In the US there is 3 types of service: TDMA CDMA and GSM. TDMA (at&t and Cingulars old system) stands for time delay multiple access. In essence what happens is your phone rapidly scrolls through 3 settings, talk, listen and wait. This was the first way that cell phone company figured out how to put more people on a tower. When you are talking the other 2 people on your frequency are listen and waiting. Now this whole talk, wait, listen process takes place thousands of times a second so there would be no way for you to tell by listening. But in fact older model cell phones do kind of work like walkie talkies.
The “D” stands for “division.” TDMA multiplexes by timeslicing (I am not familiar with the model you described so can’t comment on that) and CDMA (code division multiple access) multiplexes by digitally encoding multiple signals onto the same channel, then decoding it again on arrival to split them back out. These terms, generically, are not mutually exclusive as GSM uses a form of time division multiplexing. (There is also FMDA [frequency] which is often combined with T or C.)
I should have prefaced my comment that while I’ve been in the cellular industry for 10 years and started with at&t (TDMA) it’s been almost 6 years since I’ve done anything with TDMA.
The talk, listen, wait model was the way the engineers described it to us in retail. I will admit that that’s not my side of the industry so I may not fully understand the nuances of it.
Oh and I knew that the D stood for division. why I actually wrote delay? The world may never know.
Both TDMA and GSM use time division multiplexing. It allows multiple users to share a single radio channel. Since the channel is being used to transport digital data, the switching is transparent to the end users.
The old analog cell phones (AMPS) were also full duplex. The base to mobile and mobile to base radio links used separate frequencies, allowing simultaneous use of the transmitter and receiver.
Nextel (and its Canadian equivalent, Mike, now owned by Telus), started out as Specialised Mobile Radio (SMR) business-oriented radio services, using Motorola’s iDen protocol. They were half-duplex, with a push-to-talk button. You could also talk to more than one other radio at one time without dialing phone numbers. They were on a different frequency band and were licensed differently than cellphones.
Since they started, the Nextel phones have acquired cellular capabilities and some can operate on cellular networks. But they still have the PTT feature, and can still communicate with a previously-set-up group of other phones, no matter where they are on the network.
So if you were using Nextel, you probably were using half-duplex at some point.
My dad complains about cell phones being only half-duplex very frequently. He never seems to remember that every time he complains about it, we proceed to test it by both talking at once, and can hear each other in full duplex.
You’ve heard GOOD Muzak??
That link describes the encoding principles, but does not state that it is used in today’s mobile phones. The word “cell” is not in the article, and the only mention of “mobile” is this:
I don’t doubt that it could be used, but I would like to see a link saying it is the CODEC du jour.
Certainly, if it took a Cray 150 seconds to encode 1 sec of voice in 1983, my AMPS phone in 1987 probably didn’t use this algorithm for real-time communications.
It’s not your fault. It’s the engineering design which is not human-friendly.
See also Speech Coding
AMPS phones were analog, basically fancy walkie-talkies. They just transmitted your voice directly over the air, with no need for encoding. Digital systems are more efficient precisely because they use mechanisms like this, and they only became prevalent when the technology became cheap enough.
Here are some slides describing speech coding in GSM, the predominant worldwide cellular standard.
AMPS was FM, just like earlier analog mobile phones. The main advantage of AMPS was that it was the first cellular system. Earlier mobile phone systems could only support a limited number of subscribers due to the relatively inefficient way they used the RF spectrum and the limited number of channels allocated for mobile phones.
Vocoders (digital voice encoders) have been around for over 50 years. You don’t need a Cray-1 to digitally encode/decode voice. It can and has been done with discrete component analog and digital electronics. What modern technology has done is to shrink it and reduce its cost, much like what has happened with computers and modems.
Well, that’s just one more way to degrade the signal and one more reason why cellphone conversations are uncomfortable. I wouldn’t use one ever except the mobility is a big plus in my business.
I’ve always thought that a huge thing to do as a cell phone provider would be to offer higher-quality calls.
Ever talk on a skype to skype call? That is very clear. Probably more-so than a land-line. It requires much less effort on both ends. Maybe we wouldn’t have to shout on phones if we had some companies using higher bandwidth to provide higher voice quality?
I agree - right now all they brag about is how remote you can be and still call. One shows a guy out by the docks and another shows how many more bars you get. But they stopped talking about sound quality, I would gladly pay double if that’s what it took to get land-line quality. I talk to my daughter for about a minute on the cell and then tell her to call me back when we are both home with land lines.