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Fourpak Packet Radio Group
UK Amateur Packet Radio Conference 2004 Proceedings
Presentation 5: Wireless LAN Basics - original text - HTML version
Wireless LAN Basics For Amateurs
Anthony Martin M1FDE
15th May 2004
Wired vs. Wireless LAN
WLAN Architectures - 1
WLAN Architectures - 2
Mesh Network
Standard equipment not designed for mesh.
6. WLAN Transceivers
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| Access Point (AP)
| PCMCIA Card
| USB device
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| PCI Card
| WLAN enabled PDA
| WLAN VOIP Phone
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7. Standards
- Main types of device
- 802.11 – 1 & 2Mbps, FHSS, DSSS and IR
- 802.11b – 2.4GHz DSSS 5.5 & 11Mb/s
- 802.11a – 5GHz OFDM 54Mb/s
- 802.11g – 2.4GHz OFDM 22-54Mb/s
- Wi-Fi – certification programme
- WECA (wireless compatability certification alliance)
IEEE 802 Standards
802
Overview
| 802.1
Management
| 802.2 Logical Link Control
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802.3 MAC
10BaseT
802.3 PHY
| 802.11 MAC
(Protocol)
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802.11
PHY | 802.11a
PHY | 802.11b
PHY
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802.11 Standards
| 802.11a | 5GHz OFDM PHY
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| 802.11b | 2.4GHz CCK PHY
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| 802.11c | 802.11 bridging
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| 802.11d | International roaming
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| 802.11e | QoS/efficiency enhancements
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| 802.11F | Inter AP protocol
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| 802.11g | 2.4GHz OFDM PHY
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| 802.11h | 5GHz regulatory extensions
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| 802.11i | Security enhancements
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| 802.11j | Japan 5GHz band extensions
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| 802.11k | Radio resource measurement
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| 802.11l | Skipped (typographically unsound)
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| 802.11m | Maintenance
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| 802.11n | High throughput PHY
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Modulation
DSSS = Direct Sequence Spread Spectrum
- Uses higher-rate chipping code to widen spectrum.
- Uses more than the necessary bandwidth.
- Provides immunity to narrowband interferers
- Improves reuse of spectrum
- DSSS is based on PSK or QPSK (phase modulation)
- Spreading gain is negligible though for WLAN!
OFDM = Orthogonal Frequency Division Multiplex
- Multiple QPSK (or QAM) modulated carriers
- OFDM isn’t really spread-spectrum at all.
1 Mb/s Spreading Modulation
5.5MB/s CCK Modulation
Arbitration in WLAN
- DCF - Distributed Coordination Function
- Collision-sensed with backoff
- Contention-resolution phase - RTS/CTS
- Seizes air around both stations before traffic
- PCF - Point Coordination Function
- Access point manages timeslots
- All headers transmitted in common 1Mbps format
- Switches modulation for the payload
- Data rates auto-negotiated and adaptive
Platform Compatibility
- Desktops
- USB dongles
- PCI cards
- Ethernet-connected bridge
- Laptops
- PCMCIA/Cardbus cards
- Palmtops
- Compact-flash cards
- Embedded devices
- CPUs as small as PIC micro
Operating System Compatibility
- Large range compatible with Windows. As cheap as £12 (Recent offer at Maplin)
- Minimum Win95. **No DOS drivers supplied**. (Not to say it couldn’t be done)
- Many cards supported under Linux.
- Often have common chipset (Prism1, 2, 3)
- To support "packet sniffer" software (eg Ethereal), choice of card is further restricted, and even more OS-dependent (some cards only under XP)
- Cards have on-board Flash firmware.
- Some cards can be used as "hostAP" under Linux - use PC as an access point.
Amateur Networks
- Distributed networks
- Access points connected by fixed links
- Routed or bridged architectures
- Mesh
- Linux-based "meshbox"; plug-in-and-go
- WLAN node generally fixed to one mode of operation. Linux HostAP is the exception
- Modified access point firmware
Security
- WEP - "wired equivalent privacy"
- Easily broken. Prevents accidental access but won’t stop serious attack.
- Requires a shared secret key. Difficult to control distribution.
- 802.11i WPA (AES encryption)
- Acceptable on amateur networks?
- Encyption no, password hashes - yes
- MAC address filtering
- can be sniffed, can be cloned, useful closed door
- No Cat Networks - login methods
- What is the requirement- how secure is packet?
Specifications
- Transmit power
- Max 20dBm (100mW)
- Typ 18dBm (60mW)
- Often as low as 30mW
- Up to 2dB variation across the band
- Rx sensitivity
- About -90dBm
- -80dBm allowed by 802.11 spec
- Co-site (blocking)
- 55dB between Tx & Rx required (on different channels)
- 10m separation of dipoles
Spectrum of WLAN
802.11 Channels Overlap
802.11 Channels
Note, only channels 1 - 6 fall in the amateur band
| Channel | Frequency Mhz | Countries | Key
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| 1 | 2417 | U,E,J | U = USA
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| 2 | 2412 | U,E,J | J = Japan
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| 3 | 2422 | U,E,J | E = ETSI
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| 4 | 2427 | U,E,J | F = France
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| 5 | 2432 | U,E,J
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| 6 | 2437 | U,E,J
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| 7 | 2442 | U,E,J
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| 8 | 2447 | U,E,J
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| 9 | 2452 | U,E,J
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| 10 | 2457 | U,E,F,J
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| 11 | 2462 | U,E,F,J
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| 12 | 2467 | E,F,J
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| 13 | 2472 | E,F,J
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| 14 | 2484 | J
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2.4GHz Amateur Band Plan
| 2322.000 - 2400.000 | All modes
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| 2322.000 - 2355.000 | ATV and ATV repeater outputs
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| 2355.100 - 2364.000 | Repeater links
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| 2355.100 | Packet radio (200KHz b/w)
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| 2355.300 | Packet radio (200KHz b/w)
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| 2356.000 -2360.000 | High speed data
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| 2364.000 | Packet radio (1MHz b/w)
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| 2365.000 - 2370.000 | Repeaters
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| 2370.000 - 2390.000 | ATV
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| 2390.000 - 2392.000 | Moonbounce
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| 2400.000 - 2450.000 | Satellites
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Range
Antennas
Chip Antenna
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Peak gain 1.0dBd typ.
Average gain -2.7dBd typ.
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Boosters
YD12440 Booster
Ethernet Access Point
Linksys WET-11 access point / bridge
802.11b - 11Mb/s
Interface: 10baseT Ethernet suitable for long runs; needs extra protection for outside use
Experts - G8OTA
http://www.wlan.org.uk
Wlan-talk: community WLAN networks mailing list
Contacts and Links
If you found this interesting, please spare a thought for Paula G8PZT, who slogged for 2 solid days to unpick the Powerpoint file, edit graphics and re-make it into a crude HTML page. Maybe I'll tidy it up one day...
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