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
|
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
|
|
| Coverage |
<20
km |
<60
km |
<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.
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.
PON
Upgrades - Speed, Split & Distance
As is common with all communications networks, work on
upgradeing 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 |
- Technical
Information on FTTX From The FOA
Online Reference Guide:
- FTTH
- FTTH
Architectures, MDUs
(Multiple Dwelling Units)
- FTTH PON Protocols
- FTTH
Installation
- Customer
Premises Installation
Testing
FTTH Networks
- FTTx
Online Tutorial
- Here's
links for more information on FTTx
Case
Studies: Do-It-Yourself FTTH
- Training & Certification
Fiber
U Online FTTx Self Study Program (free)
- FOA
Certification Overview
FOA
FTTx Certification Requirements
FOA-Approved
Training Programs
Table
of Contents: The FOA Reference Guide To Fiber Optics
|
