Full duplex vs half duplex over RF

[jackj]

You guys are confusing a device with a system. A repeater is a device that is used as a part of a system but it is NOT the system. A repeater IS capable of receiving and transmitting at the same time but the system it is used in may not be capable of full duplex operation.

Furnishing you with an education is not in my job description so I'm done here.

 

[toastycookies]

I would argue the other side of that. A traditional FM repeater by itself is NOT full duplex

you would be wrong then, even by the definitions your referenced.

A traditional FM repeater is a full duplex device used in a half duplex system.

It really cannot be defined much more simply than that.

 

[majoco]

I give up.

 

[902]

Clearly this is one of those concepts that has a bunch of different meanings based on how you view it. I don't think any of them are wrong when they're framed in context.

 

[jim202]

Let me point out what a few have accepted as the definition of full duplex. In the good old days we had both IMTS and RCC (Radio Common Carrier) radio interconnect systems. Combined with the fully capable mobile that connected to the IMTS and the RSS radio systems, you could in fact have a 2 way conversation at the same time. This has been accepted by many of us on here.

Now lets stand back and look at the actual hardware involved with these 2 system. Both of them used a "REPEATER connected to a duplexer and a single antenna. We all accept this as fact. Now these radio systems then connected to the telephone system. We all accept the telephone system as being able to have conversations going in both directions at the same time.

Now we add the mobile radio into the picture. This is a radio with a live transmitter and a live receiver as long as the connection is maintained. There was a telephone type handset that provided both receive and transmit audio at the same time. There was also a duplexer and single antenna this radio used.

So in fact we now have a full duplex radio system that functions in both directions at the same time. You have the base station REPEATER, just like we have available for both Public Safety use and HAM radio operations. The base repeater is exactly the same equipment used by all these users. The only difference here is that they are not connected to the telephone system. Plus the mobile user doesn't have the type of mobile radio that can function in the full duplex mode to allow audio to flow in both directions while the radio is connected to the repeater.

Explain to me the difference in the repeater used for the IMTS, RCC and those used by the hams and Public safety. Bottom line a radio repeater is full duplex, but the rest of the system being used, is not set up for full duplex operation.

 

[Token]

you would be wrong then, even by the definitions your referenced.

While I am OK with potentially being wrong, and willing to change my stance if someone can show me a reason to (I have been wrong before, and likely will be again, anyone who thinks they have never been wrong is deluded), I don’t see how I am wrong by the definitions I referenced.

“permits simultaneous communication in both directions between the called and the calling parties”, not something a traditional FM repeater is capable of. "In a full duplex circuit, simultaneous communications in both directions is possible", not something a traditional FM repeater is capable of. “Method of operation where each end can simultaneously transmit and receive”, not something a traditional FM repeater is capable of.

The traditional FM repeater does indeed transmit and receive simultaneously, and so I can see that basis (simultaneous in both directions) for the argument that it is full duplex. However full duplex, as near as I can tell from almost every reference I can find, requires more than just that.

A traditional FM repeater is a full duplex device used in a half duplex system.

It really cannot be defined much more simply than that.

If the only requirement for full duplex is the ability to transmit and receive (the only ability a traditional FM repeater has) at the same time then why do all the references I can find use a lot more words than that?

T!

 

[Token]

<<<<<< snip for brevity some good information if you are newish to radio >>>>>>
Explain to me the difference in the repeater used for the IMTS, RCC and those used by the hams and Public safety. Bottom line a radio repeater is full duplex, but the rest of the system being used, is not set up for full duplex operation.

A well written response, I like it, and I think it displays how approaching the issue from different aspects can change perspective.

The difference is primarily the phone line connection and how that changes the functionality of the repeater hardware. Without the phone line the repeater cannot support audio flow (or communications) in both directions simultaneously. Even if you used the same kind of mobile as you have described the repeater would still only be able to support one direction of information flow. With the phone line added you now have a system configured significantly different from a traditional FM repeater, with additional audio being sampled and added in the baseband.

So a device similar to a traditional FM repeater could be full duplex, as long as the additional functionality of the baseband (or second information source, could be another set of RF inputs/outputs) interface is added, but without that functionality it is not capable of full duplex operation regardless of how you want to use it. If it is not capable of full duplex operation without modification then it is not a full duplex device.

To me the question comes down to, without changing the hardware configuration of a traditional FM repeater could it support full duplex operation? Adding a base band audio interface, or even connecting any existing such interface, would be a change in hardware configuration to the repeater itself, adding capability that does not typically exist in traditional FM repeater, as well as the associated system.

Let me see if I am getting this from your perspective, you, and possibly other folks saying the hardware of a traditional FM repeater is full duplex, are likely looking at it from the angle that the system COULD be full duplex, using much of the same hardware, if you changed the configuration.

That still comes down to the question of, a traditional FM repeater, with a single input frequency, a single output frequency, and no baseband audio interface, is it a full duplex device or not? Further, a cross band repeat capable radio, like the Yaesu FT8900R, in FM cross band repeat mode, would that be full duplex, or not?

T!

 

[mikepdx]

That still comes down to the question of, a traditional FM repeater, with a single input frequency, a single output frequency, and no baseband audio interface, is it a full duplex device or not? Further, a cross band repeat capable radio, like the Yaesu FT8900R, in FM cross band repeat mode, would that be full duplex, or not?

T!

ALL of the above examples = half-duplex, unless the
FT8900 can receive and transmit simultaneously (in band or cross-band).

It's VERY simple:
Your cell-phone is full duplex - talk and receive simultaneously.
You can interrupt one another.
Otherwise half-duplex.

But, to complicate - here's an example of a mix:
A local cab co transmits from a mountaintop on 452.225
They receive on 457.225 - but use no repeater.
The dispatcher can key down thru an entire exchange
of messages with the cab - he is in full duplex.
The cab receives on 452.225 and transmits on 457.225,
but must un-key between transmissions to hear the dispatcher.
The cab is in half-duplex.

 

[SteveC0625]

Let me offer a slightly different perspective.

The repeater by itself doesn't really matter. It's the application that determines whether the repeater becomes a full duplex device or a half duplex device. For example, if we take a stock repeater and hook it up to a duplex telephone interface, the repeater, when used in an environment like IMTS or RCC, becomes a full duplex unit as part of a full duplex system. However, if we take the same repeater, but hook it up to a typical dispatch console (police, fire, ems, etc.), then it becomes the centerpiece of a half duplex system. Since none of the field units are full duplex capable, and the dispatcher's console and interconnection to the repeater is not full duplex capable, then the system is viewed as a half duplex repeater system. In many ways, it then functions identically as a simplex system except for the enhanced rebroadcast to other field units. Dispatch console installations typically have both repeater and simplex channels. Since the environment is almost always "I talk, then you talk.", operationally, there's not much to distinguish a half duplex repeater from a simplex channel except that the field units have a much wider coverage area in unit to unit transmission. In these installations, when the dispatcher keys up, it disable his receive audio.

Older half duplex repeater systems usually had a button that allowed the dispatcher to disable the repeater. The idea was to give a bit of privacy to a communication from a field unit to dispatch. Once the public learned about input frequencies to program into their scanners, much of the "privacy" disappeared. The advent of cell phones and MDT/MCT's pretty much signaled the demise of the "repeater down" command.

So I'd maintain that by itself, the repeater is potentially full duplex capable. What environment it's installed in will alter its functionality and likely move it to becoming part of a half duplex installation since there are very few full duplex installations these days.

 

[jim202]

Let me add a comment that I failed to point out clearly before.

A cell phone is not full duplex that fits the definition by several so called experts in this conversation. Why do I say this? Well pay attention the next time your having a cell phone conversation with someone. Try to talk at the same time the other person on the other end is. I think you will find out that only one person can talk at any moment in time.

Why do I say this? I have spent some 20 years building and optimizing cellular tower equipment. You can't have both parties talking at the same time and hearing each other at the same time. The way the equipment and system works, only one person captures the talk path at any point in time. You both will not hear each other at the same time. Try it and come back here and let the rest of the group know what your testing concludes. You might be able to get a broken word, or interrupt the other person. But you will not be able to have both going at the same time no matter how hard you try. The cellular system just won't let you do it.

 

[zz0468]

Interesting thread... Here's my take on the term 'full duplex'.

It's the transfer of information in both directions at the same time.

So then, a conventional stand-alone repeater is a full duplex device operating within a half duplex system. As soon as one device in the system appears that is not full duplex, the entire system is not full duplex.

Let's complicate things a bit. I run full duplex on UHF at home, and mobile. Two radios, one transmitting on the repeater input, one receiving the repeater output. If I'm talking to someone else with similar capability through a conventional stand-alone repeater, we can interrupt each other by keying our transmitter. The other party hears the heterodyne, drops his ptt, and allows the interruption. It's not true full duplex, but it's more than half duplex. So, there are shades of gray in levels of duplex operation. I know people who actually operate that way, and it actually works if everyone is in tune with what's going on.

Now, let's add another repeater, linked to the first with full duplex links. Both operators are full duplex mobiles, both repeaters are full duplex, and the link is full duplex. Now there can be a full two way conversation, full duplex, between those two parties.

Add a third guy who's not duplex to one of the two repeaters. Now you have a full duplex system that becomes half duplex when only when one particular individual is transmitting, but is full duplex the rest of the time.

How far do we want to take it?

 

[majoco]

The other party hears the heterodyne, drops his ptt, and allows the interruption.

How do you hear the 'heterodyne' when you are in the 'transmit' mode. 'Heterodyne' between what, the repeater transmitter is on your receive frequency, there is nothing to beat with. There is no need for 'interruption' on full duplex, you can both hear each other all the time. Your transmitter is keyed on all the time and so is your partners and both your receivers are getting the others transmissions. If either of you needs to key a transmitter, then it is not full duplex.

 

[zz0468]

How do you hear the 'heterodyne' when you are in the 'transmit' mode.

In my post, I stated that I run full duplex, with one radio transmitting, and another radio receiving.

'Heterodyne' between what, the repeater transmitter is on your receive frequency, there is nothing to beat with.

The heterodyne is in the repeater receiver caused by two inputs at the same time, one being the guy talking, the other being the guy wanting to interrupt - remember, he's full duplex as well.

There is no need for 'interruption' on full duplex, you can both hear each other all the time.

I know that. You apparently completely missed the point of my post. I was trying to illustrate that there are several gray layers between half duplex, and full duplex.

Your transmitter is keyed on all the time and so is your partners and both your receivers are getting the others transmissions. If either of you needs to key a transmitter, then it is not full duplex.

*sigh*

Where does it state in the rule book that full duplex REQUIRES all transmitters be keyed continuously? What about a situation where the transmitter COULD be keyed continuously without impact to the receiver, but the operator only chooses to transmit when he actually has something to say, thus eliminating from the airwaves extraneous noises such as burps, farts, and yelling at his wife in the background?

It gets down to the fact that the definition of 'full duplex' is all about semantics, and less about the technology. There are duplex devices within non-duplex systems, and there are situational changes that can cause a system to go from duplex to non-duplex without changing the actual system hardware.

 

[Token]

Here is my issue with calling a traditional FM standalone repeater a full duplex device. Remember my original question was just about the repeater alone, not the system it might be part of.

In order for a device to be full duplex it must be capable of supporting simultaneous bi directional data (communications) flow. Bi directional does not just mean both in and out at the same time, but usably to and from each node external of the device. At least that is how I interpret most definitions of full duplex from what I would call credible sources.

This must mean, in part, that the device has the ability to receive and transmit data (communications) simultaneously. And obviously a repeater has a receiver and a transmitter pair that does work simultaneously, and so I can understand someone wanting to call a repeater full duplex based on that.

However, a traditional FM repeater is a one way device. It takes a signal in, does something to it (essentially changes the frequency), and immediately puts the signal back out. But it only does this in one direction and if in FM mode only one usable signal at a time. There is not bi directional data flow or simultaneous communications possible within the architecture of this single device. If this is a correct interpretation then it is less full duplex than a single op amp is (and I don&#8217;t think many would argue a single op amp is full duplex). Signal in, something is done to it, signal out, one direction only and in the case of an FM repeater one signal at a time.

Now, the folks who think a traditional standalone FM repeater is a full duplex device are probably tired of this train of thought, &#8220;dang it, I told him it was a full duplex device, and he just won&#8217;t listen&#8221;. What I am looking for is not just &#8220;is it full duplex or not&#8221;, but why also a person thinks the way they do about it. If it is full duplex, how does it fit in the definition of full duplex without simultaneous bi directional communications flow?

T!

 

[zz0468]

Perhaps where we should go with this is to define half or partial duplex, duplex, and "full duplex". In some regard, the specific meaning is subjective. One of the dictionary definitions of duplex is having two parts. I lived in an apartment duplex once. A repeater fits that definition.

Does use of the term HAVE to imply two way simultaneous end-to-end communication? My opinion is no, it does not. It's contextual... In other words, a repeater is a transceiver. Most transceivers work in one direction at a time, but a repeater obviously transmits and receives simultaneously. It's duplex, even if the overall end-to-end communication is not. Take that same piece of hardware down and replace the repeater circuitry with a speaker and a microphone, and it can participate in full duplex communication, by YOUR definition.

The definitions I was weaned on are:

Simplex- tx and rx on a single frequency.
Half duplex - Tx and Rx on different frequencies.
Full duplex - Tx and Rx on different frequencies at the same time.

And a full duplex piece of hardware is separate from full duplex communications.

 

[902]

So (for the sake of discussion), let's say that console priority on the repeater is disabled and the control system is 4-wire going to the dispatcher. Dispatcher is talking through the repeater and mobile unit breaks in and can interrupt the dispatcher, who stops talking and hears the mobile's priority traffic. THAT is making the repeater (and subsequent control systems) full duplex.

 

[zz0468]

How about a little thought experiment...

Duplex communications:

>------------->
<-------------<


Both ends have information flowing in each direction.

Now, a repeater:

-------->
--------<



Excuse the crude graphic done on my phone, but do you see the similarity with the full duplex depiction? Does a device go from being full duplex to half just because the other end goes away? Is there a requirement that the information flowing in and out MUST NOT be identical? What rulebook is that in?

There is full duplex communications. There is full duplex hardware. Full duplex communications requires that full duplex hardware be used throughout. But use of full duplex hardware does not automatically infer that full duplex communications can take place. A repeater is a full duplex piece of hardware that, in itself, does not allow full duplex communications to take place without other full duplex hardware elsewhere.

 

[zz0468]

So (for the sake of discussion), let's say that console priority on the repeater is disabled and the control system is 4-wire going to the dispatcher. Dispatcher is talking through the repeater and mobile unit breaks in and can interrupt the dispatcher, who stops talking and hears the mobile's priority traffic. THAT is making the repeater (and subsequent control systems) full duplex.

Yes, although the mobile unit is not, the repeater, console, and therefor the operator is. Some would call that half duplex, although from the dispatcher's perspective, he is fully duplex capable. The limitation is on the other end.

Replacing the mobile with a full duplex radio, or talking on an HT while listening on the mobile would allow the mobile to become full duplex, making the entire system full duplex while making no changes upon the system hardware.

 

[Token]

How about a little thought experiment...

Duplex communications:

>------------->
<-------------<


Both ends have information flowing in each direction.

Now, a repeater:

-------->
--------<



Excuse the crude graphic done on my phone, but do you see the similarity with the full duplex depiction? Does a device go from being full duplex to half just because the other end goes away? Is there a requirement that the information flowing in and out MUST NOT be identical? What rulebook is that in?

But I don’t see a repeater as flowing like that.

So, full duplex:
>- - - - - - - - - - ->
<- - - - - - - - - - -<


And in this way the device can support simultaneous bidirectional communications, which does seem to be a requirement under most definitions I can find.


But, a repeater is a one way device:
>- - - - - - - - - > >- - - - - - - - ->


Or if you prefer:
- - - - - ->
^
|
|
- - - - - -<

And because of that it cannot support bidirectional simultaneous communications.

The traditional stand alone repeater is not, in my mind, full duplex not because the other end went away, but because there was never another end and there is no other end for the data to go to.

Or is that, “there is no spoon”?

There is full duplex communications. There is full duplex hardware. Full duplex communications requires that full duplex hardware be used throughout. But use of full duplex hardware does not automatically infer that full duplex communications can take place. A repeater is a full duplex piece of hardware that, in itself, does not allow full duplex communications to take place without other full duplex hardware elsewhere.

The end of that is the sticking point for me. Short of modifying the traditional FM repeater, adding another stage of input / output such as landlines or another set of RFs, the traditional FM repeater cannot support full duplex operation through it, no matter what kind of hardware talks to it as it currently sets.

It seems to me that if you have to modify it, in your drawing if “add the other end” means to include hardware in the repeater itself, then as it sets it is not full duplex. I fully agree that with slight adaptation a traditional standalone FM repeater could be full duplex, but that does not make them full duplex until after that adaptation. If the only adaptations required were completely external to the repeater hardware then the repeater would, of course, be full duplex capable but not be being used as full duplex.

T!

 

[zz0468]

It gets back to my point that "duplex" is subjective, and there are varying degrees. A transmitter- receiver pair configured to allow simultaneous non-interfering operation is, in my opinion, a duplex device, regardless of the usage within the rest of the network.

You asked for our thoughts on the subject. Those are mine. =)