Inter-ring cross-connect for survivable multi-wavelength...

Details Inter-ring cross-connect for survivable multi-wavelength... Inter-ring cross-connect for survivable multi-wavelength...

1997-12-04

2001-05-01

Negash, Kinfe-Michael (Department: 2633)

Optical: systems and elements

Deflection using a moving element

Using a periodically moving element

C359S199200, C359S199200, C370S223000

Reexamination Certificate

active

06226111

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention generally relates to multi-wavelength optical communication networks, especially multi-wavelength optical networks. In particular, the invention relates to cross-connects between multiple rings for which the rings are designed to be self-healing to faults.
The introduction of optical fibers as the transmission medium for communication networks has slowly been altering the fundamental architecture of the networks. Originally, optical fibers simply represented a replacement for the electronic links usually carried on copper cable. The electrical signals, which otherwise would have been transmitted on copper links, were used to modulate lasers on the transmission end, and optical detectors, on the receiver end, were used to reconvert the signal to its original electrical form. That is, the use of optical fibers did not affect the fundamental architecture of the network. Also, the original application of optical fibers was to the long-distance transmissions, but its utility is becoming more obvious to the more local networks.
The existing network architecture upon which the fiber link has been imposed may be characterized as a multi-level mesh. At the level of a Local Access and Transport Area (LATA), each central office is typically connected to several neighboring offices with electrical links having capacity appropriate for that link. This architecture was implemented in hardware designed in the early 1980's and was driven by a paucity of bandwidth and the relatively slow electronics then available.
The voice traffic dominant at the time of the design of the present network is digitized into DS0 channels, each of 64 kb/s (kilobits per second). Twenty-four DS0 channels are multiplexed into a DS1 channel at 1.544 Mb/s, and, if required, 28 DS1 channels are multiplexed in a separate step into a DS3 channel at 44.736 Mb/s. These rates are not exact multiples, and bits are stuffed into the transmission stream as are necessary. Further, each link has its own clock. The result is an asynchronous network in which a high-level multiplexed signal needs to be completely demultiplexed in order to extract any signal or to substitute another low-level signal.
Optical fiber changed the equation because its intrinsic bandwidth is nearly unlimited. In a fiber network, the terminal equipment for the most part determines the bandwidth, and the cost of the link becomes relatively small compared to that of the terminal equipment. As a result, a new standard was proposed and largely accepted in the U.S.A. This standard is referred to as the Synchronous Optical Network (SONET). A closely related architecture, Synchronous Digital Hierarchy (SDH), is followed in Europe. The basic building block is called the Synchronous Transport Signal-Level 1 (STS-1) which has a bit rate of 51.84 Mb/s. The transmission is divided into frames transmitting at a frame rate of 125 &mgr;s. The frames are further divided into 810 eight-bit bytes, many of which are overhead. The STS-1 frames are carried on an OC-1 optical channel operating at the same bit rate. Higher rates are available which are multiples of those above. These are STS-N signals, which are formed by simply interleaving N STS-1 signals. Currently, OC-48 fiber links operating at 2488.32 Mb/s represent the most advanced system that is commonly deployed. For the most part, the maximum signal rate is determined by the electronic and opto-electronic equipment attached to the fiber ends and not by the fiber itself. SONET is a synchronous signal, and extracting individual bytes or lower-level channels is much easier than with an asynchronous signal.
The increased capacity of optical fiber has raised concerns about the reliability and survivability of optical networks since a single cable or equipment malfunction can impact a large amount of traffic. Cable cuts are frequent and almost impossible to avoid, whether from human or weather causes, and equipment failures resulting from central-office fires or other disasters cannot be totally eliminated. Accordingly, more survivable network architectures are sought. One architecture that offers high survivability is a self-healing ring. Several versions of this architecture are described by Wu in
Fiber Network Service Survivability
, (Artech House, 1992), pp. 123-207. The self-healing function mitigates against network disasters, but its implementation must be simple, high-speed, and highly reliable. The self-healing should be totally automatic and provide 100% restoration capability for a single fiber-cable cut or equipment failure through its ring topology and simple but fast protection switching scheme. Many self-healing ring architectures have the advantage of being able to recover from the failure of a single node, such as a hub, along with the ability to recover from a cable cut.
An exemplary unidirectional self-healing fiber ring
10
is illustrated in FIG.
1
. This is one type of self-healing ring network and is presented here to support the introductory discussion. A more complete description of the different architectures of self-healing rings will be presented later.
In
FIG. 1
, a number of nodes
12
, here illustrated as four nodes A, B, C, D, are interconnected in a ring configuration by two counter-rotating optical fibers
14
,
16
. That is, one fiber
14
forms a ring around which signals propagate in the counter-clockwise direction while the other fiber
16
forms another ring around which signals propagate in the clockwise direction. Each node
12
can be a central office, a remote distribution point within the local network, or other high-traffic node. Importantly, each node
12
is connected to each of the fibers
14
,
16
at two points, one for reception, one for transmission. The first fiber
14
is a working fiber and, in this particular architecture, carries all the traffic. The second ring fiber
16
, indicated by the dashed line, is a protection fiber. In normal operation, it is dark for Automatic Protection Switching (APS), but for Path Protection (PP) it carries some or all of the traffic nominally assigned to the working fiber
14
. The protection fiber
16
propagates whatever signals it carries around the ring
10
in a direction opposite to that of the working fiber
14
, and the choice of which fiber
14
,
16
propagates in the clockwise direction is, of course, immaterial.
The number M of nodes
12
within the ring can vary but is generally in the range of 4 to 10. A smaller number of nodes can be accommodated within a mesh architecture without the need for protection fibers or for multi-hop transmission. This difference arises because a ring network of M nodes requires W wavelengths for full mesh connectivity within the ring, where for odd values of M
W
=
M
2
-
1
8
and for even values of M
W
=
(
M
+
1
)
2
-
1
8
A larger number of nodes introduces a high excess level of traffic through each node that is not needed by that node.
It is assumed that the working and protection fibers
14
,
16
are co-located along virtually the same geographic paths so that a cable break arising from a construction accident, a weather disaster, or the like is likely to affect both of them. However, it is also assumed that the fibers
14
,
16
are routed such that the different inter-nodal portions extend for the most part along different paths so that cable breaks usually affect only one inter-nodal portion of the dual ring. Although the figures show a neat circular ring, it is to be appreciated that rings can be set up within the existing mesh network, even using existing point-to-point fibers, resulting in a more ragged shape. It is also to be appreciated that these rings can be enlarged or shrunk to a different set of nodes within the ring without necessarily laying new fiber between the nodes.
The most common fiber failure is a cable break
20
, illustrated in
FIG. 2
as occurring between nodes C and D and assumedly cutting both a portion
14
′ of the working fiber
14
and the corresponding portion
16
′ of the protection fiber
16
.

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