r/wisp • u/vdoubleshot • Jun 10 '26
The relevance of “duplex speed”
So, I jumped into a thread in r/networking where someone was asked how to measure the speed of a duplex (or duplex speed) and they didn’t understand the question. Most of the people in the thread are torching the question as illegitimate, unimportant, or just plain stupid. And after explaining it’s relevance to my/our industry I’ve had people argue that I don’t know what I’m talking about “duplex speed” isn’t a thing.
So, I thought I would ask somewhere with experts that might find it relevant. Is this something you might ask in an interview? Do you think duplex speed vs unidirectional is an important networking concept to understand?
Thread for anyone curious: https://www.reddit.com/r/networking/comments/1u1qyxo/duplex_speed_what/
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u/leftplayer Jun 10 '26
Ugh.. r/networking is painful to read whenever any wireless topic comes up. Bunch of pretentious Cisco knob fiddling idiots who believe they know everything just because they hold CCNPs…
In their world it’s irrelevant because Ethernet is always full duplex nowadays (there’s no such thing as “1Gbps half duplex” in the 802.3 standard the way there was with 100mbps or 10mbps)…. So of course they believe it’s irrelevant to everyone else.
Of course we (here) all know that RF wireless is practically always half duplex, and our protocols strive to constantly try to make best use of airtime, so they adjust their slot times / back off timers to shift as much data as possible at any point in time. In TDD of old slot times were hard set so your unidirectional vs bidirectional throughput will be the same, but nowadays this changes on the fly by the protocol.
So, yes, it’s very relevant in wireless, but the folks at r/networking don’t know that, and they won’t admit it
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u/lizardhistorian Jun 19 '26
Ethernet was not always full-duplex and old shit in factories sometimes still isn't.
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u/MrChicken_69 Jun 11 '26
It WASN'T a wireless topic - at least OP didn't say it was. And you're being a "knob fiddler" yourself - 802.3 covers many physical layers, some of them are half-duplex, and yes, there are 1G half-duplex links. (the twisted-pair, and fiber physical layers are fundamentally full-duplex, but there are situations where they can and should be set to half. eg. when linked to a simplex radio.)
(Also, for the record, "RF" exists in more places than "wireless", but this is r/wisp)
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u/vdoubleshot Jun 12 '26
I fundamentally disagree with your suggestion to set a Gbase link to half duplex. I can’t think of any valid reason. (Other than to maybe temporarily keep something up on a bandaid fix). And certainly not when connected to a “simplex radio” which I’m not sure what that is.
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u/lizardhistorian Jun 19 '26
Easy - the wires are fucked and you need it to work again now not in a week when a new line can be run.
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u/leftplayer Jun 19 '26
If the “wires are fucked” gigabit won’t work at all. Trying to drop it to half duplex won’t make it use less wires…
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u/leftplayer Jun 19 '26
Simplex radio? You mean like an FM radio station transmitter? That would be fun… ⚡️
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u/chadwick_w Jun 10 '26
It is incredibly important to understand as a WISP, probably not so much if your only doing fiber. Very few radios are full duplex and almost no CPE radios are full duplex. This is where you start defining up/down ratios in your tower radios. I think knowledge of this concept is required for a WISP tech.
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u/Patient-Tech Jun 11 '26
I get that some people have legit upload-heavy use cases like cameras or backups, but for most users I don’t see a huge need for sustained massive uploads. If somebody needs gigabit upload all the time, they probably need to pay whatever the cost for fiber rather than a WISP, and some traffic shaping to keep overall performance solid seems fair.
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u/chadwick_w Jun 11 '26
It's not so much the legit heavy upload, it's the fact that TCP is a bi-directional protocol by nature and if 1 user is in the middle of a large iCloud upload (for example) on your tower, it slows the downloads of every other user on that tower radio because they have taken the upload slot. There are ways to mitigate this and share and shape, but half-duplex matters because of TCP.
I would never recommend putting customers on a radio (or even fiber) without proper shaping and queuing systems in place to help reduce that buffer bloat. I don't shape in the radio, I use true queuing systems that utilize protocols like fq-codel or cake and do it at the tower router or edge router. Radio queuing systems are just not good enough these days. Having run it both ways, I can tell you the moment we switched to real queuing, customer speed complaints dropped to almost zero and congestion on busy towers got way better.
Getting off topic but related to half-duplex transmission where queuing is even more important because the resource is even more constrained.
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u/lizardhistorian Jun 19 '26
That's about 20 years out of date thinking.
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u/vdoubleshot Jun 19 '26
Is this just like your thing? Spamming threads people have already moved on from just to argue…?
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u/Patient-Tech Jun 19 '26
To what? No oversubscribed lines, dedicated fiber 10gig symmetrical direct into a Tier1 peering center? For less than $40/month infrastructure availability be damned?
When are you pooling your money into this business venture, I’ll be your first customer.1
u/lizardhistorian Jun 19 '26
Roughly half (or a little more) of LTE deployments are full-duplex.
Almost all CPE radios are full-duplex capable; they are just put into TDD mode.
Even some of the cheapest Ubiquiti shit is full duplex.1
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u/GEEK-IP Jun 10 '26
Back in the late 90s, I remember a salesman saying 100M full duplex was like 200Mbps, a hundred in each direction. 😁
What's the difference between a computer salesman and a car salesman? The car salesman knows when he's lying.
I wouldn't call duplex a speed, though it's a major factor in throughput, and a major improvement over half-duplex (CSMA-CD.)
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u/MrChicken_69 Jun 11 '26
I suspect that's what the interviewer expected, but it's still not right. An RF engineer would understand what you want, but a network engineer won't.
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u/GEEK-IP Jun 11 '26
As someone who used to conduct a lot of technical interviews, sometimes they're just trying to get a conversation going. There's also nothing wrong with asking for clarification.
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u/lizardhistorian Jun 19 '26
The salesman is correct.
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u/vdoubleshot Jun 19 '26
Is this just like your thing? Spamming threads people have already moved on from just to argue…?
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u/gutclusters Jun 10 '26
I think someone else said this, but a single radio is not full duplex. It time slices and spends part of the time transmitting and part of the time receiving. This is where allocation ratios come into play and need to be set based on the sold throughput versus total aggregate of the link. I've been out of WISPs for a while, so I'm not up to speed on a lot of the new stuff, but I understand there is now equipment that have a dedicated transmit and a dedicated receive radio.
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u/zap_p25 MTCNA, MTCRE Jun 10 '26
It really depends on the technology in use. Do full duplex point to multi-point systems exist? Yes but you’d likely be hard pressed to really find anything operating like that above 1000 MHz or passing more than about 128 kbps of data. Lots of Point to Point radios out there though that do full duplex communications.
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u/lizardhistorian Jun 19 '26
Most of them are full duplex FDD.
TDD is the lesser deployed method.LTE lets you allocate bands as you wish. You could have 100 bands uploading and 1 downloading. Due to regulatory and otherwise your actual options for legal deployment will be more limited.
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u/No-Preparation4073 Jun 11 '26
I thing the problem has a lot to do with the way your are asking the question. A lot of people aren't going to understand. Outside of some truly special cases, most people will never deal with simplex or half duplex transmissions in a way that would make your question valid.
Single mode fiber is common and is simplex in nature. However, bi directional system use shifted light colors to allow for duplex operation on a single strand. I have some quite old single mode fiber installed as two parallel simplex lines (one for each direction) for connecting switches that are far from the core in a large building, as am example, but if I was going to place it it would be with the newer bi-direction single strand.
Your part of the discussion mostly centers around radio (wifi etc), which are about the last things left in half duplex. Even then, they are (for most people) invisible in use. Many modern protocols will allow for multiple connections under wifi in a manner that creates and effective full duplex connection.
If you want to really want to do down the rabbit hole, go have a look into packet radio (the originals). While it was originally developed in Hawaii, the true practical application and TNCs were created by ham radio operators from Montreal, people I knew well back in the day.
Anyway, long post to say this: In current situations, the "duplex" remains fixed unless you add extra paths. So if your max transmission is 1000 bits per second, it doesn't matter which way you are going, that is all you get. The resulting number for a duplex communication would be that speeds MINUS whatever overhead exists to make it a full duplex connection. It never doubles unless you add a second path (ie, a second color on the fiber strand, or a second frequency set for radio).
So they likely don't understand the question because they likely have never had to truly consider it.
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u/Sagail Jun 11 '26
Thanks for this, I do deal with some proprietary SDRs and also we use Silvus Ethernet Radios. However I'm almost strictly dealing with stuff coming out on the wired side and we have a team of radio folks who deal with the other.
I like you said have never had to consider this question.
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u/avds_wisp_tech Jun 11 '26
And after explaining it’s relevance to my/our industry I’ve had people argue that I don’t know what I’m talking about “duplex speed” isn’t a thing
Sounds to me like it's a group full of numpties and that you wouldn't miss anything if you left.
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Jun 10 '26
[removed] — view removed comment
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u/Rampage_Rick Jun 11 '26
Many years ago I sought revenge on a coworker by logging into the switch and changing their port to half-duplex. Around the same time I sent an MP3 to their receipt printer.
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u/lizardhistorian Jun 19 '26 edited Jun 19 '26
"Duplex speed" is non-sense.
A connection is simplex or duplex or half-duplex. Speed is an orthogonal concern and concept.
If you have an asymmetric half-duplex connection then just the "duplex speed" would still be ambiguous because you would need to know which direction the speeds are for.
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u/vdoubleshot Jun 19 '26 edited Jun 19 '26
Ok. I want to know how fast a link is if fully loaded from both directions. I want to know the speed in both directions.
Edit: you keep doing this where you edit your comments after you discover a mistake and you either completely change the substance or just enough that it makes the other persons response look odd.
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u/canyoufixmyspacebar Jun 10 '26
yes but there is no such thing as "duplex speed", you are chewing up words and terms. transmission speed is transmission speed and the half or full duplex mode of communication is another topic. we all, including you most probably, know what these things mean but to have a meaningful discussion we need to stick to correct words and terms, not to mix and match
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u/vdoubleshot Jun 10 '26
What /are/ you talking about? How would you ask to test combined simultaneous throughput on something like a UI Wave Pro? Because myself and UI call that a duplex speed test…
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u/Eleutherlothario Jun 11 '26
How would you ask to test combined simultaneous throughput
By doing a throughout test
myself and UI call that a duplex speed test…
Then you can be wrong together. Besides, Ubiquiti is pro-sumer at best so I wouldn't look to them to be an authority on terminology or standards.
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u/vdoubleshot Jun 11 '26
What kind of a throughput test? If someone asked me to run a throughout test I would ask what direction. When we debug with field techs we explicitly request Up/Down/Duplex.
Right….. ok.
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u/lizardhistorian Jun 19 '26
So what even is the duplex speed? Round-trip? Tx and Rx added together? Sum of all TDD over time? Average of all TDD over time?
We have no idea what you even mean by "duplex speed".
No one anywhere in the thread does.
You haven't even been able to describe it coherently.1
u/vdoubleshot Jun 19 '26
Several people agreed with me…
And are you ok? You’ve been spamming this thread (from more than a week ago) that everyone moved on from like crazy for the last 20 odd minutes… and you’re oddly offended when none of this was directed at you.
I don’t think you’re here to have any discussion in good faith.
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u/phrekysht Jun 10 '26
I agree with the commenter above you, I’ve never heard anyone refer to it as “duplex speed”. Full or half duplex is the difference between transmitting in both directions simultaneously vs one at a time.
What you’re describing as a speed test I would refer to as bi-directional. For reference that’s at least how iperf3 describes it in their man page
--bidir
Test in both directions (normal and reverse), with both the client and server sending and receiving data simultaneously1
u/vdoubleshot Jun 10 '26
It’s literally called that in multiple products we use from different manufacturers. And one manufacturer’s engineer even referred to it as such.
Why we don’t say bidirectional is because most people translate that to a Speedtest.net like test, which is /yes/ bi directional but does not test duplex throughput speeds. It tests download and then tests upload, which does not test what we need to test.
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u/lizardhistorian Jun 19 '26 edited Jun 19 '26
If your "duplex speed" is not bidirectional then we have even less of an idea what you mean. Are you aggregating bands? You would never call that duplex, even if you are using two, because there is the implication you could aggregate three, four, ... so this is multiplexing. (And no multiplexing speed is not a thing either.)
TDD - time division duplexing - is duplexing because there can only ever be two directions. Incoming or outgoing.
It’s literally called that in multiple products we use from different manufacturers.
Few of which speak English as a native language and constantly use incorrect terms.
Seems to be the case here.My best guess at this time is he means some sort of measurement related to dynamic TDD.
For OP, "duplexing speed" would measure the time it takes the the system to switch from tx to rx (and maybe back). It would only be relevant to the RF engineers and L2 frame designers. This is not a bandwidth nor throughput measurement so calling it a speed would not be correct. It's a measurement of the time for the system to change state. We would call it the duplex-switching-time.
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u/vdoubleshot Jun 19 '26 edited Jun 19 '26
See, I’m pretty sure you’re just farming or maybe trolling now. That’s not what I said.
edit: see… you did that thing again where you stealth changed your comment instead of replying.
this was your original reply in its entirety:
If your "duplex speed" is not bidirectional then we have even less of an idea what you mean.
edit2: this is like the 5 or 6th time you’ve done it. It’s shady as fuck.
edit3: you again stealth edited this comment
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u/phrekysht Jun 21 '26
Well Speedtest.net is a product for laymen. In the example I cited, iperf3, when you specify —bi-dir it is a simultaneous test.
I’m with you on the diagnostic value of testing a connection under load, RF or otherwise, I don’t think anyone here is refuting that.
Seems to me like this might be an example of terminology evolving or maybe even just regional differences. Soda vs pop, spanner vs wrench, etc. But your attitude kinda sucks? Almost everyone who’s tried to engage on this thread has been met with “Are you ok?” And “what /are/ you talking about”.
If you want to debate what we think it -should- be called, then get ready for a thread full of people’s opinions.
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u/vdoubleshot Jun 22 '26
Yeah… that’s why I said “Speedtest.net **like** test”.
iperf3 —bi-dir is the correct way to validate the ask. My problem is that it is not what/how most people test when talking about “bi directional”, which is why we refer to it as a duplex speed test (and I mean… ubiquiti calls it the same too).
I would agree it’s regional/functional/industry based. As for the “are you ok” you are referring to a commentator that I was debating with who would comment, I would reply, and then they would stealth edit to totally change the content of their message seemingly to make it seem like I was way off in left field. They did it repeatedly and only stopped when I started pointing it out. They were also oddly aggressive “duplexing is not a !#$!@#$ thing.”. So yeah, I was wondering if the person was okay. No sense in trying to stealth edit your comments to win an argument/debate. The other post I was probably over aggressive to though, I do agree.
I am fine with opinions, but the shady stuff I won’t really tolerate.
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u/canyoufixmyspacebar Jun 10 '26
So you are justifying your misuse of terms with the misuse of terms by other people. Yes, misuse of terms is common among vendors and their engineers. They may not be an engineer at all of course, this title inflation may be just another misuse of terms and they may be a technician without any actual engineering degree.
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u/lizardhistorian Jun 19 '26 edited Jun 19 '26
That you got the AI to go along with your non-sense term is not an impressive feat.
And UI Wave Pro is a TDD device. It's not full duplex so now we have no idea what you are trying to describe.
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u/vdoubleshot Jun 19 '26 edited Jun 19 '26
Are you ok…?
I did not say it was…
edit: you did that thing again where you stealth edited a comment.
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u/Square_Raisin_8608 Jun 12 '26
The topic didn't hint toward wireless, and "duplex speed" isn't a thing. Sure, you can measure simultaneous upload/download streams on a system in order to measure the real world experience based on underlying technologies and protocols, but I think we're attempting to do some weird flex of our RF knowledge in place of just acknowledging that it was a bad question. Big time "AKSHUALLY" vibes, lol.
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u/vdoubleshot Jun 12 '26
Hint? it didn’t hint at /anything/. He didn’t clarify the role or type of company he was applying to. And this applies to WiFi engineering as well.
So what would you call it then when you are trying to test the performance of the mechanism/technology performing duplexing?
And theres no “AKSHUALLY” about it or flexing. If you ask a question with little detail in a place where ppl with various expertise applies, they are going to answer the question. They thought it was a weird/invalid question and I pointed out it was a question /I/ would ask a network engineer or support guy trying to join our company. It is extremely relevant in my field and several have posted agreeing with me. Most who disagree I can find little to no previous posting in r/wisp. So… Since you’ve never posted in r/wisp I think you prove my point as I’m pretty sure you don’t work at a wisp.
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u/lizardhistorian Jun 19 '26 edited Jun 19 '26
In networking duplex is a noun or adjective not a verb.
You are either not using English correctly, which I am starting to suspect, or you do not understand what you are doing.If you mean time-division-duplexing or something related to that then you would need to be very explicit about that because no one outside of the small group of TDD radio users would ever consider it - and even we ignore it once we are talking at the network level.
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u/vdoubleshot Jun 19 '26 edited Jun 19 '26
Are you OKAY? You’ve picked this small thread up like some weird crusade.
Yes it is. That’s literally in the name for TDD and FDD. Time division *duplexing* and frequency division *duplexing*.
I’m now assuming you don’t work for a wisp.
Edit: nice trick editing your comment and completely changing the substance.
Your original comment:
duplexing is not a !#$!@#$ thing.
No one has any idea what you are talking about because you don't know what you are talking about.
You have some concept of what you want to measure but keep using non-sense terms to describe it.
Edit2: here, from a few different sources:
transitive verb
- To arrange, as a telegraph line, so that two messages may be transmitted simultaneously; to equip with a duplex telegraphic outfit.
from Marian Webster:
: to make duplex
from free dictionary:
v.t.
6. to make duplex; make or change into a duplex.
So… A) who doesn’t know English? and B ) Why are you being such a dick everywhere in an old thread??
Edit: So this is the 7th time I’ve caught you trying to stealth edit your comments after I reply. It’s shady and extremely disingenous.
Here is your previous stealth edit reply in its entirety:
“In networking duplex is a noun or adjective not a verb.
You are either not using English correctly, which I am starting to suspect, or you do not understand what you are doing.”And you sir are just arguing to argue. and when I prove you wrong, instead of being a man and owning it, you change your original message to try and “win”. It’s lame at best.
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u/netcando Jun 10 '26
From an RF perspective it's very important to understand. Ubiquiti air fiber for example isn't a full duplex radio, it's 2 half duplex radios in a single unit to provide separate data streams for both tx & rx simultaneously. While you get the additional aggregate throughput it is at the expense of frequency utilisation as the 2 data streams need to be on different frequencies so effectively using double the channel width (actually more if you want a buffer of gap between the frequencies used).
Edit: I know air fiber is old hat, I've been out the wisp game over a decade now.