GPON Fibre ("FTTP") - What is it and how does it work?

GPON - What is it and how does it work?

With Openreach's ambition to have an all fibre network, end to end to every home and office within the next 15 years, fibre is one of the technologies that we'll all need to be familiar with, just as we learnt about analogue modems, ISDN, ADSL, VDSL and cable modems in previous times. Fibre has been available in the UK for some time using various technologies and it already connects every VDSL street cabinet back to the Openreach network but GPON is the now the largest growing medium. 

Although Openreach are the largest provider (by number of lines), there are several other new entrants also digging up the streets to lay fibre such as HyperOptic and G.Network. All of these companies will run their new trunks to the kerb and then connect up your home or office once you order a line.   This is viable in urban areas where take-up or subscriber density is likely to be high but the economics are less favourable for a telco who wants to provide rural service.  Government subsidies may be needed to encourage that.

Fibre To The Premises (FTTP) or Fibre To The Home (FTTH) are two terms used to describe a full end-to-end fibre delivery.  Despite some ambiguous (and sometimes misleading) marketing, if you have VDSL broadband you don't have fibre. With both VDSL and G.Fast, fibre in the street connects the main network to Openreach's street cabinets, but delivery into your home or office is still over copper lines. VDSL and G.Fast are only FTTC - Fibre To The Cabinet.

GPON (Gigabit Passive Optical Network), designated as ITU-T standard G.984.2 is a medium for delivering data services (including digital TV, Internet and phone services) over fibre optic cables. It's an alternative to DSL and cable technologies which are the most commonly used  delivery methods for Internet and triple play (TV, VoIP & Internet).  Note that we're using the British spelling of fibre here - in American English, it's fiber. 

Fibre optic cables are made of glass fibres and unlike copper wires are not electrically conductive. Instead they carry light generated by lasers at each end. Visible light - light that humans can see - has a frequency from around 400 to 800 nanometres (nm), from violet to red.  In GPON, two frequencies are used, one for upstream, the other downstream so that data can be sent in both directions at the same time (known as 'full duplex'). Using more than one frequency on a medium is known as Wavelength Division Multiplexing (WDM). GPON uses frequencies of 1490nm downstream and 1310nm upstream which are both in the infra-red range - slightly above visible light.  In some implementations, there may also be another sownstream channel at 1550nm for TV services.

Current GPON standards in use provide a data rate of 2.488Gb/s downstream and 1.244Gb/s upstream. That's a fixed speed. Unlike cable systems GPON's speed doesn't get lower with distance, however there are maximum distances that the line can run, beyond which the signal is no longer viable. You either have 100% signal at full speed, or none.  That maximum is up to 20Km depending on the laser power in use however that does depend on the number of splits - more on that later.  Next generation GPON, known as 10G GPON or XG-PON will provide up to 10Gb/s in both directions. It will use the same fibre cables so only the equipment at each end will need to be replaced.

.

Passive vs. Active

The 'P' in GPON standard for 'Passive'. Passive means that once the optical signal leaves the street cabinet there is no active processing of the signal before it reaches the ONT - the fibre connection is completely passive and needs no power, maintenance or electronics along its whole run of up to 20Km (unless it suffers damage of course).

A Passive Optical Network compares to an Active Optical Network (AON) which has much greater range than GPON but has active processing units  along its path which receive, process and re-transmit the signal, splitting it up in order that the each customer gets the right data (their own data) and a dedicated fibre connection. In a GPON network, one fibre feeds many premises (home and offices) on a street. Each node typically feeds up to a maximum of 64 endpoints (homes).

GPON Components

It's important to be familiar with the name of each component in a GPON connection as we'll be referring to them often here and their names aren't intuitive:

• The Optical Line Terminal (OLT) is the fibre provider's equipment in the street.  It's the equivalent of the 'ADSL DSLAM', the 'VDSL MSAN' or the 'Cable CMTS' (although an ADSL DSLAM is normally back at the telephone exchange, not in the street).  An OLT will typically have up to 72 ports, each of which can serve up to 64 clients because each port can be split - explained in 'Splitters' below.
  
  
• The Optical Network Terminal (ONT), also known as the Optical Network Unit (ONU) is inside the customer premises and is the equivalent of the 'modem' on cable/DSL services.  It takes the optical fibre input from the street and converts it to an Ethernet output for connection to your network (LAN), normally via a router/firewall. (Fun fact: ONT the ITU-T's term, ONU is the IEEE's term...you'd think they could just agree and avoid confusing people!). For the rest of this article, we'll use the term ONU as it's more distinctive and you can use the 'U' to remind you it's the User's equipment (not the 'Telcos').  The ONU can be combined into one box with the router and Wi-Fi base, or a separate box which then connects to your router/Wi-Fi base (more common).
  
  
  Left : Fibre Router ONU (artistic impression!).          Right : Simple Fibre to Ethernet ONU
  
  

• Splitters. A single GPON port on the OLT, provides one fibre feed which serves many customers. In order to do this, the fibre travels down the street and outside each of the premises being served, the optical signal is split into two identical signals onto two fibres using a type of prism. One fibre goes into the house and the other fibre continues down the street to the next house where it splits again and then continues on further.   In this way, GPON is very economical compared to, say, DSL technologies, which require one cable per customer to run back down the street. 50 customers means 50 cables - GPON requires just one.  For the upstream signal, the prism works in reverse, combining one or more signals sources into one output.

  When you split a light signal into two, each of the two new outputs have reduced power - they are each less bright than the source as there is finite energy in the source. The reduction is typically around 3.6dB loss, depending on how the connection is terminated and the quality of the components. That gives slightly less than half each because the splitter's material will attenuate the signal a little.  Conversely, when you combine two concurrent light sources, you increase the intensity, however, in GPON, only one subscriber (ONU) is transmitting at a time, as you'll learn later.

  A splitter is a very simple device: It has one socket on one side going to two (or more) sockets on the other side. In the middle is a glass prism which splits the signal into multiple identical outputs. Light of different frequencies refracts at a slightly different angle which is why a prism or raindrops cause rainbows when the white light is split into its components. In GPON, there is only a single downstream frequency in each direction, so the prism is designed to split that single frequency into multiple directions. The prism also works in reverse, allowing each of the split nodes to transmit back upstream onto the single fibre feed. Upstream, only one subscriber transmits at a time (see DBA, covered in part 2) so the upstream signals from different subscribers don't collide with each other.  As well as in the street to each subscriber, you can use splitters within a building to split further to different companies in a multi-occupancy building, or to multiple optical devices within a home, though local copper Ethernet or Wi-Fi is more commonly used within premises.

  
  

  

• Cables. The fibre cable itself consists of multiple flexible glass fibre strands within a protective outer sheath.  In the street, each cable will typically contain 96 fibres which then split off to smaller cables of 12 fibres each, then split again into cables of 6 fibres to each house or apartment building.   Multiple fibres are run to each house in order that there are spares (in case of damage) but also so that adjacent houses can be linked up without having to run more trenches/cables across front gardens or driveways.  A larger apartment or office building may have cables with more fibres. Although a single fibre light signal can be split and split again (see 'splitters' above), with each split, the signal gets weaker and weaker so multiple fibres to a larger building may be better to maintain adequate signal strength.
  
  
• Plugs and sockets.  GPON plugs use SC connectors. These connect two fibres together by pressing the two polished tips of the plug and socket together. That tip is the Physical Connector (PC). It's important that its contact surfaces are clean and unscratched. If not, an air gap may exist and light refracts differently in air vs. fibre and the speed of light varies in different medium (when we speak about the speed of light being "fixed" we're referring to light within a vacuum).
  

  
  SC Fibre connector into Fibre ONU

• SFP Module.  The ONU at the subscriber's premises may have the optical interface built-in, in which case the fibre cable goes right into the ONU or the fibre interface it may be provided by a plug-in SFP type module which plugs into an SFP slot on the ONU.

GPON SC Sockets

Within all fibre connectors, ST, SC, LC or any type, there is a centre ferrule - that is the light conducting pin which connects to the fibre. The ferrule has a spring in order that there is tension to hold it tightly against the mating connector to ensure that the connecting ferrules are as close and tight as possible to each other.

An SC connector can have different quality grades - standard PC, SPC (Super Physical Connector), UPC (Ultra Physical Connector) in increasing level of polishing and APC (Angled Physical Connector) which has an angled tip which making the best physical contact. They are all compatible with each other apart from APC which must only be connected to another APC connector (male to female) - it would otherwise damage the other connectors which have flat ends.

GPON services normally use APC connectors - the correct APC cable is generally provided by your service provider with their modem, but otherwise, be sure you're using the right cable termination type. PC, SPC, UPC and APC types all use the same SC plug; only the central ferrule varies.  The fibre cable into your building may be multistrand, to allow for replacing a damaged fibre or future expansion but only a single fibre strand is used into each connector.

The GPON cable is terminated in a wall box with an SC socket known as the OTO (Optical Telecommunications Outlet). The quality of all plugs and sockets contributes to the loss along the total fibre run, as well as signal loss each time the signal is split.  Normally with optical fibres, unless you're close to the maximum ranges, the quality of a joint has a lot of tolerance but with GPON, as you're splitting repeatedly, you want to lose as little signal as possible. Ideally, the in-street splits use permanently fused splices (that's the two fibres fused together - nothing to do with electrical fuses) and plugs/sockets are only used within the subscriber premises to connect to the ONU where plugging in or out is required when installing or replacing equipment. A fused splice has no air gap and doesn't rely on fibres pressed against each other so, if well made, a splice should introduce negligible signal loss.

No matter how well the fibre ends are polished and clean, no connection made just by touch will perfectly pass all light. On a single mode connector, where there is one light signal and it runs parallel to the fibre edges, there will always be some reflection when it hits the end of the connector where it meets the mating connector (plug or socket). If the light hits the end 'straight on' any reflection will bounce back in the source direction which could damage or interfere with the source laser. With the Angled Physical Connector (APC), because the mating angle is 8 ° any reflection would be back at 352 ° (or -8 °) so it would just head into and be absorbed by the fibre cladding.  Note, that 8 ° is relative to the surface of the end of the fibre ferrule in the connector.

Shared Bandwidth

As mentioned earlier, a single GPON port on an OLT will feed many subscribers, splitting off to each as the fibre travels down the street. With a fixed line speed of 2.488Gb/s, that means that each of those clients on that fibre is sharing the same bandwidth. If every one of 64 customers were downloading full speed at once, that's just 38Mb/s each. Based on typical usage in 2021, 38Mb/s is still enough to watch 4K video but in reality, all customers are not running full speed at once and an OLT port will rarely actually serve 64 clients. The allocation of bandwidth to each subscriber is controlled by a system called DBA which is covered in part 2.

Security on GPON

As there is only one downstream laser at the OLT (the telco's headend), everyone is receiving the same light and can see the same data stream.  Your data and all of your neighbour's data is contained within that same 2.488Gb/s data stream.  If you take your router and plug it in next door, you'll receive your data, not theirs. Therefore, in order that only you can decode the data intended for you and other customer's data remains secure, the OLT and your ONU will create an AES encryption association and all data will be encrypted with that. This is a very similar system as used for DOCSIS cable-modem systems, which also share media with neighbours.

Conclusions

So, this has been a summary of how GPON works, its differences to other 'broadband' technologies and what you might expect from it.  In the UK, Openreach, the main provider of lines, has stated its intention to use full fibre as its medium of choice, whenever possible for new connections.  DSL remains the most common method due to the ease of installation without having to run new cable into each home but with increasing bandwidth demands predicted over the coming years, GPON and later fibre technologies will be necessary.

In part 2 of this article, I'll go into more technical detail about how the ONU establishes and maintains a connection and provides diagnosis to the subscribers and provider.

I hope you've found this article useful - please do share a link on social and business networks / media and do make any comments below.

---

Tags