FTTxtra

Five Reasons for Local Loop Fiber

John Bartell — Thu, 11 Mar 2010 21:43:12 +0000

Often we take for granted that fiber optics is a good match for broadband access. Copper has served us well, and it can provide several to tens of Mbps, but fiber optics is the future. Here are the top five reasons fiber is well-suited to broadband access services.

Fiber is inexpensive, and cost is everything in delivering local loop broadband. The cost of typical fiber cable is outweighed by the cost of installing that cable and other costs associated with delivering broadband service.
Fiber provides tremendous bandwidth. Bandwidth is the primary reason fiber is replacing copper in the local loop. Fiber has been shown to deliver thousands of Gbps in lab applications, and it is certainly under no stress providing the rates used in local loop applications. Copper local loops cannot provide anything approaching the bandwidth of fiber local loops. The constraining factor for local loop applications is the cost of the optical components required to provide a certain bandwidth. These components must operate in the rigorous environment that local loop equipment experiences, not just the friendly environment of the lab. Optical transceivers are much easier to manufacture to inside requirements of 0 – 50C than to the OSP requirements of –40C to + 65C, so the OSP rated components are more expensive. As rates increase (for instance from 2.5 Gbps to 10 Gbps), the cost difference can become even more prominent.
Fiber provides great immunity to crosstalk. With twisted pair copper local loops, crosstalk is always a factor to consider in link design. With fiber, crosstalk issues between different fibers are all but nonexistent. Light just does not couple between different fibers.
Fiber optic cables are very small allowing many to be packaged into a single, compact cable. In a multi-strand cable, the space taken up by a fiber is substantially less than that of a single copper local loop. Fiber optic cables are smaller but carry much more information than twisted pair copper cables used in the local loop.
Fiber is a very low loss medium. Optical splitters and cost effective transceivers can be used in fiber access networks because of the low losses of fiber. Splitters introduce a high level of attenuation that can only be marginally improved, and a typical 32x splitter introduces 16-17 dB of loss (15 dB of loss is a perfect 32x splitter). This is more than half of the typical link budget in a PON system. If fiber attenuation was considerably higher at the wavelengths used in local loop systems, then the cost of the necessary components would make implementing PON systems impractical.

Protection in GPON Systems

John Bartell — Tue, 09 Mar 2010 00:24:34 +0000

Protection (also known as redundancy) in a GPON System is a feature desired by those customers needing continuous operation of their network connections. Protection is optional in the GPON standard. The method of operation is not specified in detail, so vendor implementations will be proprietary, at least initially. Three primary methods are described in the standard: OLT Duplex, Full System Duplex, and Dual Parented Duplex. All of these can provide uninterrupted data transmission during and after a switch to the redundant equipment.

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.

Power Leveling on PONs

John Bartell — Thu, 17 Dec 2009 16:36:39 +0000

Power leveling attempts to equalize the signal levels an OLT receives from its associated ONUs. An OLT receives separate bursts of transmission from each of its associated ONUs during an upstream frame transmission time, and each ONU signal will experience a different level of attenuation on the PON depending on the distance from the OLT.

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).

Extending GPON’s Reach

GPON’s Dynamic Bandwidth Assignment (DBA)

John Bartell — Wed, 16 Dec 2009 22:11:01 +0000

Dynamic Bandwidth Assignment (DBA) is used on Passive Optical Network’s to allow better use of upstream bandwidth, and it is especially good for dealing with bursty upstream traffic. GPON and many other PON technologies use Time Division Multiple Access (TDMA) for managing upstream access by ONUs, and at any one point in time, TDMA provides unshared timeslots (fraction of the upstream bandwidth over time) to each ONU for upstream transmissions.

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.

ONU or ONT?

John Bartell — Thu, 03 Dec 2009 20:51:33 +0000

In the ITU-T standards, ONU is the generic name for devices installed at subscriber premises to convert PON interfaces to Ethernet, POTS, and other interfaces, whether the device is serving one or more subscribers. The current version of G.984 does acknowledge the need to differentiate between single user and multi-user devices.

In this book, to avoid ambiguity, ONU is used to describe a device that serves multiple subscribers, such as a DSLAM with a PON uplink. ONT describes a device serving a single subscriber such as one that is installed on a single family home. An ONT is typically a device highly cost-optimized to serve a single subscriber. An ONU typically has a radically different architecture and is often more similar to a DSLAM or Ethernet Switch than a single family ONT.

In the ITU-T standards, Optical Network Unit (ONU) is the generic name for a device installed at a subscriber’s premises to convert fiber access interfaces to Ethernet, POTS, and other interfaces, whether the device is serving one or more subscribers. The current version of the G.984 GPON standard does acknowledge the need to differentiate between single-user and multi-user devices.

In common usage, an Optical Network Terminal (ONT) serves a single subscriber premises such as a stand alone house. An ONT has no need for security between its few interfaces and sells for perhaps a few hundred dollars. A typical indoor ONT is shown below.

ONU is used to describe a device serving multiple subscribers such as those in an apartment building or shared office building. It has a radically different architecture and is more similar to a DSLAM or Ethernet Switch than a single family ONT. An ONU can sell for several thousand dollars, depending on subscriber density, but often offers a lower per-subscriber cost than an ONT because common components are shared.

Fiber to the Cabinet or FTTCab

John Bartell — Mon, 16 Nov 2009 13:31:19 +0000

What used to be called Fiber to the Node or FTTN is now called Fiber to the Cabinet or FTTCab in the ITU standards. The acronym is obviously quite close to that for Fiber to the Curb or FTTC. Luckily, both of these architectures are related with the main difference being exactly where in the OutSide Plant (OSP) the fiber-fed DSLAM is installed. FTTCab has the Digital Loop Carrier (DLC) and Digital Subscriber Line Access Multiplexer (DSLAM) installed installed at a local loop concentration point known as the Feeder Distribution Interface or FDI and typically serving several hundred subscribers. FTTC has the DLC and DSLAM installed at the curb, and it typically serves 8-12 subscribers from a single installation.

A typical network architecture for FTTCab is shown below. A FTTC architecture would have fiber from the Wire Center all the way to the Distribution/Drop interface, which is typically installed at the curb.

The picture below is a FTTCab installation near my house. The rear beige cabinet is a Digital Loop Carrier (DLC) system, which houses telephony electronics. The two beige cabinets in front of the DLC house adjunct DSLAMs providing DSL services. The green cabinet to the right is the Feeder Distribution Interface, which provides copper pair concentration. FDI concentration is typically 2 distribution pairs for each feeder pair. In this case, the feeder copper pairs are only a few feet in length since they are terminated at the two DSLAM cabinets shown in the picture.

DLC and FTTCab

WiMAX Overview

admin — Tue, 03 Nov 2009 13:44:41 +0000

Standardized by the IEEE and currently capable of rates of up to 15 Mbps, WiMAX (Worldwide Interoperability for Microwave Access) is a form of 4G broadband wireless access. WiMAX delivers broadband wireless access to rural areas with with relatively modest investments compared to DSL, fiber, and HFC.

Subscribers can use WiMAX service with either an indoor (typically integral to the WiMAX modem) antenna or with a separate outdoor antenna. Indoor antennas place greater demands on the carrier’s network because of additional losses experienced within a structure. To support indoor antennas, a carrier typically requires more base stations for ubiquitous coverage or has limited coverage where indoor antennas can be used.

The IEEE is developing a new version of WiMAX known as 802.16m or WiMAX-m that provides for access rates of up to 1 Gbps. The IEEE 802.16m working group produced this Draft IEEE 802.16m System Description Document, which provides an overview of where the WiMAX broadband wireless standard is going. To increase the bandwidth available to a subscriber, the new IEEE 802.16m WiMAX standard will specify multiple carriers. The WiMAX Forum is dedicated to increasing the acceptance of WiMAX networks by wireless broadband providers and WiMAX broadband service by subscribers.

Clearwire is a major provider of WiMAX services in the US and offers downstream rates of up to 2 Mbps. Serving almost 50 markets, Clearwire offers service for as low as $29.99 per month according to the Clearwire website, which provides information on service plans.

Extending GPON’s Reach

John Bartell — Mon, 02 Nov 2009 12:15:52 +0000

Carriers have two fundamental options for extending the reach of a Gigabit PON (GPON) network. One option is to deploy GPON Optical Line Terminals (OLTs) outside of Central Offices (COs). Another option is G.984.6 mid-span extenders, which increase a GPON network’s reach to as much as 60 km. For carriers that only deploy GPON OLTs from their Central Offices (COs), 20km is sufficient to serve only a fraction (perhaps half) of their subscribers. The following diagram shows a basic GPON access architecture without any reach extension.

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.

Active Fiber Overview

John Bartell — Fri, 30 Oct 2009 12:27:44 +0000

Point-to-Point (P2P, also known as Active Fiber) is the simplest of all three Fiber To The Home fundamental architectures. With a point-to-point network architecture, a fiber (typically only a single fiber) is installed from each subscriber’s house directly into the Central Office serving that subscriber. This architecture has the advantage of simplicity, but it does require terminating lots of fiber cables in the Central Office (CO).

Active Fiber Architecture

With Active Fiber, the CO contains a high port count aggregation device (one port per subscriber) known as an Optical Line Terminal or OLT. A single optical fiber connects each subscriber’s house to the Central Office. An Optical Network Terminal (ONT) is installed either on the side of the subscriber’s house (typical in the US) or inside the subscriber’s house (not typical in the US).

Advantages

Active Fiber has an advantage in that no port is shared in any way, thus troubleshooting problems on the network is greatly simplified. With this simple architecture, optical problems can be easily isolated. Additionally, this architecture has the highest bandwidth potential. Links are easily upgraded to higher rates (requires new optics and electronics on both ends however), and each additional fiber linearly adds more aggregate bandwidth to the network.

Active Ethernet Overview

John Bartell — Wed, 28 Oct 2009 13:13:04 +0000

Active Ethernet is one popular way to deliver massive amounts of bandwidth to subscribers over fiber and is a major contender for Fiber to the Home (FTTH) based broadband Internet rollouts. In the US, it is more popular with the smaller telcos than with the larger ones.

Active Ethernet competes with Passive Optical Network in all it various flavors, which include Ethernet PON or EPON, 10G EPON, Broadband PON or BPON, Gigabit PON or GPON, and Wave Division Multiplex PON or WDM PON. Another very similar broadband access technology is called Point-to-Point (P2P) Ethernet or Active Fiber. The difference between Active Ethernet and Active Fiber is that Active Fiber has no switching gear outside of the Central Office (CO or wire center).

Active Ethernet Network

The essentials of an Active Ethernet network are shown in the diagram below. Ethernet aggregation equipment is installed in the CO and in the OutSide Plant (OSP). An Ethernet Optical Network Terminal (ONT) is installed at each customer premises. The Ethernet aggregation device that connects directly to ONTs is a temperature-hardened Ethernet switch and called the Optical Line Terminal (OLT).

Active Ethernet Standards

Active Ethernet is specified in IEEE (US) standard 802.3ah for Ethernet in the First Mile.

Disadvantages of Active Ethernet

The name indicates a disadvantage in that “active” electronics must be deployed in the outside plant (OSP), and OSP electronics are more expensive to deploy and maintain than CO electronics. This is one of the advantages of PON, but at the same time PON access bandwidth is shared amongst all the subscribers served by a single splitter.

Active Ethernet imposes the cost of a network transceiver dedicated to each subscriber. The advantage of having a separate network transceiver dedicated to each subscriber comes at the cost of proliferation of one of the most expensive components in an optical broadband access network. An Active Ethernet access network will have roughly two times the optical transceivers of a Passive Optical Network.

Advantages of Active Ethernet

Active Ethernet is easier to manage. A network transceiver (in the ONT) is dedicated to each subscriber so that the carrier has an unhindered view of each subscriber’s dedicated equipment.

Active Ethernet is a relatively mature technology with a variety of equipment choices. This give carriers a choice of network and subscriber equipment. Since the interfaces are standardized, a carrier even has a choice of deploying a multi-vendor broadband access network.

With Active Ethernet, sharing of bandwidth happens only on the uplinks to the aggregation equipment. Its key advantage is that tremendous bandwidth is available directly and unshared to each subscriber, at least 100Mbps and perhaps 1Gbps on each Optical Network Terminal. Active Ethernet is one of the most future proof access architectures.