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I've read¹ that microwave is a faster medium of transmission of data than infrared.

But considering that infrared waves have higher frequencies (and a little bigger frequency range too) than microwave, shouldn't they facilitate faster transmission rate? Isn't it the same reason why microwave is said to be faster than radio waves of lower frequencies?

I guess the reason has to do with the amount of research and investment put into the technologies used. Infrared has the disadvantage of being extremely short ranged, so maybe it wasn't invested into as much as microwave was, with respect to transmission speeds?


¹ I read in a sample question paper

Question:

Out of the following, which is the fastest wired and wireless medium of transmission?
Infrared, coaxial cable, optical fibre, microwave, Ethernet cable

Answer:

Wired- optical fibre
Wireless – microwave

question paper
answer key
(question number 2)


Edit : I'm pretty sure the question is meant to be about data transfer rate and not latency

Edit 2: Based on my syllabus, I'm also pretty sure the question is about wireless infrared

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  • 1
    "Isn't it the same reason why microwave is said to be faster than radio waves?" Microwave is radio wave. For example, microwave ovens use 2.4 GHz, the same as Wi-Fi.
    – Ron Maupin
    Mar 29 at 17:01
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    They need to clarify what they mean by "fastest". Are they looking for latency (propagation time), or overall bandwidth (bit rate)? The same fiber cable can transmit 100Gbps or 400Gbps, but both take the exact same amount of time for a photon to move from end to end. (the bit rate obviously depends on the encoding.)
    – Ricky
    Mar 29 at 17:48
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    Given the low quality of the test, I wouldn't be surprised if by "infrared" they mean "the infrared transmission technique as used in your TV remote control".
    – jcaron
    Mar 30 at 9:02
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    @Balu There are so many false and/or outdated things in that textbook it's not even funny. But they are thinking mostly about the infrared as used in TV remote controls (see the paragraph just above). IIRC that was also used for communication on some PDAs. But the simple fact they write "PDA" just shows how much this is outdated.
    – jcaron
    Mar 30 at 9:34
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    You have stumbled on one of the reasons that all "education, certification, or homework" questions are explicitly off-topic here. A lot of outdated or outright false information is taught. If you use answers you get here, you may get it wrong for the course. even if the answers you get here are correct. It is also difficult for us to get the correct context (as you may have noticed by the various answers for this question) because we do not have the books and have not heard the lectures.
    – Ron Maupin
    Mar 30 at 12:23

4 Answers 4

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As others have pointed out, both are (almost) the same in terms of propagation speed. I'm assuming the difference is in data rate.

Microwave comms are based on circuitry which can be specifically designed for various targets, one of which is bandwidth efficiency. Because of the high bandwidth available, modulation can be designed for higher rates in the microwave region than at lower frequencies, which are more cluttered and have smaller amounts of contiguous bandwidth available. Since we continue to drive the carrier frequencies of our signals higher, the higher frequencies are allocated according to more modern needs (among other things, wide contiguous channels).

Infrared (IR) has even greater bandwidth, but we can't access it currently with purpose-designed circuitry. Instead, we use a laser, LED, or such optical device with its own inherent characteristics. Such devices can't be easily tweaked for efficient bandwidth usage; for instance, a laser takes some time to get the beam pumped up. As such, the bandwidth of the signal depends on the device physics more than the design intent.

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Faster in terms of propagation delay? @Zac67 answer is good.

Faster in terms of data throughput? Here, infrared has a huge theoretical advantage.

The whole microwave range consists of about 300GHz bandwidth, infrared has ~400THz. This difference can be mapped to a proportionally higher data throughput (other conditions being equal).

Of course, other conditions are not really equal - at least in regard to our communication technology being much more developed in the microwave range.

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  • So practically, is microwave still faster than infrared in the current world?
    – Balu
    Mar 29 at 17:56
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    It depends. You can get off-the-shelf 10GBit/s optical fiber cable and equipment. 1GBit/s WiFi is quite a challenge.
    – fraxinus
    Mar 29 at 18:06
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Given the context which is severely outdated when not completely false, "infrared" seems to mostly refer to short-range low-bandwidth infrared communications as used:

  • In many TV remote controls
  • For short-range line-of-sight communication between devices such as PDAs in the 1990s and 2000s, using IrDA, which at the time when PDAs were still in use, was quite slow. This has since been widely replaced by Bluetooth and/or WiFi.

Note that in general the textbook and the test are quite low quality. I suspect they expect you to answer what is written in the textbook rather than what is actually true nowadays (or theoretically true).

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I've read that microwave is a faster medium of transmission of data than infrared.

It's not. Light moves at the speed of light (c0), (pretty much) regardless of its wavelength - in reference to the propagation speed. While the medium of an electromagnetic wave matters, the wavelength is of very little to no significance (see below).

The quoted question & answer may come from a misconception because usually microwaves are transmitted over the air (very close to c0 because of the air's low density) while infrared is transmitted over fiber. Due to the fiber's optical density, light moves at c0 divided by the refractive index which is roughly 200,000 km/s (velocity factor of .67). So, microwaves over the air are about 50% faster than infrared over fiber.

In an optically dense medium, the refractive index slightly varies by wavelength (that's how a prism works), but there's no such medium that transmits microwaves.

And by the way, a good coax cable has a higher velocity factor (up to .90) than fiber. Even category-7 twisted-pair copper cable is faster (.75). And there's no such thing as an "Ethernet cable". Various Ethernet variants use(d) RG-8X or RG-58 coax, twinax, categories 3, 5, 6A or 8 twisted pair, OS1 or OS2 single-mode or OM1 though OM5 multi-mode fiber. If you're paying for that test you should reclaim your money.

[edit]Regarding (wireless) infrared vs microwave speed: air does have a refractive index of ~1.0003 (at 590 nm/yellow-orange light). For microwaves (cm-range), the refractive index may be lower by as much as 10% (~1.00027) - over a distance of 1000 m that translates to a difference in propagation delay of ~330 ns 100 ps (miscalculation corrected) or 3 cm lead for the microwave.

Also, if the question defines "faster" in respect of transmission speed/bandwidth: infrared's higher frequency (>=350 THz) make it a much better data carrier than microwaves (>2 GHz). Fiber communication using infrared is used for 400 Gbit/s Ethernet (up to 40 km), with 800G and 1.6T currently being developed.

For wireless, free-space optical (FSO) laser systems using infrared wavelengths are available for 1 and 10 Gbit/s, likely even faster. The reach is several km. Microwave commonly runs at 1 or up to 10 Gbit/s. It tops out at perhaps 100 Gbit/s over 1 km or so, with a huge price tag.

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  • Would not a higher frequency facilitate more condense packing of information? Like if we have pulses every 1 second, we can transfer at twice the speed than having pulses every 2 seconds, even the signal travels at the same speed, right?
    – Balu
    Mar 29 at 17:35
  • And in their textbook, they always mentioned infrared as a short range wireless medium, so I don't think they are talking about optical fibres
    – Balu
    Mar 29 at 17:39
  • @Balu Sorry for not being clear - for "faster" I was referring to the propagation speed only. Data bandwidth is a completely different thing where fiber surpasses everything else by orders of magnitude.
    – Zac67
    Mar 29 at 17:41
  • @Balu Infrared was used for short range in the 1980s and 1990s. Today, you can use infrared lasers for wireless, line-of-sight links - they're neither short-range (kilometers) nor low-bandwidth (>10Gbit/s).
    – Zac67
    Mar 29 at 17:43
  • Any idea about the relative speeds of infrared and microwave in modern technology then?
    – Balu
    Mar 29 at 17:54

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