Optical communications – Multiplex – Optical switching
Reexamination Certificate
2001-03-08
2004-09-14
Sedighian, M. R. (Department: 2633)
Optical communications
Multiplex
Optical switching
C398S012000, C398S019000, C398S045000, C398S050000, C398S054000, C398S056000
Reexamination Certificate
active
06792207
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to optical networks that employ wavelength division multiplexing and, more particularly, to optical networks that use optical cross-connect switches in transit and hub nodes.
Many existing long distance telecommunication systems include spans of optical fiber that link digital switches in a network. Such systems, however, often operate at a single transmission wavelength and use time division multiplexing, which can restrict expansion of the system to handle larger volumes of voice or data communications.
Wavelength division multiplexing (WDM) provides a technique to accommodate increased traffic in existing long distance telecommunication networks. WDM takes advantage of the large bandwidth of optical fibers and sends multiple communications down a single optical fiber in separate wavelength channels. As a result, a WDM system can multiply the capacity of the system compared with the use of only a single wavelength.
Other prospects for expanding the capacity of an existing network provide significant obstacles. For instance, adding new optical fibers to handle more traffic is expensive and can complicate management of the existing network. Also, increasing the bit rate of a single wavelength system can cause transmission problems, such as polarization mode dispersion or self-phase modulation. WDM can avoid these problems while using the existing fiber infrastructure.
Upgrading an existing optical network to WDM, however, may present additional problems when the multiple links between nodes in the network are not uniform. For example, two links between three network nodes may have different numbers of optical fibers.
FIG. 1
illustrates an optical network
100
having links such as
105
,
110
, and
115
interconnecting nodes such as nodes A, B, and C. Link
105
has 6 fibers (each line representing 2 fibers), link
110
has 4 fibers, and link
115
has 2 fibers. Coordinating these differences in fiber capacity presents challenges to the cross-connect switches within nodes A, B, and C.
Moreover, incorporating some links and nodes already operating with WDM into a larger WDM network presents similar problems when the different links are not uniform. For instance, if some WDM has been used between the links, the number of wavelength channels, the wavelengths themselves, or the transport standards may differ. These inequalities may exist due to the current lack of standardization in many areas of WDM communications. Consequently, optical cross-connect switches (OXCs) within the various network nodes must interface the incoming and outgoing optical fibers while maintaining compatibility with the varying optical standards used by the neighboring links.
In order to permit optical communication through an existing network like
100
, nodes A-F are equipped with OXCs whose task is to switch the optical channels coming from N input fibers to N output fibers. In general, the possible OXC architectures can be divided into two main classes: fiber-routing cross-connect switches (FR-OXCs), wavelength-routing cross-connect switches (WR-OXCs), and combinations of them. FR-OXCs are also known as optical switches.
FIG. 2
illustrates a typical scheme of a WR-OXC. The WR-OXC performs routing channel by channel, thereby allowing channels from the same input fiber to be sent to different output fibers. WR-OXC 200 includes demultiplexers
205
and
210
for separating the signals traveling in received WDM combs via optical fibers
215
and
220
. After the demultiplexing, optional 3R regenerators or transponders
225
and
230
can provide both signal regeneration and wavelength adaptation for each of the wavelengths entering WR-OXC switch
235
. Transponders
225
and
230
, if present, convert the wavelengths to a grid used particularly by switch
235
and are typically realized by electro-optic techniques. This wavelength conversion, if present, makes the large network opaque with respect to carrier wavelengths, although all-optical networks are envisioned for the future that perform all phases of transmission, amplification, and switching in the optical domain. After passing through switch
235
, where any entering wavelength may be switched to any exiting path, the respective wavelengths can pass through optional output transponders
240
and
245
, are combined in multiplexers
250
and
255
, and exit via optical fibers
260
and
265
. Optional output transponders
240
and
245
can convert the carrier wavelengths to the particular value required by the network downstream of WR-OXC 200.
In contrast to the WR-OXC, an FR-OXC has the task of switching entire optical WDM combs between input fibers and output fibers without demultiplexing the optical channels. Adding or dropping of entire WDM combs from or to another optical network entity via the FR-OXC is also possible. Like the WR-OXC, the FR-OXC is statically configured based on a routing table and is reconfigured when the traffic changes. No regeneration is present in the FR-OXC, since the individual WDM channels are not demultiplexed. It is evident that once the number of input and output optical fibers is fixed, a WR-OXC has more switching versatility than an FR-OXC. Due to this increased complexity, WR-OXCs cost more than FR-OXCs.
In a wide area network, such as
100
, each node processes and switches a certain number of local channels and a certain number of passthrough channels. The local channels are generated or terminated at destinations affiliated with the node, while the passthrough channels are routed through the node to elsewhere in the network. If the number of nodes is more than a few dozen, the majority of network nodes generally has many more passthrough channels than local channels. These majority of nodes are called transit nodes. The number of local channels exceeds the number of passthrough channels in only a few nodes, which are called hub nodes.
Patents and publications have contemplated the combination of WR-OXCs and FR-OXCs into a tandem switch structure in certain circumstances. For instance, U.S. Pat. No. 5,457,556 discloses an OXC system having a space switch with first and second inlet ports and first and second outlet ports. The first inlet and first outlet ports receive, switch, and transmit entire WDM combs within the space switch. A wavelength switch has first inlet ports connected to the second outlet ports of the space switch via a demultiplexer and first outlet ports connected to the second inlet ports of the space switch via a multiplexer. The wavelength switch receives selected WDM combs from the space switch, switches individual channels between the combs, and sends the switched channels back to the space switch.
In the '556 patent, system protection occurs at a fiber or multiplex section level. Because protection switching is accomplished without demultiplexing a WDM comb, all incoming and outgoing fibers enter and exit the tandem OXC via a space switch, i.e. an FR-OXC. Accordingly, each node in the network, which operates with fiber level protection, has at least an FR-OXC as the input and output for the links to other nodes.
U.S. Pat. No. 5,805,320 discloses an OXC having a combined FR-OXC and WR-OXC where the FR-OXC handles the bypass component for reserve optical transmission lines in a fiber-level protection scheme termed Type A. The WR-OXC is connected to the working optical transmission lines, and connection links interface the two switches together. For Type B and Type C protection schemes that operate at the channel level, the '320 patent discloses a combination of WR-OXC switches for both the working and reserve optical transmission lines without the use of an FR-OXC.
Applicants have observed that the known arrangements for OXCs within an optical network suffer from excessive complexity for fulfilling the needs of both transit nodes and hub nodes. In particular, Applicants have recognized that in an opaque optical network having a channel-level protection scheme, existing configurations of OXCs contain an overab
Bentivoglio Ravasio Filippo
Iannone Eugenio
Baker & Botts L.L.P.
Cisco Technology Inc.
Sedighian M. R.
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