Page 92 - Optical Switching And Networking Handbook

04_200023_CH03/Batesx 1/17/01 9:42 AM Page 77

SONET 77

Point-to-Point

The SONET multiplexer, the entry-level PTE for an organization [or
the equipment installed by the local exchange carrier (LEC) at the
customer’s premises to access the network], acts as a concentrator
device for the multiple lower-speed communications channels such
as DS-1 and DS-3. In its simplest form, two devices are connected
with an optical fiber (with any repeaters as necessary) as a point-to-
point circuit. As the entry-level point into SONET architecture, the
inputs and outputs are identical. In this environment, the network
can act as a stand-alone environment and not have to interface with
the public switched networks. Figure 3-19 illustrates the point-to-
point multiplexing arrangement.

Point-to-Multipoint

The next step is to consider the point-to-multipoint arrangement.
This will use a form of add-drop multiplexing to drop circuits off
along the way. In a large corporate network spanning the country (or
any subset), a single high-speed link may be employed. The SONET
add-drop multiplexer (ADM) is used for the task, dropping circuits
out without demultiplexing the entire high-speed signal. In Figure
3-20, the ADM is installed between two far-end locations so that
signals can be added or dropped off as necessary. This is a better
solution than renting three different circuits between points AB, AC,
and BC, which adds to the complexity and cost. By using a circuit
from A to B to C with ADMs, the service usually can be accommo-
dated more efficiently.

Figure 3-19 OC-N OC-N
Point-to-point
service with SONET
OC-N
OC-N