I. INTRODUCTION
AS OPTICAL networks move towards being able to cope
with elastic demands the traditional challenge of routing
and wavelength assignment becomes replaced by that of routing,
modulation and spectrum allocation (RMSA) [1], [2]. The
high dimensionality of the RMSA problem can however be
simplified due to the coupling between the dimensions caused
by the fiber nonlinearities. Thus in a nonlinear optical network
the RMSA becomes primarily a routing problem, with the
assignment of modulation format and forward error correction
(FEC) depending on the signal to noise available for a given
route and the optical spectrum to be assigned depending on the
required data rate [3]. Given the increased importance of the
routing algorithm for the nonlinear RMSA, in this letter we
investigate the impact of congestion aware routing algorithms
and quantify their performance and efficacy in delaying the
onset of network blocking for the NSFNET topology.
II. UNDERLYING ASSUMPTIONS IN
THE PROPOSED MODEL
In order to facilitate an investigation of congestion aware
routing we make the following assumptions in our analysis:
Manuscript received March 5, 2014; revised March 19, 2014; accepted
March 20, 2014. Date of publication March 21, 2014; date of current version
April 24, 2014. This work was supported by the EPSRC Programme Grant
UNLOC (EP/J017582/1).
The author is with the Department of Electronic and Electrical Engineering,
University College London, London WC1E 7JE, U.K. (e-mail:
s.savory@ucl.ac.uk).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2014.2314438
1. The client side data rate is fixed (herein we restrict our
consideration to 100 GbE assumed to be 104 Gbit/s
including framing plus an overhead for FEC).
2. Transceivers can vary modulation format and FEC.
3. Channels are Nyquist shaped, having a rectangular spectrum
of width equal to the symbol rate.
4. There are negligible guard bands between channels.
5. The network employs single mode fiber and is periodically
amplified by a lumped erbium doped fiber amplifier
(EDFA) with a bandwidth of 5 THz.
6. The spacing between amplifiers is fixed throughout
the network (herein we consider 100 km span
length).
7. No optical dispersion compensation is employed and the
fiber plant is the same over the entire network.
8. The Gaussian noise model [4], [5] is valid and
the nonlinear interference (NLI) adds incoherently
such that the total NLI is proportional to the path
length.
9. Primary source of noise is from the EDFA within
the links and that losses at the nodes may be
neglected.
10. In the link where blocking occurs the spectral utilization
is sufficiently high that the nonlinear impairments
correspond to 100% spectral utilization.
11. We consider the network to become blocking at the point
when the first blocked demand occurs.
III. PROPOSED ALGORITHM FOR NONLINEAR RMSA
The proposed algorithm for RMSA in a nonlinear
elastic network utilizing Nyquist pulse shaping is as
follows:
1. Determine the optimum signal power spectral density
given the fiber and amplifier parameters.
2. For a pair of nodes, select the shortest path that avoids
the link with the highest spectral usage (determined by
measuring the total optical power which is proportional
to spectral usage).
3. For this path determine the total number of amplifier
spans (100 km herein) in order to determine the received
signal to noise ratio (SNR).
4. For this SNR, determine the maximum net spectral
efficiency (NSE) based on known relationship between
SNR and NSE for a range of polarization division
multiplexed formats with Nyquist spectra where variable
rate FEC is also included.
5. Finally determine the gross symbol rate and assign
spectrum to serve the demand between the two
nodes.