Page 309 - Optical Communications Essentials

Optical Networks

Optical Networks 299

IP/ATM/SONET
IP Packet-over-SONET
ATM IP IP-over-WDM

SONET SONET IP/MPLS
WDM (Optical adaptation layer)
Physical layer

Figure 17.16. Progression of network layering methodologies
moving from IP/ATM/SONET to packet-over-SONET to IP-
over-MPLS.

of DWDM. However, recently the trend is to format all voice, video, and data as
IP packets instead of first encapsulating them in ATM cells. With this “IP-over-
SONET” structure, the SONET network protection mechanisms are still in
place but the high ATM overhead (at least 5 B in a 53-B cell) is eliminated,
thereby reducing the number of management levels from four to three.
A further trend is aimed at bypassing the SONET/SDH layer, thereby com-
bining the IP and SONET layers into one network layer based on multiprotocol
label switching (MPLS). This “packet-over-WDM” scheme would provide faster
provisioning of services and eliminate one electronic bottleneck, so that then
there would be only two levels of management. However, there are major fram-
ing and fault recovery concerns with this approach. Since an IP packet contains
only source and destination IP addresses, to map IP onto a wavelength requires
an intermediate step of encapsulating the IP packet into a transport protocol in
order to attach a header that contains source and destination physical
addresses. This could be a protocol such as Ethernet, ATM, or SONET. After the
packet is encapsulated, it is inserted into the modulation format of the wave-
length being used. For readers interested in details on this, see the referenced
MPLS papers (5 through 7) in IEEE Communications Magazine and IEEE
Network.

17.5. Optical Transport Networks (OTNs)

Emerging next-generation transport networks are referred to as optical trans-
port networks (OTNs). In these networks it is envisioned that DWDM-based
dynamic optical elements such as optical cross-connect switches and optical
add/drop multiplexers (OADMs) will have full control of all wavelengths. In
addition they are expected to have full knowledge of the traffic-carrying capacity
and the status of each wavelength. With such intelligence these networks are
envisioned as being self-connecting and self-regulating. However, there are still
many challenges to overcome before such completely intelligent optical net-
works are feasible.

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