Abstract:
A unique WDM channel plan matrix, an optical reconfigurable service ring architecture, and method for operation in a metropolitan network that provides WDM wavelength networking functionalities such as provisional wavelength services, optical circuit switching, optical layer 2 packet switching, optical storage switching, and optical layer 3 packet routing are disclosed. The optical reconfigurable service ring is designed with capabilities for wavelength/channel routing, tuning, add/drop, optical circuit switching, optical layer 2 packet switching, optical storage switching, and optical layer 3 packet routing functionalities without requiring any immature and expensive widely tunable lasers and reconfigurable OADMs. In one embodiment, sixteen wavelengths are dynamically distributed among five optical add/drop nodes using commercially available narrowly tunable transmitters and fixed optical add/drop modules.

Description:
BACKGROUND INFORMATION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to the field of optical networks, and particularly to constructing a reconfigurable service ring (RSR) in metropolitan optical networks.  
           [0003]    2. Description of Related Art  
           [0004]    At present, metropolitan networks interconnect service nodes by using dense wavelength division multiplexer (DWDM) and optical add drop multiplexing (OADM) technologies. The drive is for the capacity boost, per bit dollar cost reduction, and for the fundamental shift in replacing the voice-centric, hard-to-scale, and slow-to-provision digital network with a data-centric, scalable, and easy-to-provision optical network.  
           [0005]    An OADM optical ring network typically consists of a central hub and two to eight OADM nodes interconnected by optical fibers. Optical wavelengths are multiplexed together at the hub using DWDM multiplexer (mux), then transmitted along the optical fiber ring, with certain number of wavelengths being dropped and added back at each OADM node. The composite DWDM signal back to the hub is demultiplexed using DWDM demultiplexer (demux) and each wavelength goes to an optical receiver. There are two types of OADM ring technologies: one is fixed OADM where fixed wavelength transmitters are used at the hub and each OADM node adds/drops pre-defined fixed wavelengths using totally passive optics; the other is reconfigurable OADM (ROADM) where tunable wavelength transmitters are used at the hub and each OADM node can configure or change its adds/drops wavelengths using various active optical means. Fixed OADM ring networks are widely deployed in the metropolitan networks as data traffic boom creates the demand for more bandwidth and protocol transparent transport platform, and passive fixed OADM modules are technologically mature since it uses the same technologies as the widely deployed DWDM mux and demux. However, reconfigurable OADM ring offers more flexibility and savings for network planning and operation. No truck rolls are needed when carrier needs to change wavelengths among the OADM nodes. As metropolitan data service heats up and carrier&#39;s spending is tightly restricted due to the bust of telecom bubble, these dynamic reconfigurability and operation expense (OPEX) savings become more and more important. Unfortunately, ROADM has not been widely deployed because today&#39;s ROADM product all suffers the shortcomings of high cost and unproven technology risk, because they require widely tunable lasers and ROADM module, both are not yet widely deployed in the field.  
           [0006]    Accordingly, it is desirable to have an OADM solution that has about the same cost level of today&#39;s fixed OADM but offers the operation flexibility and savings of ROADM.  
         SUMMARY  
         [0007]    The invention discloses a unique WDM channel plan matrix, an optical reconfigurable service ring architecture, and method for operation in a metropolitan network that provides WDM wavelength networking functionalities such as provisional wavelength services, optical circuit switching, optical layer 2 packet switching, optical storage switching, and optical layer 3 packet routing. The optical reconfigurable service ring is designed with capabilities for wavelength/channel routing, tuning, add/drop, optical circuit switching, optical layer 2 packet switching, optical storage switching, and optical layer 3 packet routing functionalities without requiring any immature and expensive widely tunable lasers and reconfigurable OADMs. In one embodiment, sixteen wavelengths are dynamically distributed among five optical add/drop nodes using commercially available narrowly tunable transmitters and fixed optical add/drop modules.  
           [0008]    Advantageously, the reconfigurable service ring or RSR in the present invention produces a more flexible yet low cost OADM ring, with the capability of optical switching/routing. The present invention also advantageously does not rely on the cost-forbidden reconfigurable OADM modules and the expensive and not-yet-commercial ready widely tunable lasers.  
           [0009]    Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1A is a graphical diagram illustrating the WDM channel definition in accordance with the present invention; FIG. 1B is a graphical diagram illustrating the RSR channel definition and channel plan example in accordance with the present invention.  
         [0011]    [0011]FIG. 2 is an architectural diagram illustrating a RSR in accordance with the present invention.  
         [0012]    [0012]FIG. 3 is a graphical diagram illustrating a RSR channel plan matrix in accordance with the present invention.  
         [0013]    [0013]FIG. 4 is a tabular diagram illustrating an example of a metropolitan network service demand change in accordance with the present invention.  
         [0014]    [0014]FIG. 5A is a graphical diagram illustrating a RSR channel plan matrix for one service demand in accordance with the present invention; FIG. 5B is a graphical diagram illustrating a RSR channel plan matrix for another service demand in accordance with the present invention.  
         [0015]    [0015]FIG. 6 is a graphical diagram illustrating RSR technology applications to optical circuit and packet switching and optical packet routing in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]    [0016]FIG. 1A and FIG. 1B are graphical diagrams illustrating the channel definitions and channel plan example used in the current invention. In FIG. 1A, a WDM channel λ 0    14  is defined by its center frequency f 0    11   a  along with a passband BW p    12  and a guardband BW g    13 . This WDM channel definition is widely used in optical WDM communications. The key to the current invention is that in addition to WDM channel  10 , a RSR channel  20  is defined. The C and/or L and/or S wavelength band in optical communications is divided into x number of equally spaced sections with y GHz frequency spacing between adjacent sections. Each section is defined as a RSR channel. Unlike a WDM channel, which is defined by frequency f 0    11   a , passband BW p    12 , and guardband BW g    13 , a RSR channel is defined by the WDM channels it contains and skips. FIG. 1B shows an example of 16 RSR channels spaced at 200 GHz  34  apart across C band. The WDM channels λ 1,1    25 , . . . λ 16,5    32  in this case are spaced at 25 GHz  33  apart. So each RSR channel contains 8 WDM channels. Due to limitations of optical demux technologies, a RSR channel may choose to exclude or skip certain number of WDM channels at its spectral edges. These skipped WDM channels serve as guardband to ease the demand on RSR channel demux. In FIG. 1B, 3 WDM channels out of 8 are skipped, shown in dashed lines. The RSR channel in this case is 8 skip 3, implying 5 WDM channels are included in 1 RSR channel. So RSR Channel  1   21  includes 5 WDM channels λ 1,1    25  till λ 1,5    26 , and RSR Channel  2   22  (or just Channel  2  for short) includes λ 2,1    27  till λ 2,5    28 . The n skip m RSR channel plan solely depends on demux technology feasibility and product cost factor. For example, if 25 GHz RSR demux is chosen for FIG. 1B, then the RSR channel becomes 8 skip 0, including all 8 WDM channels within its range.  
         [0017]    Although an 8 skip 3 RSR channel includes 5 WDM channels, it uses only 1 of the 5 WDM channels at a time. Therefore, a RSR channel has the same passband and guardband as the WDM channels it includes, only that its center frequency has several possible values. Depending on network planning, a RSR channel can be provisioned (i.e., tuned by software) to any one of its WDM channels. One way to implement this is to use a temperature tuned DWDM transmitter. A DWDM DFB (distributed feedback) laser has a temperature tuning efficiency of about 10 GHz/° C. To tune the laser transmitter across 5 WDM channels with 25 GHz spacing, the laser temperature only needs to be tuned by 10° C. Currently, the commercial DWDM lasers all offers an operating temperature range minimum from 0 to 50° C. So it is quite feasible to tune off-the-shelf commercial DWDM lasers by 10° C.  
         [0018]    [0018]FIG. 2 is an architectural diagram illustrating a reconfigurable service ring configuration  40  for metropolitan networks. The RSR  40  includes one gateway hub  41 , five service nodes N 1   42 , N 2   43 , N 3   44 , N 4   45 , N 5   46 , and optical fiber spans  69 , 70 , 71 ,  72 ,  73   74  in between nodes and hub. At the hub  41 , up to  16  RSR channels are multiplexed together and the composite DWDM signal is sent clockwise along the ring. At each service node, a fixed OADM drops 16 WDM channels from the 16 RSR channels. The fixed OADM at each service node drops/adds different set of WDM channels. In FIG. 2, service node N 1   42  drops/adds the first WDM channel in all 16 RSR channels: λ 1,1    65 , λ 2,1    66 , till λ 16,1    68 ; and service node N 5   46  drops/adds the fifth or the last WDM channel in all 16 RSR channels: λ 1,5    61 ,λ 2,5    62 , till λ 16,5    64 . Since each RSR channel only uses one WDM channel at a time, each RSR channel is dropped/added by only one service node; depending on which WDM channel it is tuned to. If RSR channel  1   49  is tuned to its first WDM channel λ 1,1    53 , it will reach service node N 1   42 ; if it is tuned to its fifth WDM channel λ 1,5    54 , it will reach service node N 5   46 . After going through all 5-service nodes, the composite DWDM signal goes back to gateway hub  41  and the 16 RSR channels are demultiplexed.  
         [0019]    This RSR configuration  40  combines the wavelength selective nature of fixed OADM nodes with the narrow band tunability of RSR channels to constitute a service ring that can be reconfigurable via software. The advantages of RSR are as follows: 
         [0020]    First, RSR offers much more flexibility, with quite minor cost increase, than current fixed OADM ring configuration, where each WDM channel is fixed to go to its designated OADM node. RSR allows all of its RSR channels to go to any nodes on the ring by tuning to different WDM channels;  
         [0021]    Secondly, RSR is at low risk in technology as well as cost effective comparing to existing reconfigurable OADM ring configurations. ROADM rings require expensive and immature ROADMs (which normally consist of optical switches and tunable filters) and widely tunable transmitters. Both are not yet field proven. Whereas RSR only requires fixed OADM based on passive WDM technologies such as thin-film filter or fiber Bragg grating etc. and narrowly tunable transmitters which can be implemented via temperature tuning of commercial DWDM lasers;  
         [0022]    Thirdly, RSR allows better wavelength tolerance for its passive WDM mux, demux, and OADMs. In RSR architecture, RSR channels are spaced much further apart than the narrowly spaced WDM channels. In the example shown here, RSR channels are 200 GHz apart, and WDM channels are 25 GHz apart. A RSR channel is tuned to only one of its WDM channels, its WDM channels do not need to be precisely on its center frequency, since all adjacent WDM channels will not be occupied. The alignment of transmitter and WDM channels can be done during initialization of the system, where all transmitters are tuned to locate and remember all the WDM channels it includes. This will ease greatly the demand on passive mux, demux, and OADM manufacturing yield, and as a result to reduce their cost. The only exception is that if RSR channels are x skip 0, with no guard WDM channels in between RSR channels, then all edge WDM channels need to be precisely on grid to avoid adjacent channel interferences;  
         [0023]    Fourthly, RSR has no channel interference problem during its reconfiguration. For existing ROADM scheme, when tunable transmitter is tuned from channel A to channel B for reconfiguration, its wavelength will sweep through traffic-bearing live channels in between channel A &amp; B. To avoid interferences, these in between channels need to shutter themselves off right at the moment when the tuning transmitter hits and quickly re-open to avoid traffic loss. Not a trivia task to perform. RSR has no such problem, because RSR channels are only tuned among its inclusive WDM channels. Each RSR channels have different inclusive WDM channels. So the reconfiguration tuning never sweeps through any traffic bearing live channels. No synchronized receiver quick shutter is needed;  
         [0024]    Last but not the least, RSR architecture can be used for optical circuit and packet switching including optical storage switching and optical packet routing, in addition to wavelength provisioning. Since all RSR transmitters can reach all nodes on the ring via setting to specific corresponding wavelength, it can read destination node from each packet&#39;s header, determined the destination node of this packet, and tune/switch the laser to the corresponding wavelength, thus switch/route packets to their designated nodes. In case of circuit switching, the RSR transmitter can switch each TDM (time division multiplexing) time slot to its destination node by switching to the corresponding wavelength according to optical cross connect (OXC) provision. The key for successful implementation of optical switching/routing on RSR is the wavelength switching or tuning speed of transmitter in comparison to the speed of input traffic signal. For wavelength service provisioning, temperature tuned transmitter of several seconds tuning time is fast enough. For optical switching/routing on RSR, transmitter wavelength needs to be switched much faster (one order of magnitude) than the speed of traffic signal. Possible candidate is ultra-fast nanosecond speed tunable laser such as distributed Bragg reflector (DBR) lasers. The capability of optical switching and routing of RSR means that RSRs deployed in metropolitan areas can use much smaller capacity centralized OXC and/or core router, thus further reducing the cost and maintenance (capex and opex) of the overall network. 
         [0025]    [0025]FIG. 3 80  to FIG. 5B  95  illustrates an application example for the FIG. 2  40  RSR architecture in metropolitan networking. Follow the 8 skip 3 RSR channel plan of FIG. 2  40 , FIG. 3  80  illustrates the channel plan or wavelength grid matrix for this RSR. 16 RSR channels are implemented by 16 transmitters  81  λ 1  , λ 2 , till λ 16 . Each transmitter can be tuned to 1 of 5 wavelengths which are located on 5 corresponding service nodes  82  N 1 , N 2 , N 3 , N 4 , N 5 . Please note that although 80 wavelengths are present on the matrix, only up to 16 wavelengths may be used at a time, since there are only 16 transmitters. Furthermore, a fixed OADM at each node can be implemented by following the wavelength or channel plan per its column in FIG. 3  80 .  
         [0026]    [0026]FIG. 4 85  is an example of service demand change from A to B. Service demand A requires that node N 1 , N 2 , N 3 , N 4 , N 5  to have 3, 1, 5, 4, 0 channels, respectively. And service demand B requires that node N 1 , N 2 , N 3 , N 4 , N 5  to have 4, 2, 3, 4, 1 channels, respectively. FIG. 5A  90  illustrates the wavelength matrix for service demand A: 13 transmitters  91  λ 1 , λ 2 , till λ 13  are tuned to 13 different channels to meet the demand. Node N 1  has λ 1,1 , λ 2,1 , and λ 3,1 ; N 2  has λ 4,2 ; etc. No transmitter reaches node N 5 . To reconfigure this RSR to satisfy service demand B, the operator only needs to set the 13 transmitters per FIG. 5B  95 . The two extra transmitters on node N 3  λ 8,3  and λ 9,3  are tuned to node N 1 λ 8,1  and node N 2  λ 9,2 , respectively. And one more transmitter λ 1,4  is turned on to serve node N 5  λ 14,5 . FIG. 6  100  illustrates the implementations of RSR for optical circuit and packet switching, and optical packet routing. The input TDM or cell or packet signal  101  to the first RSR transmitter  107  is first electronically de-multiplexed, and header information extracted for cell/packet signals  102 . Then the destination node where source information needs to be routed to, is determined  103  for each TDM circuit (time slot) or data packet according to their OXC configuration, or layer 2 switching such as Ethernet, or storage switching such as FC/iSCSI, or layer 3 routing such as IPv4/IPv6  104 . The destination node can also be determined by feedback control of optical performance monitor and/or protection trigger  104 . If the circuit/packet needs to go to the first service node N 1   109 , the transmitter  107  will switch to λ 1,1    108  to perform the switching/routing.  
         [0027]    The above embodiments are only illustrative of the principles of this invention and are not intended to limit the invention to the particular embodiments described. Although the RSR configuration and operation method described in the present invention is in the context of a metropolitan network, the RSR is applicable to other networks including a long haul network and an access network. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the appended claims.