30-10-2012, 05:05 PM
MICRON – A Framework for Connection Establishment in Optical Networks
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Abstract
Traffic grooming in optical networks has gained
significance due to the prevailing sub-wavelength requirement of
end users. Optical networks get upgraded to the latest technology
slowly with time with only a subset of nodes being upgraded to
the latest technology. The networks are thus comprised of nodes
employing heterogeneous switching architectures. In this paper,
we develop a framework called Methodology for Information
Collection and Routing in Optical Networks (MICRON) for
connection establishment in optical grooming networks with heterogeneous
switching architectures. We illustrate with examples
the information that may be collected from the links, and operators
that may be used to obtain information along a path. The
information can be used to select a path dynamically depending
on the network status. We complete the MICRON framework by
providing a generic channel assignment procedure that could be
employed to implement different channel assignment schemes.
Various routing and channel assignment algorithms can be
developed from the proposed framework. The framework may be
easily implemented with simple traffic engineering extensions to
the already existing routing protocols in the wide-area networks.
INTRODUCTION
Optical communication employing wavelength division
multiplexing (WDM) has emerged as a viable solution for
satisfying the ever-increasing quest for bandwidth due to
emerging Internet applications. WDM divides the available
fiber bandwidth into multiple wavelengths each of which
operates at peak electronic speed. Present day networks have
a transmission capacity of 40 Gbps (OC-768) on a wavelength.
However, the user requirements are of sub-wavelength
capacity, typically ranging from 155 Mbps (OC-3) to 622
Mbps (OC-12), and rarely in the range of a few gigabits per
second. Hence, alternatives for provisioning of sub-wavelength
traffic in WDM networks has received significant attention in
the recent past. One approach to provisioning sub-wavelength
traffic is to divide a wavelength into time slots that would allow
multiple traffic to be time multiplexed on the wavelength.
However, employing TDM at high transmission speeds in
long-haul networks requires stringent synchronization across
the network. Hence, Code Division Multiple Access (CDMA)
approaches may be employed to share a wavelength bandwidth
across multiple users. The resulting multi-wavelength multitime
slot or multi-code network is referred to as a WDM
grooming network.
Node architecture
A WDM grooming network with heterogeneous network
architecture is modeled as a Trunk Switched Network (TSN).
The TSN model was introduced in [22] to enable modeling
of multi-wavelength optical networks and evaluate them for
blocking probability. The MICRON framework is developed
for the TSN model, therefore we describe the TSN model in
detail.
A TSN is a two-level network model in which every link
in the network is viewed as multiple channels. A node in a
TSN groups the channels with similar characteristics in a link
into groups called trunks. The definition of a trunk at a node
depends on the switching resources available at a node. We
illustrate the notion of trunks with an example. Consider a
WDM grooming network where every link has four fibers,
three wavelengths per fiber and two time slots per frame (F =
4, W = 3, T = 2). Fig. 1 shows the channels on a link.
The shapes represent the time slots and the shades represent
wavelengths.
EXAMPLE NETWORK
Consider the example network shown in Fig. 4 illustrating
two paths from node 1 to 5. Let the nodes be connected
using three fibers each carrying three wavelengths and two
time slots per wavelength. Also assume that nodes 1, 3, 6,
and 7 are wavelength-level grooming nodes; nodes 2 and 5
are time-slot-level grooming nodes; and node 4 is a fullgrooming
node. Wavelength-level grooming nodes view the
link as 3 wavelength trunks (denoted by W1, W2, and W3)
with 6 channels in each, time slot-level grooming nodes view
a link as two time slot trunks (denoted by T1 and T2) with
9 channels in each, and a full-grooming node views a link as
one trunk (denoted by F1) with 18 channels.
Sub-trunk assignment
At the end of the forward pass, the destination node has
the path information vector for the different probed paths and
selects a path based on a certain path selection algorithm. Once
a path is chosen, a sub-trunk has to be selected on every link
of the path in order to complete the channel establishment.
The sub-trunk assignment is carried out as follows. (1) the
destination node first selects the trunk at its node where the
connection would terminate; and (2) every node along the path
selects the output trunk at its previous node. If a link connects
node i and j, then the node j selects the output trunk at node
i, hence the sub-trunk assignment on the link i–j.
Consider the information matrix represented in Fig. 6 and
operator (; +). The path information vectors obtained at
different nodes are shown in Fig. 9.
Modeling blocking trunk switches
The framework assumes that the trunk switch employed
at every node has full-permutation switching capability. This
implies that any channel on a trunk on a link can be switched to
any other channel on any output link but within the same trunk.
In such a scenario, a connection cannot be accommodated on
a trunk only due to lack of capacity on the trunk and not due
to switching. All-optical implementation of full-permutation
switches would require a large number of stages of switching,
hence may not be practical due to power and synchronization
issues. Hence, simpler but blocking switching architectures
that involves fewer stages of switching may be considered for
implementation.
Multicast tree establishment
Multicast connection establishment is accomplished using
the destination initiated request approach mentioned above.
Consider the network shown in Fig. 12 in which node 0 is the
multicast source and nodes 5 and 7 are the destination. Fig. 13
shows the expanded view of the network.
The multicast tree that needs to be established spans all
the links and requires node 1 to route the incoming traffic
along two paths. In order to facilitate multicast connections,
the incoming signal must be copied and transmitted along two
different paths. In order to provide a cost-effective solution,
the copying may be restricted within a trunk. In such a case,
the trunk in which the connection terminates at node 1 must
reach all the destinations. In MICRON, such a multicast path
is established in three steps: (1) The multicast request is
forwarded along the tree, (2) the path information vector on
the reverse path is obtained, and (3) the sub-trunk assignment
is made along the forward path
CONCLUSIONS
In this paper, we develop a framework for connection
establishment in a WDM grooming network with heterogeneous
switching architectures. We illustrate with examples the
various information that could be collected from the links and
various operators that could be used to obtain information
along a path. The information collected may be used to
select a path dynamically depending on the network status.
We complete the framework by providing a generic channel
assignment procedure that could be employed to implement
different channel assignment schemes