In the thoughts I wrote up about the iPhone 5, I had mentioned that the call quality seemed worse to me on Verizon. [I said](http://brooksreview.net/2012/09/iphone-5-2/):
>Call audio quality seems worse when you are in an area with low reception, worse than with the previous iPhone. What I suspect is that this is a difference between AT&T and Verizon and not a difference in the iPhone itself, but I cannot be sure.
I received a long and detailed email from a reader, who wished not to be named or the email shared, that pointed me in the direction of looking at CDMA versus GSM technologies. The hint that I was given was the CDMA doesn’t limit the users a cell tower can receive, instead CDMA drops the bandwidth to each person to accommodate the traffic, whereas GSM limits the users and fixes the bandwidth each user can have. That made a ton of sense, so I did some digging to see what in the world is actually happening. I started with LTE and found this note on the [LTE Wikipedia page](https://en.wikipedia.org/wiki/LTE_(telecommunication)) under voice calling:
>While the industry has seemingly standardized on VoLTE for the future, the demand for voice calls today has led LTE carriers to introduce CSFB as a stopgap measure. When placing or receiving a voice call, LTE handsets will fall back to old 2G or 3G networks for the duration of the call.
So from everything I could gather, in a lot of cases, Verizon calls are dumping back to the CDMA networks. Ok, so what can we unearth about CDMA. [Again, Wikipedia](https://en.wikipedia.org/wiki/Cdma#Advantages_of_asynchronous_CDMA_over_other_techniques):
>There is no strict limit to the number of users that can be supported in an asynchronous CDMA system, only a practical limit governed by the desired bit error probability, since the SIR (Signal to Interference Ratio) varies inversely with the number of users. In a bursty traffic environment like mobile telephony, the advantage afforded by asynchronous CDMA is that the performance (bit error rate) is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization. […]
>In other words, asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. CDM (synchronous CDMA), TDMA, and FDMA systems cannot recover the underutilized resources inherent to bursty traffic due to the fixed number of orthogonal codes, time slots or frequency channels that can be assigned to individual transmitters.
This is why at sporting events your AT&T service goes tits up while Verizon users can still actually use their devices — albeit damned slowly. Basically, the anonymous emailer was correct, CDMA is one big shared pipe with a finite amount of resources and no cap on the users.
Now, I tried to verify the claim the GSM does not act this way, but couldn’t find any kind of resource. I did however [come across this page, titled](http://www.nordicgroup.us/ssub/voicequal.htm): “Voice Quality CDMA versus GSM”. The page seems to confirm that CDMA quality will degrade with the more simultaneous users on it, but that overall call quality is in parity between the two networks. ((Full LTE voice calling should be way better sounding.))
So what I have learned is: “call quality” is a dodgy bugger. All things being equal the two standards should have relatively equal “call quality”. However, if CDMA gets congested, the call quality will drop. Whereas if GSM gets congested, you just won’t be able to make a call — roughly speaking.
There’s obvious advantages to both. If you know more, please share.