The NBN data rates are determined by the GPON infrastructure, which provides 2.488 Gbps downstream (from the network to the subscriber), 1.2 Gbps upstream. This GPON capacity is shared between 32 users.
If all 32 subscribers were active, and the GPON capacity was shared equally between them (both scenarios unlikely), then the per subscriber downstream data rate would be around 77 Mbps. In practice, the GPON capacity would be shared unequally. Given the 2.488 Gbps downlink capacity, individual subscriber peak rates of either 100 Mbps or 1 Gbps are therefore achievable.
Hence the GPON FTTP infrastructure meets the NBN 100 Mbps/1Gbps peak rate objectives.
A key GPON feature is the relatively high uplink capacity, 1.244 Gbps between 32 subscribers, as compared to the 1 Mbps uplink for current ADSL2+ services. This GPON uplink capacity is reflected in the higher speed plans offered by NBNCo, e.g. 100 Mbps downlink/40 Mbps uplink, 1 Gbps downlink/400 Mbps uplink.
XG-PON – the NBN upgrade
The next version of the GPON standard, known as XG-PON, provides 10 Gbps downstream, 2.5 Gbps upstream. This is a four fold increase in downstream capacity compared to GPON, and, like GPON, the capacity is shared between 32 subscribers.
The XG-PON standard was finalised in 2010, which also saw successful field trials by US operator Verizon. GPON and XG-PON can both be carried on the same fibre cable, hence XG-PON infrastructure can be introduced gradually to an existing GPON network. Given to 9.5 year NBN rollout timeframe, XG-PON infrastructure will likely comprise a significant portion of the final NBN, allowing multi gigabit peak rate services to individual subscribers.