OLT Duplex redundancy has two separate GPON OLT interfaces to a dual input optical splitter. A standard GPON ONT is installed on the customer premises. When one of the interface modules fails, the GPON OLT switches to the other line card and fiber interface. The advantage of this protection method is that it does not require a second GPON ONT, or even additional hardware in the GPON ONT, which is the most expensive hardware component providing GPON service to a subscriber.

Full Duplex System redundancy is similar to OLT Duplex redundancy, but it adds dual Optical Distribution Networks (ODNs). Although fiber optic cables and optical splitters have very low failure rates, technicians will sometimes remove in-service jumpers in Central Offices. In an unprotected system, this causes a failure for affected subscribers until the jumper is reinstalled. The Full Duplex System protection option, along with Dual Parenting (described below), protect against this and other ODN problems. This redundancy method requires a special GPON ONT with dual interfaces, or two GPON ONTs on the subscriber premises, both of which are expensive configurations.

Dual Parented Duplex provides for wholly separate GPON OLTs and ODNs. It is a highly robust type of redundancy, and it is the only one discussed in G.984 that provides potential geographic diversity of the GPON OLT. Note that like with Full Duplex System redundancy, a special GPON ONT with dual uplinks is required. Another option is two separate GPON ONTs, which can be standard GPON ONTs. This does require a special router configuration in the customer’s network.

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1. GPON Chip Vendors
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Besides distance from the OLT, another factor that affects the OLT’s received signal levels is variations is ONU transmit level. Each ONU may transmit at a different nominal level owing to use of different optical transmitters and variations in manufacturing for the same type of transmitters.

ONUs can be anywhere from 0 to 20 km from the OLT, and the attenuation of the intervening fiber makes the received light arrive at very different levels at the OLT. Typical fiber used in access networks (such as SMF-28)  introduces about 0.35 dB of loss for each kilometer of distance at 1310nm, which is the transmit wavelength for ONUs on a GPON. With a differential range of 20km on a PON, an ONU at 20 km of distance from the OLT may experience 7 dB of additional loss compared to an ONU at 0 km. An ONU situated far from an OLT (say greater than 15 km) can use a higher optical transmit power, and an ONU close to the OLT (say less than 5 km) can use a lower optical transmit power to achieve approximately the same receive signal level at the OLT. The graphic below shows how power leveling can reduce the range of optical levels received at the OLT.

There are two methods for initiating a change of power level in an ONU: the ONU can initiate the level change or the OLT can initiate the level change. The ONU will initiate a level change when that ONU responds to a certain number of serial number requests without a message from the OLT assigning an ONU ID. And it will repeat this process until it receives a proper message assigning it an ONU ID. The OLT will request that an ONU change its transmitted power level when the OLT measures an unacceptably high BER for transmissions received from this ONU.

Power leveling  reduces the range of signal levels an OLT must handle, and it can allow for lower cost optics at the OLT. Without power leveling, the OLT must be able to handle a greater range of receive signals while maintaining a very low bit error rate (BER).

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1. Extending GPON’s Reach

DBA allows upstream timeslots to grow and shrink based on the distribution of upstream traffic loads and operates on a timescale of milliseconds. Of course, the total of all timeslots on a PON cannot be greater than the length of a single upstream frame.

DBA functions on Transmission Containers (T-CONTs), which are upstream timeslots, and each is identified by a particular AllocID. An ONU must have at least one T-CONT, but most have several T-CONTs, and each corresponds to a particular upstream time slot on the PON. Without DBA support in the OLT, upstream bandwidth is statically assigned to T-CONTs, cannot be shared, and can be changed only through a management system.

There are two methods to determine the bandwidth requirements of ONTs. The first, called Status Reporting (SR) DBA, involves explict T-CONT buffer status provided by the ONTs. With this method, the OLT solicits T-CONT buffer status, and the ONUs respond with a report for each assigned T-CONT. The other method is known as Traffic Monitoring (TM). With TM DBA. the OLT imputes how much bandwidth is required by monitoring the number of idle frames sent in a particular T-CONT. A GPON OLT must support both methods, but ONUs need not provide any support for DBA; the OLT will just use Traffic Monitoring DBA for ONUs that provide no support for DBA.

DBA provides a way for an OLT to oversubscribe its upstream bandwidth and provides an effective increase to average bandwidth available to each of its ONTs. But DBA brings with it the possibility of a carrier not being able to satisfy all the upstream bandwidth granted to ONTs. Static bandwidth assignment does not have this problem, but it does not allow for any more than the 1.244 Gbps of the upstream to be assigned to the ONTs. The bottom line is that DBA allows more ONUs on a single GPON, and it allows more bandwidth to be assigned to each ONU, with no changes to the optics in the OLT or the ONUs.

© 2009, The Product Group LLC. All rights reserved.

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2. GPON Tutorial
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Remote GPON OLT

One robust option for extending the reach of GPON networks is to install remote GPON OLTs, but this does involve placing relatively complex electronics outside of protected COs. This option provides the most flexibility because the GPON OLT can be deployed in an OutSide Plant (OSP) cabinet or Controlled Environment Vault (CEV) up to 100km or more from the CO. The distance is limited only by the uplink optics chosen for the OLT. The following diagram shows how remote GPON OLTs are deployed in OSP cabinets.

And the picture below shows an actual OSP cabinet and power pedestal.

Mid-Span Extenders

Mid-span extenders are a carrier’s other option. These devices increase GPON network’s reach to as much as 60km, which is the logical limit of GPON’s transmission convergence layer (related to maximum delay between OLT and ONT). G.984.6 intends to allow compatibility with existing ONTs and OLTs as much as possible. The diagram below shows a mid-span extender in a GPON access network

G.984.6 specifies two technology options for mid-span extenders. They may be based on optical amplifiers, which merely provide gain in optical power. Or they may be Optical-Electrical-Optical (OEO) regenerators, which receive the optical signal, convert it to an electrical signal, retime and reshape the signal, and finally convert it to an optical signal for retransmission. Mid-span extenders are bidirectional devices, and the two basic technologies could be mixed (e.g., OEO in upstream and OA in downstream) in a hybrid device.

Mid-span extenders have some disadvantages. They are electronic devices requiring periodic maintenance, which invalidates the passive in Passive Optical Network. They are likely to be incompatible with existing GPON OLT implementations, though compatibility could be achieve with simple changes in GPON OLT parameter values. Mid-span extenders may limit next generation PON deployments unless these technologies are specifically considered in the design of these devices.

Another issue with mid-span extenders is that they will not allow an OTDR full visibility on a GPON local loop, though this is not unique to mid-span extenders. Optical splitters have such high losses (~17db for 32x splitter), and these losses are experienced twice by the OTDR signal (once in each direction for a total of ~34db loss for 32x splitters), that OTDRs can not measure attenuation beyond a splitter either.

Products

Alphion markets their PON.ext mid-span extender. A variety of vendors offer GPON OLTs, and all should be remotely deployable in an environmentally controlled enclosure such as a CEV. For an OSP cabinet deployment, a key criterion is the operating temperature range of the equipment, which should operate reliably from -40 to +65C.

© 2009, The Product Group LLC. All rights reserved.

Related posts:

1. GPON Standards Revealed
2. GPON Tutorial in 200 Words
3. GPON Tutorial

Each of the ITU GPON documents in the G.984 series (G.984.1, G.984.2, G.984.3, G.984.4, G.984.5, G.984.6) is discussed and linked below.

G.984.1, General Characteristics

G.984.1 is a general overview of GPON as specified in the G.984 series of ITU-T standards. It covers network architecture, interfaces, rates, reach, split ratios, and redundancy, among other topics. It is easy reading compared to the other documents in this series.

G.984.2, Physical Media Dependent (PMD) Layer Specification

G.984.2 covers optical specifications for GPON implementations. However, B+ and C+ are the most commonly implemented optical specifications, and neither is covered in this document. It does have some good general information on optics, but available implementations follow the specifications in the two amendments to this document.

G.984.2 Amendment 1 is the specification for GPON B+ optics, which is by far the most common type of optical interface available on GPON equipment. G.984.2 Amendment 2 (a draft document) specifies GPON C+ optics, which add an extra 4 dB to the loss budget on a GPON network compared to B+. Additionally, it covers optical layer supervision and a change to the downstream extinction ratio.

G.984.3, Transmission Convergence Layer Specification

Covering framing, upstream Time Division Multiple Access (TDMA), Physical Layer OAM (PLOAM), Dynamic Bandwidth Allocation (DBA), Optical Network Unit (ONU, equivalent to ONT) activation, Forward Error Correction (FEC), and security, G.984.3 is the core of the GPON specifications. It is quite long and technical, and it is essential reading for vendors of GPON hardware and software. G.Imp.984.3, Implementators’ Guide for ITU-T Rec. G.984.3, clarifies G.984.3 and allows vendors to more easily achieve interoperability.

G.984.4, ONT Management and Control Interface Specification (OMCI)

OMCI, specified in G.984.4, “hides” the ONTs from external network management systems and allows an OLT and all its associated ONTs to be managed as a single entity with a single IP address.  G.Imp.984.4, Implementor’s Guide for ITU-T Rec. G.984.4, clarifies provisioning and management and allows for easier multi-vendor interoperability.

G.984.5, Enhancement Band

G.984.5 specifies optical wavelength usage to support RF Video overlay and next generation PON services like 10G GPON.

G.984.6, Reach Extension

G.984.6 provides specifications for Mid-Span Extenders that provide additional reach on GPON networks. These devices are either regenerators (Optical Electrical Optical or OEO) or optical amplifiers (OA) and are used between an OLT and its optical splitter. Without requiring changes to existing ONT implementations, these devices provide a maximum reach of up to 60 km on a single GPON network . The diagram below shows a G.984.6 Mid-Span Extender in a GPON access network.

G.984.6 Reach Extension

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Gigabit Passive Optical Network (GPON) is a high bandwidth shared fiber access technology that is used around the world for Fiber to the Home (FTTH) and, at least in North America, is thought by many to be the successor to BPON. GPON technology is especially popular with large US-based telcos, especially Verizon, though it is used by the MSOs as well (often for serving business customers as a complement to their Hybrid Fiber Coax (HFC) networks). 10G GPON is a higher speed version of GPON that has yet to be standardized by the ITU-T. EPON, 10G EPON, WDM PON, and BPON are Passive Optical Network (PON) technologies that are also vying for carrier attention.

GPON is standardized by the ITU-T in its G.984 series (see end of this article for details), but widespread interoperability between different vendors’ equipment has not materialized. Basic data transmission is readily achievable. Managing a multi-vendor GPON solution is quite an operational challenge, however.

GPON Architectures

There are three main components in a GPON access network (other than the fiber itself). The GPON Optical Line Terminal (OLT) is the network concentrator, usually installed in a Central Office (CO). The splitter (or splitters) allows a single fiber from the CO to be shared among a number of subscribers. The Optical Network Terminal (ONT) serves a single residence, converting optical signals to electrical signals that can be used within the home. Note that the ITU standards call the subscriber device an Optical Network Unit (ONU), and many use ONU to mean an ONT serving several subscribers, which would be common in an installation serving a number of apartments in the same building.

GPON is specified to be a single or dual fiber system, but almost all GPON systems are single fiber like virtually all popular FTTH technologies. There is little reason to use dual fibers, although this option is indeed allowed in the standard.

G.984 allows for 60km maximum reach with 20km differential reach and up to 128 subscribers on a single GPON network. However, GPON systems typically provide only 0-20km reach owing to the cost of the optics. G.984.6 is a new ITU-T specification that provides for a Mid-Span Extender that can increase the reach of GPON beyond 20km to as much as 60km.

Many carriers use a maximum of 32 subscribers on a single GPON segment. B+ optics provide for 32x split with 20km reach. C+ optics, newly available and expensive, provide for 64x split with the same 20km reach. GPON wavelengths are 1490 nm down and 1310 nm up. RF Overlay is carried downstream on 1550nm. Forward Error Correction (FEC) potentially allows for cheaper optical transceivers, though this cost advantage in the optics comes at the cost of extra complexity and overhead (almost 10% extra overhead) to support FEC.

A GPON network can have two, three, or four wavelengths in use. Two and three wavelength systems are covered below. See my article on RFoG for a description and diagram of a four wavelength system combining GPON and RFoG.

Two Wavelength System

The following diagram shows the architecture of a basic two wavelength GPON network, which is probably the most common implementation. The downstream wavelength is 1490nm and transmits data at 2.488 Gbps. The upstream wavelength is 1310nm and transmits data at 1.244 Gbps.

GPON Network Diagram (Two Wavelengths)

The GPON Optical Line Terminal (OLT) is typically installed in a Central Office (CO), though it could be installed elsewhere. The optical splitter is installed somewhere between the CO and the subscribers. And a GPON Optical Network Terminal (ONT) is installed at each subscriber’s home. Voice, video, and data traffic must all be delivered across the single GPON downstream wavelength. A nice facet of GPON for IP video support is that its downstream is naturally a broadcast medium, and  it is very efficient for delivering multicast traffic.

The optics in the GPON ONT for a two wavelength implementation is called a diplexer. See the diplexer diagram below. Diplexers can be implemented with a three dimension bulk optic design (discrete components aligned and welded together manually) or with a Planar Lightwave Circuit (PLC) design (link to a good article by Enablence explaining bulk optics and PLCs). A PLC puts all its optical components on a silicon substrate for a two dimension design, eliminates all the complexity of dealing with a third dimension, and allows for low-cost automated manufacturing.

GPON Diplexer

Three Wavelength System

The architecture of a GPON three wavelength system is identical to that of a two wavelength system with the addition of a third downstream video wavelength on the fiber and the equipment to insert this signal into the fiber. The following diagram shows the architecture of a three wavelength RF Overlay GPON network.

GPON RF Overlay Network Diagram

Note that only up to 32 GPON ONTs are indicated for a single GPON OLT port. This is because of the RF Overlay video signal and not the GPON signal. For 20km reach and 32 subscribers on a single network, the maximum amount of light that a fiber will accept (20 dBm or 100 mW) must be inserted into the fiber by the RF Overlay video equipment, and any additional optical power is just wasted. New C+ optics allow for 64x splits and 20km reach for the GPON signal, but this is no help for the RF Overlay video signal. The RF Overlay transmit signal is already at maximum for 32x split and 20km and only the receiver sensitivity can be improved. This may come in time.

The transceiver in the GPON ONT for a GPON RF Overlay video implementation is called a triplexer (see diagram below). Triplexers are more expensive than diplexers and generally are implemented with a three dimensional bulk optic design (discrete components aligned and welded together manually). PLC circuits not as common for triplexer implementations though Enablence describes a PLC triplexer.

GPON Triplexer

The OLT does nothing with the third RF Overlay wavelength other than filter it out. The ONTs merely convert the 1550nm optical signal to an electrical signal for delivery throughout the home over 75 ohm coax. The thorniest issue to solve with a three wavelength signal is how to get the upstream data for controlling the RF Overlay signal to the headend. One option is to convert the traffic into IP and send it upstream on the 1310nm upstream wavelength, though this method has some limitations. A more robust (and more expensive) option is RFoG, which is described in my article on RFoG.

Transmission

The ITU GPON standard allows up to 2.488Gbps symmetric transmission, but almost all GPON systems are 2.488Gbps down/1.244Gbps up. Both downstream and upstream bandwidth is shared although in different ways.

Downstream from the OLT to the ONTs is broadcast with an ONT grabbing only traffic addressed to it. Upstream is Time Division Multiple Access (TDMA) with each ONT transmitting in turn (with perhaps multiple turns per ONT). A single ONT can have multiple upstream timeslots, and each timeslot can be a different size. Additionally, Dynamic Bandwidth Assignment (DBA) allows for real-time changes in upstream timeslot sizes to accommodate varying traffic conditions. A typical implementation has one upstream timeslot for management, one for voice, and one for data traffic for each ONT. A GPON network with 32 ONTs may have about 100 upstream timeslots in use.

GPON natively supports Ethernet (GFP), ATM, and TDM, but most systems run just Ethernet (sort of like a souped up EPON). Upstream and downstream frames are transmitted 8000 per second (the downstream frame is twice the size of the upstream frame), which provides a nice 8 kHz signal to the ONTs for POTS service (required for good fax speeds).

OMCI

ONT Management and Control Interface (OMCI) is the management protocol used between the OLT and the ONTs. With OMCI, external management systems do not have to communicate directly with the ONTs. OMCI allows a single IP address to be used to manage an OLT and, through OMCI, all of its associated ONTs. This is very efficient for IP address conservation, and it reduces the load on a management system, but it does require the implementation of a technology-specific management protocol. If there is a VoIP implementation in the ONTs, it is likely they will require separate management and IP addresses anyway. Owing to the popularity of VoIP in these systems, IP address conservation with OMCI is of dubious benefit in many GPON implementations.

Standards

GPON is standardized by the ITU-T in its G.984 series. The list below provides links to the relevant ITU-T GPON standards.

• G.984.1, General characteristics, [A general overview. Easy reading.]
• G.984.2, Physical Media Dependent (PMD) layer specification, [Optics, mostly irrelevant.]
  • G.984.2 Amendment 1, [B+ optics, most common implementation.]
  • G.984.2 Amendment 2, [C+ optics and some tweaks. Allows 64x splits with 20km reach.]
• G.984.3, Transmission convergence layer specification, [The protocols. Pretty darn technical.]
  • G.Imp.984.3, Implementators’ Guide for ITU-T Rec. G.984.3,
• G.984.4, ONT management and control interface specification, [OMCI. Yawn.]
  • G.Imp.984.4, Implementor’s Guide for ITU-T Rec. G.984.4,
• G.984.5, Enhancement band, [Next generation PON compatibility.]
• G.984.6, Reach extension. [Increased range using an active mid-span extender.]

© 2009, The Product Group LLC. All rights reserved.

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2. GPON Overview in 10 Items
3. 10G GPON Tutorial

Luckily, there are now two vendors of GPON analyzers: TraceSpan and Tecnalia/TELNET-RI. TraceSpan, first with a GPON analyzer, provides protocol analyzers for a variety of broadband access technologies. Their GPON analyzer is known as the GPON Xpert and is shown below.

Tecnalia and TELNET-RI, both Spanish companies, developed the GPON Doctor, shown below.

GPON analyzers provide vendors of GPON hardware an independent view of GPON standards compliance. Carriers use these analyzers to evaluate a particular GPON implementation and interoperability between OLTs and ONTs. However, the big issue with interoperability between GPON OLTs and ONTs is management of the multi-vendor solution, and using these analyzers to verify OMCI management traffic is only a fraction of what is necessary for multi-vendor management to be accomplished. The data these analyzers provide does, however, provide some visibility into what it will take to achieve interoperability with a particular vendor’s products.

© 2009, The Product Group LLC. All rights reserved.

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GPON Network Diagram

© 2009, The Product Group LLC. All rights reserved.

Related posts:

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• Broadcom

• Broadlight

• Conexant Systems

• Cortina Systems

• Freescale Semiconductor

• Ikanos Communications

• PMC-Sierra

• TranSwitch

• Vitesse Semiconductor

• Xelerated

© 2009, The Product Group LLC. All rights reserved.

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“Two important factors impacting demand for Access equipment are broadband subscriber additions and network upgrades,” said Tam Dell’Oro, President of Dell’Oro Group. “Total broadband subscriber additions have been lower since the peak year of 2006, and are having a negative impact on equipment demand, especially for a slower-speed technology such as ADSL. On the positive side are the upgrade projects that are being driven by competition, increasing internet traffic, government incentives, and the desire by operators to enable new revenue-generating services such as TV over broadband. These upgrade projects increasingly drive demand for higher-speed PON, VDSL, and Cable DOCSIS 3.0 equipment,” Dell’Oro added.

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