FTTH Fiber to the Home Overview

The leading FTTH technology is PON or Passive Optical Network technology. This approach differs from most of the telecommunications networks in place today by featuring "passive" operation. Active networks like DSL, VDSL and cable have active components in the network backbone equipment, in the central office, in the neighborhood network infrastructure, and in the customer premises equipment. PONs have only passive light transmission components in the neighborhood infrastructure with active components only in the central office and the customer premises equipment.

The elimination of active components means that the access network consists of one bi-directional light source and a number of passive splitters that divide the data stream into the individual links to each customer. At the central office, the termination point is in PON optical line terminal (OLT) equipment. At the customer premises, the termination point is in optical network terminals or ONTs also called optical network units or ONUs. These are in the customer premises equipment, or CPE. Between the OLT and the ONT/ONUs is the passive optical network comprising fiber links and passive splitters and couplers.

What's in a Name?

The FTTH market has a plethora of acronyms. Some are meaningful and some are redundant, but here are some of the key ones:

FTTH - Fiber to the Home

A description of the PON-based broadband access network technology that uses fiber optics running all the way from the Internet backbone to the home or premises. Some times other acronyms, FTTx, FTTP, or FTTB are used, but these are essentially interchangeable. FTTH is becoming the catch-all descriptor for all fiber to the home, premises, business and "x" technologies.

PON - Passive Optical Network

The fiber based network infrastructure between central office and the home.

OLT, ONU, ONT - Optical Line Terminal, Optical Network Unit,

Optical Network Terminal

The fiber link is terminated in the central office at a Optical Line Terminal or OLT. OLT devices are the semiconductors that perform that function. They interface to the fiber link connecting the central office equipment to the customer premises equipment or CPE. In the CPE, the fiber link is terminated by an Optical Network Unit, or ONU, or by an Optical Network Terminal, or ONT. These terms have the same meaning, but ONU is IEEE terminology and ONT is ITU-T terminology.

APON, BPON, GPON, EPON, and GE-PON Explained

These represent three flavors of PON technology. APON and BPON are the same specification which is commonly referred to as BPON. BPON is the oldest PON standard, defined in the mid-1990s and while there is an installed base of BPON, most of the new market deployment focus is now on EPON/GE-PON. GE-PON and EPON are different names for the same specification, that is defined by the IEEE 802.3ah Ethernet in the First Mile standard ratified in June 2004. This is the current standardized high-volume solution for gigabit PON technologies. GPON is now being standardized as the ITU-T G.984 recommendation and is receiving interest in North America and elsewhere, but with no final standard. GPON devices have just been announced, and there is no volume deployment as yet.

Differences Between BPON, GPON and GE-PON

One important distinction between the standards is operational speed. BPON is relatively low speed with 155 Mbps upstream/622 Mbps downstream operation. GE-PON/EPON supports 1.0 Gbps symmetrical operation. GPON promises 2.5/1.25 Gbps asymmetrical operation.

Another key distinction is the protocol support for transport of data packets between access network equipment. BPON is based on ATM, GE-PON uses native Ethernet and GPON supports ATM, Ethernet and WDM using a superset multi-protocol layer.

BPON suffers from the very aggressive optical timing of ATM and the high complexity of the ATM transport layer. ATM-based FTTH solutions face the problems posed by the provisioning (requires ATM-based central office equipment), complexity (in timing requirements and protocol complexity) and subsequent cost of components. This cost is exacerbated by the relatively small market for traditional ATM equipment used in the backbone telecommunications network.

GPON is still evolving; the final specification of GPON is still being discussed by the ITU-T and FSAN bodies. But by definition, it requires the complexity of supporting a multiple protocols through translation to the native Generic Encapsulation Method (GEM) transport layer that through emulation provides support for ATM, Ethernet and WDM protocols. This added complexity and lack of standard low-cost 2.5/1.25 Gbps optical components has delayed industry development of low-cost, high-volume GPON devices.

GE-PON or Ethernet in the First Mile has been ratified as the IEEE 802.3ah EFM standard and is already widely deployed in Asia. It uses Ethernet as its native protocol and simplifies timing and lowers costs by using symmetrical 1 Gbps data streams using standard 1Gbps Ethernet optical components. Like other Ethernet equipment found in the extended network, Ethernet-based FTTH equipment is much lower-cost relative to ATM-based equipment and the streamlined protocol support for an extended Ethernet protocol simplifies development. Specific to PMC-Sierra, the GigaPASS architecture that is designed for gigabit throughput for GE-PON devices is adaptable for use with GPON devices as well.

The important characteristics of each PON technology are defined by two important standards bodies, the IEEE and the ITU. Click here for more information on PON standards.