Optical line switching system

Details Optical line switching system Optical line switching system

1999-12-23

2002-11-05

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

Switch

C385S024000, C359S199200, C359S199200

Reexamination Certificate

active

06477288

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical communication system, in particular, to an optical network with fault recovery capabilities.
2. Discussion of the Background
The explosive growth of the Internet, which has millions of users, has and will continue to generate enormous data traffic over the backbone networks of the Internet. These backbone networks correspondingly require greater bandwidth in the transmission paths to support the many user applications, from email to streaming video. By the same token, any disruption along the transmission paths of the backbone networks would adversely affect countless numbers of users. Such a disruption may stem from a cable cut, or some other fault (e.g., equipment failure).
The effects of the disruption range from increased response times to data loss. When the traffic is mission critical, any loss of information is unacceptable. Hence, along with greater bandwidth demand from the users, service providers are required to guarantee extremely high network reliability, which in turn requires spare capacity.
It has been recognized that fiber optic cables hold the promise of being able to rapidly transport vast amounts of traffic. Thus, development in optical communications networks has steadily progressed. SONET (synchronous optical network), which is promulgated by the American National Standards Institute (ANSI), has emerged as an accepted standard defining the transport and management of data traffic over fiber optic transmission systems. An important feature of SONET is its ability to automatically recover from network faults.
A conventional SONET ring network, as schematically shown in
FIG. 16
, utilizes four optical fibers to transport data, in which two optical fibers
1
a
and
1
b
are designated as working, while the other two
2
a
and
2
b
are considered protection fibers. The fibers
1
and
2
connect five SONET nodes, N
1
-N
5
. The optical signals within the working fiber pairs
1
a
and
1
b
are normally transmitted in opposite directions, as indicated by the arrowheads. Similarly, within the protection fiber pairs
2
a
and
2
b
, the fiber line
2
a
carry optical signals in an opposite direction to the optical signals that are transmitted by fiber line
2
b
. As will be discussed more fully below, the optical signals that are transmitted via the working fiber pairs
1
are switched over to the protection pairs
2
a
and
2
b
upon occurrence of a fault; SONET nodes N
1
-N
5
are SONET add/drop equipment for adding and dropping the optical signals to and from the working optical fibers
1
a
and
1
b
, and the protection optical fibers
2
a
and
2
b.
The add/drop function within SONET is performed at the electrical level; accordingly, a SONET node must temporarily convert the optical signals into electrical signals before processing the optical signals. Based upon examination of the electrical signals, the SONET node can determine whether a fault has occurred in the network.
FIG. 16
shows an operational scenario in which all the transmission paths
1
a
,
1
b
,
2
a
, and
2
b
as well as the nodes N
1
-N
5
are fully functioning. In this example, when an optical signal is added to the node N
3
(as indicated by the arrowhead A) and is destined to node N
1
. The optical signal follows the shortest route, and thus, is transmitted in a clockwise direction via optical fiber
1
b
to the destination node N
1
. The optical signal is then dropped at node N
1
, as indicated by the arrowhead D. On the other hand, if an optical signal is added to node N
1
, as indicated by the arrowhead A, the optical signal is transmitted counterclockwise via optical fiber
1
a
to the destination node N
3
(which is the shortest route). Next, the transmitted optical signal is dropped at node N
3
.
Although not illustrated in
FIG. 16
, a node connected to another ring network is provided with cross-connect functionality to switch the optical signals over to the other network. That is, under the conventional approach, to link one optical communications network to another, an electrical cross-connect converts the optical signals into electrical signals, then reconverting some of the electrical signals, if necessary, into optical signals for transmission over the other ring network.
FIGS. 17A and 17B
illustrate the operation of the SONET ring, whereby the optical fibers between nodes N
1
and N
2
are totally and partially down, respectively. The protection function of the SONET layer works to set alternative transmission paths in the ring network. Under these fault conditions, alternative routing of the optical signals can be achieved using two protection schemes: (1) ring protection, and (2) span protection. The architecture of the SONET ring in
FIGS. 17A and 17B
resembles that of
FIG. 16
, in which five nodes are interconnected via four optical fibers (i.e., an working pair
1
and a protection pair
2
).
FIG. 17A
shows an alternative routing scheme known as “ring protection”, in which all four optical fibers (i.e., working optical fibers
1
a
and
1
b
and protection optical fibers
2
a
and
2
b
) simultaneously fail. Under this scenario, a fault Os occurs between the node N
1
and the node N
2
in the ring network. The ring protection system sets an alternative route by detecting faults in the fiber links, in this case, between the nodes N
1
and N
2
. Based upon detection of fault Os, the ring protection system connects the working optical fiber
1
a
to the protection optical fiber
2
b
, and the working optical fiber
1
b
to the protection optical fiber
2
a
. According to the ring protection scheme, the optical signal added in the node N
3
, as indicated by the arrowhead A, is carried by the working optical fiber
1
b
. The shortest route to the destination node N
1
is in the clockwise direction via node N
2
. Upon the optical signals reaching node N
2
, node N
2
recognizes that both the working pairs
1
and the protection pairs
2
are disconnected due to some fault. Accordingly, node N
2
switches the optical signal over to the protection optical fiber
2
a
. The optical signal, thereafter, is transmitted in the counterclockwise direction to the node N
1
, where the optical signal is dropped (as indicated by the arrowhead D) from the protection optical fiber
2
a.
In a converse situation, node N
1
adds an optical signal to the protection optical fiber
2
b
. After the optical signal is transmitted clockwise, it temporarily passes through the destination node N
3
so that node N
2
may switch the optical signal over to the working optical fiber
1
a
. Node N
2
transmits the optical signal in a counterclockwise direction and drops the optical signal from the working optical fiber
1
a
at destination node N
3
, as indicated by the arrowhead D.
FIG. 17B
shows the operation of an alternative routing scheme known as “span protection”. As shown, a fault Os occurs between the node N
1
and the node N
2
in the ring network, in which the working optical fibers
1
a
and
1
b
experience failure. Under this scenario, the span protection system detects a fault between nodes N
1
and N
2
, involving both working optical fibers
1
a
and
1
b
. After detection of this fault Os, the span protection system utilizes protection optical fibers
2
a
and
2
b
in that particular span where working lines
1
a
and
1
b
failed.
In the example of
FIG. 17B
, node N
3
adds an optical signal to the working optical fiber
1
b
, and transmits the optical signal clockwise to node N
2
. Node N
2
then switches the optical signal over to protection optical fiber
2
b
, transmitting the optical signal in a clockwise direction, and dropping it in the destination node N
1
.
In the case where an optical signal is added at node N
1
to the protection optical fiber
2
a
, node N
1
transmits the optical signal in a counterclockwise direction to node N
2
. Node N
2
switches the optical signal over to the working optical fiber
1
a
, and transmits the optical signal counterclockwise, droppi

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