Passive Optical Networks (PONs) are an enigma to me in many ways. On one hand the concept goes back to late 1980s and has been floating around ever since with obligatory presentations from the large vendors whenever you visited them. Yes, for sure there are Pacific Rim countries and the odd state or incumbent carrier in the western world deploying the technology, but they never seemed to impact my part of the world.On the other hand, the technology would provide virtual Internet nirvana for me at home with 100Mbit/s second available to support video on demand to each member of my family who, in 21st century fashion, have computers in virtually every home of the house! This high bandwidth still seems as far away as ever with an average speed of 4.5Mbit/s downstream bandwidth in the UK. I see 7Mbit/s as I am close to a BT exchange. We are still struggling to deliver 21st century data services over 19th century copper wires using ATM-based Digital Subscriber Line (DSL) technology. If you are in the right part of the country, you get marginally higher rates from your cable company. Why are carriers not installing optical fibre to every home?
To cut to the chase, it’s because of the immense costs of deploying it. Fibre to the Home (FTTH) as it is known, requires the installation of a completely new optical fibre infrastructure between the carrier’s exchanges and homes. Such an initiative would almost require a government led and paid for initiative to make it worthwhile – which of course is what has happened in the far east. Here in the UK this is further clouded by the existing cable industry which has struggled to reach profitability based on massive investments in infrastructure during the 90s.
What are Passive Optical Networks (PONs)?
The key word is passive. In standard optical transmission equipment, used in the core of public voice and data networks, all of the data being transported is switched using electrical or optical switches. This means that investment needs to be made in the network equipment to undertake that switching and that is expensive. In a PON, instead of electrical equipment joining or splitting optical fibres, fibres are just welded together at minimum cost – just like T-junctions in domestic plumbing. Light travelling down the fibre then splits or joins when it hits a splice. Equipment in the carrier’s exchange (or Central Office [CO]), and the customer’s home then multiplex or de-multiplex an individual customer’s data stream.
Although the use of PONs considerably reduces equipment costs as no switching equipment is required in the field and hence no electrical power feeds are required, it is still an extremely expansive technology to deploy making it very difficult to create a business case that stacks up. A major problem is that there is often no free space available in existing ducts pushing carriers to a new digs. Digging up roads and laying the fibre is a costly activity. I’m not sure what the actual costs are these days, but £50 per metre dug used to be the cost many years ago.
As seems to be the norm in most areas of technology, there are two PON standards slugging it out in the market place with a raft of evangelists attached to both camps. The first is Gigabit PON (GPON) and the second is Ethernet PON (EPON).
About Gigabit PON (GPON)
The concept of PONs goes back to the early 1990s to the time when the carrier world was focussed on a vision of ATM being the world’s standard packet or cell based WAN and LAN transmission technology. This never really happened as I discussed in The demise of ATM but ATM lives on in other services defined around that time. Two examples are broadband Asynchronous DSL (ADSL) and the lesser known ATM Passive Optical Network (APON).
APON was not widely deployed and was soon superseded with the next best thing – Broadband PON (BPON) also known as ITU-T G.983 as it was developed under the auspices of the ITU. More importantly APON was limited to the number of data channels it could handle and BPON added Wave Division Multiplex (WDM) (covered in Technology Inside in Making SDH, DWDM and packet friendly). BPON uses one wavelength for 622Mbit/s downstream traffic and another for 155-Mbit/s upstream traffic.
If there are 32 subscribers on the system, that bandwidth is divided among the 32 subscribers-plus overhead. Upstream, a BPON system provides 3 to 5 Mbits/sec when fully loaded.
GPON is the latest upgrade from this stable, uses an SDH data framing standard and provides a data rate of 2.5Gbit/s downstream and 1.25-Gbit/s upstream. The big technical difference is that GPONs are based on Ethernet and IP rather than ATM.
It is likely that GPON will find its natural home in the USA and Europe. An example is Verizon who is deploying 622Mbit/s BPON to its subscribers but is committed to upgrade to GPON within twelve months. In the UK, BT’s OpenReach has selected GPON for a trial.
About Ethernet PON (EPON)
EPONs comes from the IEEE stable and is called IEEE 802.3ah. EPONs are based on Ethernet standards and derives the benefits of using this commonly adopted technology. EPON only uses a single fibre between the subscriber split and the central office and does not require any power in the field such as needed if a kerb-side equipment was required. EPON also supports downstream Point to Multipoint (P2MP) broadcast which is very important for broadcasting video. As with carrier-grade Ethernet standards such as PBB, some core Ethernet features such as CSMA/CD have been dropped in this new use of Ethernet. Only one subscriber at a time is able to transmit at any time using a Time Division Multiplex Access (TDMA) protocol.
Typical deployment is shown in the picture below, one fibre to the exchange connecting 32 subscribers.
EPON architecture (Source: IEEE)
A Metro Ethernet Forum overview of EPON can be found here.
The Far East, especially Japan, has taken EPON to its heart with the vast majority being installed by NTT, the major Japanese incumbent carrier, followed by Korea Telecom with 100s of thousands of EPON connections.
There is still lots of development taking place in the world of PONs. On one hand 10Gbit/s EPON is being talked about to give it an edge over 2.5Gbit/s GPON. On the other, WDM PONs are being trialled in the Far East which would enable far higher bandwidths to be delivered to each home. WDM-PON systems allocate a separate wavelength to each subscriber, enabling the delivery of 100 Mbits/s or more .
Only this month it was announced that a Japanese MSO Moves 160 Mbit/s using advanced cable technology (the subject of a future TechnologyInside post).
DSL based broadband suffers from a pretty major problem. The farther the subscriber is away their local exchange, the lower the data rate that can be supported reliably, PONs do not have this limitation (well technically they do but the distance is much greater). So in the race to increase data rates in the home PONs are a clear cut winner along with cable technologies such as DOCSIS 3.0 used by cable operators.
Personally, I would not expect that PON deployment will increase over and above its snail-like pace in Europe at any time in the near future. Expect to see the usual trials announced by the largest incumbent carriers such as BT, FT and DT but don’t hold your breath waiting for it to arrive at your door. This has been questioned recently in a government report where the lack of high-speed internet access could jeopardise the UK’s growth in future years.
You may think so what – “I’m happy with 2 – 7Mbit/s ADSL!”, but I can say with confidence that you should not be happy. The promise of IPTV services are really starting to be delivered at long last and encoding bandwidths of 1 to 2Mbit’s really do not cut the mustard in the quality race. This is case for standard, let alone for high definition TV. Moreover, with each family member having a computer and television in their own room and each wanting to watch or listen to their own programmes simultaneously, low speed ADSL connections are far from adequate.
One way out of this is to bond multiple DSL lines together to gain that extra bandwidth. I wrote a post a few weeks ago – Sharedband: not enough bandwidth? – who provides software to do just this. The problem is that you would require an awful lot of telephone lines to get 100Mbit/s which is what I really want! Maybe I should emigrate?
Addendum #1: Economist Intelligence Unit: 2007 e-readiness rankings