What the future holds for next-generation PON
technologies
October 1, 2017
Passive optical network technologies help user
organizations achieve higher bandwidth, costeffective speed upgrades, flexible network
topology and legacy system compatibility.
By Qing Xu, Belden
The only way to benefit from emerging applications like the Internet of Things (IoT),
4K video streaming and next-generation wireless is by having the right network
backbone in place. These developments help all types of businesses be more
productive and efficient, but they also have high local area network (LAN) bandwidth
requirements.
Passive optical network (PON) technology, including Ethernet passive optical
networks (EPONs) and Gigabit passive optical networks (GPONs), is being deployed
globally in optical access networks for broadband network architecture such as FTTx
(fiber-to-the-X) architecture that supports the emerging applications you keep
hearing about.
Leveraging the mature technical solutions developed for PONs, passive optical LANs
(POLANs) can sometimes offer advantages over traditional LANs for higher
bandwidth, cost-effective speed upgrades, flexible network topology and legacy
system compatibility, to name just a few.
In this article, we introduce next-generation PON technologies, and illustrate the
POLAN deployment and future migration path.
The Full Service Access Network (FSAN) Group developed the initial Gigabit
Passive Optical Network architecture. ITU-T continued the work and
standardized newer generations of PON.
Ethernet PON (EPON) is a 1-Gbit/sec PON standard ratified as part of the IEEE
802.3’s Ethernet in the First Mile project. The 10G EPON standard was also
developed to support 10G/10G symmetric downstream and upstream speed.
PON is a network that only uses fiber and passive components like splitters and
combiners (splitting ratio of 1:32 or higher) rather than active components like
amplifiers, repeaters or shaping circuits.
It makes use of wavelength division multiplexing (WDM) technology to increase the
bandwidth per fiber and supports point-to-multipoint networks.
PON-enabled FTTx deployment is the most popular residential and business
broadband access technology. As of 2017, there are already more than 300 million
fiber-to-the-home (FTTH) subscribers globally. Multiple services, such as video, audio
and internet (broadband and wireless), can be supported over the same thread of
singlemode fiber.
Several components make up PON architecture.
• Optical line terminals (OLTs)are the endpoint hardware devices in a PON. As
active central aggregation equipment, they are located in the main
crossconnect/equipment room. They replace multiple Layer 2 access switches
in telecommunications rooms.
• Optical distribution networks (ODNs)distribute signals to network users. These
passive non-wavelength-selective optical splitters/couplers can be mounted to
the rack, wall, ceiling or floor. They are typically seen in split ratios of 1:16, 1:32,
and up to 1:128 based on link distance and power class. Upstream (U/S) and
downstream (D/S) signals use different wavelengths over one singlemode fiber
thread.
• Optical network terminals (ONTs/ONUs) are active end devices with a small
switch at the access point (e.g. work area, hotel room, etc.). They converge
services like Ethernet, Power over Ethernet, Plain Old Telephone Service, Voice
over Internet Protocol, RF video, Internet Protocol TV, and videoconferencing
to end-user ports.
Passive optical networking is deployed in fiber-to-the-x networks, in an
architecture that combines copper and fiber cable. A PON facilitates fiber to the
curb (FTTC), fiber to the node (FTTN), fiber to the building (FTTB), and fiber to
the home (FTTH).
POLAN standards: GPON and EPON
POLAN is adapted from the PON standards for indoor LAN applications; therefore,
there is no dedicated standard body for POLAN deployment. Instead, several
standard bodies provide guidance. 1) Full Service Access Network (FSAN), the
International Telecommunication Standardization Sector (ITU-T), the Institute of
Electrical and Electronics Engineers (IEEE) and Data Over Cable Service Interface
Specification (DOCSIS) provide system guidance. 2) ANSI/TIA-568 and BICSI TDMM
13 provide components and cabling guidance. 3) The Association for Passive Optical
LAN (APOLAN) promotes POLAN technology and deployment.
The first PON architecture was initially developed by the FSAN working group, which
was formed by major telecommunications service providers and system vendors.
ITU-T continued the work, and standardized newer generations of PON.
Because ODN costs can be as high as 70 percent of total network deployment, it’s
essential for each PON standard to be not only backward compatible, but also
forward compatible.
To support high data rates and new service types, GPON, XG-PON1, XGS-PON and
NG-PON2 can now coexist over the same ODN, with additional coexistence elements
(CEx). Different generations of ONTs and OLTs can coexist over the same fiber
because there is no wavelength overlap.
GPON, XGS-PON, and NG-PON2 overlay on the same optical distribution
network.
There is also an allocated wavelength range reserved for RF video; optical timedomain reflectometer (OTDR) measurements can be performed in-band without
service interruption.
Ratified as part of IEEE 802.3’s Ethernet in the First Mile project, 1G EPON is a
1-Gbit/sec Ethernet passive optical network standard. The 10G EPON standard was
also developed to support 10G/10G symmetric downstream and upstream speed.
Currently, the IEEE 802.3ca task force is working on 25G/50G/100G EPON
standards development. All EPON standards are developed to be backward and
forward compatible to support legacy service and new higher-speed service over the
same ODN.
Radio frequency over glass (RFoG) network design was developed by the Society of
Cable Telecommunications Engineers (SCTE) under IPS 910 and later became
ANSI/SCTE 174.
Upgrading from GPON to XSG-PON typically is carried out as a four-phase
process. Once the network is completely upgraded, some legacy ONT devices
likely remain in operation for designated applications.
Mainly deployed in North America, backward compatibility with RFoG is also
required in some PON deployments and upgrades.
Different PON systems can overlay on the same ODN with a CEx that supports PON
system forward/backward compatibility.
• Multiple OLTs of different generations and service types can be overlaid over
the same fiber.
• In the downstream, signals from multiple OLTs are combined at CEx and
simultaneously sent to the ONT side. Each ONT receives its signal with a
wavelength filter in the optical module. ONTs also have wavelength block filters
(WBF) to block future PON wavelengths.
• In the upstream, wavelengths from different ONTs are separated at CEx and
WDM mux/demux to different OLTs.
PON system coexistence allows the reuse of existing fiber resources for fast
deployment without a new investment; a system upgrade to high-bandwidth services
can be done by simply replacing or adding OLT line cards.
POLAN deployment and migration
The step-by-step process of deploying a POLAN is: 1) the first splitter is installed in
the splitter cabinet next to the OLT rack; 2) the second splitter is installed on each
floor for a dedicated distribution group; 3) ONTs in the work zone can be standalone
devices like WiFi routers, or conversion devices equipped with RJ45 outlets to
provide data services and PoE.
Upgrading a typical POLAN from GPON to XGS-PON takes place in four phases.
Phase 1: Deploy the ODN, install the GPON OLT line cards and gradually add GPON
ONT devices at endpoints.
Phase 2: The GPON OLT line cards reach full capacity; additional services need to be
added with new XGS-PON equipment and end devices.
Phase 3: Install the XGS-PON OLT line cards and add the CEx to the same ODN;
install the new XGS-PON ONT devices with the legacy GPON OLT line cards and
some GPON ONT devices still in operation.
Phase 4: Add/replace with new XGS-PON ONT devices gradually until the system is
fully upgraded. Some legacy ONT devices will still be in operation for designated
applications.
Deployment challenges of in-building POLAN
When comparing POLAN to traditional LAN technologies, it is not necessarily a
“better” replacement solution for all the new access network deployment; instead it
offers numerous technical and economic advantages over traditional LAN, when
space and distance challenges need to be addressed, and ability to carry multiple
services become a critical requirement.
POLANs are most suitable for large premises and public venues (hospitality, sport
venues, campus, conference centers, hospitals, military bases, etc.) with more than
250 endpoints (drops). On the other hand, it is wise to be aware of potential
challenges when reviewing a POLAN deployment for your next project.
System interoperability—EPON and GPON are not interoperable, and most active
equipment vendors offer proprietary systems that will not be supported by other
vendors. Your choice of a POLAN partner is very critical to your future success.
Limited POLAN ecosystem—POLAN is still in its infancy while traditional LAN is
dominant in the market. This means there are not many knowledgeable IT
professionals in the POLAN space, which could create a maintenance challenge if the
deployment has not been planned and conducted properly.
Distributed ONT/ONU devices—These end devices are distributed throughout your
network and may be accessible to end users. This could increase the risk of having
ONT/ONU equipment manipulated or damaged.
Lack of dedicated POLAN standards—FSAN, ITU-T, IEEE, and DOCSIS provide
system guidance; ANSI/TIA-568 and BICSI TDMM 13 provide components and
cabling guidance. Unlike traditional LAN, however, there is no dedicated standards
body that covers this particular indoor network architecture, therefore POLAN
deployment is usually based on best practices rather than standard guidance.
These next-generation PON technologies are set to help us achieve higher
bandwidth, cost-effective speed upgrades, flexible network topology and legacy
system compatibility by having the right network backbone in place.
Proper planning can help create a robust blueprint that allows you to migrate through
several upgrade paths without having to replace your structured cabling. Although
these technologies offer several positive attributes, it’s important to be aware of
potential deployment challenges. Vendor partners can help you navigate this
landscape, ensuring your network meets your needs long into the future.
Qing Xu is technology and applications manager for optical fiber systems with Belden
(www.belden.com).