Introduction to PON Networks

A passive optical network (PON) allows to eliminate all active components between the telecommunications service provider and the client, introducing in its place passive optical components to route traffic across the network. The use of these passive systems, whose main element is the optical splitter device- reduces the cost of installation and maintenance.
PON networks are the basis on which it is building the new networks for broadband access over fiber optics to the home. This new technological approach is known as FTTH (Fiber to the home).

Structure of a PON-FTTx network

A passive optical network is a multi-point network consisting essentially of:

· OLT (Optical Line Terminal)Located in the operator's facilities.

· Singlemode fiber optic network with a tree-branch topology

· Optical splitter.

· ONTs (Optical Network Terminal),Located in the client's home.

The transmission is performed between OLT and multiple ONTs using common fiber optic network. In this fiber network optic splitters are present which are responsible for routing the signal from the OLT to each of the ONT.

Downstream channel

The downstream channel of a PON network uses a wavelength of 1490nm and behaves as a point-multipoint transmission system. In this environment the OLT sends signals and the optical splitter is responsible for providing to all the ONT. Once the information has come to the ONT can only be open those pieces of information to a specific ONT user. The rest of the information remains encrypted.

Upstream channel

A PON's upstream channel behaves in a model of point to point communication. Here, each ONT transmits content to the OLT at 1310nm. It is therefore necessary to use arbitration transmission channel models by use of TDMA (Time Division Multiple Access) for each ONT send the information in different times. This control is done from the OLT. At the same time, all users are synchronized through a process known as "Ranging."

Advantages of passive optical networks

· More bandwidth for the user. The current GPON technology can provide up to 2.5 Gbps for each 64 users. Currently work is being done on standards to raise the bandwidth up to 10Gbps.

· Increased coverage and quality of service. From the operator's network it is possible to extend a PON to a distance of 20 km. With DSL-based technologies a maximum 5.5-km are being reached. Furthermore, PON networks are immune to electromagnetic disturbances origin.

· Cost savings associated with network deployment. Besides the cost savings the use of optical fiber versus copper, the tree-branch topology of the fiber network, alongside the use of transmission systems monofibra, significantly reduces costs.

PON-FTTH Protocols

• ITU-T G.983

  • APON (ATM ((Asynchronous Transfer Mode) Passive Optical Network)

  • BPON (Broadband PON)

• ITU-T G.984

  • GPON(Gigabit-capable PON):

• IEEE 802.3ah

  • EPON(Ethernet PON)

  • GEPON

• IEEE P802.3av

  • 10GEPON

• WDM PON: Still in definition

ITU-T G.984 G-PON

GPON (Gigabit Passive Optical Network) is the most widespread PON protocol in FTTH networks in Europe and in United States. In 2003-2004 was approved by the ITU-T under the recommendations G.984.1, G.984.2, G.984.3, G.984.4 and G.984.5.

GPON Objectives

This standard was created to cover new demands on the network:

· Multiservice transportation: TDM voice, synchronous SONET/SDH transport, Ethernet (10/100 BaseT), ATM,...

· Multirate: Support multiple bitrate within the same protocol, including symmetrical speeds of 622 Mb/s, 1.25 Gb/s, and asymmetric 2.5 Gb/s in the downlink and 1.25 Gb/s in upstream.

· Maximum range of 20 km, although the standard has been prepared to reach up to 60 km.

· OAM end to end.

· Protocol level security for the downlink due to the multicast nature of PON.

· The maximum number of users that can hang from a single fiber is 64 (the system is ready to provide service to up to 128).

Features and Techniques

· Data multiplexing: Both channels of information (downstream and upstream) travel in the same fiber using different wavelengths. This scheme uses a WDM (Wavelength Division Multiplexing) multiplexing. 

· Downstream - TDM: TDM technology is used (Time Division Multiplexing). All data are transmitted to all the ONTs. Each ONT filters the received data and is only able to access those data that are directed toward it. It is possible to encrypt the traffic that is ongoing between OLT-ONT to be inaccessible to a second ONT modified to behave as a spy. 

· Upstream - TDMA: TDMA technology is used (Time Division Multiple Access). The OLT monitors the upstream channel, allocating transmition  time windows to each ONT. It requires a media access control to avoid collisions and to allocate bandwidth among users. The perfect synchronization of the packets upstream to the OLT is needed in order to reconstruct the GPON frame. For this reason it is necessary for the OLT knows the distance to each ONTs in order to take into account the delay experienced by the information from the user. 

· User ID: GPON provides a mechanism that allows the OLT to identify each ONT in the same fiber network. For this reason, each ONT has a unique serial number which is known by the OLT. 

· Remote configuration of the ONTs: One of the main challenges that resolves GPON technology is the remote user equipment. This ensures a significant cost savings resulting from the maintenance since it is not necessary to intervene in the customer's home.

To do so, within the standard GPON it has developed a protocol called OMCI (ONT Management and Control Interface). This protocol allows remote configuration of the ONTs. For each ONT provides a management channel between OLT and ONT. Includes management, performance, alarm monitoring, fault and performance. The OMCI protocol is one of the key aspects to ensure interoperability between manufacturers. There are various mechanisms of OMCI information transmission..

· Transport Protocols: The GPON standard provides two options regarding the transport protocols that can be used:

· ATM is used by the UPON and BPON, which is a continuity solution.

· GEM (GPON Encapsulation Method): This is a new protocol defined for use in the G.984s GPON.

· Implementing Multicast: Multicast is a protocol used for television broadcasting. Not to be confused with the video on demand service. This protocol, integrated in the ONT, OLT and decoder, enables the user to select the television channel that receives at each moment.

Problems in PON and GPON networks

Different terminals Sharing the media 

As discussed above, the optical media in PON is common and multi-point communication techniques are used. This fact has important implications when defining how to arbitrate the traffic on the upstream channel. For this reason it is necessary to establish a TDMA scheme in which the OLT is responsible of arbitrating the time when each ONT can send. At each downstream frame a field is included (BWMAPs) which indicates a time reference for each ONT to know when to issue data. This time reference is dynamic and variable, the OLT is responsible of deciding the allocation of bandwidth for each ONU always fulfilling:

· Each ONT at minimum upstream bandwidth set up. 

· Minimum roundtrip and minimum latency. 

However, there is an extra degree of complexity, the temporal reference gap that exists between the different ONTs:

· Each ONT uses the arrival time of the start of the frame as temporal reference for their bandwidth allocations.

· Each ONT can be at a different distance from the OLT, the propagation time OLT to ONT is different and what follows that the same downstream frame comes at different time instants for each ONT.

· In turn, each upstream frame sent by each ONT will have a different propagation time.

For the above reasons, its is established a process of "Ranging" that allows to calibrate the distance of each ONT: Each ONT receives a delay time to be applied when the broadcasting in the upstream channel starts to avoid collisions or errors in the line.

This ranking process is complex because it requires creating "windows of silence," to measure the delay of a specific ONT and end assigning that delay to the ONT. 

The creation, use and "respect" the windows of silence are important points to check for the proper functioning of the GPON Network: Without this it is impossible to start any negotiation process.

In addition, a malfunction in the window of silence system is very difficult to diagnose since the equipments since the semantics of a data received depends on the time it is received.

Power division and Optical Budget

The structure of a PON network is a fiber that is divided into more fibers using optical power dividers or splitters. At this point appears the concept of "degree of splitting", defined as the number of divisions that have the fiber to reach an ONT. The degree of splitting indicates the percentage of optical power that is coming to an ONT.

For example, suppose a PON in which there is a 1:16 splitter and from each branch hangs a 1:4 splitter. The degree of splitting is 64. That means you get to each ONT 1/64 of the optical power emitted by the OLT (and vice versa, the OLT will get 1/64 of the optical power emitted by the ONT).

Adding the attenuations due to connectorization, merging and distance in the fiber, it appears comes that the attenuation in a GPON circuit can be very high:

· Splitting Grade 1 to 64: Approximate Attenuation - 18.5 dB

· 20 km fiber: approximate attenuation (1310 nm): - 6.5 dB

Total attenuation: -25 dB.

Assuming that the sensitivity of the optical modules is about -27 dBm and the power that can be broadcast is around the 2dBm, this leaves a total of between -2 dB to -4 dB of permitted attenuation due to mergeing or connectorization.

GPON Interoperability problems

One of the challenges of the market of high-speed access is the price per subscriber. The economic advantages of PON type deployment have presented, but we must bear in mind another point: The cost of the ONT or terminal client.

With the aim of reducing the price of the ONT it is important that any OLT is able to interact with any ONT regardless of manufacturer. However, GPON has a number of characteristics that may difficult the interoperability of manufacturers for any of the following reasons:

· Commercial implementations from earlier versions of the standard: Although it is now in a very mature state, there is still no final version of the standard. This has led to "early adopters" manufacturers to have implemented versions of the standard that may differ greatly from the current one. 

· Temporal complexity of the negotiation process: The processes of detection and ranging are very sensitive. A delay or advancement of microseconds can cause both equipments not to trade. 

· Misinterpretation of the standard: GPON standard is complex and has undergone major changes. It is easy for different manufacturers to interpret the standard differently causing problems in negotiations. 

· OMCI, a very broad standard: OMC management layer is designed to remotely configure all the functionality of an ONT. In addition, the standard defines a very broad set of OMCI entities that can be combined in various ways to establish the same services. Two manufacturers may be able to offer the same services but using different GPON OMCI entities. 

· Heterogeneity among operators: Each carrier deploying a GPON solution selects a subset of OMCI to deploy their services, this means GPON custom hardware implementations of the specific OMCI layer for each operator.

As can be inferred, interoperability is an important requirement in any GPON network. When problems of interoperability appear, identification of the causes is a difficult diagnostic challenge that usually requires a source of information and analysis beyond the GPON hardware manufacturers affected.

GPON in the real world: A shared world

The economic, legal and technological developments that occur in FTTH-GPON deployments make this world a shared place. We will see why and the consequence:

In general we see that it appears the need to be able to diagnose any problems in a GPON system from the following angles:

· Interoperability:

· In the process of negotiation

· Interpretation of messages according to the standard

· Support for the same OMCI entities

· Respect for the temporal parameters of the protocol.

· In most cases unable to access one of the terminals: We need a source of direct information from the fiber.

· Independent from manufacturer: Do not deformed by the misinterpretation of the standard.

· Without access to the central facilities: Because they can belong to another operator, and therefore no direct information.

· Unable to access first-hand information or "reliable" because it has been generated by another operator or competitor. For example, SLAs or performance of services.

GPON Doctor

What is GPON Doctor?

GPON Doctor is a passive analyzer independent from manufacturer of FTTH GPON protocol oriented to Interoperability analysis:

• Analyzer: Connects to a point on the fiber distribution in a FTTH GPON network:

  • Captures data at bit level from the fiber in both upstream and downstream.

  • Interprets the data stored throughout the protocol's control information at OAM, PLOAM, OMCI and DBA level.

  • Analyzes data and bulids the state machine and topological states of the different elements that are on the network: (ONTs/OLT).

  • Applies a set of rules of assessment to identify any negotiations that have not followed the protocol ITU-T G.984.x.

• Passive: Captures data from the fiber in a transparent way, does not alter or intercept the data that moves across the network.

• Independent Manufacturer: Not based on any ONT/OLT chipset from any manufacturer. In this way their results are independent of specific commercial implementations.

• FTTH GPON protocol: protocol defined by the FSAN and currently used in the deployment of FTTH as ITU-T G.984.x recommendation. Oriented to interoperability: The protocol analysis is aimed at finding possible errors in the negotiation and transfer between devices GPON due to non compliance with standard. It's perfect for evaluating implementations of network elements and assess its GPON interoperability with devices:

  • From the same manufacturer

  • From different manufacturers

General Technical Features

• Capture system

  • Propietary capture hardware deployment independent from commercial chipsets implementations of private producers. 

  • Ability to sniff on a non-invasive way a GPON network at any point. 

  • Distance synchronization system: Calculate the distance from the OLT and the analyzer is synchronized with the upstream and downstream channels. 

  • Able to capture the GPON network control data in both upstream and dowsntream, including: 

    • OAM 

    • DBRu-DBA 

    • PLOAM 

    • OMCI 

  • Captures of information control up to 30 and 60 minutes (Depending on machine configuration and type of traffic). 

• Analysis System 

  • CST-GGS (Cold Start Topology Guess GPON System) capable of reconstructing the network topology from the control messages and negotiation: 

    • ONTs hanging from the intervened fiber. 

    • Negotiation state 

    • State machine for each network element. 

  • PERB (Protocol Evaluator Rule Based): From a capture and topology deduction applies a series of control rules that allows to: 

    • Detect abnormal situations in the negotiations. 

    • Verification of the standard by the ONTs OLT. 

    • Assessment level of interoperability between devices. 

• Storage 

  • You can dump the capture on the hard disk incorporated for later analysis.

  • The capture is stored in binary format.

  • Capture exportable to XML to use with other applications. 

  • The capture can be dumped into an external storage system. 

• Equipment

  • Autocontent equipment: do not need keyboard or mouse. 

  • High-resolution 11' touch screen incorporated

  • Internal storage system.

  • Finished in white or black aluminum.

  • Windows XP Embedded  Operating System

OMCI Diagnosis and analysis

Able to capture all the OMCI information in a PON to:

• Create a status composition of the OMCI entities residing in an ONT with the value of its attributes.

• Verify that the entities used are defined in the standard and used correctly.

The OMCI layer and the support of the OMCI management entities in ONTs is the main source of problems for interoperability between GPON equipments. 

Applications

• Fundamental tool in the deployment of a GPON network.

• Analysis of incidents and malfunctions in an established GPON network Troubleshooting of GPON Interoperability problems between different manufacturers in a network operator

• Assessment of compliance with the protocol during the development of GPON OLTs/ONTs

• Analysis of GPON protocols from the Ethernet

Use cases

• GPON 984.x Interoperablidad Test
  • Verification of the negotiation process

  • Verification of the correct use of the entities and in a correct way.

  • Compliance with the time parameters

  • Detection of uses of outdated standard

  • Using non-standard messages.

  • Detection of the point where it stops negotiating

  • Able to generate report of the errors found.

• Diagnosing problematic PONS

  • Identification of the terminal causing an incidence

  • Detection of errors in the line: FEC, CRC

  • Diagnosis of situations of optical budgets exceeded.

• Refine problems in a GPON PON.

  • Identify wich part of the PON is causing the problem.

  • Help the cleaning and monitoring of a PON

• Knowing the state of the network and all its components (ONTs)

  • From the data captured, can make an interpretation that allows to know the state of the elements connected to the PON.