Page 311 - Optical Communications Essentials

Optical Networks

Optical Networks 301

a group of lightpaths is set up together and kept in place in the network for long
periods of time. In dynamic routing a lightpath is set up for each connection
request as it arrives and is released after the requested call is over.

17.6. FTTx

As noted in Sec. 17.1.2, an access network refers to the connections that extend
from the central office to a neighborhood and further to individual businesses
and homes. This network often is called the last leg or the last mile. Traditionally,
copper wires were used as the transmission medium in the access network,
since using optical fibers cost-effectively in these transmission spans is a major
challenge. However, various means of using fibers in the access network have
been explored. These schemes are known by the all-inclusive term fiber-to-the-x
(FTTx), where x is some letter designating at what point the fiber terminates
and copper wires (or wireless links) again take over.
Figure 17.17 shows the reach of various FTTx schemes. These are defined as
follows:
■ Fiber to the neighborhood (FTTN) refers to the optical fiber running from the
CO to a main distribution frame (MDF) located in a building or to an outdoor
shelter in a neighborhood. FTTN implies that another medium such as coaxial
cable will carry the communication signals from the MDF to the users inside
buildings within the neighborhood.
■ Fiber to the curb (FTTC) refers to optical fiber cable running directly from the
CO to the outdoor shelters on curbs near homes or any business environment.
Another medium (typically copper wires) will carry the signals the very short
distance (typically about 100m) between the curb and the user inside the
home or business.
■ Fiber to the building (FTTB) refers to optical fiber running from the tele-
phone company central office to a specific building such as a business or an
apartment house. Inside the building an optical network unit (ONU) converts
the optical signal to an electrical format for distribution over a wire network
to the occupants.

Central MDF Curbs Large Home
Large
Central
office MDF Curbs building Home
building
office
FTTN
FTTC
FTTB
FTTH
Figure 17.17. The reach of various FTTx schemes.

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