In the current
electronic networking world there are principally three transport
services available to users/customers in various forms. These are
While all three network transport
services are available today, users are moving away from circuit
services toward datagram (packet) and virtual circuit services in
order to efficiently accommodate bursty data communications traffic.
Plain old telephone
ISDN (BRI, PRI),
PDH, (T1/E1, T3/E3),
Virtual circuit services
transfer mode (ATM).
Figure 1 -
Service layers of a broadband network with possible position of an
services in many different ways. In small and private networks where
a dedicated infrastructure can be provisioned, the network can be
optimized for the specific service requirements of the users.
However, when public infrastructures are needed to connect a variety
of private networks or users together, it is difficult for a single
networking technology to provide all the required services. Multiple
service requirements are generally provided via a layered
architecture such as that shown in Figure 1 (so called overlay
networks). Also shown is the possible position of an optical network
(ON) layer in an overall broadband network (BBN) architecture. The
figure indicates that an ON could support packet service directly,
through an ATM layer or an ATM-over-SONET/SDH architecture, among
other choices, such as IP over SONET/SDH.
Today, the bottom
layer of most BBNs consists of single-wavelength point-to-point
optical fiber links, and the bandwidth available on these fibers
using existing transmission technologies is being consumed by
transmission technologies have the capability to significantly
increase optical link bandwidths:
time division multiplexing (OTDM).
Optical code division
systems divide the fiber bandwidth into a large number of optical
wavebands, each of which carry information at electronic rates (~ 10
divide the fiber into a smaller number of broader wavebands
(possibly only one) and use short pulses to transmit at very high
rates (> 100 Gb/s) per waveband. These pulse streams are then
optically demultiplexed down to slower electronic rates for
switching and distribution to users.
Whilst WDM and
OTDM techniques partition the available spectrum and time to
different users, respectively, OCDM techniques multiplex users
simultaneously andasynchronously (or synchronously) across the same
spectrum and timeslot through a unique code.
systems are currently the most promising for BBNs because WDM
technology is far more mature than OTDM and OCDM. Therefore we
ignore OCDM in the sequel, since it is still at research stage.
enables very-high-speed optical logic which can be used to provide
enhanced digital services and functions such as packet routing,
signal regeneration, forward error correction, bit error rate (BER)
measurements for network management, stream encryption, and so
description of these technologies is presented in the next chapter.
Here, we focus on optical networking and services
Figure 2 - Service layer taxonomy.