Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1999-01-13
2002-09-17
Jung, Min (Department: 2663)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S535000
Reexamination Certificate
active
06452945
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 electrical crosspoints to implement the add/drop multiplexing (ADM) function in optical fiber communications systems using frequency-division multiplexing (FDM).
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the areas of networking including the Internet, telecommunications, and application areas which rely on networking or telecommunications, there is an increasing demand for capacity for the transmission of digital data. For example, the transmission of digital data over a network's trunk lines (such as the trunk lines for telephone companies or for the Internet), the transmission of images or video over the Internet, the distribution of software, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically requires the high speed transmission of large amounts of digital data. Typical protocols which are intended to support such transmissions include the SONET and SDH protocols. As applications such as the ones mentioned above become more prevalent, the use of these and similar protocols and the corresponding demand for transmission capacity will only increase.
Optical fiber is a transmission medium which is well-suited to meet this increasing demand for transmission capacity. 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 digital data over optical fibers.
However, 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, for example transmitted by different users, are combined into a single high-speed channel for transport across the fiber. Similarly, when the high-speed channel reaches the destination for one of the low-speed data streams contained in it, the low-speed data stream must be extracted from the rest of the high-speed channel. The low-speed data streams shall be referred to as tributaries. Once multiple tributaries are combined into a high-speed channel, the corresponding portion of the high-speed channel shall be referred to as a “low-speed channel” that occupies a “low-speed slot” within the high-speed channel. Thus, a high-speed channel contains a number of low-speed slots, each of which may be occupied by a low-speed channel. Furthermore, each low-speed channel corresponds to a tributary, or possibly a group of tributaries.
A typical optical network consists of nodes which transmit high-speed channels to each other over optical fibers. The tributaries may be fed to and received from these nodes via a number of communications channels, including branch fibers, metal conductors, or even wireless communications channels. In addition to transporting low-speed channels through the node (the “pass-through” function), nodes typically also combine incoming tributaries to the high-speed channel (the “add” function) and extract outgoing tributaries from the high-speed channels (the “drop” function). These functions are commonly referred to as add-drop multiplexing (ADM).
Increasing the ADM functionality of the nodes in a network enhances both the applicability and the reliability of the network by increasing the number of applications, network configurations, and types of protection mechanisms that may be implemented by the network. For example, as described above, basic add, drop, and pass-through functionality supports the addition and extraction of tributaries to and from low-speed slots within high-speed channels. This enables a variety of network configurations, including point-to-point, linear chain, ring, and ring-to-ring configurations. More advanced ADM functionalities include drop-and-continue, in which a low-speed channel is both dropped as a tributary from one high-speed channel and continued on a low-speed slot of another high-speed channel; broadcast, in which a low-speed channel is dropped from a high-speed channel but then broadcast to multiple tributaries rather than just a single tributary; and multicast, in which a single tributary is added to multiple low-speed slots in one or more high-speed channels. These functionalities enable additional services, such as video and other Internet applications, to be deployed on top of the network configurations listed above. The added flexibility also facilitates the use of redundancy and the reconfiguration of the network with minimal disturbance to the on-going operation of the 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. Two widely used approaches to combining low-speed channels are wavelength division multiplexing (WDM) and time division multiplexing (TDM). In WDM, 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 concernin 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 tributary, as is typically the case, the total number of tributaries is also limited. Furthermore, if the bandwidth capacity of the fiber is to be used efficiently, each tributary must have a relatively high data rate due to the low number of tributaries, 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 tributaries, 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 tributaries 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 tributaries, 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 all-optical approaches, the channels typically are not regenerated as they pass through each node in the network and will continuously deteriorate until they reach their final destination. As a result, the entire network must be designed assuming
Coward James F.
Elmer Augustus
Ho Minnie
Kou Abraham
Newell Laurence J.
Fenwick & West LLP
Jung Min
Kestrel Solutions, Inc.
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