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Can toggling airplane mode actually improve your 3G data speeds?

WiWavelength

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by Andrew J. Shepherd
Sprint 4G Rollout Updates
Friday, August 17, 2012 - 1:14 PM MDT

 

CDMA1X and EV-DO carrier channels are shared resources. In CDMA1X, many subscribers share the same carrier channel, their individual traffic kept theoretically orthogonal by code division. Likewise, in EV-DO, individual traffic is separated by time division. But what happens when Sprint (or any other CDMA2000 network provider) has deployed greater than one CDMA1X and/or EV-DO carrier channel on a given cell site? How does your handset determine which carrier channel to utilize?

You might like to think that your handset would automatically choose the least loaded CDMA1X and/or EV-DO carrier channel. But that is not really the case. Instead, when multiple carrier channels are available, each cell site broadcasts a channel list message of the available carrier channels on that site. Upon receiving this list of multiple carrier channels, each handset then invokes a hashing algorithm to select which carrier channel to use. Think of it like a multi lane highway, but each car must choose a particular lane based on the car's license plate number.

For CDMA1X, the hashing algorithm -- which is a kind of pseudo random number generator -- is seeded with the handset's ESN or the subscriber's MDN/MSID (i.e. phone number). Unless the subscriber changes devices or phone numbers, these values remain static, hence the carrier channel hash is quite predictable. And Sprint, for reference, seems to use MDN/MSID based hashing. Nearly a decade ago, I built a spreadsheet that emulates the CDMA1X hashing algorithm, downloadable as an XLS file.

However, for EV-DO, the carrier channel hash is not quite so outwardly predictable. To seed the hashing algorithm, EV-DO uses a session number, which obviously varies from data session to data session. Each time that a handset powers up, crosses a SID/NID boundary, or even toggles airplane mode, for example, generates a new EV-DO data session, hence a new session number. And it is this session number that determines the output of the hashing algorithm.

To demonstrate this process, I positioned myself in one location about a quarter of a mile distant from the north sector of a local cell site. Over the course of several minutes, I grabbed three screen caps of the EV-DO engineering screen on one of my handsets. In between each screen cap, I cycled airplane mode at least once, each cycle generating a new data session. In the span of four minutes, I was able to get my handset to hash to each of the three EV-DO carrier channels deployed on this site. When I arrived at the site, my handset hashed to PCS 0175, which is the third EV-DO carrier channel (F3) in the channel list message. The second and third hashes after toggling airplane mode several times were to PCS 0150 (F2) and to PCS 0100 (F1). See the Channel Number field depicted in the screen caps:

 

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In addition, here is a raw RF look with a spectrum analyzer at the seven CDMA2000 carrier channels deployed on this cell site sector:

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The four CDMA1X carrier channels are PCS 0050, PCS 0075, PCS 0125, and PCS 0200. As is oft the case, the three aforementioned EV-DO carrier channels -- PCS 0100, PCS 0150, PCS 0175 -- are distinguishable by their slightly higher RF power output. Furthermore, for those curious, PCS 0025 (at the far left of the graph) and PCS 0225, PCS 0250, and PCS 0275 (at the right of the graph) are fallow spectrum on this site. If deployed, PCS 0025 would be the next EV-DO carrier channel (F4), PCS 0275 the final EV-DO carrier channel (F5), while PCS 0225 and PCS 0250 would be additional CDMA1X carrier channels.

Back to the hashing algorithm, while it attempts to distribute users more or less evenly among available EV-DO carrier channels, it does not take into account several other factors, such as loading and backhaul. For example, if you are stuck on a carrier channel and sector with a few data hogs who have stronger signal than you do, your data speeds will likely suffer as the "fair and proportional" scheduler integral to the EV-DO airlink attempts to maximize total throughput by allocating greater time slots to the users with better signal quality. Additionally, backhaul may not be distributed evenly among deployed carrier channels, so it is possible that some carrier channels may have inherently greater data capacity than others do.

Another benefit of rehashing to a different carrier channel is that you may be able to connect to a closer cell site. Because not all cell sites have the same number of deployed EV-DO carrier channels, carrier channel hashing is an imperfect process. To illustrate, the cell site (call it cell site "A") that I detailed above for this trial has three EV-DO carrier channels (F1, F2, F3), as duly noted. But the adjacent cell site to the north (call it cell site "B") has only two EV-DO carrier channels (F1, F2). A handset that hashes to F3 on cell site "A" will cling to carrier channel PCS 0175 even as it moves north well into the coverage area of cell site "B." Interference will not be a problem, as cell site "B" does not transmit PCS 0175, but signal strength (and data speeds) will diminish until cell site "A" drops below a network defined threshold, at which point the handset will handoff to cell site "B" and hash to PCS 0150. This can require substantial movement and/or time. So, if you always want the most crisp EV-DO handoffs, you can try to ensure that your handset always hashes to F1, the EV-DO carrier deployed on essentially every site in the market.

To conclude, by no means is airplane mode a panacea for slow 3G data ills. EV-DO carrier channel deployment and backhaul can vary from site to site, while loading can also vary from site to site, even from minute to minute. And EV-DO networks in some cities are just generally overloaded. But if you are at work, in a restaurant, at a park, etc., and find yourself with unbearably slow 3G data or lower than usual signal strength for that location, try toggling airplane mode. A 30 second on/off cycle of airplane mode will start a new data session and could get your handset to rehash to another EV-DO carrier channel that is on a closer site, has better backhaul, and/or is currently less loaded.

 

Sources: Qualcomm, author's field data

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Where else are you getting information like this? A little over my head as the only hashes I deal with have corned beef or potatoes in them...sometimes both. But the main thrust of the post was well articulated and easily understood. Good work AJ!

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Hey, if someone else is your dealer, you can probably get hashish. But if S4GRU is your addiction, we can definitely supply you with the lowdown on the hash.

 

;)

 

AJ

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AJ,

 

I did this yesterday in McPherson cause my data was unusable, then cycled and it became awesome so makes alot of sense!

 

Rickie

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Maybe I wasn't leaving my phone in airplane mode long enough but I could never get it to flip over in the past. I would have to do an update profile. Totally makes sense now though. I always thought the Site told the phone which channel to use on the evdo side as I have tried removing some channel scans from the evdo side of the prl. I understand why our channel 100 sucks all the time in the places where the towers have 2 carriers..The nearby sites don't have 100 so the phone is hanging on to the channel at the very low signal. A bunch of poor signal phones on the cell brings it to its knees..The poor tower spacing here gets us in another double whammy. Wonder if I can negative out channel 100...hmmmm

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With Sprint planning to implement DO Advanced, will this in theory keep a a user on the best available tower/carrier?

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I always thought the Site told the phone which channel to use on the evdo side as I have tried removing some channel scans from the evdo side of the prl. I understand why our channel 100 sucks all the time in the places where the towers have 2 carriers..The nearby sites don't have 100 so the phone is hanging on to the channel at the very low signal.

 

So, in the New Orleans market (or, at least, in Baton Rouge), EV-DO F2 is PCS 0100, correct? What is EV-DO F1? PCS 0075?

 

Regardless, removing or negating an EV-DO carrier channel assignment in the PRL should not have much, if any effect. As long as you keep the local EV-DO F1 carrier channel assignment in the PRL, then your handset will locate EV-DO. Once it does, it will still see the multiple EV-DO carrier channels in the channel list message, so will still invoke the hashing algorithm. As such, you still have about a 50/50 chance of ending up on EV-DO F2.

 

Basically, the PRL controls only system acquisition. But once a native system is acquired, the PRL is set aside and the hashing algorithm determines the carrier channel on which the mobile will idle.

 

AJ

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With Sprint planning to implement DO Advanced, will this in theory keep a a user on the best available tower/carrier?

 

Yes and no. CDMA2000 base stations already have the ability to redirect a mobile from one CDMA1X carrier channel to another (or, seemingly more rarely, one EV-DO carrier channel to another).

 

To illustrate, your handset may hash to PCS 0025, so it idles on that CDMA1X carrier channel. Your handset cannot choose to move to another CDMA1X carrier channel simply because PCS 0025 is heavily loaded. However, when a base station sets up a traffic channel for your handset, the base station can determine to set up the traffic channel on PCS 0050 and redirect your handset to that carrier because PCS 0025 is already heavily loaded.

 

This is a CDMA2000 network management capability that is rather old. In fact, it predates CDMA2000 and goes back to cdmaOne. But its implementation may certainly be vendor and/or network dependent.

 

Now, you might wonder why mobiles do not simply search for the best carrier channel on their own. This is for several reasons. Mobiles already have their hands full continually searching for the best cell site sector. Adding carrier channel search would slow the process and degrade battery life.

 

Furthermore, both the BTS and MSC expect the mobile to hash to a specific carrier channel, so all paging and traffic are directed to the mobile on only that channel. If the mobile were constantly, unpredictably switching carrier channels, the network overhead required to keep track of the mobile would increase significantly.

 

Lastly, consider the tragedy of the commons. If every mobile of its own volition were actively to seek out the best carrier channel, then numerous mobiles would quickly converge on that best channel, likely rendering it no longer the best channel, as loading had just greatly increased. So, mobiles would then swarm like locust to the carrier channel that had become the new best carrier channel. And the process would oscillate out of control. Instead, the hashing algorithm averages out mobiles across available carrier channels and prevents this problem.

 

AJ

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So, in the New Orleans market (or, at least, in Baton Rouge), EV-DO F2 is PCS 0100, correct? What is EV-DO F1? PCS 0075?Regardless, removing or negating an EV-DO carrier channel assignment in the PRL should not have much, if any effect. As long as you keep the local EV-DO F1 carrier channel assignment in the PRL, then your handset will locate EV-DO. Once it does, it will still see the multiple EV-DO carrier channels in the channel list message, so will still invoke the hashing algorithm. As such, you still have about a 50/50 chance of ending up on EV-DO F2.Basically, the PRL controls only system acquisition. But once a native system is acquired, the PRL is set aside and the hashing algorithm determines the carrier channel on which the mobile will idle.AJ

 

The typical channels for 4159/51 are 25 for 1x and 75 for EV.

 

I kinda figured as much as I've tried playing around with the channels in a ACQ record before. I even went as much as one time removing the Cellular channels from the VZW EV scan and just left in the PCS ones. It still went to the cellular side.

 

I can never seem to just toggle airplane mode on my EVO LTE, after toggling the 3G connection never comes back. Its just falls on its face. A reboot fixes it.

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Nice article! I've used this method over the years, and know that it works, now I know WHY it works, which makes my tiny retired engineer mind happy. :)

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I work a stones throw from woodfield mall in Schaumburg and my 3g speeds are always horrible (network congestion, sub 300 kb). I've been toggling airplane mode today, however I always get channel 325 for EVDO. Could it be that such a congested tower would only have one carrier? 1x is on 350. Is anyone familar with what channels Sprint is able to deploy in the Northwest suburbs of Chicago (Schamburg, Elgin)? Thanks.

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I work a stones throw from woodfield mall in Schaumburg and my 3g speeds are always horrible (network congestion, sub 300 kb). I've been toggling airplane mode today, however I always get channel 325 for EVDO. Could it be that such a congested tower would only have one carrier? 1x is on 350. Is anyone familar with what channels Sprint is able to deploy in the Northwest suburbs of Chicago (Schamburg, Elgin)?

 

Sprint has two PCS A-F block licenses in Chicago: PCS D block 10 MHz and PCS E block 10 MHz. That allows Sprint six potential CDMA2000 carrier channel assignments: PCS 0325, 0350, 0375, 0725, 0750, and 0775.

 

Whenever I visit a different market, I try to track typical CDMA1X and EV-DO deployed carriers, and I was in Chicago about eight months ago. The carrier channel assignments that I noted around the city were as follows:

 

CDMA1X: 0350, 0375, 0775

EV-DO: 0325, 0725

 

The only available carrier channel assignment that I did not encounter while I was in Chicago was PCS 0750, so I cannot say whether it is typically deployed as CDMA1X or EV-DO.

 

Regardless, Sprint is somewhat limited in the number of carrier channels -- especially EV-DO carrier channels -- that it can deploy in Chicago. A good rule of thumb seems to be that a site in Chicago will have no greater than three EV-DO carrier channels, and many will have only one or two.

 

Now, in the case of the Woodfield Mall area in Schaumburg, Sprint has no fewer than four sites surrounding the mall. Chances are, jman, that your EV-DO serving site -- likely, your closest site -- has only one EV-DO carrier channel deployed.

 

AJ

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Wow, thank you for the detailed reply. I guess from the outside looking in, it doesn't make sense that they wouldn't use the existing 0325 and an additonal 0725 carrier for EVDO as well.

 

You mentioned that, in Chicago, a tower would have no more than three carrriers, but I believe you mentioned only two channel possibilities. EV-DO: 0325, 0725. Can you please explain what the channel assignments would be for three EVDO carriers on a tower? Thank you in advance.

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