![](bg6.png)
strategy while the other is named as RWSA-D
derived from decentralized strategy.
In simulations, two types of network topology
are used. As shown in Figure 3(a), a regular ring-
lattice topology represents TWSR-OSN with a
single constellation (such as LEO), which consists of
4 orbits with 8 satellites at each orbit, where each
satellite node has OISLs with four adjacent satellite
nodes at the same orbit and at the adjacent orbit,
meaning the whole network has 32 nodes and 64
OISLs. Figure 3(b) depicts an irregular network
topology, representing optical satellite networks
with hybrid constellations of LEO and MEO etc,
which has 10 nodes and16 OISLs.
Figure 3: (a) Regular network topology, (b) Irregular
network topology.
Assuming that requests for subcarrier channels
arrive randomly in Poisson Process with the network
arrival rate
, traffic intensity among all satellite
node pairs is distributed uniformly, while multiple
subcarrier paths between a pair of nodes are
permitted. After subcarrier paths are set up, its
services time depends on negative exponential
distribution with a mean value of
. The average
service time in TWSR-OSN is set to an unit time, so
it total network load is
. Once a subcarrier
channel connection request is rejected, it is
discarded immediately without a waiting queue
scheduling mode. In order to ensure the network into
a stable running status, 1
×
10
6
connection requests
are generated.
For comparison with the proposed RWSA
algorithm, two routing and wavelength assignment
algorithms are employed, including fixed routing
and First-Fit wavelength assignment algorithm
(FR/FF), fixed alternate routing and random
wavelength assignment algorithm (AR2) which
determines two alternate routes for each pair of
nodes by its basic costs.
A: TWSR-OSN with uniform wavelength
distribution and the same link costs
In TWSR-OSN with uniform wavelength
distribution and the same link costs, wavelength
number at each OISL is set to
||||
ij
W W
. For
simplicity, the basic cost
a,b
C
and
a,b
δ
of each edge in
any link
a,b
lL
in
L-G
are all set to 1.
Based on the regular network topology above,
the network performances of TWSR-OSN is
simulated firstly, where wavelength number of each
OISL is set to 4, 8 and 16 respectively, while each
wavelength has 16 subcarrier channels. By RWSA-
D algorithm, Figure 4 describes the network
performances with different wavelength numbers.
As expected, with the increasing wavelengths, its
network blocking probability is degraded sharply at
the same traffic load, while its network throughput
increases greatly at the same blocking probability.
For example, when the blocking probability is 0.01,
its throughput with 8 wavelengths is over 3 times of
that with 4 wavelengths, while its throughput with
16 wavelengths is over 8 times, that is, the network
throughput appears to increase rapidly at a nonlinear
rate, which reveals its advantage of the network
performance improvement by introducing two-
layered wavelength/subcarrier routing in optical
satellite networks.
Figure 4: Network performance of TWSR-OSN with
different wavelength numbers in the regular network
topology.
It is assumed that the network traffic load is
fixed, and its channel number in each ISL is set to
||||32
WS
, and the number of wavelengths |W|
changes from 1 to 32. Figure 5 illustrates the
network blocking probability by different
algorithms. The simulation result shows that AR2
algorithm appears better network performance with
lower blocking probability than FR/FF algorithm
obviously; meanwhile, once the wavelength number
|W| exceeds 4, the performance of RWSA-D
algorithm is far superior to two other algorithms.
For a given irregular network topology of
TWSR-OSN, the total channel number of each OISL
is set to
||||16
WS
, while wavelength number |W|
is set to 4 and 16 separately. In Figure 6, the
blocking probability curve of RWSA algorithm
based on SL-G is compared with other algorithms,
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