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City Fibre
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Old 29-04-2021, 08:21   #31
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Re: City Fibre

GPON stands for Gigabit Passive Optical Networks. GPON is a point-to-multi point access mechanism. Its main characteristic is the use of passive splitters in the fibre distribution network, enabling one single feeding fibre from the provider's central office to serve multiple homes and small businesses.

Any better now Chris?
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Old 29-04-2021, 10:52   #32
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Re: City Fibre

Quote:
Originally Posted by Chris View Post
Sorry ... groupon and frogs and what the what now?

Anyone got a helpful glossary to hand here? This all sounds like it ought to be interesting, if only I could follow it...
Others have already done a decent job of explaining but I'll throw my hat in the ring!

HFC or "Hybrid Fibre Coax" is the legacy VM system, whereby you have various "levels" of node, usually powered, with fibre stopping at a certain point and powered distribution amplifiers from that point onwards in the network.

At the end, you have runs from the final node to the customer houses, you'll have a powered distribution amplifier with 1 cable in, a load of coax cables out, some at higher power levels than others. The higher power ports are used for longer coax runs, the very longest coax runs will need higher quality, thicker coax, on the highest power output from the amp.

The equipment in the home from the wallbox is usually passive, and so splits the power between your SH and your TIVO box. If you have multiroom and a load of TIVO boxes, sometimes you'll need a powered splitter in your house. Sometimes there is too much power, and you'll need an attenuator.

Copper Coax cable has, in the same way as any copper cable, an amount of attenuation and loss, and a range of frequencies it is most suited for. Don't forget, all the TV channels are being broadcast at all times on the coax, so if there are 200 TV channels, at 10mbit/s each, then there is 2Gbit/s of TV coming down that cable, in addition to bandwidth for internet.

RFoG, or Radio Frequency Over Glass, literally replaces all the coax (except the 6ft from the wallbox on your house to your SH3.0) with fibre. This means that all the issues of power levels, powered cabinets, etc can (mostly) be removed. There is a powered converter in the wallbox, that the customer supplies power for, that converts the fibre to coax. From the customer perspective, they are using the same signals, the same frequencies, and there is really no advantage to them over HFC (as of today), except perhaps less risk of downtime etc. I've certainly seen evidence of a much lower pre-rs error rate on RFoG installs - although these errors are corrected anyway on the HFC equivalent, it does show the system has more margin of error.

From VM's perspective, RFoG is about future proofing. So far it's been used on new rollouts only, and it's a case of "why would we install coax when fibre has more potential". It also means most of the cabinets can be unpowered, as fibre can be split passively. This saves a fortune, you don't have to get the power company to install cables etc to the nodes etc, but certainly for the customer, it does nothing to solve issues like DOCSIS having a high latency etc. The head end (which was fibre anyway in HFC) doesn't need to change, and the home equipment doesn't need to change.

However, fibre opens up another possibility. If I shine a red light down a fibre, I see a red light at the other end. If I put a prism at each end, and shine a blue light into one side of the prism, a red light in the other, I can split them out again at the other end. This is WDM or "Wavelength Division Multiplexing", using one fibre for multiple signals. This is already actually used in RFoG areas, there are usually 4 "colours", 4 frequencies of DOCSIS used and there will be a colour coded label in the converter in the wallbox on each house showing which frequency it is using.

GPON, Gigabit Passive Optical Network, is the system used by BT Openreach FTTP, and by Cityfibre. This delivers 2.488 Gbits/s download speed, and an upload speed of 1.244 Gbits/s, which can be split between up to 64 users, but in practice split ratios tend to be much lower. I think BT mostly aim for 32:1 but in practice it's usually a bit better than that. I believe Citfibre are aiming for 8:1.

You can see, that at 8:1, if all 8 CF users tried to upload at 1000mbit/sec, they wouldn't be able to, as the network only has 1.2gbit between those people, this is a similar issue to local contention on DOCSIS, but honestly far less likely to occur.

There is then XGPON, which is 10GBit/s down, 2.5GBit/s up, shared between however many users, and XSGPON, (the S standing for Synchronous) with 10GBit/s up and down. Note that, in theory, VM could use WDM as described above to run this over the same fibre cable as RFoG for DOCSIS. Allowing them to use the same TV boxes tuning into the same DOCSIS signals, while also providing far far more bandwidth (yes downstream, but more so upstream) than DOCSIS can offer, but they couldn't do this in HFC areas.
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Old 29-04-2021, 11:14   #33
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Re: City Fibre

Quote:
Originally Posted by rtho782 View Post
Others have already done a decent job of explaining but I'll throw my hat in the ring!

HFC or "Hybrid Fibre Coax" is the legacy VM system, whereby you have various "levels" of node, usually powered, with fibre stopping at a certain point and powered distribution amplifiers from that point onwards in the network.

At the end, you have runs from the final node to the customer houses, you'll have a powered distribution amplifier with 1 cable in, a load of coax cables out, some at higher power levels than others. The higher power ports are used for longer coax runs, the very longest coax runs will need higher quality, thicker coax, on the highest power output from the amp.

The equipment in the home from the wallbox is usually passive, and so splits the power between your SH and your TIVO box. If you have multiroom and a load of TIVO boxes, sometimes you'll need a powered splitter in your house. Sometimes there is too much power, and you'll need an attenuator.

Copper Coax cable has, in the same way as any copper cable, an amount of attenuation and loss, and a range of frequencies it is most suited for. Don't forget, all the TV channels are being broadcast at all times on the coax, so if there are 200 TV channels, at 10mbit/s each, then there is 2Gbit/s of TV coming down that cable, in addition to bandwidth for internet.

RFoG, or Radio Frequency Over Glass, literally replaces all the coax (except the 6ft from the wallbox on your house to your SH3.0) with fibre. This means that all the issues of power levels, powered cabinets, etc can (mostly) be removed. There is a powered converter in the wallbox, that the customer supplies power for, that converts the fibre to coax. From the customer perspective, they are using the same signals, the same frequencies, and there is really no advantage to them over HFC (as of today), except perhaps less risk of downtime etc. I've certainly seen evidence of a much lower pre-rs error rate on RFoG installs - although these errors are corrected anyway on the HFC equivalent, it does show the system has more margin of error.

From VM's perspective, RFoG is about future proofing. So far it's been used on new rollouts only, and it's a case of "why would we install coax when fibre has more potential". It also means most of the cabinets can be unpowered, as fibre can be split passively. This saves a fortune, you don't have to get the power company to install cables etc to the nodes etc, but certainly for the customer, it does nothing to solve issues like DOCSIS having a high latency etc. The head end (which was fibre anyway in HFC) doesn't need to change, and the home equipment doesn't need to change.

However, fibre opens up another possibility. If I shine a red light down a fibre, I see a red light at the other end. If I put a prism at each end, and shine a blue light into one side of the prism, a red light in the other, I can split them out again at the other end. This is WDM or "Wavelength Division Multiplexing", using one fibre for multiple signals. This is already actually used in RFoG areas, there are usually 4 "colours", 4 frequencies of DOCSIS used and there will be a colour coded label in the converter in the wallbox on each house showing which frequency it is using.

GPON, Gigabit Passive Optical Network, is the system used by BT Openreach FTTP, and by Cityfibre. This delivers 2.488 Gbits/s download speed, and an upload speed of 1.244 Gbits/s, which can be split between up to 64 users, but in practice split ratios tend to be much lower. I think BT mostly aim for 32:1 but in practice it's usually a bit better than that. I believe Citfibre are aiming for 8:1.

You can see, that at 8:1, if all 8 CF users tried to upload at 1000mbit/sec, they wouldn't be able to, as the network only has 1.2gbit between those people, this is a similar issue to local contention on DOCSIS, but honestly far less likely to occur.

There is then XGPON, which is 10GBit/s down, 2.5GBit/s up, shared between however many users, and XSGPON, (the S standing for Synchronous) with 10GBit/s up and down. Note that, in theory, VM could use WDM as described above to run this over the same fibre cable as RFoG for DOCSIS. Allowing them to use the same TV boxes tuning into the same DOCSIS signals, while also providing far far more bandwidth (yes downstream, but more so upstream) than DOCSIS can offer, but they couldn't do this in HFC areas.
Pretty concise.

BTW BT Openreach has never existed as a company. It's just Openreach.
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