FTTH PON: Passive Optical Network
A PON system utilizes a passive splitter that takes one input and splits it to "broadcast" signals downstream to many users. This reduces the cost of the system substantially by sharing one set of electronics and an expensive laser with up to 32 homes. Upstream, the passive splitter acts as a combiner to connect all users to the same shared PON port. An inexpensive laser is used for the home to send signals back to the FTTH system in the central office.
Triple Play Systems
Most FTTH systems are so-called "triple play" systems offering voice (telephone), video (TV) and data (Internet access.) To provide all three services over one fiber, signals are sent bidirectionally over a single fiber using several wavelengths of light.
BPON, or broadband PON, was the most popular PON application in the beginning. BPON uses ATM as the protocol. ATM is widely used for telephone networks and the methods of transporting all data types (voice, Internet, video, etc.) are well known. BPON digital signals operate at ATM rates of 155, 622 and 1244 Mb/s.
Downstream digital signals from the CO through the splitter to the home are sent at 1490 nm. This signal carries both voice and data to the home. Video on the first systems used the same technology as CATV, an analog modulated signal, broadcast separately using a 1550 nm laser which may require a fiber amplifier to provide enough signal strength to overcome the loss of the optical splitter. Video could be upgraded to digital using IPTV, negating the need for the separate wavelength for video. Upstream digital signals for voice and data are sent back to the CO from the home using an inexpensive 1310 nm laser. WDM couplers separate the signals at both the home and the CO.
BPON architecture with analog TV
GPON, or gigabit-capable PON, uses an IP-based protocol and either ATM or GEM (GPON encapsulation method) encoding. Data rates of up to 2.5 Gb/s are specified and it is very flexible in what types of traffic it carries. GPON enables “triple play” (voice-data-video) and is the basis of most planned FTTP applications in the near future. In the diagram above, one merely drops the AM Video at the CO and carries digital video over the downstream digital link.
EPON or Ethernet PON is based on the IEEE standard for Ethernet in the First Mile. It uses packet-based transmission at 1 Gb/s with 10 Gb/s under discussion. EPON is widely deployed in Asia. The system architecture is the same as GPON but data protocols are differenet.
PON System Specification Summary
||IEEE 802.3ah (1 Gb/s)
IEEE 802.3av (10Gb/s)
||155, 622 Mb/s, 1.2 Gb/s
||155, 622 Mb/s, 1.2, 2.5 Gb/s
||1.25 Gb/s, 10.3 Gb/s
||155, 622 Mb/s
||155, 622 Mb/s, 1.2, 2.5 Gb/s
||1.25 Gb/s, 1.25 or 10.3 Gb/s
||Ethernet over ATM/IP or TDM
||RF at 1550 or IP at 1490
||RF at 1550 or IP at 1490
|Max PON Splits
|~13dB (min) to 28dB (max) w/32 split
||~13dB (min) to 28dB (max) w/32 split
RFOG: CATV's FTTH
CATV operators were the first broadband providers using a HFC (hybrid fiber coax) system with cable modems using RF signals. Today, some CATV operators see a need for a system to provide fiber to the home, which has lead to the development of RFOG (RF over Glass.) CATV standards have looked at PON architectures and the SCTE has proposed a standard for deploying a broadcast architecture of analog signals similar to PONs called RFoG for RF (radio frequency - i.e. FM) over Glass. RFOG is basically nothing more than an all-fiber HFC/cable modem system built with less expensive components now available thanks to the volume pricing of components used in FTTH. It’s designed to operate over a standard telco PON (passive optical network) fiber architecture with short fiber lengths and including the losses of a FTTH PON splitter.
There is one interesting aspect of this approach. Now telcos and CATV companies can deliver the same services over the same cable plant using totally different technologies. But that means that office or apartment building owners, developers or even whole towns that might be considering installing FTTH infrastructure themselves and leasing the fiber to a service provider can have a choice of service providers. One cable network can support either CATV or telco systems – or even someone else for that matter. That opens up a big market for private fiber optic systems.
WDM and PON
Obviously, PON networks use WDM (wavelength-division multiplexing) with different wavelengths upstream and downstream. But the PON architecture can easily support more wavelengths, allowing greater bandwidth to the user but allocating one wavelength to a user or a group of users or greater security by having each user have their own wavelength. WDM PON architectures are being developed by many companies but no standards exist for them yet.
Other Uses For PONs
PONs offer low cost connectivity for a large number of users with high security and relatively low management needs. Some PON suppliers have been promoting PONs as an alternative to LANs (Local Area Networks), which are especially attractive to organizations with large numbers of users. Passive Optical LANs are claimed to be less expensive than traditional copper cabling for LANs but offer virtually unlimited future expansion. See Premises/Networks for more information on POLs.
- Technical Information on FTTX From The FOA Online Reference Guide:
- FTTH Architectures, MDUs (Multiple Dwelling Units)
- FTTH PON Protocols
Testing FTTH Networks
- FTTx Online Tutorial
- Here's links for more information on FTTx
- Training & Certification
- FOA Certification Overview
FOA FTTx Certification Requirements
FOA-Approved Training Programs
Table of Contents: The FOA Reference Guide To Fiber Optics