# Do cell phones actually work only one way at a time, like walkie talkies? **URL:** \ **Category:** Factual Questions\ **Created:** [May 6, 2008, 10:40pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358 "2008-05-06T22:40:35Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![t-mobileguy](https://avatars.discourse-cdn.com/v4/letter/t/65b543/32.png) [@t-mobileguy](https://boards.straightdope.com/u/t-mobileguy)\ **Post date:** [May 7, 2008, 1:46pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/21 "2008-05-07T13:46:40Z")
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. ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [May 7, 2008, 2:24pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/22 "2008-05-07T14:24:24Z")
[QUOTE=Colophon] Not that I doubt you, but do you have a cite for this? It seems like it would make transmitting non-voice sounds something of a lottery. [/QUOTE] > **[Code-excited linear prediction](https://en.wikipedia.org/wiki/Code_Excited_Linear_Prediction)** > > Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algorithms, such as residual-excited linear prediction (RELP) and linear predictive coding (LPC) vocoders (e.g., FS-1015). Along with its variants, such as algebraic CELP, relaxed CELP, low-delay CELP and vector sum excited linear prediction, it is currently the ... ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [May 7, 2008, 2:50pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/23 "2008-05-07T14:50:08Z")
[QUOTE=t-mobileguy] 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. [/QUOTE] 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.) ---
**Author:** ![t-mobileguy](https://avatars.discourse-cdn.com/v4/letter/t/65b543/32.png) [@t-mobileguy](https://boards.straightdope.com/u/t-mobileguy)\ **Post date:** [May 7, 2008, 3:21pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/24 "2008-05-07T15:21:04Z")
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. ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [May 7, 2008, 4:06pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/25 "2008-05-07T16:06:36Z")
[QUOTE=t-mobileguy] 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. [/QUOTE] 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. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [May 7, 2008, 5:52pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/26 "2008-05-07T17:52:10Z")
[Nextel](http://en.wikipedia.org/wiki/NEXTEL_Communications) (and its Canadian equivalent, Mike, now owned by Telus), started out as Specialised Mobile Radio (SMR) business-oriented radio services, using Motorola’s [iDen](http://en.wikipedia.org/wiki/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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 7, 2008, 5:54pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/27 "2008-05-07T17:54:37Z")
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. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [May 7, 2008, 6:04pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/28 "2008-05-07T18:04:20Z")
[QUOTE=Balthisar] So that’s why Muzak over the cell phone sucks so bad. [/QUOTE] You’ve heard GOOD Muzak?? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [May 7, 2008, 6:05pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/29 "2008-05-07T18:05:28Z")
[QUOTE=Absolute] > **[Code-excited linear prediction](https://en.wikipedia.org/wiki/Code_Excited_Linear_Prediction)** > > Code-excited linear prediction (CELP) is a linear predictive speech coding algorithm originally proposed by Manfred R. Schroeder and Bishnu S. Atal in 1985. At the time, it provided significantly better quality than existing low bit-rate algorithms, such as residual-excited linear prediction (RELP) and linear predictive coding (LPC) vocoders (e.g., FS-1015). Along with its variants, such as algebraic CELP, relaxed CELP, low-delay CELP and vector sum excited linear prediction, it is currently the ... [/QUOTE] 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: > [@](#): > > …more efficient ways of implementing the codebooks and improvements in computing capabilities have made it possible to run the algorithm in embedded devices, such as mobile phones. 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. > [@Cub Mistress](#): > > this “stepped-on” conversation is what happens to me. I can’t seem to break the habit of speaking while others are speaking (in casual friend-to-friend conversations.) I get a little frustrated at myself. It’s not your fault. It’s the engineering design which is not human-friendly. ---
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [May 7, 2008, 7:05pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/30 "2008-05-07T19:05:39Z")
[QUOTE=Musicat] 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. [/QUOTE] [Intterestingly it’s true](http://www.pcmag.com/encyclopedia_term/0,2542,t=speech+codec&i=51838,00.asp). > [@](#): > > Definition of: speech codec > > Also called a “voice codec” or “vocoder,” it is a hardware circuit that converts the spoken word into digital code and vice versa. It comprises the A/D and D/A Seeconversion and compression technique (see G. standards). If music is encoded with a speech codec, it will not sound as good when decoded at the other end. > > Specialized for Voice > A speech codec is an audio codec designed for human voice\*\*. By analyzing vocal tract sounds, a recipe for rebuilding the sound at the other end is sent rather than the soundwaves themselves\*\*. The speech codec is able to achieve a much higher compression ratio, which results in a smaller amount of digital data for transmission. When telephones were first digitized in the early 1960s, they generated digital streams of 64 Kbps (see PCM). Since then, speech codecs have reduced voice to as little as 5 Kbps and less. See audio codec, sampling, codec examples and codec. See also [Speech Coding](http://en.wikipedia.org/wiki/Speech_encoding) > [@](#): > > Speech coding is the application of data compression of digital audio signals containing speech. Speech coding uses speech-specific parameter estimation using audio signal processing techniques to model the speech signal, combined with generic data compression algorithms to represent the resulting modeled parameters in a compact bitstream. > > The two most important applications of speech coding are mobile telephony and Voice over IP. > > The techniques used in speech coding are similar to that in audio data compression and audio coding where knowledge in psychoacoustics is used to transmit only data that is relevant to the human auditory system. For example, in narrowband speech coding, only information in the frequency band 400 Hz to 3500 Hz is transmitted but the reconstructed signal is still adequate for intelligibility. > > Speech coding differs from other forms of audio coding in that speech is a much simpler signal than most other audio signals, and that there is a lot more statistical information available about the properties of speech. As a result, some auditory information which is relevant in audio coding can be unnecessary in the speech coding context. In speech coding, the most important criterion is preservation of intelligibility and “pleasantness” of speech, with a constrained amount of transmitted data. > > It should be emphasised that the intelligibility of speech includes, besides the actual literal content, also speaker identity, emotions, intonation, timbre etc. that are all important for perfect intelligibility. The more abstract concept of pleasantness of degraded speech is a different property than intelligibility, since it is possible that degraded speech is completely intelligible, but subjectively annoying to the listener. > > In addition, most speech applications require low coding delay, as long coding delays interfere with speech interaction. ---
**Author:** ![Absolute](https://avatars.discourse-cdn.com/v4/letter/a/b2d939/32.png) [@Absolute](https://boards.straightdope.com/u/Absolute)\ **Post date:** [May 7, 2008, 7:41pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/31 "2008-05-07T19:41:11Z")
[QUOTE=Musicat] 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. [/QUOTE] 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. [http://www.uio.no/studier/emner/matnat/ifi/INF5080/v04/mkt05b-gsm.pdf](http://www.uio.no/studier/emner/matnat/ifi/INF5080/v04/mkt05b-gsm.pdf) ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [May 7, 2008, 8:42pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/32 "2008-05-07T20:42:31Z")
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. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [May 7, 2008, 10:58pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/33 "2008-05-07T22:58:07Z")
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. ---
**Author:** ![Merkwurdigliebe](https://avatars.discourse-cdn.com/v4/letter/m/74df32/32.png) [@Merkwurdigliebe](https://boards.straightdope.com/u/Merkwurdigliebe)\ **Post date:** [May 7, 2008, 11:24pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/34 "2008-05-07T23:24:02Z")
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? ---
**Author:** ![Consonants](https://avatars.discourse-cdn.com/v4/letter/c/c37758/32.png) [@Consonants](https://boards.straightdope.com/u/Consonants)\ **Post date:** [May 28, 2008, 2:29pm UTC](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358/35 "2008-05-28T14:29:25Z")
[QUOTE=Merkwurdigliebe] I’ve always thought that a huge thing to do as a cell phone provider would be to offer higher-quality calls. [/QUOTE] 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. [Previous page](https://boards.straightdope.com/t/do-cell-phones-actually-work-only-one-way-at-a-time-like-walkie-talkies/448358.md?page=1)