEEE 802.11b wireless LAN
(WLAN) standard is similar to that for classic IEEE
802.3 (Ethernet) LANs
Similarities:
LAN with limited geographic coverage
multiple stations, with 48-bit MAC addresses
shared transmission medium (broadcast technology)
CSMA-based Medium Access Control protocol
comparable data rates (11 Mbps vs 10 Mbps)
 
Background (2 of 2)
 
But there are also many distinct differences:
wireless (air interface) versus wired (coax)
wireless propagation environment (multipath)
higher error rate due to interference, etc.
successful frames are ACKed by receiver
mobile stations versus fixed stations
half-duplex versus full-duplex operation
“hidden node” and “exposed node” problems
potential asymmetries of links
CSMA/CA versus CSMA/CD
multiple data transmission rates (1, 2, 5.5, 11)
 
Some WiFi Features
 
Infrastructure mode vs “ad hoc” mode
Access Point (AP) sends “beacon frames”
Mobiles choose AP based on signal strength
Multiple channel access protocols supported
CSMA/CA (DCF);  PCF;   RTS/CTS
MAC-layer can provide error control,
retransmission, rate adaptation, etc.
Direct Sequence Spread Spectrum (DSSS)
signal spread across 14  22-MHz channels
 
Where Does Wireless RF Live?
 
 
902-928 MHz
 
2400-2483.5 MHz
 
5725-5850 MHz
 
802.11/802.11b,g
 
802.11a
 
Bluetooth
 
Cordless Phones
 
Home RF
 
Baby Monitors
 
Microwave Ovens
 
Old Wireless
 
ISM (Industrial, Scientific, Medical) band
 
Telnet, FTP, Email, Web, etc.
 
IP, ICMP, IPX
 
TCP, UDP
 
        Logical Link Control - 802.2
(Interface to the upper layer protocols)
 
MAC
 
802.3, 802.5, 
802.11
 
Physical Layer Convergence Protocol
 
LAN: 10BaseT, 10Base2, 10BaseFL
 
WLAN: FHSS, DSSS, IR
 
Application
 
Presentation
 
Session
 
Transport
 
Network
 
Data Link
 
Physical
Wireless lives at
  Layers 1 & 2
       only!
 
Protocol Stack View
11 Mbps bandwidth
shared
” by all devices
in the Cell!
 
Wireless Cells
 
1
 
6
 
11
 
1
 
1
 
11
 
Multiple Wireless APs
 
Carrier Sense Multiple Access with 
Collision Avoidance
 
 Device wanting to transmit senses the medium (Air)
 
 If medium is busy - defers
 
 If medium is free for certain period (DIFS) - 
transmits
 
How CSMA-CA works:
 
Latency can increase if “air” is very busy! Device
has hard time finding “open air” to send frame!
 
*
 DIFS - Distributed Inter-Frame Space (approx 128 µs)
 
Medium Access Control (MAC)
 
*
 SIFS - Short Inter-Frame Space (approx 28 µs)
 
 Every frame is acked - except broadcast and multicast!
“Air” is free
for DIFS  time
period
Receive ACK
that frame was
received intact!
send frame
 
source
 
dest
 
others
 
DIFS
 
SIFS
All other
devices
must defer
while “air” is
busy
 
data
 
ack
 
NAV: defer access
 
MAC Protocol (Cont’d)
 
MAC-Layer Retransmission
 
If no ACK received “right away”, then the sender
retransmits the frame again at the MAC layer
indicates frame (or ACK) was lost/corrupted
very short timeout (e.g., 1 msec)
exponential backoff (doubling) if repeated loss
Typically recovers before TCP would notice
Max retransmission limit (e.g., 8)
May do MAC-layer rate adaptation or frame
fragmentation if channel error rate is high
 
Other MAC Protocols Supported
 
Point Coordination Function (PCF)
AP polls stations in turn to see if frames to send
useful for real-time traffic
Request-To-Send/Clear-To-Send (RTS/CTS)
reservation-based approach (ask permission)
useful for very large frames
useful for solving the “hidden node” problem
request asks for clearance (permission) to send
request also indicates time required for transmit
 
Frame Formats
 
Two frame formats available:
long preamble
short preamble
Configuration option for NIC and AP
 
Variable-size frames (max 2312 data bytes)
 
16-bit Cyclic Redundancy Code (CRC) for error
checking of frames
 
Long Preamble = 144 bits
 
 Interoperable with older 802.11 devices
 
 Entire Preamble and 48 bit PLCP Header sent at 
1 Mbps
 
128 bit Preamble
(Long)
 
16 bit
Start
Frame
Delimiter
 
Signal
Speed
1,2,5.5,
11
Mbps
 
Service
(unused)
 
Length
of
Payload
 
16 bit
CRC
 
Payload
0-2312 bytes
Transmitted at 1 Mbps
Transmitted at X Mbps
 
Frame Format (Long Preamble)
 
Short Preamble = 72 bits
 
 Preamble transmitted at 1 Mbps
 
 PLCP Header transmitted at 2 Mbps
 
 more efficient than long preamble
 
56 bit
Preamble
 
Payload
0-2312 bytes
 
16 bit
Start
Frame
Delimiter
 
Signal
Speed
1,2,5.5,
11
Mbps
 
Service
(unused)
 
Length
of
Payload
 
16 bit
CRC
Transmitted
 at
2 Mbps
 
Frame Format (Short Preamble)
 
Even More Features
 
Power Management
mobile nodes can “sleep” to save power
AP will buffer frames until client requests them
AP can use virtual bitmap field in beacons to indicate
which stations have data waiting
Security
Wired Equivalent Privacy (WEP)
not very secure at all!
 
Summary
 
IEEE 802.11b (WiFi) is a wireless LAN technology that
is rapidly growing in popularity
Convenient, inexpensive, easy to use
Growing number of “hot spots” everywhere
airports, hotels, bookstores, Starbucks, etc
Many deployments now have IEEE 802.11g (54 Mbps)
or IEEE 802.11a (also 54 Mbps)
Some deployments have IEEE 802.11n (> 100 Mbps)
U of C WLAN has about 1000 WiFi Access Points (APs)
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IEEE 802.11b wireless LAN standard shares similarities with Ethernet LANs but also has distinct differences due to its wireless nature. It operates in the industrial, scientific, and medical RF bands, supporting features like infrastructure mode, multiple channel access protocols, and DSSS signal spread. The protocol stack involves layers 1 and 2, with cells managed by access points using specific channels. This standard allows for mobile devices to connect and roam within a WLAN environment.

  • Wireless LAN
  • IEEE 802.11b
  • RF bands
  • Access Point
  • Protocol Stack

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  1. WiFi Networks: IEEE 802.11b Wireless LANs Carey Williamson Department of Computer Science University of Calgary

  2. Background (1 of 2) In many respects, the IEEE 802.11b wireless LAN (WLAN) standard is similar to that for classic IEEE 802.3 (Ethernet) LANs Similarities: LAN with limited geographic coverage multiple stations, with 48-bit MAC addresses shared transmission medium (broadcast technology) CSMA-based Medium Access Control protocol comparable data rates (11 Mbps vs 10 Mbps)

  3. Background (2 of 2) But there are also many distinct differences: wireless (air interface) versus wired (coax) wireless propagation environment (multipath) higher error rate due to interference, etc. successful frames are ACKed by receiver mobile stations versus fixed stations half-duplex versus full-duplex operation hidden node and exposed node problems potential asymmetries of links CSMA/CA versus CSMA/CD multiple data transmission rates (1, 2, 5.5, 11)

  4. Some WiFi Features Infrastructure mode vs ad hoc mode Access Point (AP) sends beacon frames Mobiles choose AP based on signal strength Multiple channel access protocols supported CSMA/CA (DCF); PCF; RTS/CTS MAC-layer can provide error control, retransmission, rate adaptation, etc. Direct Sequence Spread Spectrum (DSSS) signal spread across 14 22-MHz channels

  5. Where Does Wireless RF Live? ISM (Industrial, Scientific, Medical) band 902-928 MHz 2400-2483.5 MHz 5725-5850 MHz Old Wireless 802.11a 802.11/802.11b,g Bluetooth Cordless Phones Home RF Baby Monitors Microwave Ovens

  6. Protocol Stack View Application Telnet, FTP, Email, Web, etc. Presentation Session TCP, UDP Transport IP, ICMP, IPX Network Logical Link Control - 802.2 (Interface to the upper layer protocols) Data Link MAC Wireless lives at Layers 1 & 2 only! 802.3, 802.5, 802.11 Physical Layer Convergence Protocol Physical LAN: 10BaseT, 10Base2, 10BaseFL WLAN: FHSS, DSSS, IR

  7. Wireless Cells Access Point coverage area is called a Cell Access Point Channel 6 ESSID: NAI 11 Mbps bandwidth shared by all devices in the Cell! Range per Access Point is 100m In Canada/US, there are eleven 802.11 channels Only channels 1, 6 and 11 are non-overlapping Computers can roam between cells

  8. Multiple Wireless APs 1 1 6 11 11 1

  9. Medium Access Control (MAC) Carrier Sense Multiple Access with Collision Avoidance How CSMA-CA works: Device wanting to transmit senses the medium (Air) If medium is busy - defers If medium is free for certain period (DIFS) - transmits Latency can increase if air is very busy! Device has hard time finding open air to send frame! * DIFS - Distributed Inter-Frame Space (approx 128 s)

  10. MAC Protocol (Contd) others source dest Air is free for DIFS time period DIFS NAV: defer access data send frame All other devices must defer while air is busy SIFS Receive ACK that frame was received intact! ack Every frame is acked - except broadcast and multicast! * SIFS - Short Inter-Frame Space (approx 28 s)

  11. MAC-Layer Retransmission If no ACK received right away , then the sender retransmits the frame again at the MAC layer indicates frame (or ACK) was lost/corrupted very short timeout (e.g., 1 msec) exponential backoff (doubling) if repeated loss Typically recovers before TCP would notice Max retransmission limit (e.g., 8) May do MAC-layer rate adaptation or frame fragmentation if channel error rate is high

  12. Other MAC Protocols Supported Point Coordination Function (PCF) AP polls stations in turn to see if frames to send useful for real-time traffic Request-To-Send/Clear-To-Send (RTS/CTS) reservation-based approach (ask permission) useful for very large frames useful for solving the hidden node problem request asks for clearance (permission) to send request also indicates time required for transmit

  13. Frame Formats Two frame formats available: long preamble short preamble Configuration option for NIC and AP Variable-size frames (max 2312 data bytes) 16-bit Cyclic Redundancy Code (CRC) for error checking of frames

  14. Frame Format (Long Preamble) Long Preamble = 144 bits Interoperable with older 802.11 devices Entire Preamble and 48 bit PLCP Header sent at 1 Mbps Transmitted at 1 Mbps Signal Speed 1,2,5.5, 11 Mbps Length of Payload 16 bit Start Frame Delimiter 16 bit CRC Service (unused) Payload 0-2312 bytes 128 bit Preamble (Long) Transmitted at X Mbps

  15. Frame Format (Short Preamble) Short Preamble = 72 bits Preamble transmitted at 1 Mbps PLCP Header transmitted at 2 Mbps more efficient than long preamble Transmitted at 2 Mbps Transmitted at 1 Mbps Transmitted at X Mbps Signal Speed 1,2,5.5, 11 Mbps Length of Payload 16 bit Start Frame Delimiter 16 bit CRC Service (unused) Payload 0-2312 bytes 56 bit Preamble

  16. Even More Features Power Management mobile nodes can sleep to save power AP will buffer frames until client requests them AP can use virtual bitmap field in beacons to indicate which stations have data waiting Security Wired Equivalent Privacy (WEP) not very secure at all!

  17. Summary IEEE 802.11b (WiFi) is a wireless LAN technology that is rapidly growing in popularity Convenient, inexpensive, easy to use Growing number of hot spots everywhere airports, hotels, bookstores, Starbucks, etc Many deployments now have IEEE 802.11g (54 Mbps) or IEEE 802.11a (also 54 Mbps) Some deployments have IEEE 802.11n (> 100 Mbps) U of C WLAN has about 1000 WiFi Access Points (APs)

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