Low-loss WSS rduces costs in London
regional network study
The recent explosion in access bandwidth for Internet and other
services is driving carriers to provide a greater level of optical
layer flexibility in regional and long-haul networks. The
flexibility is needed to solve a wide variety of issues plaguing
carriers and service providers. They urgently need to further
automate and accelerate provisioning of customer services along with
providing new types of revenue-generating wavelength services. In
addition, they need to lower both capital and operating costs while
improving network resiliency and availability.
The solution is to capitalize on the power of optics to
inexpensively switch large amounts of bandwidth at the physical
layer, and operators are taking first steps towards this by
installing reconfigurable optical technologies. Wavelength-selective
switch (WSS) technology is a key building block that enables
carriers to meet these diverse needs. The technology has matured to
the point where WSSs are poised for large-scale deployment in
long-haul and even regional networks.
A WSS is a component in a network switching node that is able to
switch individual DWDM traffic wavelengths dynamically among
different fibers without any optical-to-electrical (OE) conversions.
Today’s WSSs are typically 1×5 or 1×9, which means they have one
input fiber and five or nine output fibers, respectively. Since WSSs
are all-optical devices, they can also be used in reverse to switch
waves from multiple input fibers to a single output fiber. To build
up a network wavelength switch, WSSs can be interconnected together
to make a nonblocking wavelength switch among many fibers.
Recent studies have shown that the WSS specifications, especially
loss, are critical to the network economics and performance.
Emerging WSS technologies promise to lower this loss from the
current 6- to 9-dB range to less than 3 dB. For example, a WSS based
on piezoelectric technology that is currently in final development
is projected to have less than 3 dB of optical loss.
A comprehensive study of a transparent WSS backbone using a
London regional reference network showed that even small reductions
in WSS loss can dramatically lower overall network losses, thereby
improving performance and reducing costs. The concept of “cost of
loss” was used to quantify how WSS performance affects cost. The
results of this London regional network study are applicable to a
wider range of regional, long-haul, and larger metro networks and
show the dramatic impact lower WSS losses can have on network
performance and cost.
London regional network
To quantify the impact of lower WSS loss, we studied adding a
reconfigurable optical layer using WSSs to a possible London
regional network. This real-world network has many attributes, such
as a large number of nodes and diverse traffic patterns, that make
it an ideal case study for understanding the impact of WSS loss on a
wide range of regional and long-haul network applications. The
reference model has 38 switching nodes with connectivity degree 2 to
6 and 62 interconnecting transport links, and a realistic traffic
model was assumed.
The overall network is shown in Fig. 1. For each transport link
we took into account the number of wavelengths, wavelength
assignments, fiber lengths, loss, chromatic dispersion, and
polarization-mode dispersion (PMD). For each switching node we took
into account the number of interconnection fibers and the amount of
add/drop traffic terminating at the node.
We introduce the concept of “cost of loss” to understand the
impact of higher WSS loss on the overall network. First we calculate
the total loss impact that the WSS introduces in the network on both
the add/drop and through traffic and then we determine a cost
penalty based on the amplification costs required to overcome this
loss. There are several ways to translate this loss into network
costs. For this study we used typical costs seen by service
providers for optical amplification, which is about $1,670 per
decibel of gain.
In this study we compare the impact of through traffic and
add/drop WSS losses of 3, 6, and 9 dB on the overall network. Legacy
WSS and waveblocker technologies can have losses up to 9dB or more
depending on number of fibers that are intended to interconnect in
the node. WSS technologies available today typically have losses in
the 6- to 9-dB range while emerging WSS technologies are expected to
be less than 3 dB. For loss values and costs of other network
components used in this study, we used typical industry norms.
The study shows that lowering the WSS loss can yield significant
network savings and performance improvements. Each additional 1 dB
of WSS loss adds about 450 dB to the total loss of the London
regional network. The loss due to the WSS was calculated for each
node based on the number of through paths and the number of add/drop
traffic channels. The overall network loss is calculated by adding
up the individual node losses.
Figure 2 shows a comparison of the overall contribution of the
WSS to the total network for WSS losses of 3, 6, and 9 dB. This
clearly demonstrates that even small amounts of extra WSS loss can
add thousands of decibels of total loss to the overall network. The
6-dB and 9-dB loss WSSs add an extra 1,332 dB and 2,664 dB of loss,
respectively, to the overall network. These figures are so high
because the WSS is located in the core of the network, and every
signal passes through a WSS when entering or exiting the network and
passes through two WSSs in every node while traversing the network.
In this particular network, the average node degree is 3.3, giving
on average 6.6 WSSs per node, and about 250 WSSs in total.
There are several ways to compensate for the extra WSS loss in
the network. Regardless of how it is done, making up for these large
amounts of extra WSS loss is expensive. Simply increasing the
optical amplifier gain and output power will compensate for the loss
but will increase the cost of the amplifiers, increase the nonlinear
impairments, and reduce overall system reach and performance.
Alternatively, adding extra amplifiers would increase amplification.
This alternative adds considerable cost since the amplifiers need to
be powered, monitored, and maintained. Also, adding a large number
of extra active components like optical amplifiers lowers the
overall network reliability and availability. Simply increasing the
transmitter output power or receiver sensitivity would also add cost
and could have performance implications.
These extra WSS losses add significantly to overall network
costs. Using a typical service provider optical amplifier cost of
$1,670 per decibel of gain, loss compensation adds about $750,000
per decibel of WSS loss. For the 6-dB and 9-dB WSSs cases,
compensation adds over $2.25 million and $4.5 million, respectively,
above the cost of using the 3-dB WSS. In the case of the 9-dB WSS,
the cost of compensating for the WSS loss is higher than the overall
WSS equipment costs.
Unlike optical fiber losses and other component losses that
cannot be lowered due to material properties, physical laws, or cost
constraints, there is no fundamental reason why WSS optical losses
cannot move lower in the near future. In addition to saving money in
near-term deployments, the emerging ultralow-loss WSS technology
also enables innovative architectures that are not currently
possible with higher-loss technology.
The impact of WSS loss in this study of the London regional
network showed significant savings that can be achieved by even
small reductions in WSS loss. The metric of “cost of loss”
quantifies the impact of WSS loss on network performance and cost.
This study shows that for higher-loss WSS technologies the cost of
using these WSSs in the network can be comparable to the overall WSS
The results of this London regional network study can be applied
to a wide range of regional and some long-haul network deployments.
Also, the underlying traffic patterns and connectivity of this
real-life network make a powerful statement about the impact of WSS
technology in typical network deployments. Using lower-loss WSSs
directly translates to lower bottom line network costs. As WSS and
other component costs continue to come down over time, the value of
lower WSS loss will become increasingly important to the viability
of network deployments.
Richard Jensen is director of network architecture at Polatis
(www.polatis.com). He can be reached at firstname.lastname@example.org.
Andrew Lord is responsible for optical core R&D at BT
(www.bt.com). He can be reached at email@example.com.
February, 2008 Author(s) : Richard Jensen Andrew
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