Wireless LANs (Geier Book, Chapter 2) Radio Waves Infrared Light Carrier Currents

Wireless LAN Configuration

Radio-Based Wireless LANs Most widely used method Adv: penetrating walls and other obstacles with little attenuation. Disadv: security, interference, etc. 3 approaches: ISM band Narrow band Spread spectrum

ISM Bands In 1985, FCC modified part 15 to stimulate the use of wireless networks. ISM=Industrial, Scientific, and Medical Unlicensed, you can freely install and move.

ISM Spectrum Availability Only 2.4 GHz is the world-accepted ISM band. 902 MHz is easier in manufacturing.

Narrow Band Modulation Concentrate all transmission power in a narrow range of frequency. Efficient use of radio spectrum; save bandwidth. ex: television, AM, FM Adv: for long distance transmission (e.g. metropolitan area) Disadv: noise can easily corrupt the signals Need to obtain FCC licenses to coordinate the use.

Spread Spectrum Modulation Definition: “spread” a signal’s power over a wider band of frequency.

Spread Spectrum Disadv: This contradicts with the goal of conserving bandwidth. Adv: less susceptible to electrical noise (especially from narrow band sources) In World War II, US Army uses spread spectrum to avoid hostile jamming. (invented by Hedy Lamarr, an actress) To spread a signal, there are two ways: direct sequence (DSSS) frequency hopping (FHSS)

Direct Sequence Spread Spectrum (DSSS) Use bit sequence to represent “zero” and “one” (Fig. 2-5) Also referred to as “chipping code”. Longer chipping codes are more resilient to noise. Minimum length = 10 (by FCC) IEEE 802.11 uses 11 chips per data bit.

Frequency Hopping Spread Spectrum (FHSS) Data is modulated by carrier signals that hop from frequency to frequency as a function of time, over a wide band of frequencies.

FHSS (cont.) Hopping Code: to determine the order of hopping frequencies The receiver must “listen” to incoming signals at the right time at the right frequency. FCC regulation: at least 75 frequencies, with max. dwell time 400ms. Adv.: very resilient to noise. Orthogonal hopping codes: a set of hopping codes that never use the same frequencies at the same time (can be on-line adjusted by software). Allow multiple wireless LANs to co-exist.

Forms of Radio-based Wireless LAN Wireless Local Bridge Infrastructure Ad Hoc

Infrared Light-based Wireless LAN Wavelength range: 0.75 ~ 1000 u ultraviolet < visible lights < infrared < radio typically operating at 0.82u Adv: higher security: does not propagate through opaque objects, keeping signals within rooms and building no interference from microwave ovens and radio transmitters Disadv: directional, not suitable for mobile applications Two types of infrared light LANs: Diffused Point-to-point

Diffused Infrared-based LAN Example: TV remote controller Example: three stations using a ceiling Just like using a flashlight, and talking to people using Morse Code. A person wants to send must look at the ceiling to make sure that no other people are sending. The receiver looks at the packet header to check for incoming packets.

“Point-to-Point” Infrared LAN System Each link has a pair of transducers (uni-directional): up to 75 ft. connect as a token ring (what is token ring??) Each transducer can talk with a IEEE 802.5 token ring board. data rate: 4 and 16 Mbps); Good security: Resilient to electrical noise. No mobility, little fault-tolerant capability

Carrier Current LANs Use power line as a medium to transmit data. Fig. 2.15 The electrical power companies provide 110V, 60Hz alternating current; the signal can support 1~2 Mbps data signals. Adv: adopt already-existing cables Products are being seen/proposed in the market.

IEEE 802 Families IEEE? Most notable standards: IEEE membership info: a non-profit professional org. founded in 1884; now has 320,000 members in 150 countries; Most notable standards: IEEE 802 family (802.2 LLC, 802.3 Ethernet, 802.5 Token Ring, etc.) IEEE membership info: Open to anyone. It’s FREE!! Must pay meeting fees. Membership Category: voting member, nearly member, aspirant member, sleeping member.

Task groups of 802.11 802.11a: Specification enabling up to 54 Mb/s to be achieved in the 5 GHz unlicensed radio band by utilizing OFDM 802.11b: Specification enabling up to 22 Mb/s to be achieved in the 2.4 GHz unlicensed radio band by utilizing DSSS 802.11c: Provides required information to ensure proper bridge operations, which is required when developing access points 802.11d: Covers additional regulatory domains, which is especially important for operation in the 5 GHz bands because the use of these frequencies differ widely from one country to another

Task groups of 802.11 802.11e: Covers issues of MAC enhancements for QoS, such as EDCF service differentiation and HCF 802.11f: Provides interoperability for users roaming from one access point to another of different vendor 802.11g: Specification enabling up to 54 Mb/s to be achieved in the 2.4 GHz unlicensed radio band 802.11h: Dynamic channel selection and transmission power control 802.11i: Specification for WLAN security to replace the weak Wired Equivalent Privacy (WEP) 802.11k: Radio resource measurement for 802.11 specifications so that a wireless network can be used more efficiently