FTTH101
Background
Until quite recently information services have been delivered to the home over copper wires. The telephone used the century-old twisted copper pair, with a required bandwidth of only some 3000 Hertz. Initial Internet connection was by "dial-up" over this same pair, thus blocking use of the phone while connected to the internet. Then along came DSL (Digital Subscriber Line) which managed to super-impose an "always on" Internet connection without knocking out the phone.At each step the available internet bandwidth increased, but the high capacitance of the twisted pair meant that the higher the speed, the shorter the distance the signal could be sent.
Similarly television was captured by an antenna on the roof, first by the aluminium Yagi antennas for VHF and UHF, then supplemented by the small round dishes for capturing satellite broadcasts.
Again the TV signals traveled over copper coaxial cable to the TV sets in each home.
As communal living became more popular, we have witnessed the evolution of attractive landscaped and secure housing estates and comfortable to luxurious apartments in high-rise buildings.
It is of course inefficient and costly to provide say a TV antenna for every house or apartment in these dwellings. Hence the birth of CATV or Community Antenna TV, sharing many viewers between one or a small number of "communal" antennas.
The problem here is that coaxial copper signal cable is "lossy" and each metre of cable, each splitter, each component in the CATV network introduces signal loss which has to be compensated for by using amplifiers to boost the signal.
We end up with a tidy but costly and somewhat fragile distribution system. There are many active electronics components throughout the network, any of which can fail and disrupt the signal distribution.
My first exposure to fibre optics and FTTH (fibre to the home) was not for the phone/Internet application, but indeed for TV distribution.
A property developer friend bemoaned the high cost and low reliability of traditional CATV systems. I had read about "triple play FTTH" (phone, Internet and TV) over fibre optics, and we agreed to investigate the option.
As I delved into the subject I came to realise the massive potential that fibre optics networks and FTTH offered for future information delivery.
Standards
Fibre optics communications networks have been used by major Telcos for some 20 years already. It is only recently that the cost has reduced to the point where it has become affordable for use in networks right up to individual dwellings (FTTH -Fibre To The Home).
Some people think that the minute you live close to a fibre optic cable, you can tap into it and pull off all the required services. This of course is not true. You certainly need to have either a "wholesale" fibre not too faraway for what is called "backhaul", or in some cases a high-speed wireless network serving the same purpose. Fibre has vastly higher performance and is preferred to a wireless solution for backhaul.
Between this "wholesale fibre" or "fat pipe", you must install a distribution and management system which controls the services, shares the resources, monitors performance, paramaterises the subscribers' requirements and combines the different services from different sources. It is in fact the gateway between the wholesale providers (there could be several) and the end-user.
Typically this equipment is lodged in a small building called a "Headend" somewhere conveniently located near the centre of the area to service. At the moment, a single headend can provide connection of FTTH to subscribers in a radius of between 10 and 20 Km, depending on the choice of equipment and network topology.
Now this is where the sometimes difficult choice of standard to use comes into play. Currently there are three, and there will be more soon. More on this later.
How does it work?
How do you send data and TV down a glass fibre? It's not a conductor like the old copper cables. Very simply we do it by shining light down the glass fibre and picking it up at the other end. A bit like a battery torch light you send Morse Code with, but a hell of a lot faster.
White light of course is composed of a combination of many different colours, each having it's own wavelength or frequency. Remember VIBGYOR in High school physics class? Violet, Indigo, Blue, Green, Yellow Orange and Red. But wait, there's more! Light extends beyond the humanly visible range into the infra-red and the ultra-violet.
In fact the wavelengths chosen currently for FTTH are in the infra-red range and can't be seen by the eye. Other fibre optics systems such as POF (Polymer or Plastic Optical Fibre) use light in the visible red range as do the fibre audio connectors on some hi-fi systems.
The choice is really a combination of the cost and performance of the LASERS used as the light sources, and the transmission characteristics of the fibre optic cable.
But what is quite fascinating is that you can send several different wavelengths (or colours) of light at the same time down a single fibre, and there is almost no interaction between them. In other words, you can have (right now) three totally different signals carrying three totally different kinds of data down the one fibre. These are combined at the headend and separated at the other end of the fibre in each dwelling.
The three commonly used light wavelengths are 1310 and 1490 nanometres for the data streams, (one for each direction) and 1550 nm for the TV or RF overlay signals.
So you can imagine, as far as the internet is concerned, that the headend acts as a controller, a gateway aggregating the subscribers' access to the internet via the wholesale backhaul fibre.
However because of the huge capacity of the fibre connection, you are not now limited to the speeds that DSL can deliver over the old copper wires.
How about telephone calls? Well this is usually handled now by VOIP or Voice Over Internet Protocol, and the telephone calls just become data packets over the data link. Exactly how Skype etc works.
Telcos are themselves going over to VOIP solutions right throughout their networks, abandoning little by little the old PSTN standard.
How about TV?
Many people just take for granted that TV will be delivered also over the data link as IPTV or IPVideo. This is not necessarily the case and IPTV is far from being a perfect solution. You can usually only stream one or two channels at a time over the data link, with very finite time lags to switch channels. Also the TV signal requires substantial bandwidth (more and more with HDTV and now even 3DTV), thus potentially hogging the other data activities.
The option of RF Overlay is very attractive. The two systems can perfectly co-exist.
RF Overlay uses that third light wavelength (1550 nm) exclusively and offloads up to 6 or 7 Gigabits per second of equivalent data from the data channel. This is a huge saving.
Also RF Overlay is a Multicast system, making ALL channels available instantly at the subscribers' dwelling. No time lag, no degradation of signal quality, you get your Free to Air AND Pay TV services all delivered unmodified to your TV set or STB (Set top box).
This system replaces the old coaxial CATV system but with none of the reliability and fragility problems.
No more ugly antennas hanging off roofs and balconies!
The antennas are hidden away, perhaps on the headend building, and you only need one per service. This can be a larger professional model which reduces substantially the problems of satellite rain fade etc.
So RF Video overlay is an excellent option for TV distribution, and you can see in another section how we have developed a pretty smart way of switching multiple satellite signals by the subscriber over the FTTH network. (see Bktel 4SAT section).
Standards part 2
So here we are back on the subject of standards.
The simplest, earliest standard is called P2P or Point to Point, Active Ethernet.
Simple because it usually runs a fibre from the headend right to the subscriber's home. You have two or three small lasers (for the two or three wavelengths) for each subscriber with no sharing of resources.
The main advantage is that you don't share the available bandwidth at the headend with other users.
The biggest disadvantages are that you concentrate a massive number of fibres coming into the headend that all have to be terminated and tidily placed and maintained. The other disadvantage is that P2P consumes more power per user than the other options. A very recent study by Alcatel-Lucent in Europe demonstrated that P2P used up to 6 times the power per user compared to GPON.
With carbon footprints all the rage, this is a pretty serious consideration.
The other two standards are called PONs (Passive Optical Networks), split into two flavours: EPON and GPON.
Both PON systems use passive optical splitters to reduce the fibre lengths and to reduce the number of fibres coming into the headend. The splitters have no active electronics and are extremely reliable.
EPON is based on an IEEE standard and GPON on an ITU standard.
All three standards have been installed around the world and all do a good job.
GPON seems to be taking the lead however, as one "trunk" fibre exiting the headend can be split and shared between up to 64 users, soon to be 128 users I believe.
My personal opinion is that PONs are fine for the normal home user, P2P would have advantages for a business or user requiring much wider bandwidth.
But it's a moving target, standards are being reviewed and upgraded, and new standards using many more than 3 wavelengths per fibre are in the pipeline (DWDM - Dense wave division multiplexing).
I think what's important to remember is that over time, the electronics in the headend and the home could be swapped out, upgraded, but the fibre in the ground will virtually NEVER have to be changed.
I jokingly compare it to the old telephone we played with as kids: the piece of string and two empty jam tins. You may decide to change the jam tins, but you will never have to change the piece of string. It will always do the job, even as more and more data is pumped up and down it.
There is a lot of quite passionate debate about which standard is best, usually fired by manufacturers who have a vested interest in one or other. The choice needs to be decided stoically: installation cost (CAPEX), running costs (OPEX), bandwidths required etc.
I hope this introduction to FTTH is useful for starters. I welcome any corrections and constructive criticism.