How Fast is the NBN?

The April 2009 media conference which first announced the NBN called it a “new superfast network”. A 100 Mbps peak speed was proposed, later upgraded to 1 Gbps, available to subscribers connected to the NBN “Fibre to the Premises” (FTTP) network. This report outlines how the NBN FTTP infrastructure delivers the proposed speeds.

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Background

Starting from the 1980’s, high speed fibre optic transmission systems have been used within “core” networks, e.g. data links between major cities. The last decade has seen greatly increased penetration of fibre transmission systems into “enterprise” networks, e.g. high speed data connections to corporate buildings.

The NBN takes fibre rollout a step further, with individual houses connected to fibre cables. Hence the term “Fibre to the Premises” or FTTP. FTTP uses a “Passive Optical Network”, so named because network nodes merely redirect individual light signals, a simpler process than electronic switching used in other network types (e.g. core networks, Local Area Networks).

NBN GPON Architecture

The NBN FTTP infrastructure is based on the international “Gigabit Passive Optical Network” or GPON standard, otherwise known as G.984.

The GPON architecture is shown in fig 1. A single fibre connects the Optical Line Terminal (OLT) to the Fibre Distribution Hub, which then splits the signal amongst multiple subscribers (usually 32). Each subscriber has an Optical Line Terminal (OLT) which terminates the fibre cable. The OLT device will be the NBN equivalent of the current broadband modem. The Optical Line Terminal (OLT) is a large network infrastructure device, in the NBN a single OLT will serve up to 38400 subscribers. The NBN term for the Optical Line Terminal in Fibre Access Node or FAN.

fttp-gpon

The capacity of a GPON fibre link between the NBN Fibre Access Node and the Fibre Distribution Hub is shared between 32 subscribers, each receiving the same signal, due to the splitter. A GPON rollout involves many fibres, each NBN Fibre Access Node may connect to over 10000 fibre cables. GPON cables operate at full rate for distances of up to 20 km, roughly 10 times greater than the distance for full rate DSL networks. The NBN maximum fibre span is 15 km.

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