February 2009 Archives

IEEE 802.11n is the new international standard for wireless Local Area Networks, incorporating new smart antenna technologies (MIMO - Multiple In and Multiple Out) permitting a 5x performance and 2x coverage improvement for WLANs. While this new technology is now becoming the de facto standard in consumer and enterprise networks, it has not yet made an appearance in outdoor, metropolitan scale networks derived from WiFi technology. Many of these same MIMO techniques are being incorporated in both WiMax and in LTE for cellular. Sadly neither is being produced in much volume as yet and fixed WiMax networks do not incorporate MIMO technology. There has been much dispute about whether the specifics of 802.11n designed for indoor networks would apply to outdoor networks and bring the economy of scale of 802.11 to outdoor applications. We (Novarum) decided to test the effects of .11n on outdoor performance. We found dramatic improvements in using indoor 802.11n technology outdoors - so much so that 802.11n has become the recommended baseline for new network deployments. First, let’s review the key pieces of technology incorporated in 802.11n and how it might affect outdoor performance.
Maximal Ratio Combining Receiver combines signal from multiple paths to maximize SNR. We can see a 3-4 dB receiver link budget improvement even to legacy clients. Dramatically improved receiver signal strengths and dramatically reduced packet error rates. More reliable use of higher level encoding methods increasing link performance.
Transmit Beam-forming Modulate phase and amplitude from multiple antennas to create phased antenna array pointing increased performance at the destination node. 7-8 dB gain possible with omnidirectional antennas Decreased inteference, increased capacity, decreased deployment cost. Likely directional antenna performance with omnidirectional antennas substantially increasing network performance and decreasing deployment cost.
Spatial Multiplexing Use the redundant paths created by multipath to increase throughput by transmitting data in parallel paths. Probably not compelling outdoors since high SNR needed for parallel data paths. However, increasing reliability by taking advantage of multipath around deep fades.
Channel Bonding 20 and 40 MHz channels in both 2.4 and 5 GHz bands 20 MHz channels legacy compatible while 40 MHz channels double throughput, mostly useable in the 5 GHz band.
Protocol Improvements Packet aggregation Modest overhead reduction and performance improvement for streaming media and bulk transfers
Cost Reduction Indoor WiFi network demand is for low cost, dual band (2.4 and 5 GHz) simultaneous radios at commodity prices The availability of these dual band 3x3 MIMO chipsets drives the cost of multiradio outdoor units down
In the course of Novarum’s Wireless Broadband Review in 2007 and 2008, we examined over 25 deployed WiFi networks (including all major vendors), 46 deployed 3G cellular networks and 4 fixed pre-WiMax networks. In the case of the WiFi networks, we noted the dramatic effects that client selection had on network performance, coverage and ultimately user satisfaction. We both examined 802.11n clients against the installed multivendor base of 802.11g infrastructure and constructed our own testbed from early outdoor 802.11n components to evaluate the effect of 802.11n when deployed in the infrastructure itself. It is important to recognize that in almost all outdoor WiFi networks, the client access uplink is the weakest link in the communication chain. Legacy 802.11b/b clients experience VERY high packet retry rates of between 100 and 1000% and there are often deep multi-path fades of between 10-30 dB within a few tens of feet. The WiFi protocol is VERY good at masking these effects - instead they are most commonly seen indirectly - by lower throughput and higher delay variance. These effects are seen even for deployments of very high access node density of 50 nodes per square mile or more. These deep fades and very high packet retry rates made mobility difficult, dramatically effect throughput, make packet delay variance so high as to make streaming media difficult and materially decrease the overall capacity of these networks. The improvements that 802.11n provides outdoors astonished us - particularly for a technology that has been disparaged as inappropriate for outdoor deployment. Deploying IEEE 802.11n technology has dramatic effects outdoors - both with legacy systems and even more compellingly with green fields deployment. Let’s summarize the facts of what we found in Novarum’s experiments:
  • 100% throughput improvement of 802.11n WiFi clients with legacy 802.11g outdoor infrastructure;
  • 100% throughput improvement of legacy 802.11g WiFi clients with new 802.11n outdoor infrastructure;
  • 200% throughput improvement of 802.11n client with 802.11n outdoor infrastructure;
  • Similar coverage of 802.11n clients and infrastructure in the 5.4 GHz band as for legacy 802.11g clients and infrastructure in the 2.4 GHz band - making the 5.4 GHz band useful for client access;
  • 25% decrease in access latency and a dramatic improvement in latency variance;
  • a low power 802.11n client has the same throughput and coverage as a high power 802.11g with 10x the power and antenna; and
  • coverage to smartphones at low power and with poor antennas dramatically improves.
These results have a dramatic impact on outdoor wireless networks - bringing the benefits of MIMO technology at consumer price-points. We can expect that 802.11n technology will dramatically improve outdoor WiFi networks.
  • 4-800% increase in system capacity and throughput
    • 2-300% improvement in spectral efficiency through increased link budgets, reduced packet errors, increased modulation rates and improved fading performance
    • effective client access to the 200 MHz of the 5.4 GHz band
  • 802.11n clients dramatically improve legacy 802.11g networks and new 802.11n networks dramatically improve legacy 802.11g clients.
  • Streaming media applications will perform as we expect and will be much easier to deploy.
  • Better backbone designs by reducing the interference of the backbone mesh through beam-forming antennas rather than omnidirectional broadcast.
  • Decreased deployment cost due to decreased node cost. Possibly dramatically.
While not optimally designed for outdoors, 802.11n will SUBSTANTIALLY increase the performance and customer satisfaction of outdoor wireless networks. We can expect the first product announcements of outdoor WiFi networks incorporating 802.11n shortly and can expect that all major vendors of outdoor WiFi equipment to be shipping by the end of 2009. Novarum recommends that all new outdoor WiFi networks use 802.11n products in their infrastructure and strongly recommends 802.11n clients wherever possible.
Two of the important issues in large scale wireless have been:
  1. Can a given technology provide a usable data communications service and
  2. How much does it cost to deploy such a service.
A useful network service provided at an affordable price are necessary preconditions for a successful network offering. Many of the early muni WiFi networks were hampered by the double whammy of both a poor service AND the higher cost to deploy than expected. In seeking to answer this, Novarum structured its’ Wireless Broadband Review to provide some of this information. During 2007 and early 2008, we tested cellular, WiFi and pre-WiMax networks in these cities: Anaheim CA (2x), Brookline MA, Chico CA, Cupertino CA, Daytona FL, Eugene OR, Galt CA, Longmont CO, Madison WI, Minneapolis MN, Mountain View CA (2x), Palo Alto CA, Philadelphia PA (2x), Portland OR (2x), Raleigh NC, Rochelle IL, St. Cloud FL (2x), Santa Clara CA, Sunnyvale CA, and Tempe AZ (2x). In several cities we tested twice to detect changes in traffic and improvements in network service over time and experience. We discovered that, on average, all of these networks have similar performance and coverage, but that the best of the WiFi networks substantially outperformed the best of either the cellular AND pre-WiMax networks. Our test was an apples to apples comparison of performance (delay, uplink throughput, downlink throughput) and availability (percentage of tested locations with service within the advertised service area) for all of the major network technologies:
  1. ATT (Cingular), Sprint and Verizon cellular data networks
  2. A number of metro WiFi networks using equipment by BelAir, SkyPilot, Strix, Tropos, and
  3. Four of ClearWire’s pre-WiMax networks.
We tested outdoor coverage in an average of 20 locations per city - testing all networks with the same traffic load and in the same location and time. One of the important determinants of good performance is a good client modem - and we tested with a variety of client modems. For today’s thoughts, we’ll look at standard USB external modems for each of the cellular data networks, a higher power WiFi modem (noting that current 802.11n modems appear to perform on par with these higher power clients), and a desktop directional CPE for the pre-WiMax ClearWire networks (no portable device was available) at the time. We would expect the WiMax modem (AC powered, directional antenna) to have the advantage in performance. To our surprise, with similar client modems, averaged over good and bad networks, WiFi networks delivered almost 3x better performance than cellular networks and materially better performance than pre-WiMax networks - with similar levels of availability of service over the promised coverage area for all three network technologies.
Network Delay (msec) Uplink (kbps) Downlink (kbps) Availability
Average Cellular 340 195 507 89%
Average pre-WiMax 174 169 1124 83%
Average WiFi 113 767 1286 85%
If we look at the best, and most recently deployed WiFi network, we see performance and availability superior to the best the cellular data networks (by a factor of 3!) AND the best of pre-WiMax networks we measured - by at least a factor of 2.
Network Delay (msec) Uplink (kbps) Downlink (kbps) Availability
Best Cellular 192 612 980 100%
Best pre-WiMax 190 164 1129 100%
Best WiFi 63 2062 2949 100%
The measured performance demonstrate that WiFi networks materially outperform cellular data networks AND pre-WiMax networks - and do it with similar service area coverage. And likely lower deployment costs.