What Are The Differences between SONET,SDH and DWDM?

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When
compared with traditional SONET or SDH, the next generation of DWDM (Dense
Wavelength Division Multiplexing) is seen as a simple architecture with high
scalability, capacity add/drop, multiple ring terminations, multi-services, and
multiple fabrics. Then, on comparison of SONET vs SDH vs DWDM, what are their
differences exactly?

 

SONET
vs SDH

SONET/SDH
is the dominant technology deployed in most metro and long distance networks.
It refers to a group of fiber optic transmission rates that can transport
digital signals with different capacities. Since their emergence from standards
bodies around 1990, SDH and its variant SONET (used in North America), have
tremendously enhanced the performance of telecommunications networks based on
optical fibers. The basic unit of SDH is synchronous transmission module
level-1(STM-1). The basic unit of SONET is Optical Carrier level-1(OC-1). The
other rates OC-3,OC-12,OC-18,OC-24,OC-36,OC-48,OC-96 and OC-192 are derived
from this basic rate.

 

PDH
vs SDH/SONET

TDM-based
networks of PDH (Plesiochronous Digital Hierarchy) and SDH/SONET have long
served as standard transport platforms for cellular traffic. PDH and SDH/SONET
are optimized to deal with bulk voice circuits with maximum uptime, minimal
delay and guaranteed service continuity. SDH was created to replace the PDH
system for interoperability between equipment from various vendors. The signal
hierarchy defined several line rates among which STM-1 (155 Mbps), STM-4 (622
Mbps), STM-16 (2.5 Gbps) and STM-64 (10 Gbps) and STM-256 (40 Gbps) happen to
be widely adopted.

 

DWDM
Basics

DWDM
is considered as one of the best technologies to increase bandwidth over an
existing fiber plant. It enables one to create multiple “virtual fibers” over
one physical fiber. It does this by transmitting different wavelengths (or
colors) of light down a bit of fiber. DWDM was initially adopted by long
distance carriers because the spending in amplification, dispersion
compensation, and regeneration composed most of the network equipment cost in
regional and national SONET
networks. DWDM became increasingly popular in metro networks when the
local exchange carriers grew their networks. Other than fiber exhaustion,
traffic volume is the major economic factor for deploying DWDM technology in
metro networks.

 

DWDM
Channels Frequencies

DWDM
operates in the range between 1530 and 1565 nm, the so-called C-band that
corresponds to the low-loss window of the optical fiber. This is the range
where the Erbium-Doped Fiber Amplifier (EDFA) is operative. A grid of allowable
wavelengths/frequencies of operation is per ITU-T, centered in a frequency of
193.1 THz, or a wavelength of 1553.3 nm, and all sorts of frequencies spaced at
multiples of 25 GHz (=0.2 nm) around that center frequency. Commercial systems
might have channels at 2.5 Gbps, 10 Gbps, and 40 Gbps (in which the latter are
recently becoming commercial) in addition to combinations of these in the same
system. The higher the bit rate, the larger needs from the power budget meaning
that the lasers must have better signal to noise figures, the amplifier spacing
needs to be reduced, amplification must be higher for example, by employing two
DWDM Optical Amplifiers in series. Typically 64 DWDM channels at 10 Gbps reach
a maximum distance of around 1,500 km with an amplifier spacing close to 100
km. Long-distance transmission systems beyond 1,500 km and up to 4,500 km will
also be commercially available using advanced and much more expensive systems.

 

DWDM
Technology Application

The
DWDM layer is protocol and bit rate independent, which means that it can carry
ATM (Asynchronous Transfer Mode), SONET, and/or IP packets simultaneously. WDM
technology may also be used in Passive Optical Networks (PONs) which are access
networks in which the entire transport, switching and routing happens in
optical mode. With the inclusion of recent 3R (reshape, retime, retransmit)
devices, internal towards the DWDM system, circuits utilizing only DWDM
equipment can now be built that may span the country. New performance
monitoring capabilities happen to be built into these devices so that
maintenance and repair of the link can be done. With DWDM as the transmission
method, the bandwidth of the existing fiber plant is maximized.

 

SONET
for Past

As
expected, the SONET scenarios have a low initial cost. When the traffic volume
is low, a SONET architecture is much more economical compared to DWDM
architecture. FS’s modeling indicates that when designing a SONET overlay
network with OC-3, OC-12, OC-48 and Gigabit Ethernet demands so when the design
requires under 4-10 OC-192 rings, a SONET network is a perfect choice.

 

DWDM
for Now and Future

As
the traffic volume grows, DWDM will eventually prevail and become the choice of
the network technology. Timing of this crossover is responsive to such things
as the span distances, pricing and interface density. The differences between
demand types mostly are caused by the design efficiency of these two
technologies’ interface cards in terms of density and price. FS’s study also
shows span distances usually trigger the extra requirement for regenerators,
optical amplifiers and DCMs in the routes. Long span distance has a tendency to
favor a DWDM architecture because of the efficient utilization of fibers and
optical bypass capabilities at intermediate nodes.

 

Additionally,
higher fiber cost and situations in which fiber constraints are enforced will
lead to more consideration for DWDM than SONET because DWDM saves a tremendous
amount of fiber within the optical network. DWDM systems could be planned for a
large number of channels, however, pay-as-you-grow strategy can be used and
channels added based on demand in FS. The amplifier distance and overall power
budget of the system needs to be calculated for the final quantity of channels
right from the start.

 

Conclusion

Different
alternatives and their economic impact in designing exactly the same network is
definitely an interesting study. SONET point-to-point performs better still.
These results might not apply in all situations. However, the implication is
the fact that, in large network designs, the most optimized network may not
necessarily be a single architecture. One part of the network may adopt rings
while another part implements point-to-point. Usually, the core part of the
network will justify a DWDM architecture.