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.

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

Related posts:

1. Three Fundamental Architectures for FTTH
2. Active Ethernet Overview
3. Active Ethernet Tutorial

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.

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

Related posts:

1. Active Ethernet Tutorial
2. Active Fiber Overview
3. Three Fundamental Architectures for FTTH

A compelling factor in choosing a broadband technology is what local loop facilities are already installed. The majority of the cost for a new local loop is just installing the medium to reach the customer, and much of this effort  in no way follows Moore’s law.

Four common mediums for delivering broadband services are twisted pair copper, hybrid fiber coax (HFC), fiber optics, and wireless. These are discussed below.

Twisted Pair Copper

The telcos have a tremendous amount of already installed copper local loops, and they will do everything possible to use this copper for delivering broadband. Avoiding installing new local loop (for instance fiber) to a subscriber can eliminate $1000 or more of capital, so telcos are keen to use their copper for DSL. However, competition is sometimes forcing telcos to overbuild fiber just to effectively compete (Verizon FiOS is a good example). The diagram below shows the architecture of a typical DSL access network serving residential subscribers in single family and multi-dwelling units.

The major players in DSL these days are ADSL2+ and VDSL2. VDSL2 is the relative newcomer, and its chipsets typically support a fallback mode to ADSL2+ because of VDSL2’s much shorter reach than ADSL2+. Together, these two technologies just about wring out all the broadband that a copper local loop can deliver. Further improvements will have to rely on multi-pair techniques like Dynamic Spectrum Management (DSM).

Both ADSL2+ and VDSL2 are technologies used for Fiber to the Node (FTTN) and Fiber to the Curb (FTTC). FTTN and FTTC are hybrid technologies combining fiber and DSL for delivering broadband rates in the tens of megabits per second. Their attraction is avoiding that final fiber link to a subscriber’s premises.

VDSL2

Though rates typically delivered are much lower, VDSL2 can deliver broadband at rates of up to 100 Mbps over local loop copper. Its greatest limitation is reach. It really does not provide any advantage in bandwidth over ADSL2+ beyond about 3500 feet, and it effectively does not function beyond about 6000 feet. Many VDSL2 chipsets support a fallback mode to ADSL2+ to extend their reach.

ADSL2+

ADSL2+ is an evolution of ADSL, which has been a broadband workhorse for the telcos for over a decade.  ADSL2+ reaches up to 18000 feet with useable bandwidth, and its top rate is about 24 Mbps over short distances. The advantage ADSL2+ has over VDSL2 is cost, though the difference is very small when considering the entire cost of delivering broadband to a subscriber. Another advantage is that, since its complexity is lower, ADSL2+ heat dissipation is substantially lower than VDSL2’s. This lower heat dissipation is a significant advantage in OutSide Plant (OSP) cabinets, which tend to be heat constrained.

Fiber Optics

Essentially, there are three fundamental architectures for delivering fiber directly to the home: point-to-point, switched, and Passive Optical Network (PON).  All three Fiber to the Home (FTTH) architectures require an aggregation device in the CO (the Optical Line Terminal or OLT), and all three require an optical to electrical converter (Optical Network Terminal or ONT) in or on the home. These three architectures differ mainly in what type of device (if any) is installed between the CO and the home.

Active or Point-to-Point Fiber

Active or Point-to-Point (P2P) Fiber is the simplest of all three fiber broadband architectures. With an Active Fiber network, a fiber (typically only a single fiber) runs directly from a subscriber’s house into the Central Office (CO) serving that subscriber.  This architecture is simple, but it does require dealing with a large number of fiber optic cables in the CO. The CO contains a high port count aggregation device (the Optical Line Terminal or OLT) which has one port per subscriber.

Each subscriber house has an Optical Network Terminal (ONT) installed. It is placed either on the side of the house, which is typical in the US, or inside the house, which is not typical in the US, but more common in Europe and Asia. Active Fiber has an advantage in that all access ports are dedicated and unshared, and troubleshooting access network problems is simple.

With Active Fiber, problems can be easily identified.  And, this simple architecture can deliver the most bandwidth of any fiber access technology. Each fiber added increases the aggregate bandwidth of the access network.

Active Ethernet

Tremendously decreasing the number of fibers that must be terminated in the CO, an Active Ethernet architecture has many of the advantages of active fiber in terms of scaleability and bandwidth. However, Active Ethernet requires active electronics to be installed in the OutSide Plant (OSP), which adds capital and operational costs. This equipment is necessary to aggregate fibers delivered directly to subscribers.

PON

Similar in architecture to the Active Ethernet architecture, a Passive Optical Network (PON) architecture, however, requires no OSP electronics. A completely passive optical splitter replaces the OSP Ethernet switch. The optical splitter combines the light from the ONTs and divides the light from the OLT. Optical splitters instead of active electronics reduces the cost of aggregation since the optical splitter is quite inexpensive, need no local power, and requires almost no maintenance of any kind. Up to 32 subscribers are typically served per fiber, though higher ratios are possible. See diagram below for the architecture of a typical PON access network.

Common types of PON deployed today are BPON, EPON, and GPON. On the horizon are 10G EPON and 10G GPON. WDM PON is another PON technology, though it operates differently. It uses an Arrayed WaveGuide (AWG) to divide wavelengths for individual delivery to subscribers instead of an optical splitter.

Hybrid Fiber Coax (HFC)

The cable companies have spent billions upgrading their access networks to Hybrid Fiber Coax (HFC). They will do everything possible to use these networks for delivering broadband services.

DOCSIS

DOCSIS is an upgrade to Hybrid Fiber Coax (HFC) networks to deliver high bandwidth broadband Internet service. Cable TV companies use DOCSIS to compete with telcos for broadband access subscribers. The fundamental attraction to DOCSIS is that, since it uses the existing HFC network to deliver hundreds of megabits of bandwidth, it avoids expensive network upgrades.

There are three major versions of DOCSIS: versions 1, 2, and 3. Both DOCSIS 1 and DOCSIS 2 are limited to a single analog channel for downstream bandwidth, but DOCSIS 3 allows bonding of several channels. Four bonded analog channels allows for downstream rates of over 100 Mbps. Note that an analog channel can be used for DOCSIS, or for an analog TV channel, but not both (of course), so the more channels dedicated to DOCSIS, the fewer analog TV channels available.

DOCSIS 3.0 requires upgrades to the Cable Modem Termination System (CMTS) and the cable modems, but other than this, the HFC network requires no changes.  The only change is in equipment that is installed in the headend and on the subscriber premises.

RFoG

RF over Glass (RFoG) is a more drastic upgrade to a cable company’s network, and it implements a passive optical network (PON) to replace a Hybrid Fiber Coax (HFC) network. It is one technology known as DOCSIS PON or DPON. With RFoG, the headend multiplexer is replaced by a Wave Division Multiplex (WDM) device that delivers the RFoG wavelengths on separate fibers to other devices in the headend.

A fiber splitter serves a neighborhood or portion of a neighborhood. A fiber NID is installed at each house. See below for a diagram of a typical RFoG installation.

The attraction of RFoG is that it is compatible with HFC headend gear (except for the HFC fiber mux), and it is compatible with HFC subscriber equipment, including all those STBs. The RFoG NID delivers RF coax signals identical to those used for HFC. Legacy management systems can still be used when RFoG replaces an HFC network.

4G Wireless

4G wireless technologies deliver broadband Internet rates over wireless connections. These should be popular in rural areas as they can serve a large geographic area for relatively little capital compared to the wired alternatives. 4G deployments are just beginning. WiMAX is already deployed, and ClearWire has an extensive nationwide WiMAX network.  Long Term Evolution (LTE) is quite popular today, with LTE-Advanced following and providing up to 1 Gbps of throughput.

There are two forms of 4G wireless broadband: mobile and fixed. Mobile is, like it sounds, for users on the go. Fixed wireless broadband provides higher bandwidth and is competition for DSL, DOCSIS, and Fiber access technologies to serve residential and business locations.

WiMAX

WiMAX (Worldwide Interoperability for Microwave Access) is a broadband wireless technology standardized by the IEEE and currently capable of rates of up to 10-15 Mbps. A new version of WiMAX known as 802.16m or WiMAX-m provides rates of up to 1 Gbps.  WiMAX uses OFDMA for both the uplink and downlink. Clearwire is deploying WiMAX and will continue to do so for the next couple of years owing to agreements it has in place with its backers.

LTE

LTE (Long Term Evolution) is a competitor to  WiMAX and is being standardized by the 3GPP (3rd Generation Partnership Project), which operates under authority of the ITU.   LTE provides low latency and high data rates compared to 3G technologies and is similar to WiMAX. LTE-Advanced is is a new version of LTE providing data rates up to 1 Gbps.  The 3GPP is still working on the LTE-Advanced specification, and services based on this new technology should be available in perhaps a few years.

Like WiMAX, LTE uses OFDMA for its downlink. However, a big difference between LTE and WiMAX is that LTE uses SC-FDMA for its uplink instead of OFDMA. A big advantage of SC-FDMA is its lower power consumption compared to OFDMA, so LTE should allow greater battery life in handsets/terminals.

A Cornucopia, but Some Still Go Hungry

Carriers have a plethora of choices when it comes to delivering broadband. What they deploy often has more to do with where they have been, rather than where they would like to go. Many subscribers, especially rural ones, would be happy to get anything at all.

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

Related posts:

1. Broadband Wireless Overview
2. Active Fiber Overview
3. Active Ethernet Overview

One common characteristic of Fiber to the Home network architectures is the use of bidirectional transceivers (BIDI) allowing the use of a single fiber to serve each home. One wavelength is used for downstream (CO to home), and another wavelength is used for upstream transmissions. Bidirectional transmission on a single fiber increases the cost of optical transceivers somewhat, but it reduces the quantity of fiber (and labor to splice the fiber) needed to serve a home by half.

Point-to-Point FTTH

Point-to-Point (P2P, also known as Active Fiber) is the simplest of all three FTTH fundamental architectures. With a P2P 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). The CO contains a high port count aggregation device (one port per subscriber) known as an Optical Line Terminal or OLT. 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). P2P has an advantage that no port is shared in any way, thus troubleshooting problems on the network is greatly simplified.  With P2P, 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.

Switched FTTH

A switched FTTH architecture (commonly Active Ethernet) has many of the advantages of P2P, but it dramatically reduces the number of fibers terminated in the CO. Of the three architectures, it has the potential to have the fewest fiber terminations in the CO, but it requires the largest investment in the OutSide Plant (OSP). To aggregate fibers delivered directly to subscribers, a switched architecture requires switches be installed in secured cabinets between the CO and the subscriber homes.

Passive Optical Network FTTH

A Passive Optical Network or PON network architecture is similar to the switched architecture, but it requires no OSP electronics. Instead, an optical splitter is used in place of the OSP switch. The splitter divides the light coming from the OLT, and it combines the light coming from the ONTs. This greatly reduces the cost of OSP aggregation since the splitter is inexpensive, requires no power and very little, if any maintenance. Maximum concentration of subscribers is generally limited to 32 per fiber (with a 32 port splitter) delivered to the CO, though up to 64x and even 128x splits are possible. BPON, EPON, and GPON are common types of PON networks in use today. 10G EPON and 10G GPON are new technologies to be deployed in the next few years. WDM PON is similar, but instead of a splitter, it requires an Arrayed WaveGuide (AWG) to divide wavelengths for individual delivery to subscribers. Note that a PON may support RF overlay analog video, which is generally not feasible in the other two architectures.

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

Related posts:

1. Active Fiber Overview
2. Active Ethernet Tutorial
3. Active Ethernet Overview

Point-to-Point (P2P) Ethernet/Active Fiber

P2P Ethernet/Active Fiber is just 100Mbps or 1Gbps fiber Ethernet delivered directly from a Central Office (CO).  Active Ethernet is essentially P2P Ethernet/Active Fiber delivered from aggregation equipment installed outside the CO.  An Active Fiber/P2P Ethernet network is shown below with the Optical Line Terminal (OLT) installed in the CO and Optical Network Terminals (ONTs) installed at each subscriber’s house.

Active Ethernet

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

Optics

Two single fiber and two dual fiber interfaces are commonly used in Active Ethernet networks as described below.

Dual fiber (not bad, less expensive transceivers but lots of fiber)

• 1000BASE-LX – 1 Gbps over two single mode fibers (1310 nm in both directions)
• 100BASE-LX -100 Mbps over two single mode fibers (1310 nm in both directions)

Single fiber (better, same rates, half the fiber, but more expensive transceivers)

• 1000BASE-BX – 1 Gbps over one single mode fiber (1490 nm from network to subscriber and 1310nm from subscriber to network)
• 100BASE-BX – 100 Mbps over one single mode fiber (1490 nm from network to subscriber and 1310nm from subscriber to network)

Advantages and Disadvantages

What are the advantages of Active Ethernet?  1) Large bandwidth is available directly and unshared to each subscriber, at least 100Mbps and perhaps 1Gbps on each Optical Network Terminal (though shared on the uplinks to the aggregation equipment), 2) It is easier to manage because a network transceiver (in the ONT)  is dedicated to each subscriber, and 3) It is a relatively mature technology with a variety of equipment choices.

What are the disadvantages of Active Ethernet? 1) 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, and 2) Active Ethernet imposes the cost of a network transceiver dedicated to each subscriber.

Lightwave has an article comparing Active Ethernet to PON written by World Wide Packets, now part of Ciena.

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

Related posts:

1. Active Ethernet Overview
2. Active Fiber Overview
3. Three Fundamental Architectures for FTTH