Last Mile Lexicon

The crucial last mile * What's there now * How it's changing * What's ahead * Who's involved

November 20, 2000

20 Min Read
Last Mile Lexicon

The last mile, first mile, local loop, access network: Whatever you choose to call it, the meaning's the same -- that part of the telecom network that links users with broadband services.

And it's a crucial part. Today's optical networks may be big enough and fast enough to span the globe, but their ultimate worth will be gauged by how well they deliver the broadband goods to residential and business customers. Without subscribers, costly optical gear becomes a carrier liability, not an asset.

Unfortunately, the last mile is the most primitive portion of the telecom network. Sources estimate that fewer than 5 percent of all office buildings and homes in the U.S. have fiber connectivity. The majority of broadband customers are battling for bandwidth over old-fashioned copper-based PSTN and leased lines.

Getting these users up to optical speed is a key priority for vendors and service providers interested in staying profitable. The result is a rapidly changing landscape for business and residential access.

In this report, Light Reading sets out to describe this evolution in basic terms. First, we describe the present architecture used to deliver fiber-based broadband services to business customers. Then we talk about how this is changing and give our take on what tomorrow's business last mile will look like. We then do the same for the residential network.

To help things along, we've constructed a chart defining key elements of the access network and who makes them. And we've compiled a glossary to help readers stay on track. A list of vendor Websites is provided for easy lookups.

Read the report sequentially, or click on any of the pages hyperlinked below:

Business Fiber Today
Business Fiber Tomorrow
Residential Fiber Today
Residential Fiber Tomorrow
Last Mile Glossary
Vendor List
NOTE: We'll be treating the last mile chiefly as it applies to North America. Although there are similarities in Europe, different issues may apply.

Today's business fiber networks have two characteristics that make them complicated and expensive for handling broadband data traffic:

  • Reliance on Sonet rings in the fiber-based core and edge portions of the service provider network;

  • Copper connectivity in the access portion of the network.

Business Fiber Today Let's delve into both issues, taking Sonet first. While Sonet is great for voice networking, it's got some key drawbacks when it comes to running data and voice to business customers. The main reason for this is its rigid architecture, which relies on graduated series of redundant, fixed-bandwidth circuits to transmit data across fiber optic links.

Sonet circuits are created by multiplexing specified numbers of synchronous transport level 1 (STS1) signals, which operate at 51.84 Mbit/s and are roughly equivalent in bandwidth to DS3 channels -- the 45-Mbit/s signaling increments that are used in traditional TDM-based telephone networks.

Add/drop multiplexers (ADMs), attached to the carrier's core network by digital access crossconnect switches (DACS), are used to take STS1s and concatenate them into specific types of Sonet Optical Carrier (OC) circuits, set up in ring fashion to assure that each circuit will be automatically backed up in case of a fiber break or equipment failure.

Because the Sonet signaling hierarchy is rigidly defined, carriers don't have much flexibility in divvying up bandwidth for specific types of services. All customer requests must be met within the increments of the Sonet standard. It's not possible, for instance, to cut an OC3 in half if an ISP customer of a carrier's carrier wants a limited supply of bandwidth.

Sonet's rigidity also makes filling bandwidth orders a costly and complex process. Skilled fiber optic technicians must manually configure distinct channels at each add/drop multiplexer in the portion of the Sonet network where service is being activated. Corresponding changes must be made to ADMs that bring Sonet services to customer sites.

At those customer sites, the second big drawback of today's broadband networks must be addressed: Businesses must deploy a series of devices in order to convert Sonet signals to electrical bandwidth for use in their copper-based networks.

As shown in the diagram, many large metropolitan business customers have ADMs in their basements or data centers. These devices take Sonet bandwidth and convert it to DS3 increments. These DS3s in turn are fed into devices called M13 multiplexers that in turn convert DS3s into multiple DS1 increments for use in traditional copper-based TDM networks. From there, the DS1s are converted to T1 format for use in business applications.

Additional devices are required to support the different protocols and modulation schemes required to support data, voice, and video.

Large multisite metro area businesses, for instance, might have a full-fledged Class 5 telephone switch installed at headquarters, with multiple T1 circuits terminating in CSU/DSUs and PBXs at remote locations. Data traffic is typically handled via routers. And ATM switches are used to convert bandwidth to handle multimedia applications or to simultaneously handle voice, data, and video.

There are many variations on this scenario. Carriers who specialize in selling bandwidth to other service providers, for instance, typically install ADMs in outdoor cabinets or controlled environmental vaults (CEVs) buried under streets or in manholes. These ADMs are linked to fiber muxes, which convert multiple DS3s to TDM-based leased lines that use copper media to serve business customers.

Optical networking techniques are set to change access to business fiber networks by eliminating the need for Sonet and economically extending the carrier fiber insfrastructure closer to customers.

Business Fiber Tomorrow In emerging scenarios, new kinds of devices are linking the Sonet network with the business customer, replacing the complicated series of traditional ADMs that now prevail.

Leading the pack are so-called multiservice provisioning platforms (MSPPs) from a growing list of vendors. Attached to the edge switches and sometimes directly to DACS, these platforms take Sonet bandwidth in one end and apply it to a variety of dynamically definable interfaces at the other. The bandwidth is then picked up by routers, switches, and integrated access devices (IADs) for further distribution to business customers.

MSPPs are meant to be deployed in carrier central offices and outside plants. They're also showing up in the basements of buildings, particularly when used with new-style multitenant building local exchange carriers (see The Big Idea).

Besides short-cutting the Sonet hierarchy, a key feature of MSPPs is their support of DWDM (dense wavelength-division multiplexing) and their ability to interact with routers and switches also equipped with DWDM, either in the CO or at the customer premises.

MSPPs also support Layer 2 or 3 data intelligence, giving carriers the flexibility to add and subtract bandwidth on command -- a requirement for provisioning services like virtual private networks (VPNs). (For more on MSPPs and the vendors who make them, see Sonet Goes POP.)

Ultimately, MSPPs provide a way for carriers to meet business customer requirements faster and more easily than they can at present. For one thing, MSPPs offer a way to deliver Ethernet from the carrier network -- an increasingly crucial feature for doing business in the metro space (see Metro Optical Ethernet).

MSPPs also give carriers a way to deploy DWDM to extend their fiber economically. For instance, since DWDM can be used to run bandwidth over multiple channels in one fiber strand, it eliminates the costs associated with installing separate dedicated fiber channels with multiple amplifiers.

There are other means of extending the fiber broadband network (with or without MSPPs) that also are starting to get attention. One of these is the passive optical network (PON), a solution that's still in the experimental stages, mostly at RBOCs like BellSouth Corp. (NYSE: BLS) and SBC Communications Inc. (NYSE: SBC) that have a burning need to make the most of existing fiber.

In PONs, light broadcast from a special switch called an optical line terminal (OLT) is sent down a length of fiber, then split by a coupler into multiple connections for use in businesses or homes (see PONs: Passive Aggression). PON OLTs can be housed in central offices, in the outside plant, or in the basements of telco customers. Some PONs support so-called coarse wavelength-division multiplexing, in which four to eight distinct channels are carved out of a single fiber strand.

Carriers also are looking to wireless technologies to expand the reach of fiber. A number of approaches have emerged in this area, incuding the use of free space optics, millimeter/microwave technology, and other licensed and unlicensed techniques. (For a full description of these, see Wireless Wonders.)

All of these emerging solutions for the business local loop are still largely in the developmental stages, and because of this they have some kinks that need to be worked out. MSPPs, for instance, often feature multiple switching fabrics in order to support a broad range of traffic types. This can make them costly as well as complex. PONs are still limited in distance and capacity. And wireless devices face a range of obstacles, such as sensitivity to environmental forces.

Still, tomorrow's business network looks to be shaping up as a less complicated, more flexible, and exponentially more powerful environment for all kinds of online services.

Residential broadband access is a potentially huge market for carriers. Sources estimate there are more than 1.2 million high-rise residential buildings in the U.S. today, fewer than one percent of which have fiber connectivity. Added to the huge number of single-family dwellings, the market is sizeable indeed.

Two key problems are holding back the progress of optical connectivity in the residential market:

  • An almost total lack of fiber to customer premises;

    • Multiple connections from different providers.

      A glance at the existing landscape clearly shows the problems involved.

      Residential Fiber Today Most of today's residential networks were built with voice access in mind, so plenty of copper wiring is in place between the telcos' points of presence and most homes in North America. These voice circuits are served by Class 5 telephone switches fed by Sonet rings in telco central offices or underground CEVs (controlled environmental vaults). ADMs in the carrier plant are used to break out multiple analog connections to neighborhoods for residential use.

      Most telcos use existing copper facilities to extend data services to their customers. They do this by using techniques such as digital subscriber line (DSL) to move high-speed digital data over analog phone lines. DSL is implemented via DSLAMs (DSL access multiplexers), devices that control the modulation and flow of voice and data between the carrier network and multiple copper connections. DSLAMs are typically housed in the central office or CEV.

      DSLAMs can be standalone devices or modules that fit into chassis called digital loop carriers (DLCs). DLCs convert carrier fiber to copper-based services and typically reside in the CO or CEV next to a splice case, a device that opens up multiple fiber connections on the carrier side for conversion to copper on the customer side.

      DSL is still limited in distance and speed, compared with fiber. Most DSL services today support about 1.5 Mbit/s data delivery -- less than a tenth of what fiber can carry. Also, most carriers don't offer DSL services for video, although some trials are in progress.

      Video and high-speed data also are offered by cable TV operators that own coaxial cable connections to multiple residences. These links use radio-frequency signals to support bursty video traffic on connections that are shared among many home subscribers.

      To offer high-speed data, cable TV operators replace the coax cable with fiber in the links between their headends (the equivalent of a central office in a cable TV network) and the neighborhood boxes that serve multiple subscribers. This creates a hybrid fiber coax (HFC) network.

      To access the HFC net, a customer installs a cable modem at his or her PC to access data over the coax connection to the home. Alternatively, a set-top box can be used to split the coax link into separate connections for TV and Internet access. Back at the cable TV operator's headend, platforms called cable modem termination systems (CMTSs) manage multiple links back to residential cable modems and set-top boxes.

      The end result of this setup is that residential customers who want broadband services must typically pay two different subscribers for one to three separate connections into their homes. This can run some customers close to $200 per month -- the typical limit most carriers say residential customers are willing to pay for services. That puts a ceiling not only on what service providers can offer customers, but on customers' ability to pay for the services.

      The residential last mile is changing fast, as carriers and cable operators see the need to offer fewer but more powerful connections to individual homes.

      Residential Fiber Tomorrow In this emerging scenario, devices are evolving to help carriers offer a broader range of services over fewer connections. DSLAM vendors are exploring video over DSL specs. DLC vendors like Alcatel SA (NYSE: ALA: Paris: CGEP:PA) and Marconi Communications PLC (Nasdaq/London: MONI) are recasting their gear to do much more than just handle multiple DSL links, which was their traditional role. Marconi, for example, says its Disc*s series supports voice, Ethernet, cable TV services, and DSL via separate modules that fit into a single chassis.

      By developing new multitasking equipment, carriers can reduce the costs of maintaining lots of equipment in order to offer different types of services. The savings can be passed on to customers in reduced costs that make it more practical to have several copper or coax links to the home. Alternatively, the savings can be used to help extend the broadband fiber infrastructure.

      While it's still too expensive to bring "fiber to the home" or to run individual, dedicated, fiber optic links to homes that now have copper, a number of service providers are looking at ways of bringing the fiber in their networks closer to homes through the use of broadband wireless and PON technology.

      This alternative is often called "fiber to the curb," because it involves extending fiber in the carrier network to regional or neighborhood hubs housed in outdoor cabinets or underground CEVs next to large residential buildings or developments. These locations can then be outfitted with wireless gear or with PON OLTs and splitters. This helps bring optical bandwidth to individual floors of a building or to groups of homes where no fiber exists, or where many users must share a single fiber connections.

      While it's doubtful that cable TV operators will concede to let traditional telcos dominate the market for broadband home services, it's probable that in the future, new techniques will encourage some outfits to become "multiservice operators," offering voice, data, and video services over single high-speed connections. These new MSOs will strike deals with cable TV operators to obtain rights of way over the cable network, or to bring new services to existing cable TV subscribers.

      ADM: Add/drop multiplexer. A device that manages multichannel Sonet or TDM links, picking up and assigning individual channels to a particular group of channels with a specific destination.

      Cable access router: An access router designed to deliver Ethernet services over cable TV networks. Most cable access routers are compatible with DOCSIS specs (q.v.).

      Cable modem: An external device or card that links to a PC to bring Ethernet data services from the cable TV network to SOHO (small office/home office) customers or residential users.

      CMTS: Cable modem termination system. Device in the headend of a cable TV network that channels data traffic to residential customers over the hybrid fiber coax network. CMTSs achieve their task by applying quadrature amplitude modulation (QAM, q.v.) to the radio frequencies used for TV broadcasting. This technique conforms to the DOCSIS specifications (q.v.) and allows home or small business users to access the CMTS via an Ethernet link to their cable modem.

      CEV: Controlled environmental vault. A large, underground water- and weatherproof enclosure containing telecom transmission gear.

      CWDM: Coarse wave division multiplexing. The technique of running a small amount of wavelengths (typically four or less) over a single fiber optic connection. (See DWDM

      DLC: Digital loop carrier. A device that links services from carrier fiber to subscriber copper. DLCs are usually housed in the point of presence office or in an outdoor cabinet about 9,000 to 12,000 feet from a cluster of residences. Each DLC serves 600 to 2,000 subscribers

      DOCSIS: Data over cable systems (or service) interface specification. A series of specs that define how data can be carried in Ethernet format over cable TV lines. Originated and regulated by Cable Television Laboratories Inc. (Cablelabs), a consortium of North and South American cable TV operators.

      Dropline: The portion of the access network, usually copper, that directly serves customers.

      DSLAM: Digital subscriber line access multiplexer. A device that controls the modulation and flow of voice and data between the carrier network and multiple copper connections.

      DSL: Digital subscriber line. A technique for running high-speed data over analog copper phone lines and supporting voice and data on the same connection. There are different flavors of DSL for transmission of two-way voice, video, and data.

      DWDM: Dense wavelength-division multiplexing. The technique of running many (at least four or more) individual wavelengths of light over a single fiber optic connection. DWDM wavelengths typically operate at 1310 nanometers or 1550 nm, depedning on the number of wavelengths or "channels" driven over the fiber.

      Fiber multiplexer (fiber mux): A device that converts multiple DS3s from fiber to copper for business use.

      Fiber node: Also known as a hub in cable TV network parlance. Generally, the location in the cable TV provider's network where fiber is converted to coax for delivery of service to home or small business users. Usually housed in a box mounted outdoors on a stand or pole.

      Fiber to the curb: Refers to the scenario when fiber is pulled from the CO to a so-called fiber node or to a high-rate digital terminal in an outdoor cabinet alongside homes or office buildings.

      FTTH: Fiber to the home. Refers to providing last-mile fiber connectivity directly to residential users, instead of relying on coax or copper. Today's FTTH deployments are generally limited to some residential PON (q.v.) trials being undertaken by RBOCs (regional Bell operating companies) such as BellSouth, although a range of smaller independent telcos and CLECs also have installed FTTH lines.

      Fiber to the node or neighborhood: A general term that refers to the scenario when fiber is pulled to a larger hub or ONU (q.v.), in which fiber is linked to multiple copper or coax lines, typically serving about 200 residential or small business customers with a radius of about 3,000 to 4,000 feet.

      GR-303: A Telcordia interface that provides a generic, nonproprietary link between any conforming vendor's central office switches and third-party DLCs (q.v.).

      Headend: The cable TV provider's equivalent of a central office. The location where TV and data broadcast signals originating in the provider's core network are transmitted to and from a network of hubs or nodes. Most headend locations serve multiple hubs in a radius of about 100 miles. A headend is typically housed in a building or data center and can be shared among many providers.

      HFC: Hybrid fiber coax. A network in which a cable TV operator has replaced the coax connections that link its headend to neighborhood nodes with fiber -- typically to support data connectivity for its customers.

      IAD: Integrated access device. A CPE device, typically ATM-based, that converts Sonet bandwidth to a variety of local interfaces for use on customer networks.

      M13 multiplexer: A multiplexer specifically designed to aggregate up to 28 DS1 channels operating at 1.544 Mbit/s into DS3s operating at 44.736 Mbit/s.

      MSO: Multi-service operator. A term coined for a new breed of service provider that offers cable TV, voice telephony, and data services.

      Multimode fiber: Fiberoptic cable with a typical core diameter of 25 to 200 microns. This core is larger than that of singlemode fiber and allows for relatively inexpensive coupling with cheaper light sources. Multimode fiber's chief drawback is that it only supports short distances -- up to about 2 kilometers.

      MSPP: Multiservice provisioning platform. A system that combines Sonet connectivity, DWDM, and standard Ethernet and CPE interfaces with Layers 2 and 3 data intelligence in order to help carriers provision a range of services dynamically to customers. Used to bypass the Sonet network.

      OLT: Optical line terminal. A switch used in PONs (q.v.) to manage the two-way multiple shared connections created by the PON splitters. Also known as the headend equipment connected to a PON.

      ONU: Optical network unit. A generic term meaning any device that converts optical signals over fiber to copper-based electrical signals, typically within 1,000 feet of a residence or business. ONUs are key elements of "fiber to the curb" approach to access. The ONUs in that case sit inside outdoor controlled environmental vaults or remote terminals.

      Pair gain: The situation that results when a set number of twisted pair wires serve more subscribers than their physical capacity permits, as a result of the fact that subscribers don't use the pairs at the same time.

      PON: Passive optical network. A network in which bandwidth traveling over fiber is shared among multiple users, via the use of splitters. PONs are typically viewed as an economical alternative to running dedicated fiber to home or business customers.

      QAM: Quadrature amplitude modulation. The technique used by modems and other gear to squeeze digital data into analog format for transmission on copper phone lines.

      Set-top box: Device used to extend the capabilities of a user's TV to handle voice, data, or more TV channels

      Singlemode fiber: Fiber optic cable made of a single strand of glass, typically 5 to 25 microns in diameter, which supports transmission rates over distances ranging from about 15km to 45km. Most carrier networks are comprised of singlemode fiber, which is converted to multimode fiber through CPE (customer premises equipment) converters inside routers or in small boxes on customer premises. Singlemode fiber used to be significantly more expensive than multimode fiber, but the difference has lessened with the increased use of singlemode.

      Following is a list of hyperlinks to Websites of vendors mentioned in this article that are involved in last-mile technology:

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