deploying and managing ip over wdm networks - Index of

2.3 Current Transport Networks
Network and Service Evolution Scenarios 15
Asynchronous transfer mode (ATM) has been the dominant transport technology
for most wide area packet-switched networks for the last five years. ATM,
combining transmission, switching, and multiplexing functions based on fixedlength
cells, appeared a promising solution not only for core networks, but also
for access networks. The characteristic of being asynchronous as opposed to the
synchronous transfer mode (STM) of TDM was considered ideal for Internettype
data networks, whose traffic comes most of the times in bursts. In addition,
the characteristic of being connection oriented matched the concept of traditional
circuit-switched-based voice calls. Other features, such as the sophisticated
traffic control and management, the support of user-defined QoS classes,
and the use of optical fibers for the transportation of the cell-based traffic, presented
ATM as the all-in-one network solution.
Nevertheless, ATM was not accepted in the LAN environment, where
100BaseT Ethernet was significantly cheaper, more convenient, and more
importantly, supported thousands of IP applications running on top. On the
other hand, neither ATM PC cards nor native ATM applications reached the
real end users and were only part of research projects. The interworking of IP
with ATM gave the opportunity to use the “popular” user-friendly IP applications.
However, this pushed ATM to the WAN networks, most of the time not
even taking into account the ATM QoS classes, but rather sticking to the usual
“best-effort” IP service. The continuously growing demand for IP applications
and the support for real-time and nonreal-time multimedia applications had as
an immediate effect the demand for higher IP bandwidth.
As far as the STM, the SONET/SDH transmission protocol was the evolution
of PDH networks based on TDM technology. All incumbent telecom
operators currently operate SDH networks. SDH technology has significant
advantages supporting high and variable transport rates, sophisticated management,
automatic protection techniques, and easy path provisioning schemes
using add/drop capabilities. SDH succeeded in satisfying the needs and growing
demand for better quality mass plain old telephone system (POTS) service and
integrated services digital network (ISDN) telephony. In parallel, as a result of
the smooth interworking with the ATM protocol, SDH captured part of the
ATM or other type data traffic into its network.
PDH, beyond the lower bit rates, lacks the significant advantages of SDH
and thus suffers from the lack of network management functions, easy end- toto-end
path provisioning, and automatic recovery. This makes it expensive,
inflexible, and inconvenient.
The outcome, based on the different needs regarding the services offered
to the users and the corresponding traffic type, was that there was a balance
mainly between the three different protocols, IP, ATM, and SDH, each one