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Fiber in Broadband Access Networks

John Bartell — Tue, 20 Oct 2009 12:18:26 +0000

The most expensive element in delivering broadband Internet services over fiber optic cable is the access network, and its architecture largely determines how much bandwidth can be delivered to a subscriber. In brownfield installations (with copper local loop already installed), the most expensive element is the labor to install fiber.

There are three elements to a typical broadband fiber access network: the feeder portion, the distribution portion, and the drop cables to the subscribers’ premises. Where fiber is deployed in place of copper determines the fundamental architecture of the access network. For telcos, the three major architectures are Fiber to the Node (FTTN), Fiber to the Curb (FTTC), and Fiber to the Premises (FTTP), and each has a different proportion of fiber. FTTN and FTTC are hybrids combining fiber with copper local loops and are attractive owing to their cost-efficiency in brownfield installations. FTTP is entirely fiber from the wire center to the subscriber.

FTTN

If only the feeder portion of the access network has fiber optic cables, and copper local loops are served with DSL (Digital Subscriber Line) from the Feeder Distribution Hub, then the architecture is known as Fiber to the Node or FTTN (BT confusingly refers to this architecture as Fiber to the Cabinet or FTTC). The feeder portion of the access network is from the wire center (or Central Office) to a Feeder Distribution Hub. A typical network architecture for FTTN is shown below.

The picture below is of a FTTN installation about one mile from my house. TThe 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 (typically 2 distribution pairs for each feeder pair, which in this case are only a few feet in length).

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FTTC

If fiber is installed in the feeder and distribution portions of the access network, and DSL copper from there to businesses and homes, then this architecture is known as Fiber to the Curb or FTTC. The distribution portion of the broadband access network distributes fiber from the Fiber Distribution Hub to the drop portion of the network, which usually begins at the curb. The diagram below represents a typical FTTC network architecture.

FTTP

If fiber is installed in the feeder, distribution, and drop portions of the broadband access network, then the architecture is known as Fiber to the Premises or FTTP. FTTP is known as Fiber to the Home (FTTH) when serving residential subscribers and as Fiber to the Building (FTTB) when serving buildings such as multi-tenant office buildings. With FTTB, fiber enters the building, but subscribers are served directly with DSL or Ethernet. FTTB is especially common in Japan and Korea, which lead in broadband rates.

Common FTTP technologies include the various PON alternatives (Broadband PON or BPON, Gigabit PON or GPON, Ethernet PON or EPON, and Wave Division Multiplex PON or WDM PON), Active Ethernet, and Active Fiber. With PON, an optical splitter (Arrayed WaveGuide with WDM PON) is installed at the Feeder Distribution Hub with possibly another splitter at the curb. With Active Ethernet, an OutSide Plant (OSP) Ethernet switch is installed at the Fiber Distribution Hub. With an Active Fiber network, only fiber cables and cross-connects are installed in the access network between the wire center and subscribers’ premises.

In brownfield applications, FTTP is the most expensive alternative for a broadband access network. The most costly element per subscriber is the fiber drop cable serving that subscriber’s premises, and the attraction of FTTC is that it avoids this cost. In greenfield installations, FTTP is not significantly more expensive (if at all) compared to copper or coax. A representative FTTP architecture is shown in the diagram below.

Hybrid Versus FTTP

All three fiber access architectures can deliver broadband rates to subscribers. The two hybrid alternatives are generally less expensive, especially in brownfield installations, but also less capable. With fiber rates measured in Gbps, and DSL in Mbps, more fiber equates to greater potential broadband rates. FTTP, of course, provides the ultimate in broadband access.

Fiber to the Curb (FTTC) Overview

John Bartell — Fri, 11 Sep 2009 06:30:57 +0000

Fiber to the Curb (FTTC), like Fiber to the Node (FTTN), is a fiber and copper local loop architecture for delivering broadband Internet to subscribers. With FTTN, fiber is installed to copper local loop aggregation points known as Feeder Distribution Interfaces (FDIs), which tend to be up to about 5000 feet from subscribers. In contrast, with FTTC, fiber is installed to a lawn pedestal typically within 500 feet of the 8-12 subscribers it serves. This proximity to subscribers allows for much higher average DSL rates than are possible with FTTN.

FTTC’s higher DSL rates do come at a cost. Especially in overbuilds, an FTTC network is more expensive to deploy than a FTTN network. This is due to the additional fiber and more numerous equipment locations, but both are less expensive than a Fiber to the Home (FTTH) network architecture.

With FTTC, a carrier avoids laying fiber to each home, which is the most expensive part of deploying an overbuild FTTH access network. Several homes are served with a single fiber interface installed to what is essentially a small DLC placed on the lawn of a subscriber. Another cost advantage for FTTC is that it does not require an expensive Optical Network Terminal (ONT) at each home or the ONT batteries required for “lifeline” telephone service . A subscriber served with FTTC can use an inexpensive DSL modem to get broadband Internet service and regular analog phones for voice.

A typical FTTC network architecture is shown in the drawing above. Electronic pedestals are installed on the lawns of subscribers (usually near the curb, hence the name). Each pedestal serves 8-12 homes with DSL and POTS, is typically network powered, and connects to an upstream aggregator. The FTTC pedestals can connect to an aggregator installed in an OutSide Plant (OSP) cabinet as shown above, or the pedestals can be served directly from the Central Office.

BellSouth extensively deployed Tellabs/AFC/Marconi/Reltec FTTC equipment (FTTC was a real hot potato) before BellSouth became part of AT&T. Note that BT also uses the term FTTC to describe its VDSL2 deployments. However, in BT’s usage of the term, FTTC stands for Fiber to the Cabinet, and this is virtually identical to what is called FTTN in the US.

VDSL2 Versus DOCSIS Smackdown

John Bartell — Fri, 04 Sep 2009 12:23:51 +0000

Light Reading is reporting that Cox is offering 50 Mbps DOCSIS cable modem service in Arizona on Cisco gear to counter Qwest’s 40 Mbps VDSL2 service offering. Prices are $89.99 per month for Cox’s DOCSIS-based offering and $109.99 per month for Qwest’s VDSL2-based offering. The informal poll on FTTxtra indicated that the majority of participants were willing to spend as much as $100 per month on broadband access service, so the prices Cox and Qwest are charging make sense as initial conditions.

The bulk of HFC electronics are deployed in OutSide Plant (OSP) cabinets serving a neighborhood or portions of a large neighborhood, and with DOCSIS, all of this substantial investment is retained by the MSO. These node cabinets are fed with fiber and serve many homes with telephone, DOCSIS cable modem, and television services over coax. A typical HFC node cabinet (this is the actual one serving my neighborhood) is shown below.

HFC Cabinet

The picture below shows the power, telephone, and cable connections into my house. The grey box is the HFC coax NID, which is span powered from the cabinet above. The HFC NID has a coax connection to a three-way splitter installed above the NID and a two-wire telephone connection to the brown box on the top right.

Telcos often deploy VDSL/VDSL2 services in FTTN (at the node) or FTTC (at the curb) architectures. The length of some copper local loops served from a FTTN node (typically up to 5000 feet or so, depends on the telco) tend to be a bit long for VDSL2, but sometimes FTTN is a telco’s only feasible option for installing VDSL2.

FTTN VDSL2 nodes are installed in locations with access to the copper local loop, usually through what is called a Feeder Distribution Interface or FDI, shown below. The DSL electronics accessing the loops at this FDI are housed in the beige boxes located behind the FDI. The FDI houses no electronics, just cables and cross-connects. A typical concentration level at an FDI is one upstream copper pair (feeder pair) for two downstream (distribution) copper pairs, so on average, before any electronics are installed at the node, only perhaps half of the local loops delivered to subscribers are already in use.

DSL Feeder Distribution Interface (FDI)

If you want to see what an HFC network can do with broadband, do not miss the video at the end of the article on DOCSIS 3.0 showing a speed test on a Comcast DOCSIS 3.0 network.

VDSL2 Overview and Tutorial

John Bartell — Tue, 01 Sep 2009 18:24:56 +0000

VDSL2 (Very high bit rate Digital Subscriber Line 2), a performance-improved version of the original VDSL now known as VDSL1, is standardized by the ITU in G.993.2. A relatively new local loop access technology, VDSL2 provides broadband access rates up to 100 Mbps over twisted pair copper. VDSL2 is important mainly because it leverages all that expensive copper local loop in the ground to provide a compelling broadband Internet service. It is well suited to deployments on the curb (FTTC) or in a building (FTTB) and can be used for Fiber to the Node (FTTN). The diagram below shows a typical VDSL FTTC network architecture.

VDSL2 faces a rather large challenge, however, because many copper local loops are too long to make effective use of all the capabilities of VDSL2, which relies on higher frequencies that are filtered out on longer local loops. VDSL2 chipsets provide a fallback mode to ADSL2+ because of this. ConvergeDigest offers the graph below showing a comparison of ADSL2+ and VDSL2 performance over distance. According to this graph, it is obvious that VDSL2 provides only marginal benefits over ADSL2+ beyond 4000 feet and no benefit beyond 7000 feet.

VDSL2 will often be deployed at a Feeder Distribution Interface (FDI) because of easy access to local loops at this point, though some of the local loops can be a bit long for optimal VDSL2 performance. A picture of an FDI (just about a mile from my house) is shown below.

When VDSL2 is deployed at an FDI, this architecture is called Fiber to the Node or FTTN. A typical FTTN network architecture is shown in the drawing below.

VDSL2 is implemented with what are called profiles, which specify what subset of VDSL2’s total capabilities are to be used in a particular installation. Differerent profiles are optimized for different situations. Some profiles are for Central Office (CO) VDSL2 deployment, some are for OutSide Plant (OSP) cabinet deployments of VDSL2, and some are for in-building deployments of VDSL2, especially the higher rate ones. VDSL2 profiles 8a, 8b, 8c, and 8d use 8 Mhz to provide data rates up to 50 Mbps. Profiles 12a and 12b use 12 Mhz to provide bandwidths up to 68 Mbps. Profiles 17a (17 Mhz) and 30a (30 Mhz) provide bandwidths up to 100 Mbps, but are intended only for very short distances such as those common with in-building (FTTB) VDSL2 installations (see below).

VDSL2 is expected to be more successful than VDSL1, which has been around a while but has seen only limited deployments. The following table, from RubyTech, compares VDSL1 and VDSL2.

VDSL and VDSL2 Comparison

Sunrise Telecom offers this screenshot below from their MTT – ACM II to show how VDSL2 is susceptible to bridge taps. The screenshot also shows how other technologies like ADSL and ADSL2+ do not require the same range of frequencies as VDSL2 does, though VDSL2 can operate at higher levels of attenuation. A bridge tap 15 feet in length created the notch shown and centered at about 9.7 MHz.

Sunrise Telecom MTT - ACM II Screenshot

The following companies are prominent manufacturers of VDSL2 chipsets.

Ericsson provides more technical information in this article on VDSL2.

PON, VDSL, and DOCSIS 3.0 Demand Up

John Bartell — Wed, 26 Aug 2009 15:44:17 +0000

From a Dell’Oro Group press release issued today…

“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.