A T1 backhaul? If youâve got a 54Mbps connection on a T1 backhaul then thatâs the problem! Why blame the user (AKA the victim) when a provider has created an extreme constriction? The idea of then giving the unproviders who created the problem the ability to certify the applications adds insult to injury!! How can one say the user is hogging the bandwidth when the unprovider has taken 99.99% of the capacity off the table. Who is the hog?
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You can read http://rmf.vc/?n=Unbaked as to why the idea of baking in the naivetà of todayâs provider is so dangerous. Just wait till we have connected devices as the norm â http://rmf.vc/?n=MakerDisconnect.
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From: Dave Farber [mailto:dave@farber.net]
Sent: Tuesday, October 26, 2010 11:52
To: ip
Subject: [IP] Effects of BitTorrent on a wireless hotspotÂ
Begin forwarded message:From: Jon Henke <jonhenke@gmail.com>
Date: October 26, 2010 11:47:24 AM EDT
To: dave@farber.net
Subject: Effects of BitTorrent on a wireless hotspot[For your IP-consideration]
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George Ou posted this interesting piece about the impact of BitTorrent on a wireless hotspot. ÂResult: "the average latency with BitTorrent running went up to 772.3 ms which is an increase in jitter of 623.5 ms. ÂFrom further testing, I verified that the jitter was induced on the T1 backhaul and it didnât stress the 54 Mbps capacity of the Wi-Fi access link, but it could easily stress the wireless link of a 2G or 3G cell."
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Effects of BitTorrent on a Starbucks-AT&T hotspot
ByÂGeorge OuÂ25 October 2010
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I recently discussed the phenomenon of a single applicationÂaccidentallyÂtaking down a T-Mobile cell and how BitTorrent would have an even worse effect. ÂI wanted to test this with BitTorrent, but doing so on a cell tower isnât feasible at this moment and itâs not something that any wireless carrier would want me to test on their production network. ÂSo I found the next best thing during off hours at a Starbucks hotspot operated by or in conjunction with AT&T and Wayport. ÂI conducted the tests well after store closing in front of the store from my car when no one was at the store, and the BitTorrent test only ran for 141 seconds so that it wouldnât disrupt any background activity that the store might be running.
There are a lot of similarities between a Wi-Fi âhotspotâ (effectively a cell) and a wireless 3G cell, but the Wi-Fi cell has a lot more wireless capacity because it has 20 MHz of spectrum compared to a typical 5 MHz or 10 MHz cell on a 3G cell and because Wi-Fi is working at closer range with hundreds of times higher radio frequency (RF) field density. ÂPublic hotspots like the ones operated by Starbucks usually employ a T1 (symmetricalÂ1.554 Mbps) backhaul as confirmed by Figure 1 below which is similar to many cell tower backhauls. ÂThe backhauls on 3G and LTE towers are likely faster than a T1, but the 3G tower has less capacity than a hotspot running 802.11g. ÂThat means what harms a Wi-Fi hotspot will also harm a 3G cell in a similar manner.
Note that the measured speed is below â1.554â because it is measuring data throughput excluding overhead and because theyâre using the binary definition of megabit which calls for 1,048,576 bits/sec instead of a decimal definition of 1 million bits/sec.
Figure 1: Starbucks performance indicates T1 backhaul
First I had to get a baseline measurement when nothing was running. ÂFigure 2 shows that the Starbucks hotspot had a baseline latency of 148.8 milliseconds (ms) to the first pingable hop (IP address = 209.85.243.160)Âon the network.
Figure 2: Starbucks Wi-Fi â baseline latency
Next, I fired up BitTorrent and it was uploading and downloading at around 1.4 Mbps including overhead bandwidth which is about as fast as I could push it with no user defined bandwidth constraints in the BitTorrent application. ÂFigure 3 shows that the average latency with BitTorrent running went up to 772.3 ms which is an increase in jitter of 623.5 ms. ÂFrom further testing, I verified that the jitter was induced on the T1 backhaul and it didnât stress the 54 Mbps capacity of the Wi-Fi access link, but it could easily stress the wireless link of a 2G or 3G cell. ÂItâs also noteworthy that even faster 3, 5, or 15 Mbps links can be harmed by BitTorrent which would certainly bring down a typical cell tower backhaul which would harm the dozens of wireless cells served by a single tower. ÂThat would affect hundreds or thousands of users.
Figure 3: Starbucks Wi-Fi â Latency with BitTorrent active
So how would this affect other users during operational hours? ÂTheyâd be pretty upset with the massive degradation in performance not to mention the fact that BitTorrent has the ability to hog most of the bandwidth at the expense of other users and applications. ÂEven when BitTorrent is capped at a lower bandwidth, the sheer volume of signaling and payload packets per second can overwhelm a wireless network and even low bandwidth BitTorrent can cause a lot of jitter. ÂThis is why I ran the tests off hours.
On a production cellular network, there is likely little to prevent a user from running bandwidth hogging and jitter inducing applications like BitTorrent other than the âAcceptable Use Policyâ (AUP) that forbids these network disruptive applications andÂpossibilityÂsome technical mechanisms that throttle overactive users. ÂBut there are Net Neutrality advocates who want the regulators to forbid these network management practices and application restriction policiesÂbecause they want to pretend that wireless networks are the same as wired networks. ÂBut itâs clear that from an engineering perspective, such policies would be harmful to the vast majority of paying customers on the network and it would prevent the network carriers from maintaining an operational network.
Now some would argue that carriers shouldnât have blanket prohibitions against an entire set of applications because those applications might be able to operate in a non-disruptive manner. ÂBut that doesnât address the reality of how the applications measure in tests like the ones conducted above. ÂThe better approach than blunt regulatory force would be if application providers like BitTorrent worked with the wireless carriers on application certification where the application provider demonstrates that their application avoids generating jitter and avoids taking a disproportionate amount of bandwidth. ÂThe alternative is a more actively managed data network where the wireless base station actively distributes bandwidth fairly between subscribers and prevents a single application from inducing jitter. ÂThat level of network management doesnât exist today so until it does, the networks do the best they can to keep the networks running which includes prohibitions on applications.
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_______Jon Henke
202-595-4323
Twitter: @jonhenke
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