By Art Reisman, CTO, www.netequalizer.com
The trend to go all wireless in high density housing was seemingly a slam dunk just a few years ago. The driving forces behind the exclusive deployment of wireless over wired access was two fold.
- Wireless cost savings. It is much less expensive to strafe a building with a mesh network rather than to pay a contractor to insert RJ45 cable throughout the building.
- People expect wireless. Nobody plugs a computer into the wall anymore – or do they?
Something happened on the way to wireless Shangri-La. The physical limitations of wireless, combined with the appetite for ever increasing video, have caused some high density housing operators to rethink their positions.
In a recent discussion with several IT administrators representing large residential housing units, the topic turned to whether or not the wave of the future would continue to include wired Internet connections. I was surprised to learn that the consensus was that wired connections were not going away anytime soon.
To quote one attendee…
“Our parent company tried cutting costs by going all wireless in one of our new builds. The wireless access in buildings just can’t come close to achieving the speeds we can get in the wired buildings. When push comes to shove, our tenants still need to plug into the RJ45 connector in the wall socket. We have plenty of bandwidth at the core , but the wireless just does can’t compete with the expectations we have attained with our wired connections.”
I found this statement on a Resnet Mailing list from Brown University.
Despite the tremendous economic pressure to build ever faster wireless networks, the physics of transmitting signals through the air will ultimately limit the speed of wireless connections far below of what can be attained by wired connections. I always knew this, but was not sure how long it would take reality to catch up with hype.
Why is wireless inferior to wired connections when it comes to throughput?
In the real world of wireless, the factors that limit speed include
- The maximum amount of data that can be transmitted on a wireless channel is less than wired. A rule of thumb for transmitting digital data over the airwaves is that you can only send bits of data at 1/2 the frequency. For example, 800 megahertz ( a common wireless carrier frequency) has 800 million cycles per second and 1/2 of that is 400 million cycles per second. This translates to a theoretical maximum data rate of 400 megabits. Realistically though, with imperfect signals (noise) and other environmental factors, 1/10 of the original frequency is more likely the upper limit. This gives us a maximum carrying capacity per channel of 80 megabits on our 800 megahertz channel. For contrast, the upper limit of a single fiber cable is around 10 gigabits, and higher speeds are attained by laying cables in parallel, bonding multiple wires together in one cable, and on major back bones, providers can transmit multiple frequencies of light down the same fiber achieving speeds of 100 gigabits on a single fiber! In fairness, wireless signals can also use multiple frequencies for multiple carrier signals, but the difference is you cannot have them in close proximity to each other.
- The number of users sharing the channel is another limiting factor. Unlike a single wired connection, wireless users in densely populated areas must share a frequency, you cannot pick out a user in the crowd and dedicate the channel for a single person. This means, unlike the dedicated wire going straight from your Internet provider to your home or office, you must wait your turn to talk on the frequency when there are other users in your vicinity. So if we take our 80 megabits of effective channel bandwidth on our 800 megahertz frequency, and add in 20 users, we are no down to 4 megabits per user.
- The efficiency of the channel. When multiple people are sharing a channel, the efficiency of how they use the channel drops. Think of traffic at a 4-way stop. There is quite a bit of wasted time while drivers try to figure out whose turn it is to go, not to mention they take a while to clear the intersection. Same goes for wireless users sharing techniques there is always overhead in context switching between users. Thus we can take our 20 user scenario down to an effective data rate of 2 megabits
- Noise. There is noise and then there is NOISE. Although we accounted for average noise in our original assumptions, in reality there will always be segments of the network that experience higher noise levels than average. When NOISE spikes there is further degradation of the network, and sometimes a user cannot communicate at all with an AP. NOISE is a maddening and unquantifiable variable. Our assumptions above were based on the degradation from “average noise levels”, it is not unheard of for an AP to drop its effective transmit rate by 4 or 5 times to account for noise, and thus an effective data rate for all users on that segment from our original example drops down to 500kbs, just barely enough bandwidth to watch a bad video.
Long live wired connections!