Protection and restoration scheme for light-trail WDM ring networks

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Protection and Restoration Scheme for
Light-trail WDM Ring Networks
Ashwin Gumaste and Si Qing Zheng
Advanced Computer Networks and Architecture Laboratory
The University of Texas at Dallas, TX 75083
Email: ashwing@ieee.org; sizheng@utdallas.edu
Abstract: In this paper we propose a protection strategy for
light-trail [1-4] networks for ring topologies. A light-trail is a
multi-point lightpath, such that multiple users can take part in
communication along the trail, through time (differentiated)
non-overlapping connections. Dynamic connections between
source-destination pairs can be set up and torn down without
the need for optical switch reconfiguration. A light-trail hence
represents an opportunistic medium that reduces the
uncertainty of provisioning resources for dynamic connections.
However, this multi-point flow model leads to a new set of
problems in the area of protecting and restoring light-trail
based networks. Conventional link protection or path
protection techniques are not sufficient for light-trails because
of the potential of having multiple possible source-destination
pairs in the same trail over time. In this paper we propose a
hardware scheme to protect light-trails and further prove its
correctness. Further, the scheme is implemented through a
failed-traffic assignment algorithm. Subsequently we consider
alternative heuristic schemes for implementing the protection
scheme proposed. We simulate and compare the simulation
results to an analytical model developed.
I. INTRODUCTION
Contemporary optical networks deploy lightpaths [5, 10]
for bandwidth provisioning. A lightpath is an end-to-end
optical circuit that connects one source node to one
destination node. In contrast, a light-trail [1-4, 13-14, 8] is a
generalization of a lightpath, whereby multiple nodes along
a trail can take part in communication. Specifically, we can
have multiple source-destination pairs within a light-trail
forming time-differentiated connections. A light-trail thus is
analogous to a unidirectional optical bus. Though a light-
trail occupies the granularity of a full wavelength, it still
allows sub-lambda granular communication among its
participating nodes through time differentiated connections.
An out-of-band control channel is used to arbitrate the
communication within the light-trail. Once a light-trail is
created, connections among multiple nodes are dynamically
set up and torn down without the need for optical switch
(re)configuration. The out-of-band control channel does the
necessary signaling procedure for setting up and tearing
down connections within a light-trail. Clearly, a k-node
light-trail can support up to
??
?
2
connections, but at any given time at most one connection is
??
?
?
?
k source-destination pairs or
in the communicative state. The basic physical network
topology supporting light-trails is a ring. The hardware
and protocols that support light-trails for ring networks
are described in [2]. With simple extension in hardware
and protocols, mesh networks supporting light-trails are
also possible [4]. Light-trail represents a shift from
conventional lightpath, and has many advantages over
lightpaths [2-4, 14].
In this paper we consider the issue of providing
protection to light-trails within ring optical networks.
Ring topology is prominent in the metropolitan area on
account of ease of management, resiliency, and low
deployment costs (fiber relief) [17, 12]. The scenario we
are considering is when communication is interrupted due
to a fiber-cut. Protecting
connections) in ring networks is well defined [6, 7]. Since
in lightpath communication there is one source-
destination pair, the protection path is just routed across
the remainder of the ring – the graphical complement of
this lightpath in the opposite ringlet. In contrast, for a k-
node light-trail there can be
lightpaths (end-to-end
??
?
?
??
?
?
2
k possible connections
within the light-trail that need to be protected over time.
We first show that conventional re-routing does not
suffice to protect the entire light-trail. We then propose a
scheme that can create an alternative protected or healed
light-trail that is able to provide an alternate route for all
the
??
?
??
?
2
light-trail. We show how to use this scheme to create the
protection light-trail by using remnants of the failed light-
trail such that optical switching is minimized at
transponder interfaces when moving from the failed light-
trail to the protection light-trail, and once the light-trail is
protected all the
??
?
??
?
2
(without switching) be provisioned, thus preserving light-
trail properties.
??
k connections that were possible in the original
??
k connections can still seamlessly
0-7803-8956-5/05/$20.00 ©2005 IEEE.
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Lightpath
Fig. 1a. A Conceptual Lightpath
Convener
node
End node Light-trail
Fig. 1b A Conceptual Light-trail
This paper is organized as follows. In section II we briefly
describe the light-trail concept and outline the protection
problem. In section III, we describe the hardware support
for protection and outline the algorithm used. Section IV-VI
shows formulation and simulation results for protecting a
failed fiber and re-routing the traffic in minimum number of
light-trails. Section VII sums up the results.
II. LIGHT-TRAILS AND PROTECTION IN LIGHT-TRAILS
Shown in Fig. 1a is a lightpath while that in Fig. 1b is a
light-trail. A light-trail is defined on a unidirectional optical
arc of a ring, by a sequence of nodes on the arc, with its first
node and last node named the convener node and the end-
node of the light-trail, respectively. The architecture that
supports light-trails is detailed in [1-2] (also shown in Fig.
2a for convenience). This architecture employs the principle
of optical drop-and-continue; i.e., an intermediate node in a
light-trail can drop the signal (wavelength) as well as
continue the signal in one state of communication, while it
can completely drop the signal (say at the end node, in Fig.
1.b) in another state exhibiting pure drop. This drop-and-
continue principle is the quintessential shift from
conventional OADMs (Optical Add-Drop Multiplexers). In
addition the light-trail node also allows for passive adding
of the optical signal, whereby signal can be added into the
light-trail from a local transponder without optical
switching. When we combine the drop-and-continue
principle, passive add with the out-of-band control channel
over an opened optical path, we achieve dynamic
provisioning of connections between multiple source-
destination pairs in this path without the need for optical
switching. Hence a light-trail represents a dynamically
provisionable, opportunistic optical communication system
that allows multiple source-destination
connections to be set up and torn down without optical
switching. Furthermore, a collection of light-trails
represents a virtual topology that changes slowly, yet being
able to facilitate IP centric dynamic communication [1, 2, 3,
20].
Consider a ring of n nodes labeled 1 through n clockwise.
Physically, this ring is implemented by two unidirectional
fiber rings (ringlets) in opposite directions (clockwise and
independent
counter-clockwise). A light-trail of k nodes, k<n, defined
on this ring can be represented by a node sequence.
Without loss of generality, assume that the convener of
the light-trail is defined by node 1, and the light-trail is
then given as (1,2,…k)CW. By nomenclature it is
important to note, that another light-trail (1-k)CCW is then
in the counter-clockwise ring having the node sequence
(1, n, n-1, … k). We use similar notation for a connection
within a light-trail.
Shown in Fig. 2a is a specific light-trail between nodes
1 and 5 in the clockwise direction represented by (1-5)CW
on a 6-node ring. This light-trail has a maximum of 10
connections (
??
?
??
?
2
pairs embedded in it. At any given time, only one source
can transmit data. Hence in Fig. 2a, we have a
‘connection’ from node 2 to node 4. This connection is
‘heard’ by all the nodes downstream of 2 (i. e. 3, 4 and 5).
However only node 4 successfully taps the data on this
connection following the intimation of the impending data
that node 4 receives via the out-of-band control channel
ahead in time. Now, consider Fig. 2b, where a fiber-cut is
shown in the span between nodes 3-4. The result of the
fiber-cut is that the data flow in the residing connection
from 2 to 4 is impaired instantaneously. To protect this
connection, the logical (conventional) step is to re-route
the traffic from node 2 to node 4 via the counter-
clockwise fiber, thereby creating a backup connection P1
= (2, 1, 6, 5, 4), which is (graphically) disjoint with the
failed span 3-4, as shown in Fig. 2b. This form of
protection is conventionally deployed in present optical
networks and lacks novelty.
conventional loop-back will not suffice for protecting
light-trail ring networks.
To prove the invalidity of conventional loop-back
methods for light-trail networks, consider Fig. 2c, where
we have as before the light-trail from node 1 to 5 in the
clockwise sense. The fiber span between nodes 3 and 4 is
cut and we had created backup path P1 = (2, 1, 6, 5, 4) to
protect the connection 2-4. Subsequently after connection
2-4 has exhausted its data, we want to create a new
connection 1-4. Now by extending the classical definition
of protection to light-trails, once a path is protected (trail
in this case) all connections should be able to be
provisioned seamlessly oblivious to the fiber-cut.
However that is not the case as we had in Fig. 2b only
protected connection 2-4. The backup path cannot protect
this new connection request (1-4). This means that the
conventional approach of providing “fixed” alternate
routing on a per path basis does not protect the entire
light-trail along with its
??
?
??
?
2
protects the connection that resides in the light-trail at the
time of the failure. Then for the light-trail to carry about
?
?
k, k=5) or possible source-destination
Furthermore, this
??
k possible connections but just
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communication seamlessly for all the
??
?
?
??
?
?
2
kconnections, we
need a new mechanism for being able to provision any of
the
??
?
??
?
2
motivation is to maintain the characteristics of the light-trail,
i.e. to allow all the
??
?
??
?
2
provisioned without optical switch configuration once the
trail is protected. This discussion thus has identified the
problem in protecting light-trails and this can be summed
as:
•
for a given failed fiber-span, provide a protection
scheme that will suffice
(connections) that the original light-trail could suffice
for;
•
organize optical switches, and transponders in a way
that the nodes in the original light-trail are oblivious
to the protection light-trail; and
•
create protected light-trail(s) for the whole network.
??
kconnections despite the fiber cut. The central
??
kconnections to be able to be
all the demands
RING WDM NETWORK
SUPPORTING LIGHT-TRAILS
1
2
4
53
6
Light-trail
Connection
Node
Architecture
Fig. 2. a
RING WDM NETWORK
RING WDM NETWORK
SUPPORTING LIGHT-TRAILS
SUPPORTING LIGHT-TRAILS
1
1
2
2
4
4
5
5
3
3
6
6
1
P1
Fig. 2. b
RING WDM NETWORK
RING WDM NETWORK
SUPPORTING LIGHT-TRAILS
SUPPORTING LIGHT-TRAILS
1
1
2
2
4
4
5
5
3
3
6
6
Light-trail
Light-trail
Connection
Connection
NEW PATH
Cannot be provisioned
through protection P1
(CONNECTION)
P1
1
Fig. 2. c
Fig. 2: In 2a we have a light-trail (semi-permanent) represented
a thick solid arc and a connection 2-4 represented by a dashed
arc. In Fig. 2b. we show a conventional method to protect the
connection 2-4 in the event of failure using protection path P1.
In Fig. 2c, we see how this method is not possible to scale to the
other plausible connections in the light-trail system.
III. PROTECTION SCHEME AND ALGORITHM
So far we have identified the challenge involved in
light-trail protection due to its bus-like property. In this
section we shall propose a method to overcome this
challenge. However, since the challenge inherently is of a
hardware configuration nature, we shall first describe the
hardware involved in the protection mechanism.
In contemporary service provider optical networks data
is injected into the optical network by transponders that
comprise of a WDM compatible ITU grid laser. A
network also contains receive type transponders that are
able to receive data from the work or protect path.
Further, when a span fails, the injected data is
“switched” from the “working” path to a protection path.
This switching is generally done by optical switches that
are typically three port devices. The optical switches are
triggered by the network management system to define
which direction to send/receive the data and this form of
protection is popularly called optical shared-path
protection (OSPPR).
In our light-trail case (note that a light-trail node has the
peculiarity that it supports
architecture), let us assume that the 3-port optical switch
is an MZI (Mach Zehneder Inteferometer [17]) based 1x2
switch (with one input/output port connected to two
input/output ports) with the special assumption that there
can be a variable power ratio while switching between the
three ports. The assumption is valid and implementation
of such switches is discussed in [17]. We will consider
three states of such a switch:
State 1: Work Port Coupled (WPC). In this state, all the
input transmit data is directly coupled in full to the
work output port/fiber. Likewise at the receiver all
drop and continue
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the data is ‘extracted’ directly (only) from the work
path.
State 2: Protection Port Coupled (PPC). In this state all the
input data is directly coupled to the protection output
port. Likewise at the receiver all the data is extracted
directly from the protection path.
State 3: Dual Port Coupled (DPC). In this state the incoming
or outgoing data is optically combined or split from or
to the work port and protection port simultaneously. In
the transmit side, the switch splits the data, whereby
both ports get an identical copy of the signal. Likewise
at the receive side, the switch combines the data, and
hence the data arriving at either work/protection port
reaches the transponder seamlessly. When the switch is
splitting the outgoing data into the two ports it is called
DPC-add, as the data is being added into the network.
Similarly, when the switch combines the incoming data
from two different ports, then it is termed DPC-drop.
The switch DPC add and drop states are shown in Fig.
3. However, note that while combining, the data can
come from either of the two ports (work/protect) but
not both.
For the algorithm that we propose subsequently, States 1
and 3 are of special interest. State 1 is used when the
network has no fiber-cut, while State 3 is used by certain
nodes/ports that are affected by a fiber cut.
TRANSPONDER
MZI switch,
1x2 and variable
ratio
CLOCK
WISE
FIBER
COUNTER
CLOCKWISE
FIBER
1x2
50/50 splitting ratio
TRANSPONDER
RECEIVER
MZI switch,
1x2 and variable
ratio
CLOCK
WISE
FIBER
COUNTER
CLOCKWISE
FIBER
1x2
50/50 combining ratio
Fig. 3a
Conceptual description of protection
switching a network element (node).
Fig. 3b.
Fig. 3a shows dual port coupled – add, while
Fig. 3b. shows dual port coupled – drop.
Let us now briefly describe the basic idea of our
mechanism for protecting light-trails in a ring WDM
network. Consider a C-shaped light-trail (an arc of a ring).
When a fiber-cut in the span between two adjacent nodes in
this light-trail is detected, our proposed scheme forms a Ω-
shaped physical connection in order to replace the original
C-shaped light-trail. This Ω-shaped physical connection
consists of two working remnants of the original C-shaped
light-trail as well as a complete wrap-around light-trail less
the failed span in opposite fiber. Hence the curvature of the
‘Ω’ is this new wrap around, while the two horizontal base-
segments of the ‘Ω’are the remnants of the C-shaped light-
trail.
Our protection procedure consists of three steps:
1. Isolating the failed span: Let the original light-trail be
(1-k)CW. Define span i as the link connecting node i and
node i+1. Find span i which has failed in the light-trail (1-
k)CW..
2. Creating the protection light-trail (theΩ ): The
protection light-trail has three segments, which are
Segment 1: (1-i)CW (remnant)
Segment 2: (i-(i+1))CCW (created new)
Segment 3: ((i+1)-k)CW (remnant)
The original light-trail can be assumed to have the shape
of C. The combination of three segments of the protection
light-trail form the shape of Ω (shown in Fig. 4c-d).
Segments 1 and 3 are the two remnants of the original
light-trail. Segment 2, which is also denoted as
CCW
k
P
) 1 ( −
, is
known as the loop-back path with respect to span i,. The
combined protection light-trail architecture is the novelty
of this article and will be shortly described.
3. Routing the traffic: For original light-trail
−
2
convention N(1-k)CW denotes the number of nodes in (1-
k)CW. Therefore the protection light-trail should be able to
map all the flows emanating from each of the
−
2
, the nodes 1,…,i are able to carry flows to all other
CW
k)1 ( −
,
there are
??
?
?
??
?
?
)1 (
CW
kN
source-destination pairs, where by
??
?
?
??
?
?
P
nodes using both
) 1 (
CW
kN
source-destination pairs. In light-trail
CW
k)1 ( −
CCW
k
P
) 1 ( −
as well as the remnant (1-i)CW of
original light-trail (1-k)CW. The original light-trail upon the
fiber cut creates two light-trail segments (1-i)CW and
((i+1)-k)CW. However, nodes (i+1),..., k are not able to use
the light-trail
CCW
k
P
)1 ( −
anymore, as the signal flow of
P
)1 ( −
is opposite to the flow emanating from (i+1),..., k.
This means that the constructed light-trail
CCW
k
CCW
k
P
−
2
)1 ( −
k
by itself
is not sufficient to map all the
??
?
?
??
?
?
)1 (
CW
N
possible
connections within the light-trail (1-k)CW.
So we propose a simple corollary for protecting a light-
trail after a fiber-cut.
We create one light-trail exactly in the opposite
direction to the original light-trail and of length the entire
ring circumference, less the failed span (create:
CW
k
P
)1 ( −
)
and in addition to this created light-trail, we continue to
use the remnant portions of the existing light-trail ((1-
k)CW) that was affected due to the fiber-cut. The remnant
portion consists of two segments (1-i)CW, and ((i+1)-k)CW
separated by the failed span i. From a protection
convention, the protection light-trail consists of a loop-
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back light-trail across the failed span plus the two segments
that are remnants of the original light-trail. Note here that
the loop-back path and the two segments all continue to
exhibit light-trail properties of being able to insert data at
any node in a light-trail without optical switching (shown in
Fig. 4c-d).
In order to restore the entire
??
?
?
??
?
?
−
2
)1 (
CW
kN
connections,
the three segments must be made a unified medium. This is
done by changing the state of optical protection switch from
WPC to DPC (add and drop at different nodes). This allows
nodes to make use of the three Segments and hence the
unified system collectively represents the same effect as that
experienced through the original light-trail for their clients.
Fig. 4a through Fig. 4d show how the proposed scheme
works.
Thus after protection we, have the following switch
hardware states:
1. All nodes upstream of the fiber cut in span i i.e.
nodes 1…i configure their protection switches in
DPC-add mode. This way they split their
transmitting data into the protection loop-back
light-trail (Segment 2) as well as into the remnant
light-trail, Segment 1. The nodes in Segment 1 then
can communicate to their downstream nodes either
through the protection Segment 2 or through the
remnant light-trail Segment 1 itself.
2. All nodes downstream of the fiber cut in span i i.e.
nodes in Segment 3 given by (i+1), …k, configure
their protection switches in DPC-drop state. By
keeping the protection switch in the DPC-drop
state the nodes in Segment 3 (downstream of the
fiber cut) can tap data from nodes in Segment 1
through Segment 2. They can also tap transmission
from their upstream neighbors (if any) within
Segment 3 itself (as shown in Fig. 4c and Fig. 4d).
Note that once the light-trail is protected there is no
need for any further optical switching to establish
−
2
??
?
?
??
?
?
)1 (
CW
kN
connections.
Routing of connections through Ω shaped protection
light-trail:
1. Route connections that are originating from any of
the nodes in Segment 1 and destined for any of the
nodes in Segment 3 through the protection light-
trail
CCW
k
P
) 1 ( −
(which is Segment 2).
2. Route connections originating from any node in
Segment 1 and destined for any downstream node
in Segment 1 through Segment 1.
3. Route connections originating from any node in
Segment 3 and destined for any downstream node
in Segment 3 through Segment 3.
Now we show how the Ω – shaped light-trail suffices
for protecting all the flows seamlessly that would have
otherwise been provisioned through the original light-
trail.
For a k-node light-trail denoted (1-k)CW, such that the
total flow in this light-trail is,
=−
a
1
where
ab
x
represents the flow from node a to node b
along the light-trail (1-k)CW. For (1) we use the principle
that a light-trail has
??
?
??
?
2
destination pairs). By Expanding (1) for individual
connection flows we have:
k
CW
xx
) 1( 1 ) 1(
...
−−
+
?
<≤≤
−
kb
k
ab CW
CW
xkf
) 1 (
))1 ((
(1)
CW
k) 1 ( −
??
k
possible connections (source-
CW
k
CW
CWCW
k
k
k
k
k
k
xxkf
) 1 ()1 (
)1 (
1
)1 (
11
.........))1((
−−
−−
+++=−
(2)
Next let us observe the flow distribution after switching
to the protection light-trail.
The flow in Segment 1 is:
=−
CW
if
1
?
<≤≤
−
iba
k
ab
CW
x
)1 (
))1((
(3)
The flow in Segment 2 is:
??
==+
−
−
=
i
a
k
ib
k
abk
CW
CCW
xPf
11
)1 (
)1 (
))((
(4)
and the flow in Segment 3
f
(((
?
≤≤<+
−
=−+
kbai
k
abCW
CW
xki
1
)1 (
))) 1
(5)
Our protection scheme is able to successfully provide a
medium to continue the light-trail characteristics of
multiple source-destination
communicate through the same opened optical path. The
addition of the right-sides of equations (3), (4) and (5)
yield (2), the flow in the original light-trail. Segment 1
and Segment 3 exist in the original light-trail. The flow in
Segment 1 after protection remains the same (due to
splitting), while the flow in Segment 3 is reduced in the
protection light-trail.
pairs being able to
315