Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-03-23
2003-12-02
Ton, Dang (Department: 2666)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S224000
Reexamination Certificate
active
06658013
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the transmission of optical data and, in particular, to a method for ensuring the survivability of traffic travelling between adjacent rings.
BACKGROUND OF THE INVENTION
SONET, or Synchronous Optical NETwork, is now the preferred standard for optical transport of telecommunications traffic in North America. This standard has been developed and implemented over the last decade to give telecommunications carriers important benefits that are difficult to achieve using previously available asynchronous transport technology. Among the most significant of these advantages are: greater compatibility among equipment from different manufacturers; synchronous networking for improved reference timing of network elements; enhanced operations, administration and provisioning capabilities; and compatibility with any service mix including both traditional services and newer services such as Asynchronous Transfer Mode (ATM) traffic.
The SONET hierarchy is built upon a basic signal of 51.84 megabits per second (Mbps), known in the art as a level-1 synchronous transport signal frame, denoted STS-1 and sometimes referred to as a “time slot”. A byte-interleaved multiplexing scheme can be applied to multiple STS-1 frames, resulting in a digital signal having a rate of N times the basic rate, where N is typically 1, 3, 12, 48 or 192. The optical form of an STS signal is called an optical carrier (OC) and thus an STS-N signal and an OC-N signal have the same rate.
The STS-1 frame has a portion of its capacity used for delivering payload, while the remaining portion is devoted to overhead. The payload refers to the data or traffic part of a signal, while the overhead consists of signalling and protocol information. The use of the SONET overhead allows communication between intelligent nodes in the network, which enables administration, surveillance, provisioning and control of the network to be carried out from a central location.
Today's SONET transport networks typically employ a number of different topologies to satisfy important objectives such as network simplicity, cost containment, bandwidth efficiency and survivability. For instance, an optical hubbing configuration may be used to eliminate the need for a costly and complicated arrangement consisting of several back-to-back network elements. Another example is the deployment of self-healing rings to assure survivability of traffic around the ring through the provision of redundant communications paths.
By way of example, a two-fiber bidirectional line-switched ring (2F BLSR) is a survivable SONET transport architecture that protects against cable cuts and node failures by providing duplicate, geographically diverse paths for each service to be delivered. In a 2F BLSR, the two fibers carry unidirectional traffic in opposite directions and the bandwidth of each unidirectional fiber is split between working traffic and protection traffic.
A service path is provisioned through a 2F BLSR by selecting endpoint network elements and one or more STS-1 time slots linking the service entry and exit points. Although two communication paths are available around the ring, a service reaches its destination by travelling along the working path of only one of these. Intermediate nodes on the service path, if they exist, simply pass the service from east to west (or vice versa) without modifying the STS-1 channel assignment.
In the event of a failure or degradation of an optical span, the automatic ring protection switching functionality of SONET reroutes affected traffic away from the fault within 50 milliseconds in order to prevent a service outage. Traffic is redirected by looping back STS-1 time slots across the protection path in the other direction. The normally unused protection bandwidth thus forms a logical bridge over the defective span, thereby maintaining service for all terminating and pass-through traffic.
Another characteristic of the 2F BLSR architecture is that it allows individual STS-1 channels to be reused as traffic is terminated at various locations around the ring. This feature makes the 2F BLSR architecture ideally suited to the mesh and node-to-adjacent-node traffic patterns found in interoffice networks and also in certain types of access networks. The reuse of STS-1 channels also offers important bandwidth synergies in ATM networks.
In a four-fiber (4F) BLSR, two pairs of unidirectional fibers link adjacent nodes in the ring. One fiber pair exclusively carries working traffic while the other pair serves as a protection facility. If a fault affects a working fiber along a span, traffic is rerouted along the corresponding protection fiber. If the fault affects both the working and protection fibers, automatic ring protection switching redirects traffic in a manner similar to a 2F BLSR. However, instead of looping back time slots within the same fiber pair as in a 2F BLSR, traffic is transferred from the working pair to the protection pair.
In many cases, it is desirable to exchange information not only between nodes in a ring but also between nodes located in separate rings. Using SONET, adjacent rings are easily connected to one another by virtue of arranging one or more nodes from each ring to communicate as gateway nodes. While the above-described route diversity fully protects all working traffic passing from node to node along an individual ring, service paths must nevertheless be protected on an end-to-end basis. This means that survivable inter-ring connections are required as traffic passes through the designated gateway nodes from the ring serving the entry point to the ring serving the termination point.
To this end, protection of inter-ring traffic can be provided by the SONET “matched nodes” configuration, in which redundant (i.e., duplicate) routing is provided across inter-ring boundaries. For example,
FIG. 1
illustrates a matched nodes configuration as applicable to two 2F BLSR rings wishing to communicate with one another. Node
6
in ring
2
and “matched” node
14
in ring
8
have been chosen as primary gateway nodes, while node
10
in ring
2
and “matched” node
16
in ring
8
have been configured as secondary gateway nodes.
Within primary gateway node
6
, a drop-and-continue router
4
is used for duplicating a working signal and forwarding copies of the signal to secondary gateway node
14
in ring
8
as well as to secondary gateway node
10
within ring
2
. Secondary gateway node
10
is equipped with means for forwarding the received copy to secondary gateway node
16
in ring
8
, which then sends the copy to primary gateway node
14
. Thus, under normal operating conditions, primary gateway node
14
in ring
8
receives two copies of the signal transmitted by primary gateway node
6
in ring
2
. In
FIG. 1
, the duplicate paths between the two rings are shown as a thick solid line.
Of course, both copies of the delivered service are not required at the primary gateway node
14
in ring
8
. For this reason, node
14
is equipped with a service selector
12
that chooses one of the copies as a function of signal integrity, which can be inferred from standard parameters such as the line and/or path alarm indication signal (AIS). For the explanatory purposes, it is assumed that the service selector
12
is programmed to select the “primary” inter-ring signal arriving directly from node
6
rather than the “secondary”inter-ring signal arriving via node
16
.
In the event of a failure within either ring (e.g., a fiber fault along lines B-B′or C-C′ in FIG.
1
), the matched nodes configuration provides no significant benefit, as the automatic ring protection switching facility of SONET will cause the working signals to be looped back over the appropriate protection path in the respective ring. Rather, the classical advantage of the matched nodes configuration is that inter-ring traffic is protected in the event of a failure on the inter-ring span between primary gateway nodes
6
and
14
, e.g., a fiber fault along lines A-A′ i
Caporuscio Robert
de Boer Evert
Pare Louis-Rene
Phelps Peter
Steele David
Nortel Networks Limited
Ton Dang
LandOfFree
Method and apparatus for ensuring survivability of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for ensuring survivability of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for ensuring survivability of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3096530