Best practices for optical network design

21 Nov 2008

Fiber-optic technology -- not long ago used only in long-haul networks -- has become the transmission medium of choice not only in the core, but in metro and access networks. The game-changer is global growth in consumer broadband and the need to distribute enormous amounts of content without hauling it halfway around the world.

This Telecom Insights guide to best practices for optical network design looks at access, metro and core network issues affecting fiber deployment including:

  • Recent developments in FTTX ('fiber to the whatever') deployment and passive optical networking (PON) technology in access networks
  • Factors affecting metro network use of SONET, WDM or Ethernet, and the services that will be supported by fiber deployment
  • Specific core optical network design considerations including aggregation, geography and reconfigurability.

In this series:

Fiber-optic networks: Access network design

Optical networks: Metro network design best practices

Optical networks: Core network design best practices

Fiber-optic networks: Access network design
by Tom Nolle

The rapid growth in consumer broadband seen worldwide today would not be possible without a major shift in the practices for provisioning access infrastructure. Copper loop and CATV cable were once the only means of transporting information from a provider central office or head end to the customer. Today, both these media are being 'shortened' or even eliminated by the use of fiber optics.

Fiber is not a new development in access networks. Not only has it been used for almost two decades in the provisioning of high-speed commercial/enterprise customers, service providers in the 1990s found that replacing large bundles of copper by a few fiber strands could improve service reliability and lower craft cost. BellSouth took the lead in deployment of access fiber in that period, and the move was justified completely on cost savings.

The traditional access fiber architecture has been the fiber remote, which is a high-speed fiber trunk (SONET or Ethernet) that terminates in an electro-optical multiplexer. In analog phone days, these were called 'digital loop carriers' (DLCs), and the term 'new generation DLC' was used for a time, but most such devices today deliver DSL services and so are usually called 'remote DSLAMs.' A remote DSLAM's primary benefit is to shorten the access copper to allow higher DSL speeds and improve reliability. Most providers would counsel against offering premium DSL on loops over 8,000 feet, and the highest DSL speeds may be achievable only on loops 1,000 feet or less in length.

Pushing fiber close to the customer is generically called 'deep fiber,' and various acronyms are used to indicate just how deep the fiber is. FTTH means 'fiber to the home,' which is the extreme of giving every user an optical-electrical termination. FTTC takes 'fiber to the curb,' serving a group of homes, while FTTN means 'fiber to the node' or 'neighborhood,' and allows each fiber remote to serve a larger population.

The problem with all deep fiber strategies, and the reason why providers don't simply run fiber to every home, is cost. If loops are kept to a length of 5,000 feet, a single remote can serve customers in an area of almost 2,000 acres.

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