Your guide to broadband tech
FTTP, FTTN, HFC, GPON? THE AUSTRALIAN BROADBAND SCENE CAN BE CONFUSING, SO WE’VE PUT TOGETHER THIS QUICK GUIDE TO HELP YOU COMPREHEND THE MOST IMPORTANT BITS.
[ NATHAN TAYLOR ]
OUR MULTI-TECHNOLOGY MIX NBN sure has created some confusion. With at least five different technologies in use, on top of the four major broadband techs already available, it’s no wonder that there’s a lot of misunderstanding about how things work and what the different technologies are capable of. So this month, we thought we’d break it down for you.
Before we start talking about the different technologies, we should touch on an incredibly important concept in networking — contention. We’d say that the misunderstanding (or lack of understanding) of contention is the reason for roughly 90% of false claims about broadband. It’s why people, including politicians that should know better, still make claims like 4G being as good as fibre (“It can do gigabit!”), even when it’s actually not even a contest.
Contention, in short, is competition for the network. When two devices on the same network try to transmit at the same time, they have to share network resources. If you have two Wi-Fi devices, both have to share the airwaves, so the bandwidth for each is halved. Pretty simple, really.
When it comes to broadband, you may have heard the term ‘contention ratio’. Your ISP doesn’t have enough backhaul (that’s the links that carry data from your local exchange or hub back to the ISP) and internet bandwidth to support all users going at max speed all the time. Instead, it under-provisions, expecting that, at any given time, only a certain number of users are accessing the internet at once.
The higher the contention ratio is, the worse the ISP will be at peak times.
But contention also plays a part in what’s called the local loop — that’s the ‘last mile’, the link between your home or wireless device and the hub/node/exchange/cell where your signal is joined with that of your neighbours and sent on to the ISP for delivery onto the internet. This is a huge part of what separates different services. For example, with 4G, every user connected to a given cell is competing for bandwidth, and that could be several thousand users at once so the ‘gigabit’ speed is divided amongst all those users. With ADSL, on the other hand, there is no contention between you and the exchange. The link is yours alone.
Now with an understanding of that in mind, let’s look at how the different techs stack up.
FIBRE TO THE HOME (FTTH)
FTTH is, as many have noted, the Rolls Royce of Australian broadband. If you have FTTN (fibre to the node), you’ve pretty much won the broadband lottery.
In Australia, the NBN is using what is called GPON: gigabit passive optical network. The ‘passive’ part means that, unlike FTTN, the fibre distribution hubs (FDH) — the street-side cabinets that connect to your home — require no power.
Each FDH node has a total bandwidth of 2.488Gbps downstream and 1.244 gigabit upstream. That is effectively divided amongst the homes connected to that FDH — nominally 32 homes per FDH according to the initial rollout plans. Technically, any home could connect at the full 2.488Gbps, but to prevent a single home from sucking up all the bandwidth they’re throttled to 25, 50, 100 or 1000Mbps, depending on the type of service you’re subscribed to.
The contention ratio with fibre is, thus, incredibly good. A single GPON is enough to deliver around 77Mbps to every user, all at once. So, at least in the local loop, you’ll be pretty much guaranteed to always get the full 100mbps barring some freak occurrence where every user is typing to max out their connection at once. In the future, an upgrade to XG-PON (also known as 10G-PON), which provides four times as much downstream bandwidth, would be possible, easily allowing 1Gbps subscriber services with very little
contention. Of course, there can still be contention deeper in the network, depending on how much internet bandwidth your ISP has provisioned, but locally fibre is the bee’s knees.
Cable internet looks good on paper. Technically, subscriber speeds of 1Gbps downstream are possible. But it’s the very high contention ratios that make it less capable than it might appear.
With cable, a coaxial cable is run from a node along the street sides. To connect to the network, you essentially splice into this cable. The bandwidth on this cable is shared amongst all the homes it passes, which is typically several hundred. The node itself is connected back to the parent network via a fibre cable (thus hybrid fibre-coax, or HFC).
With the current setup, a single cable segment can pass several hundred homes. With DOCSIS 3.0 (which we’re currently using) a total bandwidth of just over 1.2Gbps is possible, shared amongst all the users on a given segment (as with fibre, individual users are throttled to 30Mbps or 100Mbps to prevent one person taking over the network). Thanks to the relatively low current subscriber base, that actually works out pretty well – but if everybody whose house was passed by cable actually used it, it would be a different story.
When DOCSIS 3.1 is implemented on the NBN, the shared speed can actually go up to 10Gbps, though with the plan to put everyone in the cable footprint onto cable, the subscriber base will also likely go up (barring the building of more nodes).
DSL, FTTN AND FTTDP
Although they don’t provide nearly as much total bandwidth, the copper cables used in ADSL, VDSL (used in fibre to the node) and G.Fast (potentially used in FttDP) have one advantage — they’re non-contending. The link between your house and the exchange (for ADSL), local node (for FTTN) or micro-node (FttDP) is yours alone, though, of course, there’s still contention deeper in the network. (With FTTN, the uplink from the street-side node to the exchange varies — currently, the design document lists up to 4Gbps — and each node will service between 48 and 384 users.)
So, in general, DSL speeds in the local loop are determined by the quality of the signal your home receives, which is, in turn, a product of your distance from the exchange/ node and line noise. There’s a theoretical max, and you’ll get some fraction of that depending on your signal quality. ADSL2 can go up 24Mbps. VDSL2 (in the NBN implementation) can max 100Mbps. G.Fast can theoretically hit 500Mbps.
3G/4G, SATELLITE AND FIXED WIRELESS
Wireless technologies all share one Achilles’ heel: they’re a shared medium. So when someone says “Why do we need landlines when 4G will be able to do 1Gbps?” this is your answer. There is a reason that data volumes on mobile services are so tight: if everybody started to use 4G for everything, the whole system would collapse.
The total amount of data that an LTEAdvanced network can carry largely depends on how much spectrum the telecoms provider has purchased, and the contention ratio depends on the density and range of cell towers. What’s more, final speeds are heavily affected by signal interference and signal power loss — without a cable to shield a signal and hold it together, wireless signals degrade easily, reducing the effective bandwidth. The end result is speeds that vary pretty wildly from place to place and time to time. In a perfect situation, with nobody else using a cell, speeds of up to 100Mbps and even 1,000Mbps (for fixed position installations) are possible, but in practice they’re likely to be much, much lower.
Like 3G, satellite and fixed wireless are also dependent on a shared medium. Directional antennae and multiple access signalling can and do limit contention, but where signals overlap, there will be performance trade-offs. Nominally fixed wireless speeds go up to 50Mbps/20Mbps and satellite to 25Mbps/5Mbps, but they’re subject to the same rules of wireless contention as 3G.
With HFC, a single cable segment can service hundreds of homes.