[HN Gopher] Why does gigabit internet via coax cable not offer s...
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Why does gigabit internet via coax cable not offer symmetrical
speed like fiber?
Author : personjerry
Score : 71 points
Date : 2020-01-25 18:02 UTC (4 hours ago)
(HTM) web link (superuser.com)
(TXT) w3m dump (superuser.com)
| vkdelta wrote:
| Because simply US (upstream) frequencies are limited to 42 MHz or
| 85 MHz. They start at 5 MHz.
|
| While in most cases downstream frequencies start from 54 MHz to 1
| GHz
| electriclove wrote:
| I don't buy the 'no demand for it' line. It has never been
| offered by the big players. They don't want consumers uploading
| large amounts of data. I suspect this was originally done due to
| piracy concerns. But with so many homes having Nest products (and
| the like) that constantly upload, we need higher uplink speeds.
| christocracy wrote:
| While I stewed after the dotcom crash during early 2000s, I had a
| short 2-year gig working as a headend tech for a small cable
| company.
|
| It's not about demand, it's about climbing poles and rolling
| bucket-truck crews to replace filters and other expensive
| equipment on a massive scale.
|
| The top comment in the link there is spot-on.
| https://superuser.com/a/1519918
|
| I was involved with the "digital switchover", moving the
| traditional analog channels over to the digital QAM channels,
| potentially freeing those now duplicated analog channels for
| upload channels. Maybe more upload is technically possible but
| the work on the poles is going to be expensive.
|
| [edit] oops, pasted duplicate.
| the_mitsuhiko wrote:
| That information is quite outdated. It's more than possible to
| get symmetric speed on (EURO)DOCSIS 3.1. The main reason it does
| not happen is because there is no consumer demand for it.
| supercanuck wrote:
| There is and the government had provided resources to provide
| the supply, but the market is a duopoly.
| techntoke wrote:
| And Comcast has the seventh largest lobbying budget of any
| individual company or organization in the United States.
|
| ISPs should be regulated like public utilities.
| seganddr wrote:
| Makes you wonder about the origin of the OP.
| wongarsu wrote:
| the_mitsuhiko is from Austria (according to his keybase
| linked in his profile). But it shouldn't matter, if there's
| no consumer demand then lobbying is irrelevant.
| zamalek wrote:
| > there is no consumer demand for it.
|
| The most demanding thing more than, probably, 0.01% of
| customers do is livestreaming. If your line can _actually_
| sustain 10mbps (vs advertised /claimed), you're good. The only
| other real scenario is someone learning that they have to back
| their data up, for the first time, which is really a once in a
| lifetime usecase for fast upload.
|
| What would likely improve the experience of 99%+ users is
| latency, where fiber has consistently proven to be king (for me
| at least).
| rocqua wrote:
| Besides this, coax is 'half duplex' until you apply Frequency
| division. Hence every bit of upload costs you bandwidth on the
| download side.
|
| At the same time, fiber is full duplex so this is much less of an
| issue.
|
| The difference is that applying a voltage at the transmission
| side in coax will be noticed by the receiver, (cause electricity
| goes both ways) whereas shining a light into fiber doesn't get to
| the detector (because light only goes one way). At least, this is
| my limited understanding. Would love if anyone more knowledgeable
| could correct me.
| zamadatix wrote:
| Fiber is very often frequency divided to give full duplex on a
| single strand to customers. Or the concept is taken to the
| extreme e.g. GPONs.
| snuxoll wrote:
| GPON is time divided, not frequency divided. You have one
| wavelength for Tx and one for Rx and everyone on the PON has
| to split it.
|
| It is certainly POSSIBLE to do WDM on a PON, but you need
| separate matched pairs of transceivers for each connection
| which kind of defeats the purpose. As such, wavelength
| division is typically used over point-to-point links to get
| more discrete links over a single fiber (or pair).
| sathackr wrote:
| Both cable and FTTH(single-strand fiber to the home, such as
| Fios) are full-duplex via frequency division.
|
| Cable operates in RF bands, and frequencies are denoted in MHZ
| and sometimes GHZ(included in the linked answer)
|
| Fiber operates in optical bands and frequencies are denoted in
| nanometers(wavelength) -- downstream data, downstream video,
| and upstream data are all operated on different wavelengths on
| the same piece of glass, and they are received at both the
| transmitting end and the receiving end.
|
| They are typically separated and filtered by optical prisms to
| ensure the correct wavelength hits the correct receiver(most
| receivers are wide-band and will receive a wide range of
| optical wavelengths)
|
| The medium is typically shared at a neighborhood level, in that
| a single optical strand may be split 16 or more times to serve
| multiple houses. Bandwidth is shared on that strand by the
| houses served by it, typically via TDM(time division
| multiplexing)
|
| End users don't typically notice this because the line rates
| are higher than the maximum bandwidth package sold. IE
| downstream rate 2.5Gb/s, upstream 1.5Gb/s, max package
| available is 1Gb/s.
|
| Edit: removed incorrect statement regarding gepon/csma
| rocqua wrote:
| How come fiber can't use the same frequency for uplink and
| downlink? Coherent and focused laser light only travels in
| one direction, so I'd expect the transmission not to
| interfere with the receiver.
| sathackr wrote:
| Duplex fiber where a separate strand for TX/RX can use the
| same frequency.
|
| Simplex fiber, that is point to point, may in fact be able
| to use the same frequency, but it's not done in practice.
| Possibly due to the difficulty in directing the outgoing
| and incoming signals to different pieces of
| hardware(receiver and transmitter)
|
| GPON networks(typically used by Fios, etc...) have multiple
| passive splitters on the strand. For example, a common PON
| splitter would have 1 strand in from the OLT(optical line
| terminal, essentially the head-end), 32 strands out. The
| light received from the OLT is split x32 and sent to each
| downstream port. The devices on the downstream
| ports(ONU/ONT, Optical Network Terminal) receive this
| signal and also transmit it on the same strand. That
| transmitted signal must be coupled back into the single
| upstream port, from every attached ONT. I imagine there is
| significant difficulty getting the light to traverse the
| splitter and exit only via the upstream port. So a
| different wavelength is used so that it doesn't matter if
| the light is received by the other units, and also it can
| be more easily filtered (by wavelength instead of
| direction).
|
| My knowledge of optics is not sufficient to fully explain
| why it's easier to filter by wavelength instead of
| direction.
| amluto wrote:
| > My knowledge of optics is not sufficient to fully
| explain why it's easier to filter by wavelength instead
| of direction.
|
| Reciprocity. For most optical devices, if x% of light
| coming in port A comes out port B, then x% of light
| coming in port B comes out port A. Non-reciprocal
| materials exist, but they're unusual and may be rather
| inefficient and complicated to work with. Wavelength
| splitters are reciprocal and are relatively easy to
| construct. In fact they're cheap enough that people use
| them for art. (The usual splitter is a piece of dichroic
| glass.)
|
| The Wikipedia article is so-so:
|
| https://en.m.wikipedia.org/wiki/Reciprocity_(electromagne
| tis...
| dfox wrote:
| The reason why it is easier to filter by wavelength is
| that almost any optical path reflects significant amount
| of light back, primarily from various discontinuities
| along the fiber (connectors, splices, and also the actual
| receiver on the other end) but also from imperfections in
| the fiber itself. On copper interconnects there are
| similar issues, but for reasonably controlled cabling
| (eg. 1000-base-T) they can be minimized enough, that what
| remains is either insignificant or can be compensated for
| by DSP magic.
| supertrope wrote:
| Full duplex DOCSIS 3.1 has theoretical maximums of 10Gbps
| down and up. It relies on precise timing and echo
| cancellation to allow the cable modem and CMTS to transmit
| simultaneously on the same frequency bands. Kind of like how
| gigabit Ethernet allows both ends to transmit simultaneously
| on the same four pairs.
| Dylan16807 wrote:
| I'm sure you could do full duplex coax if you really wanted to.
| But it's not worth the effort here.
| aaronax wrote:
| I'm not sure that you would find any product on the market
| which shines the same frequency of light in both directions on
| a single fiber. All of the bi-directional on a single fiber
| products that I know of send a different frequency of light in
| each direction.
|
| Therefore it would seem strange to me to claim that a single
| strand of fiber can do full duplex, as frequency "B" of light
| could instead be used to send data in the same direction as
| frequency "A". So really you are just choosing to allocate some
| part of the overall capacity of the single fiber to return
| traffic.
| R0BERTGLICK wrote:
| That was an interesting article thanks for sharing.
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(page generated 2020-01-25 23:00 UTC)