Are land-line telephones full duplex? Are cell phones?

[VirtualLarry]

Just wondering.

Land-line telephones only have two wires, that forms one circuit, AFAIK. So I'm wondering how analog modems are full-duplex? I know that they use different frequency ranges for upstream and downstream signalling, but if they were going to maximize analog channel bandwidth, wouldn't that use up the entire frequency band in one direction? Since the max bandwidth is 56K, how do analog modems manage to have ~56K downloads, while still having 28.8 or 33.6 uploads?

Likewise with cell phones. If I want to tether, that is, use my digital GSM cell phone as a modem to access the internet, is that full-duplex or half duplex? I thought that all wireless devices were half-duplex, but maybe I'm wrong. Can you use a wireless antenna to both send and recieve a signal at the same time? Do cell phones do this?

 

[Ben90]

Im no expert, and definitely shouldn't be posting in this forum, but im fairly sure that land-line telephones are full-duplex because all the connections made by them are analog... pretty much the equivalent of a fast as hell telegraph (which is definitely full duplex)

And with cell-phones, im fairly sure that those are half duplex connections; similar to Ip phones

 

[Colt45]

well POTS isn't really full duplex. Two people can talk at the same time, sure, but there's contention.

Old 300 baud modems used FSK.. So there's two carriers, one on each end.. one ~1kHz, and one around 2kHz. The freq is shifted up or down, to be a one or zero. (i.e. when the sender sends 1800Hz, it's a 0; 2200Hz is a 1).
Since they're different freqs, you can go both ways.
Later modems added fancier modulation techniques, so that each 'symbol' was more than a single bit, but 8 or 10 bits.. i forget. and they added compression.
I seem to recall the later ones were half duplex though, as you could get more throughput by having all the pipe for half the time.

56k only worked down for some reason i can't remember. Something about the CO having fancier digital equipment, and/or power regulations. Something like that anyways, not a real limit.

All the wireless stuff I've played with is either half duplex, or has different freqs & antennas for tx/rx. I can't see any other way of doing it.

 

[Modelworks]

56k works when the CO has no A to D converter in the signal path except at the CO modem. It exploited the fact the end connection would be converted to digital to push more data ,basically skipping a step in the process. Instead of an analog stream it was pure digital downstream. If the line had a A to D converter that grouped several local calls into a single line then converted it back from D to A at the CO then it killed 56k because a direct digital connection to the modem on the other side was not possible. The reason 33.K upload was possible on the same line is that the return channel was analog in with the digital signal, both could operate at the same time.