Fibre vs HFC: Understanding the NBN technology debate

Fibre vs HFC: Understanding the NBN technology debate

With the NBN debate continuing to make headlines, arm yourself with this breakdown of Fibre, HFC, wireless and ADSL.

This week Communications Minister Stephen Conroy attacked the Coalition at the National Press Club, saying that the Coalition’s criticisms of the NBN were flawed. In particular, he argued that relying on HFC cable networks as part of the National Broadband Network (NBN) was a "dead-end solution". [Ed: read Shadow Minister for Communications and Broadband Malcolm Turnbull's rebuttal here]

One issue the comments have raised is that of shared broadband. It's a big honking asterisk that hangs over any discussion about HFC or 3G/4G as an alternative to fibre.

Okay, so what are our options?

To get a clearer idea of how these different technologies stack up, here’s a breakdown of the different speeds available with each:

  • ADSL 2+: 24Mbps maximum, degenerating at range. Dedicated.
  • VDSL2 (for fibre to the node): Most likely in the 60-80Mbps range, although technically it has a 250Mbps maximum. Dedicated.
  • 3G HSPA+: 42Mbps. Shared.
  • 3GPP LTE (sometimes called 4G or 3.9G): Telstra’s recently launched services works at around 100mbps with practical speeds (according to Telstra) of between 2Mbps and 40mbps. Theoretically, LTE can go to 326Mbps in the future. Shared.
  • LTE-Advanced (true 4G):  the specification isn’t finalised, and rollout is quite a ways away, but it’s expected to support more than 1Gbps per tower. Shared.
  • Hybrid fibre/coaxial (“cable Internet”): 100Mbps currently; the DOCSIS 3.0 specification supports speeds up to (and beyond) 300Mbps and even the NBN corporate plan acknowledges that 240Mbps is possible on the current network with HFC node-splitting. Shared.
  • Fibre to the premises: 1Gbps now; unknown upper limit (terabit speeds are certainly possible). Dedicated.

What’s that dedicated/shared thing about?


Wireless and coaxial networks are shared mediums. That means that the bandwidth is shared between every user connected to the same cell or node. You only get the listed speed if you’re the only person using the connection. If other people nearby are also using the same network, then you only get a share of the total. If, for example, you have 100 people on the same node using 100Mbps HFC, then a user’s average connection speed is only 1Mbps.

It’s even worse with wireless, as a good chunk of the bandwidth will be lost to noise and signal degradation. A 100Mbps 3G service might only deliver, say, a total of 50Mbps to its collective user base. That’s why Telstra claims a speed of between 2 and 40Mbps for its new LTE services, rather than the theoretical throughput.

This is not entirely unsolvable. One can reduce the number of users per node or cell by installing more nodes and cells. However, with HFC cable, subdividing loops is not a trivial thing, and that’s on a network that was pretty much stopped at 30% population coverage. With wireless, you could potentially have a cell tower on every corner (which isn’t going to be popular), and even then you run into diminishing returns as the mobile signals overlap.

Why does the local loop matter? Isn’t it all just shared deeper in the network anyway?


To a certain extent this is true. Eventually, somewhere on the network, you’re going to be contending with other users for limited bandwidth. The connection from your local exchange or aggregation point to your ISP won’t have enough bandwidth to support everybody running their connection at full speed; your ISP’s connection to the Internet will not be fast enough to support all its subscribers accessing the Internet at once; the Internet itself has a huge number of potential bottlenecks.

But the big difference is that this is fixable. If the link between the ISP and aggregation points or the ISP and the Internet is insufficient, then the ISP can provision extra bandwidth to improve their contention ratios. As a user you can switch to another ISP. You can’t just switch local access technologies.

To put it another way, a fast local connection removes one of the potential bottlenecks to Internet performance. Yes, it’s not the only bottleneck, but it’s the hardest to correct.

Tell me about FTTN


The Coalition strategy has largely crystalised around fibre to the node (FTTN) for the majority of the population. FTTN stretches fibre to street-side cabinets, with the last few hundred meters to be handled by the existing copper networks. It’s designed to solve the range problem of DSL (which gets slower the further away you are from the exchange).

The Coalition’s plan is to “incentivise” private companies to build such a network. According to the Coalition, that will provide practical user speeds in the range of 60 to 80mbps using VDSL technology.

What are the technical merits? Conroy said that 60 to 80Mbps was unfeasible with Australia’s current copper network and would require bonded pair copper wires (basically using two copper wires instead of one to carry the signal).

However several countries, including the UK, have plans to increase VDSL speeds to 80 and even 100Mbps using a technique called vectoring, which mitigates crosstalk on the wire. Tests by Alcatel have shown that 100Mbps at 400m from the node is possible.

The downside of FTTN is that it has very little headroom. In this writer's opinion, it’s really another stopgap measure to keep us ticking over for another decade or so. Maybe new technologies and techniques will be found to squeeze even more bandwidth out of copper, but in the long run it just can’t compare to fibre to the premises.

The great thing about fibre is that it has so much headroom - we’ve yet to find the upper limits of its capacity. Although it’s being rolled out in Australia with service offerings of 25Mbps, 50Mbps, 100Mbps and 1000Mbps, fibre can actually surpass 1,000,000Mbps (that’s one terabit) on a single cable. As reported here here and here and here, for example. Obviously those are backbone applications right now - but it shows that fibre is indeed capable of terabit (and a good deal more). For practical purposes, terabit has been demonstrated in the field as early as 2005. Right now, we already have 10Gbps fibre products commercially available.

With fibre, the current limitation is really in the signal processing electronics, not the cable itself. Better processors can add extra wavelengths and increase the carrying capacity of a fibre cable. If we want to upgrade fibre in the future, we just need to replace the electronics; the cable itself can stay.

[UPDATE] Hey wait, I heard fibre is also shared with that GPON technology!

A couple of commenters have mentioned that the PON (passive optical network) system that the NBN is using for consumer connections is also shared. That’s a fair comment, but it’s not shared the same way that wireless is.

With the NBN rollout, fibre connections are being split between multiple homes using a technique not entirely dissimilar to passive network hubs (remember those, from before we all moved to switches?). All the downstream data is actually broadcast to each of those homes, with data separation using encryption, while upstream data from those homes is multiplexed onto a single link.

In effect, a group of (probably) 32 homes are going to share a single optical connection back to the telco, and in theory they won’t all be able to max out their 100Mbps connections at once.

The thing is, though, that the contention ratio on that line is ridiculously good. Although the subscriber service is limited to 100Mbps, the total line capacity is 2.488Gbps downstream and 1.244Gbps upstream. So it’s not 100Mbps shared, it’s 2.488Gbps shared, which works out to around 78Mbps per home if 32 homes are connected. So if every one of your neighbours was trying to max out their 100Mbps connection at once, you’d still get 78Mbps. In practice, that’s never going to happen: you’ll always get 100Mbps locally.

What that might do is cause some future problems, as we want to move beyond 100Mbps. Even then, NBN co can just migrate to 10G-PON (a.k.a G.987), which provides 10Gbps between the premises.

Businesses (and very rich people, we suppose) will have access to direct fibre, so there will be no splitting or sharing for them.

Also read:

Understanding the NBN: You only get so much beer from the tap

Connecting to the NBN: this is the network boundary point

What an NBN connection looks like

Photos: here's what an NBN-install might look like in your street 

The NBN wheel of fortune spins again



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