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An Engineer Compares Gigabit to the Home Technologies

An engineer candidly compares Gigbit to the Home (GTTH) technologies including fiber, PON, G.Fast, DSL and DOCSIS.

By John Blyler, Editorial Director

A good overview of gigabit-to-the-home (GTTH) technologies has been developed by Broadcom and published by Electronic Design: “These Technologies Will Bring Gigabit to the Home.” This story covers the major implementation technologies including fiber, G.fast, Digital Subscriber Line (DSL), Passive Optical Network (PON) and DOCSIS 3.1.

I was curious how an engineer working in the broadband field would evaluate these technologies. What follows are the candid and rather technical observations from a veteran engineer who wishes to remain anonymous to safeguard their position in the industry. For the sake of clarity, it would be best to read the Broadcom article before reading the following interview. – JB

Blyler: Before we begin, how would you explain the difference between fiber and PON?

Engineer: PON uses typically 1:32, 1:64 or 1:128 fiber splitter. ITU standards define the protocol by which an Optical Network Termination (ONT) communicates with the Optical Line Terminal (OLT) – see Figure 1. Each ONT is basically allocated a time window by which is communicates up to the OLT. PON typically works well in residential environments where you have an asymmetrical environment, that is, where the upstream direction requires less bandwidth than the downstream).

Figure 1: In actually deployment, PON’s are used only for the last-mile of the connection. Components for this last mile of PON include the OLT, the fiber and splitters and the ONT.

Figure 1: In actually deployment, PON’s are used only for the last-mile of the connection. Components for this last mile of PON include the OLT, the fiber and splitters and the ONT.

Fiber is a generic term. My guess is that you probably are using it in context of a point-to-point system (PON is N:1 or many to one point). Point-to-point systems typically use active Ethernet and are less cost efficient than PON. However if this is for a business application, then (fiber) point-to-point works well since it is simpler than PON and you can achieve symmetrical bandwidth if need be.

Blyler: How do you decide which of these technologies to implement, i.e., fiber, G.fast, PON or DOCSIS 3.1?

Engineer: The selection of what you use is mostly due to cost and whether or not you want to reuse existing infrastructure.

Blyler: How about fiber deployments such as Google Fiber? Such implementations require laying new optical lines and building out the infrastructure – deployed on a test basis in a handful of cities.

Engineer: Google has invested quite a bit in key towns and has a rollout plan that does require building out new infrastructure. This rollout requires a high investment but after Google’s announcement other key players have come forth to provide a similar solutions – e.g., AT&T with 1G solutions in Austin, TX. One difference with their (Google’s) deployment is that it is a fiber into the home, i.e., the network box is placed insider the home. [Editor’s Note: This is part of a larger issue as Google wants to put additional small networking cabinets in the public right of way around the city, according to Google specifications.]

Blyler: Let’s talk about G.fast technology, which works upon existing DSL networks and cabling. DSL represents the largest segment of the wire line broadband market, far exceeding both Fiber-to-the-Home (FTTH) and cable deployments.

Engineer: G.fast is new technology that reuses aspects of DSL. The main difference between G.fast and Very-high-bit-rate digital subscriber line 2 (VDSL2) is that DSL uses TDD. You can get higher rates but right now only one profile has been defined (you can not yet get 1G unless you do bonding). I understand that Profile 212 is being worked on will support 1G with no need to support bonding. Yes G.fast reuses the existing physical plant but remember that its loop lengths are much shorter than VDLS2. That is why it’s called fiber-to-the-premise or FTTP.

Blyler: Let’s move onto PON, which is perhaps the lowest cost option over the long run particularly in single-family homes.

Engineer: By PON, I assume you are referring to GPON. (BPON is pretty old; NGPON standard is about to be sent out for consent in ITU). Yes, PON has been around for a while and can provide Gigabit services. If you follow what is happening with G.fast, the idea behind it (PON) is to backhaul G.fast over GPON. The main advantage of GPON is that it delivers services in an efficient manner, services like iptv/multicast, voice, and HSIA. In existing deployments, you do have FTTH type of deployments but I don’t believe that they take off that well in the US as in other part of the world.

Blyler: Finally, let’s consider DOCSIS 3.1, which advocates claim will allow operators to exceed Gigabit speeds on existing cable networks while simultaneously improving operational efficiencies. Some of the biggest operators in the U.S. and Europe (Comcast and Liberty Global, among them) are discussing rollouts in as little as 6 months

Engineer: I can’t comment a lot about DOCSIS. I have had to review some management DOCSIS documents. DOCSIS came into the picture last year (2014) so I’m not 100% sure how much deployment there is out there.

Blyler: Thank you.

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