Abstract:
One embodiment of the present invention provides a system that facilitates optical switching. The system starts by receiving a plurality of optical input signals. The system then divides each of the plurality of optical input signals into a plurality of wavebands that can be carried on a single optical fiber, wherein each waveband includes a predetermined subset of the wavelengths in the optical signal. Once the optical input signals have been divided into wavebands, the wavebands are then routed through a waveband switch. After being routed through the waveband switch, the wavebands are combined to form a plurality of optical output signals, where each optical output signal can possible include wavebands from different optical input signals. Additionally, some of the wavebands can be divided into wavelengths, and the wavelengths can be routed through a wavelength switch or a traffic grooming switch.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to the design of optical networks. More specifically, the present invention relates to a method and apparatus that performs hierarchical optical switching to facilitate routing of data across an optical network. 
     2. Related Art 
     The explosive growth of the Internet has resulted in the vast demand for bandwidth by network operators. Experts predict that the Internet bandwidth demand will continue to grow rapidly by doubling itself every 6-9 months over the next several years. Such large bandwidth demand, coupled with the large bandwidth that optical fibers offer, is driving the wide deployment of optical networks. However, laying optical fibers in the ground is both expensive and time consuming. To meet the ever-growing bandwidth demand, the industry has been quick to embrace a technology that can multiply the transmission capacity of the existing fibers: wavelength-division multiplexing (WDM). With WDM technology, multiple wavelengths, each carrying a stream of bits at very high speed, can be transmitted simultaneously through a single fiber without interfering with one another. 
     In a short period of a few years, WDM technology has penetrated most of the optical networks, and will continue to be deployed in the future. New advances in the WDM technology has enabled more wavelengths to be available for telecom usage within the optical fiber&#39;s usable spectrum window. At the present time, the number of wavelengths in a typical telecom optical fiber ranges from 8 to 64, while more than one hundred wavelengths in one single fiber are becoming available in the near future. 
     As the total capacity of the network multiplies with WDM technology, one major challenge that exists for network operators is to effectively manage the increased amounts of bandwidth. Network operators need to deliver end-to-end connectivity at different data rates. The typical data rate on a wavelength is OC-48 (2.488 Gbps) or OC-192 (10 Gbps), with OC-768 (40 Gbps) becoming standard in the near future. However, connection requests from users come with different data rates. According to the widely-accepted Synchronous Optical Network (SONET) standard, connection are named as OC-N, where N indicates the data rate of the connection. Typical values are OC-1 (51.84 Mbps) (also referred to as STS-1), OC-3 (155.52 Mbps), OC-12 (622 Mbps), OC-48, OC-192, and OC-768. Lower-speed connections are grouped together to fill up the bandwidth of an entire wavelength (i.e., by using time-division multiplexing, TDM). Optical switches are used to inter-connect wavelengths and/or lower-speed connections at the network&#39;s switching nodes. These switching nodes perform two main functions: (1) routing connections from upstream nodes to downstream nodes, and (2) initiating and terminating connections to and from the client network elements (such as IP routers, ATM switches, etc.) which requested the connections. 
     The current art in switching technologies provides two approaches to the construction of an optical switch: an electronic switch fabric and an optical switch fabric. With an electronic switch fabric, the incoming optical signals are demultiplexed to separate out different wavelengths. Each wavelength is then terminated by a receiver that converts the bits from an optical signal to an electrical signal. These streams of bits then feed into an electronic switch fabric, which reads the bit streams from its input ports and routes them to its output ports. Once the streams exit from the electronic switch fabric, they are converted back to optical signals, on different wavelengths, and are multiplexed back together before entering the outgoing fiber. The electronic switch fabric is typically constructed from integrated circuit (IC) switch chips, which are smaller switches themselves (e.g., a 64×64 switch chip with 2.488 Gbps per port). Such a switching system is also called an optical-electrical-optical (OEO) switch, which means that all the optical signals are first converted to electrical signals, then switched electronically, and finally converted back to optical signals. 
     An alternative to electronic switch fabric is to use an optical switch fabric. An optical switch fabric can directly switch optical signals using, for example, tilting mirrors. Hence, it can switch a whole wavelength or a group of wavelengths without reading the bits. However, it cannot switch lower-speed connections within a wavelength. 
     What is needed is an optical switch that has large capacity and capability to switch different bandwidth granularities, both on the wavelength and sub-wavelength level. It also needs to have excellent scalability, i.e., the cost, power consumption, and size of the switch should be maintained at an acceptable level as the capacity of the switch grows larger. 
     SUMMARY 
     One embodiment of the present invention provides a system that facilitates optical switching. The system starts by receiving a plurality of optical input signals. The system then divides each of the plurality of optical input signals into a plurality of wavebands that can be carried on a single optical fiber, wherein each waveband includes a predetermined subset of the wavelengths in the optical signal. Once the optical input signals have been divided into wavebands, the wavebands are then routed through a waveband switch. After being routed through the waveband switch, the wavebands are combined to form a plurality of optical output signals, where each optical output signal can possibly include wavebands from different optical input signals. Additionally, some of the wavebands can be divided into wavelengths, and the wavelengths can be routed through a wavelength switch or a traffic grooming switch. 
     In a variation on this embodiment, the optical input signals are divided into wavebands by sending the optical input signals through an Arrayed Waveguide Grating (AWG) device. 
     In a variation on this embodiment, some of the plurality of wavebands are further divided into a plurality of wavelengths, and the wavelengths are routed through a wavelength switch. Note that some of the wavebands might be routed through the waveband switch, while others are divided into wavelengths and are routed through the wavelength switch. 
     In a further variation on this embodiment, the wavebands are divided into wavelengths by sending the wavebands through an AWG device. 
     In a further variation on-this embodiment, the plurality of wavelengths is further divided into a plurality of Time-Division Multiplexing (TDM) signals, and the TDM signals are routed through a switch. Note that some of the wavelengths might be routed through the wavelength switch, while others are divided into TDM signals. 
     In a variation on this embodiment, some of the plurality of wavebands are further divided into a plurality of wavelengths, and the wavelengths are routed through a TDM traffic grooming switch. Note that some of the wavebands might be routed through the waveband switch, while others are divided into wavelengths and are routed through the TDM traffic grooming switch. 
     In a variation on this embodiment, a subset of the optical input signals are routed directly to optical output signals without being divided into wavebands. 
     In a variation on this embodiment, routing of optical signals is performed by a Micro-Electro-Mechanical Systems (MEMS) based optical switch fabric. 
     In a variation on this embodiment, routing of optical signals is performed by a micro-fluid based optical switch fabric (bubble switch). 
    
    
     BRIEF DESCRIPTION OF THE FIGURES. 
     FIG. 1 illustrates a circuit-switched network in accordance with an embodiment of the present invention. 
     FIG. 2 illustrates a hierarchical optical switch in accordance with an embodiment of the present invention. 
     FIG. 3 illustrates an Arrayed Waveguide Grating (AWG) device in accordance with an embodiment of the present invention. 
     FIG. 4 illustrates a waveband and a wavelength switch in accordance with an embodiment of the present invention. 
     FIG. 5 illustrates a hierarchical optical switch with a TDM traffic grooming switch in accordance with an embodiment of the present invention. 
     FIG. 6 illustrates a hierarchical optical switch without sub-wavelength grooming in accordance with an embodiment of the present invention. 
     FIG. 7 illustrates the process of hierarchical switching in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Circuit-Switched Network 
     FIG. 1 illustrates circuit-switched network  100  in accordance with an embodiment of the present invention. Circuit-switched network  100  contains nodes  102  to  112 . Optical switching allows for virtual connections to be made directly between all nodes in circuit-switched network  100 . For example, in FIG. 1, direct connections are represented by solid lines and virtual connections are represented by dashed lines. Node  110  has a direct connection with nodes  108  and  112  via fiber. Similarly, node  112  has direct connection with node  102  via fiber. In contrast, node  110  has a virtual connection  114  with node  102  through an optical switch at node  112 . 
     Optical Networking Background 
     For an electronic switch fabric, there are typically two types of switch chips available: grooming switches and non-grooming (crosspoint) switches. A grooming switch chip is capable of separating incoming bit streams into lower-speed streams (also called time slots) and switching these lower-speed streams. An example is a 64×64 switch chip, with 2.488 Gbps (OC-48) data rate on each input/output port. This chip can switch at a granularity of 51.84 Mbps (OC-1 or STS-1). In other words, although the input/output data rate at each port is 2.488 Gbps (OC-48), the chip can separate each 2.488 Gpbs stream into 48 51.84 Mbps streams and then switch them individually. This function is called “traffic grooming” or “time-slot interchange.” The state-of-the-art grooming switch chip available today is a 72×72 OC-48 switch chip. In contrast, nongrooming chips do not have the capability of separating incoming bit streams into lower-speed streams. The state-of-the-art non-grooming chip is a 144×144 OC-48 switch chip. 
     For an optical switch fabric, there are currently two types of products. (There are other optical switch fabric technologies as well in various stages of development and maturity, but the two discussed below seem to be the leading contenders.) The first is micro-mechanical-electrical-system (MEMS)-based switch. This type of switch is based on micro-machined tilting mirrors fabricated on silicon or other semiconductor material. They are capable of re-directing incoming light beams to different output ports, regardless of how many wavelengths there are in the beam. Over a dozen vendors are trying to build commercial MEMS switches. 
     The second type of optical switch fabric is based on moving bubbles in liquid inside an optical waveguide. When a bubble is created and moved into the course of an optical waveguide, the light traveling in the waveguide can be re-directed through reflection. The bubble switch is also wavelength insensitive. This type of switch is mainly fabricated by Agilent Technologies, Inc. of Palo Alto, Calif., which applies its ink-jet technology to the switch design. 
     Both MEMS-based switches and bubble switches can switch one or multiple wavelengths at the same time, and they are bit-rate transparent; however, they cannot switch at sub-wavelength granularity (i.e., lower-speed connections). The state of the art is a 32×32 fabric, for both MEMS-based (by OMM Systems) and bubble switches (by Agilent). 
     There are several types of optical switch systems that vary according to the switch fabric used and how switch chips/fabrics are arranged. For OEO optical switch systems, there are grooming optical switches and non-grooming optical switches. An example of the grooming optical switch is the CoreDirector from Ciena Corporation of Linthicum, Md. It uses proprietary grooming chips to construct a 256×256 OC-48 switch, with OC-1 grooming granularity. An example of the non-grooming optical switch is the Aurora switch from Tellium, Inc. of Oceanport, N.J. Its capacity is 512×512 OC-48. The architecture of these OEO switches are typically Clos networks, where switch chips are arranged in a three-stage manner to achieve full non-blocking capability. Such architectures have poor scalability, as the number of switch chips required grows much faster than the overall switch size does. As a result, the cost, power consumption, and size of the entire switching system becomes unacceptable for large switch capacities. For switch systems based on optical switch fabrics, a main constraint is the small port count and high cost of the available optical switch fabric (MEMS-based or bubble-based). A large port-count switch system will require a large number of such optical switch fabrics, and the cost of such a system will be prohibitively high. An example of an optical-switch-fabric-based switching system is the 256×256 LambdaRouter from Agere Systems, Inc. of Allentwon, Pa. 
     Hierarchical Optical Switch 
     FIG. 2 illustrates a hierarchical optical switch  200  in accordance with an embodiment of the present invention. Hierarchical optical switch  200  combines an optical switch fabric and an electronic switch fabric. As a result, it has large capacity, low cost, excellent flexibility, low power consumption, small size, and very good scalability. Hierarchical optical switch  200  includes the following parts (switch hierarchies): fiber switch  202 , waveband (a collection of wavelengths) switch  204 , wavelength switch  206 , sub-wavelength grooming switch  210 , TDM multiplexer  212 , and TDM demultiplexer  214 . 
     Note that the routing of optical signals in hierarchical optical switch  200  can be performed by any one of the following: a MEMS micro-mirror based switch; a micro-fluid based switch (bubble switch); a bulk mechanical switch; an electro-optic switch, wherein a material changes its refractive index when voltage or current is applied; a thermo-optic switch, wherein a material changes its physical dimension or refractive index when its temperature is changed; a broadcast-and-select switch, wherein the incoming signal is split and sent to multiple ON/OFF switches, and an ON/OFF switch selects which output port to allow the light to pass through; a grating-based switch, wherein gratings are used to select one or more wavelengths (the gratings can be fixed or reconfigurable); an acousto-optic switch, wherein an acoustic wave is used to construct a grating-like refractive index fluctuation in the waveguide material and select one or more wavelengths; or a polarization-based switch, wherein the incoming optical signal is polarized and passed through a polarization-sensitive device (e.g., liquid crystal material), which routes the light to different directions according to its polarization. 
     In fiber switch  202 , incoming fibers are connected to an all-optical switch fabric. This switch fabric functions as an automatic patch panel that connects incoming fibers containing bypassing traffic directly to the outgoing fibers. Fibers containing traffic that needs to be switched at a finer granularity (e.g., waveband level, wavelength level, or sub-wavelength level) or that needs to be dropped at the local node are connected to output ports that lead to the next lower level in the hierarchy (i.e., waveband switch  204 ) of the switch system. 
     In waveband switch  204 , incoming fibers, which come out from fiber switch  202 , are first connected to Arrayed Waveguide Grating (AWG) device  208  which acts as a waveband demultiplexer. The function of a waveband demultiplexer is to separate all the wavelengths carried by a fiber into several groups (wavebands). These wavebands from different fibers are then sent into the optical switch fabric, which functions as a waveband switch. At the output of waveband switch  204 , wavebands are multiplexed together and sent into fibers that lead to the next higher level of the switch hierarchy (i.e., fiber switch  202 ). In the optical switch fabric, wavebands can be switched into different fibers other than the ones they were originally traveling in. 
     Wavebands containing traffic that needs to be switched at wavelength-or sub-wavelength-level are sent to ports that lead to the next lower level of the switch hierarchy (i.e., wavelength switch  206 ) of the system. For wavebands containing only traffic to be dropped at the local node, they can be directly switched to the local TDM demultiplexer  214 , which will demultiplex the wavebands into individual wavelengths and disseminate the connections to client network elements. Similarly, if there is enough traffic to initiate a whole waveband, the local TDM multiplexer  212  can aggregate all the traffic and directly send it in a waveband to waveband switch  204 . 
     In wavelength switch  206 , incoming wavebands are first sent to AWG device  208  which acts as a wavelength demultiplexer, which separates individual wavelengths from the waveband. These wavelengths are then sent into wavelength switch  206 , which can be based on electronic switch chips or optical switch fabric. At the output of wavelength switch  206 , wavelengths are multiplexed back into wavebands, and are sent to the next higher level of the switch hierarchy (i.e., waveband switch  204 ). In wavelength switch  206 , wavelengths can be switched into different wavebands other than the ones they were originally traveling in. Wavelengths containing traffic that needs to be switched at sub-wavelength-level (i.e., traffic grooming/time-slot interchange) are sent to ports that lead to the next lower level of the switch hierarchy (i.e., sub-wavelength grooming switch  210 ) of the system. Wavelengths containing only traffic to be dropped at the local node can be directly switched to the local TDM demultiplexer  214 , which terminates the wavelengths and disseminates the connections to client network elements. Similarly, if there is enough traffic to initiate a whole wavelength connection, the local TDM multiplexer  212  can aggregate all the traffic and directly send it in a wavelength to the wavelength switch. 
     In sub-wavelength grooming switch  210 , incoming wavelengths are sent into a grooming switch, which is based on electronic grooming switch chips. At the output of sub-wavelength grooming switch  210 , wavelengths are sent back to the next higher level of the switch hierarchy (i.e., wavelength switch  206 ). In sub-wavelength grooming switch  210 , low-speed connections can be switched into different wavelengths other than the ones they were originally traveling in. For connections that need to be dropped at the local node, they can be switched to the local TDM demultiplexer  214 , which will terminate the wavelengths and disseminate the connections to client network elements. Similarly, local traffic to be sent to the network is sent from the TDM multiplexer  212  to sub-wavelength grooming switch  210  and travels up the switch hierarchies (i.e., wavelength switch  206 , waveband switch  204 , and fiber switch  202 ) before it is sent out to the fibers leaving the node. 
     TDM multiplexer  212  and TDM demultiplexer  214  are responsible for aggregating/disseminating traffic from/to the local node. 
     Fiber switch  202  can be constructed with a MEMS-based optical switch fabric, in which tilting mirrors reflect all the light coming from one fiber into another fiber. 
     Waveband switch  204  can be constructed with a MEMS-based optical switch fabric. One embodiment of waveband switch  204  is shown in FIG.  4 . Each MEMS switch fabric is responsible for switching among the same wavebands from different fibers. In other words, there is one MEMS switch fabric used for each waveband layer. This arrangement also contributes to the excellent scalability of the system. As the number of wavelengths increases, the number of the wavebands remains the same, and the size of waveband switch  204  remains the same as well. 
     Wavelength switch  206  can be constructed with available electronic cross-point switch chips (e.g., a 144×144 OC-48 crosspoint switch chip). One embodiment of the wavelength switch architecture is the fully non-blocking Clos network configuration. Alternatively, wavelength switch  206  can be constructed in a wavelength-layered fashion (also called wavelength-dilated switch), similar to waveband switch  204 . 
     Sub-wavelength grooming switch  210  can be constructed with available electronic grooming switch chips (e.g., a 72×72 OC-48 grooming switch chip). One embodiment of sub-wavelength grooming switch  210  is the fully non-blocking Clos network configuration. Alternatively, sub-wavelength grooming switch  210  can be constructed in a wavelength-layered (wavelength-dilated) fashion. 
     TDM multiplexer  212  and TDM demultiplexer  214  can be one of the current traffic aggregation products in the market. An example is the multi-service provisioning platform (MSPP). 
     Arrayed Waveguide Grating Device 
     FIG. 3 illustrates an Arrayed Waveguide Grating (AWG) device in accordance with an embodiment of the present invention. AWG  208  can take multiple wavelengths from input  302 , and transmit all the wavelengths to output ports  304  to  310  in a cyclic fashion. As a result, AWG  208  can function as a waveband demultiplexer that separates all the wavelengths from one fiber into several wavelength groups. Because AWGs are reciprocal devices, they can also function as waveband multiplexers. One main advantage of using AWG  208  is its scalability. As the number of wavelengths increases in the fiber, the same AWG can be used to group wavelengths. Other significant advantages of AWG  208  are: mature technology, passive device, and ready commercial availability from numerous vendors. 
     In another embodiment of the present invention, instead of using AWG  208 , the waveband multiplexer/demultiplexer can be constructed with other means such as coarse WDM multiplexer/demultiplexer. The latter functions similarly to wavelength multiplexer/demultiplexer, except that the coarse WDM multiplexer/demultiplexer has a much wider passband at each output. Therefore, it can separate all the wavelengths into groups or merge wavelength groups together. 
     In another embodiment of the present invention, the waveband demultiplexer can be replaced by a device that selects (or can be programmed to select) a number of arbitrary wavelengths from the incoming fiber and send them individually to wavelength switch  206  or local TDM demultiplexer  214 . Similarly, these devices can accept a number of arbitrary wavelengths from wavelength switch  206  or local TDM demultiplexer  214  and multiplex them back to the outgoing fiber. 
     For waveband switch  204 , an alternative embodiment of the present invention is to use a bubble switch or any other optical switch technology that is wavelength insensitive instead of MEMS-based switch. 
     Another embodiment of the present invention is to bypass wavelength switch  206 , and connect waveband switch  204  directly to sub-wavelength grooming switch  210 . 
     Another embodiment of the present invention is to bypass subwavelength grooming switch  210 . 
     Waveband and Wavelength Switches 
     FIG. 4 illustrates a waveband and a wavelength switch in accordance with an embodiment of the present invention. In this embodiment of the present invention, AWG  208  functions as both a waveband multiplexer/demultiplexer and a wavelength multiplexer/demultiplexer. 
     Hierarchical Optical Switch with TDM Traffic Grooming Switch 
     FIG. 5 illustrates hierarchical optical switch  500  with TDM traffic grooming switch  502  in accordance with an embodiment of the present invention. In this embodiment, hierarchical optical switch  500  is similar to hierarchical optical switch  200  from FIG. 2, but has one major difference. Instead of having wavelength switch  206  and sub-wavelength switch  210 , as in hierarchical optical switch  200 , hierarchical optical switch  500  has TDM Traffic Grooming Switch  502  which combines the functions of wavelength switch  206  and sub-wavelength switch  210 . 
     Hierarchical Optical Switch without Sub-Wavelength Grooming 
     FIG. 6 illustrates hierarchical optical switch  600  without subwavelength grooming switch  210  in accordance with an embodiment of the present invention. In this embodiment, hierarchical optical switch  600  is similar to hierarchical optical switch  200  from FIG. 2, but has one major difference. Hierarchical optical switch  600  does not contain sub-wavelength grooming switch  210 . Instead, wavelength switch  206  is connected directly to TDM multiplexer  212  and TDM demultiplexer  214 . 
     Process of Hierarchical Switching 
     FIG. 7 illustrates the process of hierarchical switching in accordance with an embodiment of the present invention. The system starts by receiving an optical input signal (step  700 ). Next, the system routes the optical signals and drops optical signals that need to be switched at a finer granularity (step  702 ). The system then demultiplexes the optical signals into wavebands (step  704 ), routes the wavebands, and drops wavebands that need to be switched at a finer granularity (step  706 ). Next, the system demultiplexes the dropped wavebands into wavelengths (step  708 ), routes the wavelengths, and drops wavelengths that need to be switched at a finer granularity (step  710 ). Dropped wavelengths are then demultiplexed into TDM signals (step  712 ), and the TDM signals are routed and multiplexed into wavelengths (step  714 ). Then the wavelengths are routed and multiplexed into wavebands (step  716 ). Finally, wavebands are routed and multiplexed into optical output signals (step  718 ). 
     The foregoing descriptions of embodiments of the present invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention. The scope of the present invention is defined by the appended claims.