Clockwork-God: Transmitting and receiving are on different frequencies negotiated by the cell tower. Sometimes there will just be interference on the receiving channel, sometimes just the the transmitting, and sometimes both. If you are in a densely populated area, your signals may be even going to different towers. There are a lot of things that can go wrong.
GerrardsClaw: Modern communication systems use digital streams of data to connect calls. In most systems the signalling (setting up a call, putting it on hold, ending a call) is sent using SIP while the voice itself is sent in RTP (real time protocol) streams. The RTP streams are one way. So you have a dream from you to your destination and they have one back.
It is possible that one of these streams gets disrupted, leading to one side hearing the call but the other not.
Edit: I’m staying dreaming!
andenq: Compare it to regular wifi: Have you ever noticed that you sometimes can detect wifi-networks with low signal strength but that you can never connect to? The wifi router usually has a much better antenna and transmitter, while your phone has a built-in much smaller one. So the wifi router is able to scream, but your phone can only whisper in response. In the cellphone case, the cell tower is the router. So person B can hear person A because the tower is able to transmit (scream) through the interference, but person A cannot hear person B, because B’s phone cannot transmit back through the same interference.
I.e. these kind of problems are usually only on one side of the call, between A and the tower, or between B and the tower (in this case between B and the tower). So if you are talking on the phone, and you can hear the other person well but he can’t hear you, the problem is most probably on your side of the call.
salyut3: Person A and person B are not actually talking to each other but rather you will be talking to the telephones companies media servers sitting in the middle of your conversation. A talks to the server, the server passes it to B, B responds back to the server, the server passes it back to A. It can depend on what technology the call you are making is based off but typically newer phone services will run of a communications protocol called SIP. SIP can even emulated other protocols so often end users are not aware they are running through a SIP service. Anyway a typical SIP topology is a lot more complex that your older PSTN or ISDN type calls (old school calls where calls route out from voice switches that may be located at large telephone exchanges) When you make a call nowadays it will need to establish a session with the telephone companies core infrastructure that will often pass through multiple servers and switches that might handle things like call features, call routing, session establishment and audio stream. That audio stream, call setup, features, routing etc are established and maintained as separate entities. That audio stream will be sent from your phone’s IP address and needs to be sent in a way that is compatible with the telephone companies infrastructure. This IP address may also need to change as it routes from your home, through a router and through the telephone companies infrastructure then back to you again. It could be routing through 5 or 6 major nodes just to get the server handing the audio stream before it gets sent back to you. There are many points of failure in modern telecommunications and are no way as reliable as the older more simpler methods of making calls. In my experience the 3 biggest causes of one way audio problems are 1. Your access – your internet has packet loss or latency for eg causing the audio stream to fail. 2. Your network or router may not be configured correctly – you may be blocking certain ports or IP’s, not natting correctly, firewall issues, router needs to be need to be reconfigured to run SIP. 3. Your telephone companies infrastructure, whether that be misconfigured on their side or hardware issues. There are a lot of bugs out there that need to be worked out that will drop the audio stream. Some network devices are not 100% compatible with each other and need vendor firmware updates to resolve. Some vendors dont seem to care and wont release fixes for well known issues either. Protip – If you are getting one way audio on a regular basis try placing the person on hold then pick them back up again to re-establish the audio stream.
coolplate: phone A connects to cell tower 1 and pnone B connects to cell tower 2.
Imagine a 1-way radio connection. In order for phone A to speak to tower 1 it has a finite amount of power sent to the radio transmitter (because it has a small battery) and so sometimes signals are very very. The tower has a lot more power attached to its receiver so it can boost weak signals, but some are just too weak still. But looking the other way around, tower 1 can send a strong signal to A, but if phone A is not picking up good signal, and can only boost it so much (due to the small battery) sometimes it can’t communicate well and info gets lost.
This works on both sets of connections, A<–> 1 and B <–> 2 you can figure out what arrangement will get you your situation.
This happens with radios as well.
It takes “very little” energy to RECEIVE a signal.
Relatively, it takes a lot more energy to SEND a signal.
There can be “imbalances” between reception and transmission resulting in being able to hear, but someone else not hearing you.
Arquill: Imagine if you were standing in an open field, and you and your friend are separated by some distance. Your friend has a megaphone, but you don’t. Right now, it’s quiet, and you can both talk to each other. You don’t need a megaphone for your friend to hear you right now. But, let’s say it gets windy. Suddenly, your voice doesn’t travel so far, but your friend has a megaphone, so you can still hear him.
This is analogous to what could happen to your cell phone. The cell towers that give you service are plugged into the electrical grid, allowing high transmission power, and have very large antennas which can amplify an already strong signal and communicate one way with your phone. However, your phone must make do on battery power, with small antennas which cannot focus very well, so the cell towers might not be able to “hear” your phone when signal quality is low.