Introduction and Market Overview: The Need for Fiber
The way people use the Internet today creates a great demand for very high bandwidth: More and more workers are telecommuting. Consumers watch multiple HDTV channels, often on several TVs in the same household at the same time. They upload and download multimedia files and use bandwidth-hungry peer-to-peer services. They play online games that demand high speeds and immediate reactivity. Web 2.0-based communities and hosted services such as social networking sites and wikis are pervasive, fostering interactivity, collaboration and data-sharing while generating a need for capacity. Bringing optical fiber to every home is the definitive response to such demands for greater bandwidth.
Bringing Fiber to the Home: Benefits of GPON
One way of providing fiber to the home is through a Gigabit Passive Optical Network, or GPON (pronounced ‘djee-pon‘).
GPON is a point-to-multipoint access mechanism. Its main characteristic is the use of passive splitters in the fiber distribution network, enabling one single feeding fiber from the provider’s central office to serve multiple homes and small businesses.
GPON has a downstream capacity of 2.488 Gb/s and an upstream capacity of 1.244 Gbp/s that is shared among users. Encryption is used to keep each user’s data secured and private from other users. Although there are other technologies that could provide fiber to the home, passive optical networks (PONs) like GPON are generally considered the strongest candidate for widespread deployments.
Why choose GPON?
When planning a fiber-to-the-home (FTTH) evolution for their access networks, service providers can choose between three generic FTTH architectures: point-to-point; active Ethernet; and passive optical networking (PON) such as GPON.
“Point-to-point” is an Ethernet FTTH architecture similar in structure to a twisted-pair cable phone network; a separate, dedicated fiber for each home exists in the service provider’s hub location. The point-to-point architecture has merits for small-scale deployments such as citynets, but is not suitable for large-scale deployments due to its poor scalability in terms of hub location space or the number of required hub locations, power consumption and feeder fibers.
An “active Ethernet” architecture is based on the same deployment model as fiber to the node (FTTN) with active street cabinets; it is therefore feasible as a complement or migration path towards FTTH for larger deployments in very high-speed digital subscriber line (VDSL)-dominated environments.
GPON is a fully optical architecture option that offers the best of all worlds. A GPON system consists of an optical line terminal (Huawei OLT) that connects several optical network terminals (ONTs) together using a passive optical distribution network (ODN). Like active Ethernet, it aggregates users in what is called the “outside plant” or OSP, which means no mess of fibers in a central office somewhere; like point-to-point, it avoids the need for active electronics in the field by employing a passive OSP device (the optical splitter). Being a passive device, the GPON splitter requires no cooling or powering and is therefore extremely stable; in fact, it virtually never fails.
How does GPON work?
GPON has been called “elegant” for its ability to share bandwidth dynamically on a single optical fiber. Like any shared medium, GPON provides burst mode transmission with statistical usage capabilities. This enables dynamic control and sharing of upstream and downstream bandwidth using committed and excess information rate (CIR and EIR) parameters. Users can be assured of receiving their committed bandwidth under peak demand conditions, and of receiving superior service when network utilization is low. While subscribers rarely require sustained rates of 100 Mb/s each, bursting beyond this to the full line rate of a PON system (about 1.25 Gb/s upstream or 2.5 Gb/s downstream in the case of GPON) is easily enabled using the right subscriber interface. This allows a GPON to be used for many years even if subscribers have a regular need to transmit beyond an engineered guaranteed limit of 100 Mb/s.
GPON was developed with the support of the FSAN (Full Service Access Network) Group and the ITU (International Telecommunication Union). These organizations bring the major stakeholders in the telecoms industry together to define common specifications, ensuring full interworking between OLTs and ONTs. The IEEE (Institute of Electrical and Electronics Engineers) has also defined a PON standard, called Ethernet PON or EPON. The EPON standard was launched earlier than GPON and has been deployed successfully. IEEE specs are however restricted to the lower optical and media access layers of networks, and full interoperability for EPON must therefore be managed in a specific case-by-case way at every implementation. Additionally, EPON runs at only 1 Gb/s, upstream as well as downstream, providing a lower bandwidth than GPON. These factors make EPON a less attractive technology choice for providers making FTTH investment decisions today.
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