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Lecture 26

CSE 123 Lecture Notes - Lecture 26: Carrier-Sense Multiple Access, Media Access Control, Frequency-Division Multiple Access


Department
Computer Science and Engineering
Course Code
CSE 123
Professor
Alex Snoeren
Lecture
26

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IEEE 802.11 Infrastructure
Bridges - different than what we know about bridges!
- We learned that bridges were “store-and-forward” devices that speak the SAME layer
- Used the SAME link layer
These bridges are NOT Ethernet cable to cable
- “Kind of” Ethernet, but also not
- Same bits of the Frame, BUT
- Runs a different media access control (MAC)
802.11 Frame Format
Issue: “arbitrary” devices listening! And may not ALL be the same!
Synchronization
- Synch, gain setting, energy detection, frequency offset compensation
SFD (Start Frame Delimiter) - 1111001110100000
Signal - data rate of the payload
Service - future use, 00: 802.11 complaint
HEC (Header Error Check) - protection of signal, service & length
| Synchronization | SFD | Signal | Service | Length | HEC | payload
^ - PLCP preamble -- | PLCP header |
(training sequence)
HOW are these bits going to be “encoded”???
WLAN: IEEE 802.11b
- Slow, and high power!
- But HUGE range
Data Rate
- 1, 2, 5.5, 11 Mbit/s
- User data rate MAX approx. 6 Mbit/s
Transmission range
- 300m outdoor, 30m indoor
- Max data rate ~10m indoor
Frequency
- 2.4 GHz channel
Physical Channels
12 channels available for use in the US
- EACH channel 22 MHz wide
- Only 3 orthogonal channels
- They are NOT “nice and separate
- Using ANY others causes interference
FDMA - 100 Mhz divided into 3 separate channels
Carrier Sense Multiple Access (CSMA)
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