Fiber

Fiber

As you've read many times in this book, fiber is an area of rapid evolution. Few other media options promise to offer the capacities of fiber. Where its deployment is possible, fiber is the clear-cut best solution. Fiber can be used on its own or in conjunction with twisted-pair, coax, and wireless to provide broadband access. FTTC is a solution in which fiber is run very close to the home to the curb and coax runs from the curb to the home. In addition, all-fiber networks can be used to deliver broadband services. Fiber to the home (FTTH) goes a step further than FTTC it brings fiber into the residence. In addition, a new generation of technologies, called passive optical networks (PONs), promises to dramatically reduce the cost of deploying FTTH.

FTTC

FTTC is also known as Switched Digital Video and Switched Digital Broadband. In deploying FTTC, the service provider has looked to the future the capability to support the time-sensitive and high-capacity requirements of interactive multimedia, interactive television, and all the other advanced applications that involve the senses. The FTTC architecture involves laying fiber that offers approximately OC-3 (that is, 155Mbps) bidirectionally from the local exchange to the host digital terminal (HDT). The HDT is involved with traffic supervision and maintenance functions over a number of downstream optical network units (ONUs), also called optical network terminations, which are where the optical-to-electrical conversion takes place. From the HDT to the ONU the downstream rates are up to 52Mbps and the upstream rates are up to 20Mbps. Each ONU serves from 4 homes to 60 homes, and twisted-pair (VDSL or another member of the DSL family) or one of the other media types runs from the ONU to the home.

In provisioning FTTC, some service providers initially laid coax as an underlay because currently the vast majority of content is old analog film archives. In order to run this rich history of entertainment over the digital FTTC network, it is necessary to digitize all that content in order to store it on digital video servers, deliver it digitally over transport, and, at the customer's premise, have digital set-tops that undigitize and decompress the video for viewing over today's sets. In those early stages of digital interactive content, it seemed reasonable to provide coax, with the intention of delivering entertainment or television services and with the thought that if the content becomes digitized to run on an integrated basis over the fiber, that coax could continue to be used for purposes of carrying power. Therefore, some FTTC deployments actually have a second wire, the coax.

The topology of FTTC is a switched star, which means there are dedicated point-to-point connections. It is not a shared infrastructure, so what bandwidth you have is yours and yours only; you have the privacy of having a dedicated link. FTTC security is best addressed with mechanisms that involve encryption, authentication, and public key exchange, but remember that it is more difficult for someone to tap into fiber than to tap into other media. Of course, it can be done, but at great expense, so the often-spoken-of security benefits associated with fiber largely address the difficulty of tapping into the fiber and the improved capability to detect that intrusion (with proper test equipment, you can see when there is a leak in the fiber).

The modulation approach used to integrate the voice, data, and video streams is Time Division Multiplexing (TDM), and the TDM signals are digitally transported over the fiber, backed by ATM-based switching. Remember that ATM provides opportunities for traffic engineering and management and the administration of QoS, which are critical for providing high-quality network services and for being able to meet the SLAs that all providers must have with customers.

Figure 13.6 illustrates an FTTC configuration. At the home, the network termination (NT) splits and controls signals so that the appropriate voice signals go to the telephones, the video programming goes to the set-top boxes, and Internet access is enabled at the PC. A twisted-pair runs to the ONU, and in some cases, a coaxial cable takes a different path onto the headend of the cable TV provider. From the ONU, fibers converge on the HDT, which manages the group of ONUs. From that HDT, private-line traffic can go over the digital cross-connect system (DCS), voice traffic can be switched through the traditional Class 5 offices onto the circuit-switched PSTN, and high-speed multimedia (QoS-sensitive traffic) can go through the ATM switches to the ATM backbone.

Figure 13.6. An FTTC configuration

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The modulation scheme for FTTC was developed by Bellcore (which is now Telcordia) and is standardized by both the ITU and DAVIC. As many as 1,400 channels are possible in this architecture. Why would you want 1,400 channels, when surfing through 50 can present a problem? In public forums worldwide be they sporting stadiums, concert halls, opera houses, or outdoor venues video cameras are being placed so that you can participate remotely in more and more functions and also so that you can control your viewing angle. Perhaps you are watching a football game, but you're not interested in the player that the film crew is filming. Instead, you want to zoom in for a better look at the team's mascot. With cameras strategically positioned throughout the stadium, you would be able to take on whatever viewing angle you wanted, but each angle would require its own individual channel. So the availability of thousands of channels has to do with the ultimate control that interactive TV promises. It's not about video-on-demand and getting programming when you want and the ability to stop it; it's about being able to control what you see when you see it, in what language you see it, and who is in it. For example, you might want to watch Gone With the Wind, but you don't want to see it with Clark Gable and Vivien Leigh. You would like to see it with Mel Gibson and Joan Chan. In the digitized interactive environment, you would go to the digital thespian bank and deposit the bits you want into the film. You could apply the soundtrack you like and apply the language that's most suitable.

The multichannel FTTC architecture promises to deliver on much more than just broadband access for Web surfing. It allows the ultimate control over the viewing environment. However, as mentioned earlier, it does require that all media streams be digitized, so you need digital video servers at the system headend and digital set-top boxes at the subscriber end. Consequently, this environment has a higher per-subscriber cost than does the simpler HFC environment. But it offers the promise of easier upgrading to interactive broadband, full-service networks; in such an environment, carriers would be better prepared to offer a multitude of new revenue-generating services over the single integrated platform. So, in many ways, FTTC is a solid solution, but it depends to a great extent on the last 1,000 feet (300 meters) being a copper-pair technology that can sustain the data rates to support the applications we're discussing. Of course, this piece of the puzzle is still under development.

FTTH

FTTH is an all-fiber option that provides a minimum of 155Mbps in both directions. As shown in Figure 13.7, it involves fiber from the service node to the optical splitter. From the optical splitter, multiple fibers fan out to terminate on single-home ONUs. So, similarly to FTTC, FTTH is a point-to-point architecture with a dedicated connection from the home to the network, which offers secure transmissions by virtue of the fact that it's challenging to intercept activity on the fiber network. This architecture is definitely suited for broadband service applications and is an integral part of the broadband future we are preparing for.

Figure 13.7. An FTTH configuration

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FTTH also provides a very robust outside plant, meaning extremely low maintenance cost over the lifetime of the system. For many operators of traditional wireline networks, 25% of the cost of operating the network goes to maintenance. Fiber-based networks require considerably less maintenance than do traditional networks, so there are long-term operational savings associated with fiber networks. This is one reason why fiber is such a favored wireline solution. In some parts of the world, fiber already justifies itself economically. However, the cost of deploying FTTH is still quite high (around US$2,500 to US$3,000 per subscriber), but the cost is predicted to drop to US$1,700 or less in the next year or so, and then fiber will begin to compete effectively with some of the other traditional wireline options, such as DSL and HFC. These figures assume new installations, where a big part of this cost is in the construction. This is always the issue with wireline: 80% of the cost of delivering the service is in the construction effort. In wireless, the situation is reversed the construction is only 20% of the cost. As the cost of deploying fiber drops, FTTH will become an increasingly attractive solution because of the bandwidth and the low noise that it offers.

PONs

The newest fiber option is the PON (see Figure 13.8). A PON allows multiple buildings to share one access line. PONs bundle together multiple wavelengths up to 32 wavelengths currently and carry them over a single access line from the carrier's local exchange to a manhole, called a controlled environmental vault (CEV), that is close to a group of customer sites. From the CEV, the wavelengths are broken out, and each one is steered into a different short length of fiber to an individual site. From the customer to the local exchange, each site is given a specific time slot to transmit, using a polling scheme.

Figure 13.8. A PON configuration

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A key benefit of PONs is that they extend the use of fiber to business customers. Also, provisioning PONs is comparatively easy because multiple fiber connections can be provisioned from a single connection in one location. PONs also offer bandwidth flexibility. Whereas SDH/SONET provides fairly rigid rates, resulting in costly upgrades, PONs allow bandwidth to be allocated quickly. Another benefit of PONs is that they consolidate leased lines. Furthermore, PONs can be configured to give exact increments of bandwidth as needed. The low deployment cost is a big incentive to using PONs and makes them a very attractive alternative to xDSL.

A key issue involved with PONs is that it's a shared-media environment. The upstream bandwidth is divided among a number of users that is, the fibers that are terminated by ONUs at the customer premises. For example, on a 155Mbps PON link with four splits, each subscriber receives 38.75Mbps, and the more customers you add, the less bandwidth each customer receives. PONs are also subject to distance limitations: According to the Full Service Access Network (FSAN) Coalition specifications, PONs have a theoretical distance limitation of about 12 miles (20 kilometers). The actual distance depends on the power of the laser used to transmit the light and the reduction in power that the light suffers along the way. There is a tradeoff between distance, bandwidth, and the number of sites supported by a single access line into the optical line termination (OLT).

The OLT is a special switch that has a nonblocking architecture. Blocking probability is an important measure of network adequacy. A nonblocking switch ensures that there is always a serving trunk available for each station, so no blocking or contention occurs. The OLT sends traffic downstream to subscribers and handles the upstream traffic as well. Downstream and upstream traffic use different frequencies on the same fiber to avoid interference. Traffic is typically sent in both directions at 155Mbps, although emerging products offer 622Mbps in both directions. Downstream, the OLT either generates light signals on its own or takes SDH/SONET signals from colocated SDH/SONET cross-connects and broadcasts this traffic through one or more outgoing subscriber ports. Upstream, the OLT aggregates traffic from multiple customer sites and uses TDM to ensure that each transmission is sent back to the local exchange over one fiber strand, without interference.

The outside plant includes passive optical splitters placed inside the CEVs. As the light that is broadcast from the OLT hits the splitter, it is deflected onto multiple fiber connections. Passive means the splitters don't need any power: They work like a prism, splitting light into the colors of the rainbow, so there is nothing to wear out or to go wrong. Today, splitters feature 2 to 32 branches, and they can be positioned to create PON star, ring, or tree configurations. PON networks terminate on ONUs, whose main function is to take the light coming from the passive splitters, convert it to a specific type of bandwidth such as 10Mbps or 100Mbps Ethernet, ATM, or T-1/E-1 voice and data and then to pass it on to routers, PBXs, switches, and other enterprise networking equipment. ONUs can be installed at the customer's data center site, in wiring closets, or in outside plant locations, where the subscribers can connect to the PON via DSL services. This enables carriers to offer PON service under existing DSL tariffs and gives customers the benefit of optical networking without installing new fiber.

PON standards emerged from the FSAN Coalition, which formed in 1995. The coalition decided to use ATM over a simple physical network, with a minimum of moving parts. By 1999 the ITU-T had approved specifications G.983.1 and G.983.2. Current trials of commercial PON equipment include projects sponsored by NTT in Japan; Bell Atlantic, Bell South, Comcast, and SBC in the United States; and Singapore Telecom.

The key issue with PONs is the availability of fiber. As mentioned in Chapter 3, there is a shortage of fiber in access networks. PONs also face distance limitations: Because they are passive meaning there is no electrical amplification or regeneration of light signals their distance is limited. Again, shared-media issues affect the actual rates that customers can achieve. Finally, product availability is a problem at this point. But we're still in early stages with PONs, and as the problems are worked out, PONs will be deployed more and more, and the revenue from PON products is expected to skyrocket over the next few years.

 



Telecommunications Essentials
Telecommunications Essentials: The Complete Global Source for Communications Fundamentals, Data Networking and the Internet, and Next-Generation Networks
ISBN: 0201760320
EAN: 2147483647
Year: 2005
Pages: 84

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