5 ms·

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, f

by rocqua 7y ago

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 7y ago
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 7y ago
  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 (you now need a mux/demux at the end which adds more attenuation to the line, as well as more expensive electronics as you went from needing one GPON port to N Gbe ports). As such, wavelength division is typically used over point-to-point links to get more discrete links over a single fiber (or pair).
  - sathackr 7y ago
    GPON is both time divided and frequency divided. There are typically 3 wavelengths.(frequency division). Two for data(downstream and upstream) and one for video (downstream). The downstream channels(both video and data) are broadcast and not time divided. The streams are filtered by the ONT. The upstream data channel is time divided to permit multiple devices transmitting without collision.
sathackr 7y ago
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 7y ago
  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 7y ago
    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.
Dylan16807 7y ago
I'm sure you could do full duplex coax if you really wanted to. But it's not worth the effort here.
aaronax 7y ago
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.
madengr 7y ago
You actually can separate out the forward and reverse waves on a coax, but it requires good impedance match on both ends, and some transformer based magic-Ts. So you can do full duplex on a single coax, sharing the same bandwidth. It’s just not practical in long runs since you’ll only get 20 dB isolation between directions. A common use is carrying Ethernet over coax when retrofitting analog CCTV cameras to IP cameras, and you don’t want to run new cable.

by rocqua 7y ago

zamadatix 7y ago

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 7y ago
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 (you now need a mux/demux at the end which adds more attenuation to the line, as well as more expensive electronics as you went from needing one GPON port to N Gbe ports). As such, wavelength division is typically used over point-to-point links to get more discrete links over a single fiber (or pair).
- sathackr 7y ago
  GPON is both time divided and frequency divided. There are typically 3 wavelengths.(frequency division). Two for data(downstream and upstream) and one for video (downstream). The downstream channels(both video and data) are broadcast and not time divided. The streams are filtered by the ONT. The upstream data channel is time divided to permit multiple devices transmitting without collision.

sathackr 7y ago

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 7y ago
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 7y ago
  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.

Dylan16807 7y ago

I'm sure you could do full duplex coax if you really wanted to. But it's not worth the effort here.

aaronax 7y ago

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.

madengr 7y ago

You actually can separate out the forward and reverse waves on a coax, but it requires good impedance match on both ends, and some transformer based magic-Ts. So you can do full duplex on a single coax, sharing the same bandwidth. It’s just not practical in long runs since you’ll only get 20 dB isolation between directions. A common use is carrying Ethernet over coax when retrofitting analog CCTV cameras to IP cameras, and you don’t want to run new cable.