Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-09-24
2003-03-04
Tong, Nina (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C370S480000
Reexamination Certificate
active
06529303
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to the field of optical fiber communications. More specifically, the invention relates to the use of frequency-division multiplexing (FDM) in optical fiber communications systems.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking, there is an increasing demand for communications bandwidth. For example, the growth of the Internet, home office usage, e-commerce and other broadband services is creating an ever-increasing demand for communications bandwidth. Upcoming widespread deployment of new bandwidth-intensive services, such as xDSL, will only further intensify this demand. Moreover, as data-intensive applications proliferate and data rates for local area networks increase, businesses will also demand higher speed connectivity to the wide area network (WAN) in order to support virtual private networks and high-speed Internet access. Enterprises that currently access the WAN through T1 circuits will require DS-3 and OC-3 connections in the near future. As a result, the networking infrastructure will be required to accommodate greatly increased traffic.
Optical fiber is a transmission medium that is well-suited to meet this increasing demand. Optical fiber has an inherent bandwidth which is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the OC protocol have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal.
One approach to address the increasing demand for communications bandwidth is to simply add more optical fiber to the current networking infrastructure. However, this is not always a viable alternative. There are many areas of the country, for example metropolitan areas, where the ducts or conduits carrying optical fiber are filled to capacity or where the fiber was originally buried directly into the ground. In addition, adding more fiber is often both prohibitively expensive and time-consuming, due to high installation costs and local resistance to the disruption caused by fiber installation. These factors therefore favor solutions which increase communications bandwidth by more efficiently utilizing the installed fiber infrastructure rather than by installing new fiber.
Furthermore, other factors also favor solutions other than installing new fiber. For example, because of its large inherent bandwidth, an optical fiber is most efficiently used when multiple users share the fiber. Typically, a number of low-speed data streams (i.e., “low-speed channels”), for example transmitted by different users, are combined into a single high-speed channel for transport across the fiber. Conversely, when the high-speed channel reaches the destination for one of the low-speed channels contained in it, the low-speed channel must be extracted from the rest of the high-speed channel. A typical optical network consists of nodes which transmit high-speed channels to each other over optical fibers. In addition to transporting low-speed channels through the node (the “pass-through” function) as part of high-speed channels passing through the node, nodes may also combine incoming low-speed channels to the high-speed channel (the “add” function) and/or extract outgoing low-speed channels from the high-speed channels (the “drop” function). These functions are commonly referred to as add-drop multiplexing (ADM).
Increasing the ADM functionality of nodes in a network increases the flexibility of the network, thus increasing the number of applications and network configurations that may be implemented by the network. For example, metropolitan networks are characterized by densely populated areas, a large number of nodes (e.g., central offices), short distances between nodes (typically less than 40 km), and lower data rates than long distance networks (typically less than 2.5 Gbps). The traffic patterns for metropolitan networks change rapidly and require dynamic interconnections at the large number of nodes, which are often remotely managed. ADM functionality allows low-speed channels to be remotely added to or dropped from a high-speed channel, thus addressing the requirements of the metropolitan network.
However, the manner in which the ADM functionality is implemented in a particular network will depend in part on how the low-speed channels are combined to form a high-speed channel. Thus, an approach which addresses the capacity problem by combining a large number of low-speed channels into a high-speed channel may not be favored if it does not readily support ADM functionality. A good approach should both increase the number of low-speed channels contained in each high-speed channel and also support significant ADM functionality.
Two widely used approaches to combining low-speed channels are wavelength division multiplexing (WDM) and time division multiplexing (TDM). In WDM or its more recent counterpart dense wavelength division multiplexing (DWDM), each low-speed channel is placed on an optical carrier of a different wavelength and the different wavelength carriers are combined to form the high-speed channel. Crosstalk between the low-speed channels is a major concern in WDM and, as a result, the wavelengths for the optical carriers must be spaced far enough apart (typically 50 GHz or more) so that the different low-speed channels are resolvable. In TDM, each low-speed channel is compressed into a certain time slot and the time slots are then combined on a time basis to form the high-speed channel. For example, in a certain period of time, the high-speed channel may be capable of transmitting 10 bits while each low-speed channel may only be capable of transmitting 1 bit. In this case, the first bit of the high-speed channel may be allocated to low-speed channel
1
, the second bit to low-speed channel
2
, and so on, thus forming a high-speed channel containing 10 low-speed channels. TDM requires precise synchronization of the different channels on a bit-by-bit basis (or byte-by-byte basis, in the case of SONET), and a memory buffer is typically also required to temporarily store data from the low-speed channels.
In the case of WDM, one approach is to implement the ADM functionality entirely in the optical domain. This avoids having to convert the high-speed channel from optical to electrical form, but has a number of other significant limitations. First, as described previously, the wavelengths for each of the optical carriers in a WDM system typically are spaced far apart (e.g. 50 GHz or more). As a result, the number of different optical carriers is limited and if each carrier corresponds to a low-speed channel, as is typically the case, the total number of low-speed channels is also limited. Furthermore, if the bandwidth capacity of the fiber is to be used efficiently, each low-speed channel must have a relatively high data rate due to the low number of low-speed channels, thus preventing add-drop at a fine granularity. For example, if the high-speed channel has a total capacity of 10 Gigabits per second (10 Gbps) and is allotted a bandwidth of 200 GHz, then current WDM systems will typically be limited to no more than four low-speed channels, each of which will be 2.5 Gbps in order to meet the overall bit rate of the high-speed channel. However, this means that the low-speed channels can only be added or dropped in blocks of 2.5 Gbps. Since many data streams occur at a much lower bit rate, such as at 155 Megabits per second (Mbps) for OC-3, it is often desirable to add and drop at a granularity which is finer than what WDM can support.
The current state of technology also limits the practicality of all-optical ADM. In al
Chang Peter H.
Coward James F.
Elmer Augustus
Ho Minnie
Kou Abraham
Fenwick & West LLP
Kestrel Solutions, Inc.
Tong Nina
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