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Wireless LAN Design Alternatives
David F. Bant and Frederic J. Bauchot, IBM Research
The best fit for IBM customer needs is a WLAN using slow FH, 2.4GHz, and TDMA.
Customer requirements (determined by surveys):
- Worlwide portable product
- Battery life
- License-free operation
- Transmission robustness and security
- Collocated networks
- Ease of management
IR and RF are complementary, not competing:
- IR suitable in small rooms
- Simple, cheap, unregulated
- Orthogonal modulation techniques can be used to isolate IR networks
- RF has wider range (outdoors) but subject to noise
- Worldwide demand for RF has created intense competition for spectrum
- Slow FH (chipping rate slower than data rate) vs fast FH: slower is cheaper (power, cxity) and relaxes hopping synchr. constraints
- Non-spread spectrum (NSS): requires license, except new U-PCS (1890-1930 MHz)
- Most PCS services will be voice-oriented; data services will be adequate for messaging, but will complement (not replace) WLANıs
- Cost impact of using high-frequency NSS: emerging technology (GaAs,Ge)
- Problems that donıt exist in NSS, and are handled with more agility by FH than DS:
- Interference with other systems in same band
- Collocation of networks
- Yielding to higher-priority users
- ISM band: FH has best interference properties since 40% of channels can be thought of as "spare" for interference mgmt.
- BUT, high-freq (5.8GHz) ISM implies multiple semiconductor technologies to implement, hence single-chip xceivers unlikely anytime soon
- 915 MHz ISM already crowded and experiencing greatest market growth
- Only 2.4 GHz band is available worldwide
- So...we should use 2.4Ghz (its one vice: microwave ovens)
Topology and MAC
Difference between contention-based peer-to-peer and TDMA BS-to-many-MHıs schemes: Centralizing control in a BS allows
Ergo, best design point is 2.4 GHz, TDMA, slow FH
- Air scheduling with knowledge of fileserver etc. needs. With peer-to-peer, all traffic is "equal".
- Access to wired network for all MHıs. With p2p, need a dedicated gateway for this.
- Security management
- Traffic scheduling to reduce power consumption at MHıs
- Can choose TDMA frame to be submultiple of FH interval. In CSMA, must defer a message if it would "cross" a hop
- End user latency in TDMA scheduling may be higher under light load, but it is much less variable than for CSMA
A design point argument for WLANs based on fitting to customer needs. Nice tables comparing the design alternatives for each component.
Almost felt like the goals were presented in such a way as to force the conclusion; the fit is just too neat....
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