Passive optical network technologies help user organizations achieve higher bandwidth, cost-effective 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 time-domain 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).