sonet 101

What are the Benefits of SONET?
NEW SERVICES THAT USE ATM
One of the important benefits of SONET is its ability to position the network for
carrying new revenue-generating services. With its modular, service-independent
architecture, SONET provides vast capabilities in terms of services flexibility.
High-speed packet-switched services, LAN transport, and high-definition television
(HDTV) are examples of new SONET-supported services.
Many of these broadband services may use asynchronous transfer mode (ATM)—
a fast-packet-switching technique using short, fixed-length packets called cells.
Asynchronous transfer mode multiplexes the payload into cells that may be generated
and routed as necessary. Because of the bandwidth capacity it offers,
SONET is a logical carrier for ATM.
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In principle, ATM is quite similar to other packet-switching techniques; however,
the detail of ATM operation is somewhat different. Each ATM cell is made up of
53 octets, or bytes. Of these, 48 octets make up the user-information field and
five octets make up the header. The cell header identifies the “virtual path” to be
used in routing a cell through the network. The virtual path defines the connections
through which the cell is routed to reach its destination.
An ATM-based network is bandwidth-transparent, which allows handling of a
dynamically variable mixture of services at different bandwidths (see Figure 3-6).
ATM also easily accommodates traffic of variable speeds. An example of a service
that identifies the benefits of a variable-rate interface is that of a video codecbased
service. The video signals can be coded into digital signals and packetized
within ATM cells.
During periods of high activity within the video camera’s field of view, the rate of
packet transfer will increase. Hence, an average rate of transfer may take place
below the maximum capacity of the physical layer. But during peaks in activity,
the rate of packetization goes above the normal maximum.