PON Fault Scenarios and Troubleshooting Basics

A PON network consists of an OLT connected via a PON splitter to multiple ONTs (one for each subscriber, up to 64 subscribers). Sometimes, a second splitter can be connected in cascade to the first splitter to dispatch services to buildings or residential areas, which has been introduced more clearly in the previous post “Understanding the Split Ratios and Splitting Level of Optical Splitters”. This post will tell about troubleshooting of a point-to-multipoint FTTH network, also defined as a PON network.

PON Fault Scenarios

Scenario 1: Simple PON (only one customer is affected)

There are three potential faults when only one subscriber cannot receive service—fault in the distribution fiber between the customer and the closest splitter, or fault in the ONT equipment, or fault in the customer’s home wiring.

Scenario 2: Cascaded PON (all affected customers are connected to the same splitter)

When all customers connected to the same splitter cannot receive service, but others connected to the same OLT can, the cause may be one of the two—fault at the last splitter, or fault in the fiber link between the cascaded splitters.

Scenario 3: All customers are affected (at the OLT level)

Whether or not the PON is cascaded, all customers dependent on the same OLT may be affected. If all customers are affected, the cause may be from of the three—fault in the splitter closest to the OLT, or fault in the feeder fiber cable of the network, or fault in the OLT equipment.

PON Troubleshooting Basics

Troubleshooting a PON first involves locating and identifying the source of an optical problem. The following picture offers a complete view of all of the possible fault locations depending on how many customers are affected, and the best location to shoot an OTDR.

Generally, most PON problems can be located using PON power meter and PON-optimized OTDR. The power meter is connected as a pass-through device, allowing both downstream and upstream traffic to travel unimpeded. It measures the power at each wavelength simultaneously and can be used for troubleshooting at any point in the network. A monitoring OTDR provides a graphical trace that enables to locate and characterize every element in a link, including connectors, splices, splitters, couplers and faults. OTDRs designed specifically for in-service PON troubleshooting exist. These OTDRs feature a dedicated port for testing at 1625 or 1650 nm and incorporate a filter that rejects all unwanted signals (1310, 1490 and 1550 nm) that could contaminate the OTDR measurement. Only the OTDR signal at 1625 or 1650 nm is allowed to pass through the filter, generating a precise OTDR measurement. In-service OTDR troubleshooting of optical fiber should be done in a way that does not interfere with the normal operation and expected performance of the information channels. Testing with the 1625 or 1650nm wavelength does just that. A PON-optimized OTDR does not interfere with the CO’s transmitter lasers because the 1650nm wavelength complies with the ITU-T L.41 Recommendation. The addition of a broadband filter, acting as a 1625 or 1650nm testing port at the CO’s WDM coupler, may be beneficial. And the quality of service provided to other subscribers serviced by the same 1xN splitter is not affected. Consequently, the technician can connect the OTDR’s 1625 or 1650nm port to the ONT and send the signal toward the CO. If a 1625 or 1650nm testing port is added to the CO, it is also possible to perform tests from the CO down to the ONT, but a 1625 or 1650nm filter may be needed at each ONT.

Summary

PON troubleshooting should first find the fault locations of the network. This post lists three types of potential fault scenarios for your reference. After knowing where the fault is, then you should use the correct tools to test and verify. PON power meter and OTDR can help significantly during the testing process. If you are confused about PON troubleshooting, hope the information in this post will be helpful.

Originially published: www.fiberopticshare.com/pon-troubleshooting-basics.html

FTTH Access Network Based on GPON

Growing demand for high speed internet drives the new access technologies which enable experiencing true broadband. This leads telecommunication operators to seriously consider the high volume roll-out of optical fiber based access networks. In order to allow faster connections, the optical fiber gets closer and closer to the subscriber. Then FTTH (Fiber To The Home) appears the most suitable choice for a long term objective because it will be easier to increase the bandwidth in the future if the clients are wholly served by optical fibers. FTTH is a future-proof solution for providing broadband services.

Passive optical network (PON) based FTTH access network is a point-to-multipoint, fiber to the premises network architecture in which unpowered optical splitters are used to enable a single optical fiber to serve multiple premises, typically 32-128. The GPON FTTH access network is highly emphasized in this article.

Components of GPON FTTH Access Network

Taking advantages of WDM (wavelength division multiplexing), PON uses one wavelength for downstream traffic and another for upstream traffic on a single fiber. The OLT (Optical Line Terminal) is the main element of the network. Placed in the Local Exchange, OLT is the engine that drives FTTH system. OLT performs the function of traffic scheduling, buffer control and bandwidth allocation. The optical splitter splits the power of the signal and enables sharing of each fiber by many users. ONT (Optical Network Terminal) is deployed at customer’s premises and connected to the OLT through optical fiber and no active elements are present in the link.

GPON FTTH Access Network Architecture

With a tree topology, GPON is able to maximize the coverage with minimum network splits, thus reducing optical power. A FTTH access network comprises five areas, namely a core network area, a central office area, a feeder area, a distribution area and a user area as shown in the following diagram.

The core network includes the ISP (internet service provider) equipment, PSTN (packet switched or the legacy circuit switched) and cable TV provider equipment. The main function of the central office is to host the OLT and ODF and provide the necessary powering. The feeder area extends from ODF (optical distribution frames) in the CO (central office) to the distribution points. Distribution cable connects level-1 splitter with level-2 splitter. Level-2 splitter is usually hosted in a pole mounted box placed at the entrance of the neighborhood. In the user area, drop cables are used to connect the level-2 splitter to the subscriber premises.

Traffic Flow in GPON FTTH Access Network

The data is transmitted from OLT to ONT in downstream as a broadcast manner and as a time division multiplexing (TDM) in upstream. The wavelength of the downstream data is 1490 nm. Core network data services transported over the optical network reaches the OLT and then distributed to the ONTs through the FTTH network by dint of power splitting. Every home receives the packets intended to it through its ONT. The upstream represents the data transmission from the ONT to OLT and the wavelength is 1310 nm. If the signals from different ONTs arrive at the splitter input at the same time and at the wavelength 1310 nm, it will lead to superposition of different ONT signals when it reaches OLT. Thus TDMA is adopted to avoid the interference of signals from ONTs. In TDMA time slots will be provided to each user on demand for transmission of their packets. At the optical splitter packets arrive in order and they are combined and transmitted to OLT.

Conclusion

This paper presents the components, architecture, and traffic flow in GPON FTTH access network. The content may not be detailed, but GPON FTTH network architecture is indeed reliable, scalable, and secure. It is a passive network, so there are no active components from the CO to the end user, which dramatically minimizes the network maintenance cost and requirements. It is a future-proof architecture.

Article source: www.fiberopticshare.com/ftth-access-network-based-on-gpon-2.html

Introduction to PON Technologies

Passive optical network (PON) is a very significant class of fiber access system in the world and it enjoys a dominant position in the access market. GPON and EPON are the two classifications of PON. The primary differences between the GPON and EPON lie in the protocols used for upstream and downstream communications. This article will introduce PON, GPON and EPON sequentially.

Passive Optical Networks (PON)

 

A PON is a fiber network that only uses fiber and passive components like PON splitters and combiners rather than active components like amplifiers, repeaters, or shaping circuits. Thus PON network costs significantly less than those using active components, but it has a shorter range of coverage limited by signal strength. An active optical network (AON) is able to cover a range to about 100 km (62 miles), while a PON is typically limited to fiber cable runs of up to 20 km (12 miles). PON is also called FTTH (fiber to the home) network.

The typical PON arrangement is a point to multi-point (P2MP) network where a central optical line terminal (OLT) at the service provider’s facility distributes TV or Internet service to as many as 16 to 128 customers per fiber line. Dividing a single optical signal into multiple equal but lower-power signals, the optical splitters distribute the signals to users. An ONU (optical network unit) terminates the PON at the customer’s home. Usually, ONU communicates with ONT (optical network terminal). The ONU/ONT may be one device.

Gigabit Passive Optical Networks (GPON)

 

GPON utilizes optical wavelength division multiplexing (WDM) so a single fiber could be used for both upstream and downstream data. A laser on a wavelength of 1490 nm transmits downstream data, while upstream data transmits on a wavelength of 1310 nm.

While each ONU gets the full downstream rate of 2.488 Gbits/s, GPON uses a time division multiple access (TDMA) format to allocate a specific timeslot to each user. It divides the bandwidth, so each user gets a fraction such as 100 Mbits/s depending on the way the service provider allocates it. The upstream rate is less than the maximum as it is shared with other ONUs in a TDMA scheme. The distance and time delay of each subscriber are determined by the OLT. Then software provides a way to allot timeslots to upstream data for each user. The typical split of a single fiber is 1:32 or 1:64, which means each fiber can serve up to 32 or 64 subscribers. Split ratios up to 1:128 are possible in some systems.

Ethernet Passive Optical Networks (EPON)

 

Based on the Ethernet standard 802.3, EPON 802.3ah specifies a similar passive optical network with a range up to 20 km. EPON uses WDM with the same optical frequencies as GPON and TDMA. The raw line data rate is 1.25 Gbits/s in both the upstream and downstream directions.

EPON technology provides bidirectional 1Gb/s links using 1490nm wavelength for downstream and 1310nm wavelength for upstream, with 1550nm wavelength reserved for future extensions or additional services. EPON is fully compatible with other Ethernet standards, so no encapsulation or conversion is necessary when connecting to Ethernet-based networks on either end. The same Ethernet frame is used with a payload for up to 1518 bytes. As Ethernet is the primary networking technology utilized in local area networks (LAN) and now in metro area networks (MAN), no protocol conversion is needed.

Summary

 

PONs are used to provide triple-play services including TV, and Internet service to subscribers. The lower cost of passive components means simpler systems with fewer components failing or requiring maintenance. The primary disadvantage is shorter range possible, commonly no more than 12 miles or 20 kilometers. As the demand for faster Internet service and more video grows, PONs are growing in popularity. The age of PON has begun. It is a new era of access network upon us.

Article source: www.fiberopticshare.com/introduction-to-pon-technologies.html

Passive Optical Network–A Superior Network Solution

With the explosive growth of Internet, the introduction of a broadband access network based on fiber-to-the-office (FTTO) and fiber-to-the-home (FTTH) has been triggered. Under this circumstance, access and metro networks should be scalable in terms of capacity and accommodation as well as flexible with regard to physical topology. Passive optical network (PON), one class of fiber access system, can deal with the various demands.

Definition

A passive optical network (PON) is a telecommunication network that uses point-to-multipoint fiber to the end-points in which unpowered optical splitters are used to enable a single optical fiber to serve multiple end-points. It consists of an optical line terminal (OLT) at the service provider’s central office and a number of optical network units (ONUs) or optical network terminals (ONTs), near end users (see below figure). PON takes advantages of wavelength division multiplexing (WDM) and uses one optical wavelength for upstream traffic while another for downstream traffic on a single-mode fiber. The upstream signals are combined at the splitters by using a multiple access protocol (time division multiple access). The downstream signals are directed to multiple users by passive optical splitter technology.

Advantages

There are two ways that the signals can be broken out in shared fiber architectures. One is active Ethernet (AE), with which the individual signals are split out using electronic equipment near the subscriber. The other one is PON, in which the signals are replicated passively by the splitter. Compared with AE, a network based on a PON system is more superior. The advantages of PON are as below.

PON incurs lower capital expenditures because it has no electronic components in the field. Also PON lowers the operational expenditures as there is no need for the operators to provide and monitor electrical power in the field or maintain backup batteries. Besides, a PON has a higher reliability because in the PON outside plant there are no electronic components which are prone to failure. Additionally, one of the most crucial features of a PON-based access network is its signal rate and format transparency. It is much simpler for a PON to upgrade to higher bit rates. Both AE and PON require upgraded electronics in the CO and customer premises, but unlike AE, PON does not need to upgrade in the outside plant as the passive splitters are agnostic to PON speed. Lastly, a PON solution has the ability to span long distances without degrading performance. The low-loss characteristics of single-mode fiber enable PON to support a maximum physical reach of 20 kilometers.

Applications

There are some applications for which PON is well suited, such as fiber-to-the-home (FTTH) delivery of voice, Internet data, and cable access broadband video. More specifically, PON is used when the applications require anticipated system to upgrade to high-security areas or where the rerouting of cable may be difficult. Or in the cases that installations involving widely dispersed nodes require long runs of fiber. And PON is utilized for the projects where costs, especially initial deployment costs, are a key concern. At the same time, using PON can help user bandwidth to be adequately managed.

By reading the above illustration, have you got a basic understanding about the passive optical network? Fiberstore, a professional manufacturer and supplier in the optical industry, has many high-quality PON products including PON splitters, optical network units and optical line terminal. Choosing a PON product in Fiberstore can help to deploy your network more efficiently.

Originally published: www.fiber-optic-components.com/passive-optical-network-a-superior-network-solution.html