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What is GPON?

06 August 2019

When it comes to full-fibre broadband, there are effectively two ways that the physical fibre network is delivered, PtP and GPON:


•    Point to point (PtP) fibre
•    GPON (Gigabit Passive Optical Network)

GPON is increasingly being used by many fibre providers, including our carrier-partners Openreach & CityFibre, to deliver full-fibre connectivity to customers. It’s predominantly being used to deliver broadband-based products, whereas leased line use the point to point (PtP) fibre alternative. There are some providers such as Gigaclear who use PtP fibre for all types of customer connections.

This article helps to explain what GPON is, dispel some myths and concerns, and convey that no matter if you have access to GPON or PtP fibre – you have a connection that will set you apart from those suffering with legacy copper services.

Providing clear and accurate information allows you to make an informed decision without being baffled by marketing buzz words.
 
GPON Introduction

GPON more-efficiently uses the physical fibre network being deployed in an area compared to point to point fibre. This can:

•    help speed up roll-out times
•    whilst decreasing roll-out costs
•    offering lower cost installs and rental to end customers
•    and still deliver a premium & future-proofed full fibre connection to the customer displacing the ‘victorian-era’ copper network

The basis of GPON is that a single fibre connection to the local exchange can be “split” and shared between multiple end customers. It achieves this with a passive (not electrically powered) optical splitter in a junction box, usually in the pavement/road manhole around a cluster of potential end customers. Think of this main fibre link from the exchange to the splitter as a “trunk line” to a business-park, street or housing estate. Once it reaches the park or estate, and a cluster of potential end customers reside, a splitter is connected. From here, multiple ‘drop’ fibre cables that run to each end customer fan out of the splitter. The quantity of fibres connecting to a single splitter/trunk cable depends on the architecture of the network. We’ll explain more below. In some networks, such as Openreach, there is also an “aggregation node” which is where multiple fibres are spliced together between various splitters and the local exchange.
 
OLT (Optical Line Termination): Located in the broadband carrier’s exchange or point of presence and serves as one end of the GPON active (powered) equipment.

Fibre Aggregation Node: Located in a chamber in the ground, this is a fibre jointing location that diverts a high-capacity multi-core fibre to various different locations in the area, and GPON splitters.

GPON Splitter: A passive optical fibre splitter that optically splits a single fibre into multiple fibres to each customer. 8:1 and 32:1 are the common maximum

GPON split ratios we see with our carriers.

ONT (Optical Network Terminator): Located at the customer’s premises which connects to the fibre cable that runs to the carrier’s network. It’s electrically powered.
 
How can a single fibre support multiple customers?

Multiplexing is the basis for how multiple customers can all share a single fibre from this splitter.

Multiplexing is where you can combine multiple signals/streams of data across a shared medium – e.g. a fibre cable. Multiplexing is how your Sky TV service can deliver 100 of channels, your 3G/4G mobile connection works, as well as your DAB radio.

In most common GPON networks, the multiplexing is achieved by assigning each customer a time-slot on when they can download and upload across the main trunk fibre. Time division multiplexing (TDM) is the technical name for this.

In some GPON networks, they can also use different wavelengths of laser light. This is called optical wavelength division multiplexing (WDM).

By combining both TDM and WDM multiplexing technologies, GPON is a technology that is very future proof, can cost efficiently deliver Gigabit+ speeds to multiple customers.
 
Giganet’s GPON-based services

Many of Giganet’s full-fibre broadband services are powered over a GPON-based network.

Please use our availability checker to see whether you can receive a full-fibre service.

Here are some example Giganet full-fibre services that use GPON technology:

Local UltraBEAM
Available across Giganet’s unbundled exchange areas from Basingstoke, Salisbury & Winchester.
Using the Openreach network.
Selection of PIR (burstable bandwidth)
Selection of CIR (guaranteed bandwidth)
Maximum of 32:1 GPON split.
UltraBEAM
Available across Giganet’s unbundled exchange areas from Basingstoke, Salisbury & Winchester.
Using the Openreach network.
Selection of PIR (burstable bandwidth)
Selection of CIR (guaranteed bandwidth)
Maximum of 32:1 GPON split.
CityFibre UltraBEAM 500 & 1000
Available across CityFibre’s network area.
Using CityFibre’s own fibre network.
500Mb/s & 1Gb/s PIR (depending on UltraBEAM 500 & 1000 respectively).
300Mb/s down & 156Mb/s up CIR.
Maximum of 8:1 GPON split.
CityFibre ELITE 200-1000 Flex
Available across CityFibre’s network area.
Using CityFibre’s own fibre network.
1Gb/s PIR.
200Mb/s guaranteed symmetric CIR.
Maximum of 5:1 GPON split.

More technical details

Optical Line Terminator (OLT)

This is the device in the carrier’s exchange or point of presence (POP) that terminates the incoming fibre from the street. It manages and controls the time-slots, prioritised information rate (PIR) and committed information rate (CIR) to each customer on the GPON fibre spur. It also encrypts traffic between customers to ensure that traffic is segregated securely between customers on the same GPON spur.

Optical Network Terminator (ONT)
This is the device at the customer’s premises which terminates the fibre at your home or business. It is powered by the mains, connects to the incoming fibre supply, and connects to your firewall or router. It’s special in that it securely encrypts the traffic between your connection and the OLT to ensure that no other customers on the same GPON spur can intercept your traffic. It also manages the time-slots.

GPON speeds
The most common GPON standard permits a capacity on each “trunk” line of 2.488 Gb/s on the downstream, and a maximum upstream capacity of 1.244 Gb/s.

This asymmetric difference between the download and upload bandwidths is what causes some carriers to offering different download speeds to the upload. However some carriers choice to engineer symmetric speeds to customers to keep things simple.

GPON split ratio

The above capacity is shared across the customers that connect to the passive optical splitter. These splitters can support a range of maximum customers, with the most common ones being 8:1 (CityFibre) & 32:1 (Openreach).

This is the maximum number of end customer connections that could be supported on each main fibre trunk link, however often not all of the 8 or 32 are occupied, therefore utilisation can be lower. It’s also worth considering that even if all slots are occupied, not all customers will be transmitting at the same time, so speeds to each customer are dynamically shared.

Committed Information Rate (CIR)

The minimum guaranteed rate that the ONT provides for the particular Ethernet service.

Prioritised Information Rate (PIR)

The maximum bandwidth rate for that the ONT allows the service to use if bandwidth is available.
 
How secure is GPON?

As a single fibre “trunk cable” is being used which is split off to multiple customers, the download signals from the exchange/point of presence is transmitted to every customer connected to that GPON spur. Clearly this would not be acceptable if there was not encryption.

GPON uses AES encryption (the same kind that SSL/HTTPS uses on websites) as a means to prevent others from eavesdropping on your data transmission. Encryption is negotiated between your ONT in your home/business and the OLT in the carrier’s exchange/point of presence.

This ensures that there’s no way that other customers can see your data.
 
Benefits of GPON over point to point fibre
  • Lower network infrastructure costs.
  • As as single fibre from the exchange to a business-park, housing estate, or street (which could be 10km+ away) could support and distribute fibre connections to up to 32 properties locally. This saves running 32 separate fibre cables all the way back to the exchange.
  • It’s future proof. With just a change of the OLT and ONT, the speeds and capacities can be increased over time without laying more expensive fibre cables. This also saves disruption of the civils too!
  • XG-PON: 10Gb/s down, 2.5Gb/s up (TDM)
  • XGS-PON: 10Gb/s down, 10Gb/s up (TDM)
  • NG-PON2 (TWDM-PON): 4 x 10Gb/s down, 4 x 10Gb/s up (TDM + WDM)
  • It could be upgraded to point to point fibre at a later date:
  • The customer connections that connect to the GPON splitter could be spliced onto a dedicated point to point fibre link to the exchange/point of presence at a later date. The fibre cable to the customer’s premises would remain untouched. Just change of optics and equipment at each end and a reconnection to the PtP fibre is needed.

Drawbacks of GPON over point to point (PtP) fibre
  • You don’t get your own point-to-point fibre between your premises and the local exchange/points of presence.
  • You are at the mercy of the PIR and CIR on GPON whereas with PtP fibre, you have a dedicated connection across the link that isn’t influenced by others.
  • PtP fibre can support upwards of 100Gb/s with the right optics at either end, and in fact much faster than this. You can also multiplex the fibre service to deliver n x 100Gb/s or faster. It’s very future-proofed.
 Compromises

As with anything in like, there are compromises. If we could all have point to point fibre and deploy this to every property and business in the UK, then this would represent the ultimate solution to upgrading the victorian copper network. However this would be extremely cost prohibitive, provide a capability that for most customers is overkill, and not quickly solve the UK’s lack of full-fibre connectivity. GPON is future-proof with the various different versions, advances and multiplexing techniques available. GPON can be upgraded to point to point fibre at a later date by re-splicing the customer drop fibres to higher-capacity fibre trunk cables to the exchange.

 



 











 

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