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Wireless LAN Design Alternatives
David F. Bant and Frederic J. Bauchot, IBM Research
One-line summary:
The best fit for IBM customer needs is a WLAN using slow FH, 2.4GHz, and TDMA.
Overview/Main Points
Customer requirements (determined by surveys):
- Worlwide portable product
- Battery life
- License-free operation
- Transmission robustness and security
- Collocated networks
- Ease of management
Physical layer
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
Radio technology
- 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
- 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
Ergo, best design point is 2.4 GHz, TDMA, slow FH
Relevance
A design point argument for WLANs based on fitting to customer needs. Nice tables comparing the design alternatives for each component.
Flaws
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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