GPON passive optical networks

The aim of the access network modernisation is a fully optical infrastructure, i.e. the FTTH (Fiber To The Home) topology. An optical modem, the ONT (Optical Network Termination), is placed at the customer's site to convert the optical signal into an electrical one.

The optical infrastructure can be built as point-to-point or point-to-multipoint. The point-to-point (P2P-point to point) option assumes the existence of a separate fiber between the Central Office (CO) and the customer. Each connected customer then occupies one optical interface at the CO. This places high demands on the capacity of the central switch as well as the number of fibres in the Optical Distribution Network (ODN).

The savings in fibre in the ODN and the reduction in the number of optical interfaces per CO are achieved by using the point-to-multipoint (P2MP) variant. The optical infrastructure then includes optical splitters that split the optical signal in a 1:N ratio. This is a passive element and the network is therefore called a PON (Passive Optical Network). The technology that can be used to connect customers via PON then ensures that each customer receives an allocated bandwidth according to the services they have purchased.

The most widespread, especially thanks to Asian countries, is EPON technology. However, in Europe, GPON, and more recently XGS-PON and NG-PON 2 are the most popular technologies. Compared to EPON, GPON has a much more sophisticated protocol, which means that about 7% of the transmission capacity is consumed in network management overhead, compared to up to 50% lost in EPON.

GPON technology - Gigabit capable Passive Optical Network uses 2 optical channels, in the direction from the network to the customer (downstream) at a wavelength of 1490 nm, and in the direction from the customer to the network (upstream) at 1310 nm. The downstream signal is shared by all optical network units (ONUs) or optical terminal units (ONTs) connected within a single optical distribution network (ODN). Each ONT or ONU then filters only the part of the information that is addressed to it. In upstream, each ONT/ONU is then assigned a specific timeslot to avoid collision of signals from different ONTs/ONUs. Thus, in downstream it is broadcasting, while in upstream it is TDMA. To prevent eavesdropping on user data destined for other units, encryption and a security key exchange system have been introduced for the transmitted frames in downstream. In upstream, this risk is not present; a potential attacker would have to directly disrupt the structure of the optical network, which the operator would be able to detect and thus prevent eavesdropping. Also, other attacks do not pose a serious risk in passive optical networks.