Page 686 - Introduction to Information Optics

670 12. Networking with Optics

and ATM public data service networks, and transmission among office sites
within or between local regions. Another technology that relies heavily on
TDM is synchronous optical network (SONET), which defines a synchronous
frame structure for synchronous transmission of multiplexed digital traffic over
optical fiber. It is a set of standards defining the rates and formats for optical
networks specified in ANSI T1.105, ANSI T1.106, and ANSI T1.117. A similar
standard, synchronous digital hierarchy (SDH), has also been established in
Europe by the International Telecommunication Union Telecommunication
Standardization Sector (ITU-T). SONET equipment is generally used in North
America and SDH equipment is generally used everywhere else in the world.
Both SONET and SDH are based on a structure that has a basic frame and
speed. The frame format used by SONET is the synchronous transport signal
(STS), with STS-1 being the base level signal at 51.84 Mbps. A STS-1 frame
can be carried in an OC-1 (optical carrier-!) signal. The frame format used by
SDH is the synchronous transport module (STM), with STM-1 being the base
level signal at 155.52Mbps. A STM-1 frame can be carried in an OC-3 signal.
Both SONET and SDH have a hierarchy of signaling speeds. Multiple
lower-level signals can be multiplexed together to form higher-level signals. For
example, three STS-1 signals can be multiplexed together to form a STS-3
signal, and four STM-1 signals multiplexed together will form a STM-4 signal.
SONET and SDH are technically comparable standards.
While both Tl and SONET/SDH still enjoy huge success in telecommuni-
cation networks decades after their introduction, they were designed to handle
voice traffic, not data traffic. During the 1980s and 1990s, voice traffic grew at
a linear rate while data traffic increased exponentially, mostly due to advances
in computer technology, data communication, and the Internet. By 2000,
telecommunication networks were carrying five times as much data traffic as
voice traffic. However, the voice-centric core network architecture model has
four layers for data traffic: IP (Internet Protocol) and other content-bearing
traffic, ATM (asynchronous transfer mode) for traffic engineering, a SONET/
SDH transport network, and DWDM for fiber capacity. DWDM has made
available the huge bandwidth of optical fiber for transmission. But the
circuit-switched core network infrastructure makes this bandwidth inaccessible
for the huge data traffic at the IP layer, since there are two much slower layers
of ATM and SONET/SDH in between. The fastest interface of commercial
ATM switches only reaches 155 Mb/s (OC-3), much slower than current
SONET gear with a top rate of 9.95328 Gb/s (OC-192). And the fastest
bandwidth manager SONET ADM tops out at OC-192, much slower than the
top DWDM trunk of 1 Tb/s. The existing four-layer approach has functional
overlap among its layers, contains outdated functionality, and is too slow to
scale, which makes it ineffective as the architecture for optical data networks.
Bandwidth demand for data traffic and the huge bandwidth capacity of
DWDM, combined with advances in optical switch and optical filter technolo-