Information Technology Reference
In-Depth Information
3.5. On-Line Hybrid Survivability (OHS) [25]
OHS offers a new approach for dynamic connection requests with multiple levels of
service such that for a new arriving connection request, the cost of the link according to the
current state of the network is adjusted immediately, and then, the primary and backup paths
are computed. The main aim is to improve network resource utilization and blocking ratio.
Three levels of services are considered in OHS: the highest level of service is represented by
level 2. In this level, when a connection request arrives at the network, primary and shared
backup paths are elected. Level 1 demonstrates the middle level of service, where the primary
path does not share any resources with the backup paths of level 2. The lowest level of service
is shown by level 0, where no survivable mechanisms are provided for the class of
connections.
When a connection request arrives, the level of the connection is first verified. If the
connection request belongs to level 2, according to Eqs. (2)-(3) the link cost is tuned and then
a least primary path is computed. If the primary path is found successfully, the link cost of the
backup path is tuned with respect to Eqs. (4)-(6), and then, a least-cost path is allocated as a
backup path. Finally, the selected primary and backup paths and their allocated wavelengths
are saved. If primary or backup paths are not found, their connection requests are blocked and
the allocated network resources are released. For a connection request belonging to level 1,
the link cost is computed according to Eqs. (2)-(3) and for a connection request belonging to
level 0 the link cost is calculated according to Eqs. (4)-(6). Then, for both levels 1 and 0, the
least primary path is determined and the found path and assigned wavelength are recorded. If
the primary path cannot be found, the connection request is blocked.
∞
0
(2)
0
(3)
(4)
(5)
∞
(6)
where
e
and
e
'
are unidirectional links. The parameter
F
e
represents free capacity,
R
e
shows
reserved capacity which can be shared by some backup paths,
C
represents the set of existent
connections in the network,
δ
is a sufficient small positive constant say 0.01 or 0.001, and
W
e
is working capacity taken by some primary paths and cannot be used for any other purposes
until the corresponding primary path is released. The parameter
denotes the required
backup resources on the unidirectional link-cost of each unidirectional link
e
'
for the backup
path of connection request passing over
e
'
[25].
