Danny Stewart

On episode 2 of Podfusion, I spoke about how my Verizon iPhone 5 had an issue where whenever it dropped from LTE to 3G, it would hold onto that 3G connection even if I re-entered an LTE area.

Apple recommended that I confer with Verizon before assuming a hardware problem. Verizon support was extremely responsive and helpful. They reconfigured some things on their end, which did improve my signal quality dramatically, but did not resolve my main issue. They then proceeded to send me a new SIM card, which also did not resolve the issue. At this point, I was told to return to Apple and attempt to swap out the phone, which I did.

The best test for this issue has been the DC Metro, which is largely underground and has lots of signal fluctuation between stations. I ride the Metro as part of my commute, so today was my first opportunity to test after the swap.

The results were interesting. At the majority of stations, the phone quickly found 3G signal after exiting a tunnel, but only at one station did the phone automatically connect to LTE. However, I toggled Airplane Mode at the other stations (my previous workaround for this issue), and saw the phone then connect to LTE, so I know that LTE signal was present at many of the stations.

When I arrived at my destination (which is underground), the phone connected to 3G as expected, as there is no LTE signal inside the station. However, there is LTE immediately outside the station. I tend to lose LTE inside the station as soon as I have lost line-of-sight with the sky. This morning, upon exiting the station, the phone did connect to LTE by itself, but it took somewhere between three and five minutes for the phone to connect to LTE after I had line-of-sight with the sky.

This has led me to think some more about this. I am fairly certain that there was a hardware issue on my previous phone, as it would never reconnect to LTE after dropping to 3G, whereas on the new phone I saw it occur twice on just the first test. I even restored my previous phone to factory defaults without using a backup and still observed the issue, so I’m as certain as I can be that this was not a software issue. This leads me to speculate on possible causes for this issue, since there is still a delay, even though it does eventually correct automatically. Here are my theories, based on the evidence I have observed.

1. This is known behavior on CDMA networks, for whatever reason (chipset, antenna, towers, protocol, etc.). I have heard reports from non-iPhone users on Verizon that their phones often take a long time to connect to LTE as well, and that restarting the phone or its radios bypasses this wait.

2. This is a problem specific to the cellular chipset Apple is using in the iPhone 5.

3. There is a bug in part of iOS 6 that handles switching between 3G and LTE networks, and this may be fixed or improved in subsequent versions of iOS.

4. This delay occurs by design in iOS 6, so that the device is conservative in switching connections. It holds onto a known stable connection until it is sure it can transition smoothly to a connection that is equivalent or better.

5. This process is affected by network activity in iOS. For example, if you are sending an iMessage or uploading a file, it will not attempt to switch connections until all operations have completed and a timed “safety margin” has passed. While not conclusive, I have observed that the connection will not change while I am transferring data over the network.

Of course, it is likely a combination of the above. Whatever the case may be, I wanted to follow up in case others were curious or having similar issues. I’ll gladly provide more data points if they present themselves. If you have knowledge of cellular radios, CDMA, or LTE and can shed some light on why this might be happening and whether it’s a real problem specific to me, please get in touch!