FOA Guide



FTTH PON: Passive Optical Network

  A PON system utilizes a passive optical 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. In the CO or head end, the OLT (optical line terminal) has a port that connects to a single fiber, transmitting data bidirectionally at different wavelengths to a splitter which connects to the ONT (optical network terminal) at multiple subscribers.

FTTX PON
PON network


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 different wavelengths of light.

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.

   APON: The first PON standard, APON, was quickly replaced by BPON because it had no provision for broadcast video and digital TV was several years away.

   BPON, or broadband PON, was the first popular PON application for FTTH but most systems have been updated with GPON. BPON uses ATM as the protocol. ATM was 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.  Video was carried using analog techniques at a different wavelength.  

   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.

FTTH BPON
BPON architecture with analog TV

   GPON, or gigabit-capable PON, is the most popular version of FTTH PONs. GPON 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.

GPON
GPON adds digital IPTV to simplify the ONT

FTTH GPON ONT
GPON ONT provides outputs for all services to subscriber

   EPON or Ethernet PON is based on the IEEE 802.3 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 different.

PON System Specification Summary

BPON GPON EPON
Standard ITU-T G.983 ITU-T G.984 IEEE 802.3ah (1 Gb/s)
IEEE 802.3av (10Gb/s)
Downstream Bitrate 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
Upstream Bitrate 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
Downstream Wavelength 1490, 1550 1490 1490, 1550
Upstream Wavelength 1310 1310 1310
Protocol ATM Ethernet over ATM/IP or TDM Ethernet
Video RF at 1550 or IP at 1490 RF at 1550 or IP at 1490 IP Video
Max PON Splits 32 64 16
Transmitter Power*
OLT: ~0 to +6 dBm, ONT: ~ -4 to +2 dBm

Power Budget*
~13dB (min) to 28dB (max) w/32 split ~13dB (min) to 28dB (max) w/32 split
~15dB (min) to 30dB (max)
~17dB (min) to 32dB (max)

Coverage <20 km 10, 20, 40, 60 km (versions) <20 km
  * There are several versions of each type that vary so these are typical ranges.

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.

RFOG

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.

10G PON Upgrades - Speed, Split & Distance
As is common with all communications networks, work on upgrading network capability and speed starts as soon as a network is introduced and PONs are no exception. GPON has been the most widely used PON scheme for both FTTx netowrks and passive optical LANs (OLANs) and GPON has been upgraded to several versions with higher transmission speeds and higher power budgets to allow greater distance, higher split capability, or both.
The assumption is that a fiber network has a lifetime of up to 40 years, so upgrades to GPON have assumed that they will use the same passive optical network architecture and fiber type (G.652 singlemode.)

Furthermore, upgrades have been designed around coexistence with current GPON networks. By utilizing different wavelengths, it is possible to have these newer, faster networks sharing the same passive optical network as the original GPON system, allowing offering higher speeds to users while continuing to serve current users without disruption. Some commercial users can take advantage of higher speeds while typical consumers are well served by GPON. One of the big advantages of the PON upgrade standards is the ability to overlay networks. Thus a city could operate one regular GPON network for consumer FTTH use and have another, faster network operating on the same cable plant independently, offering a higher level of service and security.

Upgrade PON System Specification Summary

NG-PON2 XG-PON XGS-PON
Standard ITU-T G.989 ITU-T G.987 ITU-T G.9807
Downstream/Upstream Bitrate
10/2.5, 10/10, 2.5/2.5 Gb/s 10/2.5, 10/10 Gb/s 10/10 Gb/s
Downstream Wavelength ~1596-1603 nm ~1575-1580 Either same as GPON if no current GPON or XG-PON for overlay
Upstream Wavelength ~1524-1544 ~1260-1280 Either same as GPON if no current GPON or XG-PON for overlay
Max PON Splits 64,128, 256 64,128, 256 64, 128, 256+
Power Budget*
14-29 dB (min - max) up to
20-35 dB
(min - max) in 4 versions with up to 15 dB differential optical path loss
14-29 dB (min - max) up to
20-35 dB
(min - max) in 4 versions with up to 20 dB differential optical path loss
13-28 dB (min - max) up to
20-35 dB
(min - max) in 6 versions with up to 20 or 40 dB differential optical path loss in 2 versions
Coverage 20 and 40 km versions  60 km 60 km


PON Networks - Advantages and Disadvantages
    PON networks are quite different from the typical communications network that relies on active links and switches. Here are some differences,advantages and disadvantages:
  • No electronics between central office/head end (OLT) and user (ONT) means there are no electronic components that need space for mounting, power (including uninterruptible power), service or upgrades.
  • Fewer electronic components and the infrastructure to support them make PONs much lower cost than P2P links - as much as 50% in capital expense and 80% in operating expense.
  • Because PONs are intended to carry voice, data and video, virtually any network carrying any type of traffic can use a PON. PONs are being used for Internet service, connecting cellular sites, utility grid management, erc.
  • The all passive infrastructure means that upgrades are simpler - just change out the end electronics which will run on the same cable plant. In fact the GPON upgrades to 10G can run on the same cable plant simultaneously with the lower speed GPON since it uses different wavelengths. Thus the service provider can have both low and high speed network service on one cable plant, another economic advantage of PONs.
  • PONs share fibers to the splitters so they need fewer fibers than point-to-point networks.
  • Because PONs share fibers and "broadcast" signals downstream to subscribers, it requires encryption to ensure only the intended recipient can receive the messages intended for them. Thus PONs are secure, a major advantage to organizations (governments in particular) who are concerned over security.
  • If there is any disadvantage to a PON network over P2P it is in the design phase where deciding on the location of splitters for an optimal system can be more time consuming than simple P2P links.


Technical Information on FTTX  From The FOA Online Guide
FTTH Introduction  
FTTH Architectures
FTTH in MDUs (Multiple Dwelling Units)  
FTTH PON Standards, Specifications and Protocols  
FTTH Design    
FTTH Installation 
FTTH Customer Premises Installation  
FTTH Network Testing  
FTTH Case Studies: Do-It-Yourself FTTH  
FTTH Project Management


The Fiber Optic Association Fiber To The Home Handbook: For Planners, Managers, Designers, Installers And Operators Of FTTH - Fiber To The Home - Networks
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