Internet service providers (ISPs) are increasingly deploying fibre-to-the-x (FTTx) to provide high speed broadband services using optical fibre in the access network. As compared to DSL, FTTx offers higher speeds of up to 100 Mbps, which enable operators to offer triple-play services such as high definition video, IPTV and VoIP.

However, broadband operators are divided over the selection of the network architecture for FTTx deployment. Currently, point-to-point (P2P) Ethernet and passive optical networks (PONs) are being used across the world for providing FTTx-based broadband connectivity.

P2P Ethernet is an active network architecture that provides a dedicated optical fibre connection to each customer. Although it is a simple architecture, it requires the termination of several optical fibre cables at the central office. The components utilised in the P2P Ethernet architecture are the optical line terminal (OLT) and optical network terminal (ONT). While the OLT, which is a high port count aggregation device, is located at the central office for an ISP or at the head end for a cable TV operator, the ONT is located on the customer premises. The two terminals communicate with each other through an optical signal in a full duplex mode over a single- or multi-mode optical fibre. The 100 Mbps signal, which carries content, is interfaced in a standard 100 Base-X or 1000 Base-X format.

On the other hand, PON is a point-to-multipoint network architecture, in which the ONT at the customer premises is connected to the OLT at the central office or the head end through a passive optical splitter. The splitter facilitates the division of bandwidth among multiple users. The downstream signal sent by the OLT arrives at each customer’s ONT through the optical splitter. On the other hand, since each ONT sends a different upstream signal, the time division multiple access method is used to avoid the interference of signals at the splitter. Although PON can be deployed in three configurations including tree, bus and ring, the tree configuration is preferred by most ISPs due to the lower variation in the upstream signal from the end-customer.

Further, ISPs can select a centralised or a distributed/cascading arrangement for deploying an optical splitter on the network. The centralised arrangement uses a 1x32 splitter in an outside plant enclosure. The 32 optical fibres are routed directly from the optical splitter to ONTs at 32 locations through access point connectors. This arrangement minimises the number of transmitters at the central office/head end, and increases network reliability by providing improved measurement of signal loss on the network. The distributed/cascaded split arrangement uses multiple 1x4 splitters along the network. Each of the four optical fibre cables from the splitter is routed to an access terminal where it is further split into four or eight optical fibre cables. The high number of splitters along the network can complicate network testing and impact network reliability.

The PON architecture involves transmission protocols such as the asynchronous transfer mode (ATM) and Ethernet. The broadband PON (BPON) standard is based on the ATM protocol and has been endorsed by the Full Service Access Network Group. However, the BPON architecture did not witness significant adoption due to its limited speed capabilities (622 Mbps downstream and 155 Mbps  upstream) and complexity of the ATM protocol. This has resulted in the development of a new architecture – gigabit PON (GPON). GPON has addressed the challenges faced by BPON through higher bandwidth and the use of ATM and Ethernet protocols for transmission.

The GPON architecture provides downstream and upstream speeds of up to 2.5 Gbps. Moreover, it deploys three Layer 2 networks and uses the ATM protocol for data transmission, the Ethernet protocol for voice transmission and the GPON encapsulation method (GEM). GEM facilitates efficient packaging of internet traffic by segmenting frames to provide a higher quality of service for delay-sensitive services such as videoconferencing and voice communication.

Further, GPON deploys a generic framing procedure that enables the transmission of different services, including data and voice, in their native formats, without the addition of the ATM or IP encapsulation layer.

An alternative network architecture to GPON is Ethernet PON (EPON). EPON is a point-to-multipoint architecture that deploys an OLT, an ONT and a splitter for communication between the central office and the customer premises. However, EPON transmits data in variable-size frames using Ethernet protocol, while other PON architectures transmit data in fixed-size frames using ATM protocol.

In addition to these PON architectures, wavelength division multiplexed PON (WDM PON) is being considered as the next-generation technology for FTTx. The WDM PON architecture assigns a particular wavelength to each ONT for upstream transmission and has access to the fibre at a particular wavelength. Consequently, this architecture does not involve lower transmission speeds while allowing the sharing of the fibre network. Moreover, the allocation of a separate wavelength to each ONT provides improved security and scalability. In EPON and GPON, each ONT operates at the same wavelength and has access to the fibre only for an allotted time slot, which depends on the configuration of the optical splitter. The property of operating at the same wavelength allows each ONT to be identical, which allows the change of bandwidth allotted to each ONT by altering the assigned time slot. Most WDM PON architectures use the laser-injection locking technique, which allows identical ONTs, and consequently, minimises the cost of the WDM PON architecture. Although the components of the WDM PON architecture are expensive as compared to those of EPON and GPON, WDM PON offers higher transmission speeds.

As several network architectures are available for FTTx deployments, their selection depends on factors such as cost competitiveness, bandwidth capacity, the transmission protocol used and ease of operability. A P2P Ethernet network offers dedicated bandwidth to each customer within a distance of 80 km from the central office and allows interoperability among operators due to its reliance on Ethernet protocol. However, it involves high costs and the need for an active switch aggregator. In contrast, PON architectures allow the cost to be shared among customers and do not involve the requirement of active electronic equipment in the field by using a passive splitter. Since the passive splitter does not require power, the risk of downtime decreases. This increases the reliability of the network. However, low bandwidth, complex network architecture and a distance limitation of up to 20 km are key impediments to the mass adoption of PON architectures. ISPs will need to carefully evaluate the network architecture for FTTx deployment.