OPTICAL NETWORKS Optical Metro Networks A. Gençata İTÜ, Dept. Computer Engineering 2005
Introduction Telecommunications networks are normally segmented in a three-tier hierarchy: Access, metropolitan, and long-haul. Long-haul/backbone networks span interregional/global distances (1000 km or more) and are optimized for transmission and related costs. At the other end of the hierarchy are access networks, providing connectivity within close proximity. In the middle are metropolitan (metro) networks, averaging regions between 10 100 km and interconnecting access and long-haul networks. 2
Metro Networks Today Metro networks today are based on synchronous optical network (SONET) / synchronous digital hierarchy (SDH) ring architectures. Smaller rings, e.g., OC-3/STM-1 (155 Mbps) or OC- 12/STM-4 (622 Mbps), aggregate traffic onto larger core inter-office (IOF) rings that interconnect central office (CO) locations at higher bit rates like OC-48/STM-16 (2.5 Gbps). Synchronous Transport Signal (STS) is the electrical level transmission frame structure. Optical Carrier (OC) is the optical level transmision counterpart. STS-N frame is usually carried by an OC-N transmission link. Synchronous Transport Module (STM) is the counterpart of STS in SDH terminology. 3
Metro Networks Today SONET/SDH has been very successful in delivering the first wave of end-user connectivity, namely voice. Internet data traffic growth has significantly altered the networking domains bordering the metro. Long-haul networks felt the Internet crunch first and have undergone large scale expansions using optical wavelength division multiplexing (WDM) technology. Meanwhile, in access networks, residential cable and digital subscriber loop (DSL) modems have increased user access rates from Kbps to Mbps, and other advanced technologies (such as PONs) promise to further this trend. These increased access rates are beginning to stifle voice centric metro architectures. SONET/SDH rings are experiencing capacity exhaust at even OC- 48/192 rates. 4
Need Clearly new metro solutions are required that offer superior price/performance alternatives to SONET/SDH expansion. New platforms must: offer high bandwidth scalability carry multiple protocols over a common infrastructure to reduce costs. Rapid, intelligent service provisioning and survivability are also crucial. WDM technology meets many of these requirements. WDM component technologies (amplifiers, switches, filters, lasers, fibers) are enabling network-level wavelength routing and protection over more complex multi-hop fiber topologies, such as rings or meshes. 5
WDM in Metro Area Even though WDM technology offers many benefits, its induction within metro area is more complex than in the longhaul. WDM is best suited for larger metro core networks, where scalable large granularity lambda provisioning is required, in Gbps range. At the metro edge, operators must interface with increased protocol heterogeneities/interface bit rates. The need to cost-effectively handle finer sub-wavelength capacity increments is paramount. This contrasts sharply with long-haul networks where interface bit rates are few (2.5, 10, possibly 40 Gbps). The metro edge requires integrated, intelligent optoelectronic solutions to perform multi-protocol aggregation/grooming on to larger WDM tributaries. 6
Metro WDM Solutions Optical WDM Rings: WDM rings have evolved strongly from SONET/SDH concepts. They replace timeslots with wavelengths and perform optically equivalent channel operations: add-drop, pass-through, protection. These rings offer high capacity, scalability, transparency, and permit multiple data rates. Optical bypass eliminates the need for detailed electronic knowledge of, or access to, client signals and yields significant cost savings over traditional ADM/DXC nodes. WDM ring architectures have been proposed, varying from simple several static setups (i.e., fixed nodes) to more advanced sharing schemes (i.e., dynamic nodes). 7
Optical WDM Rings The basic building block of an optical WDM ring is the optical ADM (OADM) node. Fixed OADMs, can operate on static or factory tuned wavelengths. Usually used in static rings for static traffic. static rings will become very limiting, requiring complex manual wavelength planning and yielding reduced wavelength efficiencies. Reconfigurability is a key issue (in addition to scalability), and dynamic OADM rings are solutions. Reconfigurable OADMs (ROADMs) are the building blocks. 8
Metro Edge Solutions The metro edge represents a merging between the core inter-office and the client access spaces. It is becoming increasingly evident that SONET/SDH is not the best unifying layer. Conversely, since WDM is bandwidth-inefficient for sub-gigabit line-rates, advanced electronic multiplexing technologies are needed to aggregate diverse end-user protocols onto large-granularity optical (WDM) tributaries. Next-Generation SONET/Multi-Service Provisioning Packet rings 9
Traffic Grooming in SONET SONET ring is the most widely used optical network infrastructure. In a SONET ring network, WDM is mainly used as a point-to-point transmission technology. Each wavelength in such a SONET/WDM network is operated at OC-N line rate, for example N = 192. The SONET system s hierarchical TDM schemes allow a high-speed OC-N channel to carry multiple OC-M channels. M is smaller than or equal to N. The ratio of N and the smallest value of M carried by the network is called grooming ratio. Electronic add-drop multiplexers (ADMs) are used to add/drop traffic at intermediate nodes to/from the highspeed channels. 10
Node Architecture In a traditional SONET ring network, one ADM is needed for each wavelength at every node to perform traffic add/drop. With the progress of WDM, over a hundred wavelengths can be supported simultaneously by a single fiber. It is too costly to put the same amount of ADMs (each of a significant cost) at every network node. a lot of traffic is only bypassing an intermediate node. With the emerging optical components such as optical add-drop multiplexers (O-ADM or W-ADM), it is possible for a node, to bypass most of wavelength channels optically and only drop the wavelengths carrying the traffic destined to the node. 11
Node Architecture For λ1, since there is no need to add or drop any of its timeslots, they can be optically bypassed at the node. For λ2 and λ3 at least one timeslot needs to be added or dropped. An electronic ADM is used. 12
Node Architecture ADMs form the dominant cost in a SONET/WDM ring network. Hence, carefully arranging these optical bypasses can reduce a large amount of the network cost. It is clear that using O-ADMs can decrease the number of SONET ADMs used in the network. Then the problems are, for a given low-speed set of traffic demands, which low-speed demands should be groomed together, which wavelengths should be used to carry the traffic, which wavelengths should be dropped at a local node, and how many ADMs are needed at a particular node? 13
Single-hop Grooming As shown in the figure: ADMs do not have the timeslot interchange function, wavelength conversion is not possible without additional equipment. So there are timeslot-continuity and wavelengthcontinuity constraints at nodes when only ADMs are used. We refer to these rings as single-hop rings since all the connections are direct connections. OC-M low-speed connections are groomed on to OC-N wavelength channels. 14
Example 5-node network with uniform traffic requests. A bi-directional ring with grooming ratio 2. The total number of bi-directional requests is 10 and each request is 1 unit of sub-channel capacity. (b) and (c) illustrate two ways of organizing the connections on two wavelengths. 15
Solving the Problem The general traffic grooming problem is NP-Complete. The problem has been formulated as an integer linear program (ILP). When the network is small, ILP can be solved to obtain an optimal solution. The formulation can be applied to both uniform and non-uniform traffic demands. But as the size of the network increases, the numbers of variables and equations increase explosively. So, the computation is too complex to be useful on networks with practical size. By relaxing some of the constraints in the ILP formulation, it may be possible to get some results, which are close to the optimal solution for reasonable-size networks. The results from the ILP may give some insights for developing good heuristic algorithms to handle the problem in a large network. 16
Multi-hop Grooming In single-hop grooming, traffic cannot be switched between different wavelengths (a). Some nodes may be equipped with DXCs. In (b), node 3 has a DXC installed. 17
Multi-hop Grooming Traffic from one wavelength/timeslot can be switched to any other wavelength/timeslot at the hub node. Because the traffic needs to be converted from optical to electronic at the hub node when wavelength/timeslot exchange occurs, this grooming approach is called multihop (multi-lightpath hops) grooming. There can be a single hub node or multiple hub nodes in the network. A special case is that every node is a hub node, i.e., there is a DXC at every node: point-to-point WDM ring network. 18
Dynamic Grooming Instead of using a single static traffic matrix to characterize the traffic requirement, it is possible to describe it by a set of traffic matrices. The traffic pattern may change within this matrix set over a period of time, a day or a month. The network needs to be reconfigured when the traffic pattern transits from one matrix to another matrix. The network design problem for supporting any traffic matrix in the matrix set as well as minimizing the overall cost is the dynamic grooming problem in a SONET/WDM ring. 19
Reconfigurable Design The researchers have formulated the general dynamic-grooming problem in a SONET/WDM ring as a bipartite graph-matching problem and provided several methods to reduce the number of ADMs. For a given traffic matrix, if each node can source at most t duplex circuits, we call this traffic matrix a t-allowable traffic matrix. The traffic matrix set consisting of t-allowable traffic matrices is called a t-allowable matrix set. 20
Example 5-node SONET/WDM ring network with 3 wavelengths. Each wavelength can support 2 low-speed circuits. The network configuration is 2-allowable, i.e., it can support any 2- allowable traffic matrix. Consider a traffic matrix: 1 2, 1 3, 2 3, 2 4, 3 4, 4 5, and 4 5. The traffic matrix can be supported by assigning 1 3, 2 3 on wavelength 1, assigning 1 2, 2 4, 4 5, 4 5 on wavelength 2, and assigning 3 4 on wavelength 3. For a particular traffic matrix, there may be some redundant ADMs. The configuration is able to support other potential t-allowable traffic matrices. 21
Mathematical Definition of Grooming N: Number of nodes in the network. W: Number of wavelengths. C: Grooming ratio. T: Non-uniform traffic matrix, in which t ij represents the traffic from node i to j. The traffic matrix is given. d V cw ij : Virtual connection from node i to node j on circle c of wavelength w. d represents the direction of a connection and it can be either clockwise or counterclockwise. ADM w i: Number of ADMs at node i on wavelength w. e: A link on the physical ring. 22
Single-hop Formulation 23
Formulation In formulation, d V cw ij represents whether there is a virtual connection (one unit of capacity) from node i to node j along direction d on circle c and wavelength w. The t ij represents the total amount of traffic from node i to node j. The traffic-load constraint states that the number of links from node i to node j on all circles is equal to the traffic specified in the traffic matrix. The channel-capacity constraint requires that a circle carry only one connection on any given link. The last two constraints specify that the number of connections that start and terminate at a node of a ring is bounded by the capacity of the electronic ADM at that node. 24
Solving the ILP SONET ring has a maximal size of 16 nodes. Uniform traffic and single-hop unidirectional ring. An ILP solver (CPLEX) was used. For networks of 6 nodes or less, the ILP solver can find the optimal solution in a reasonably short time of a few seconds to a few hours. When the network is larger than 6 nodes, it takes more than 6 hours to discover the optimal solution for some cases (question marks in Table). For N=4, 5 and C=12 (shaded cells), no grooming is needed since one wavelength can carry all the traffic. 25
Solving the ILP When the network size is larger than 7 or 8 nodes, we need to turn to heuristics. When the ILP solver is given a time limit of half-hour for each problem, it usually fails to find even one feasible solution when the network size is larger than 8 nodes. The general traffic-grooming problem is NP-complete. Solving the ILP directly is not practical even for moderate-size networks because of the long solution time. Several approaches have been proposed: simple greedy heuristics simulated-annealing algorithms, etc. 26
Multi-hop Grooming Problem A greedy heuristic puts all the traffic on circles sequentially, and then applies continuously the following functions. The Wavelength Combining function checks two wavelengths link by link. If the total load on any link does not exceed the wavelength capacity, then the traffic is combined into one. The Segment Swapping function finds the underutilized links in different wavelengths and combines them into one wavelength through segment swapping. 27
Statistical result comparisons for the bidirectional rings. The upper bound (no grooming), the greedy approach, the simulatedannealing approach. Numerical Results 28
Interconnected WDM Rings In order to provide large geographical coverage, multiple SONET rings can be interconnected together. The Optical Cross-connect (OXC) technology provides a convenient way for these interconnections. Two problems: How to interconnect WDM ring networks with OXCs, How to groom traffic in interconnected rings. 29
Interconnection of Rings Two rings may interconnect at either one or multiple points. Usually, two physical intersections are desired due to the fault-recovery concern. When a node failure occurs at one intersection node, the rest of the nodes should still be connected. 30
Traffic Grooming in Interconnected Rings The single-ring traffic-grooming problem was formulated mathematically as an integer linear program (ILP). Results from the research efforts in single-ring networks serve as good references and provide a strong base for the study of traffic grooming in interconnected rings. The mathematical problem specification can be extended from the work done for single-ring networks. The grooming in a multi-ring scenario is also NP-complete. Experience shows that even an industrial-strength MILP solver, CPLEX, may be incapable of solving the problem specified by single-hop formulation on a single 7-node bidirectional network with uniform traffic within a reasonable period of time. Hence, while constructing large interconnected rings efficiently, we look for heuristics. 31
OADM Optical Add-Drop Multiplexer (OADM) is now becoming commercially available. Most networks deployed by telecom service providers are implemented as (single-wavelength) fiber-optic SONET rings. It is straightforward to upgrade them to operate over multiple wavelengths by employing OADMs and appropriate terminal equipment at the network nodes, The same fiber cable plant can be reused. In such a W-wavelength ring network an OADM drops information from and adds information to the ring on a given wavelength on which it is set to operate. OADMs can significantly reduce the network cost by allowing traffic to bypass intermediate nodes without expensive O-E-O conversion. 32
OADM An OADM can be made to operate: permanently (statically or factory tuned) on a particular wavelength and is called fixed OADMs (FOADMs), or dynamically tuned using some control mechanism and is called reconfigurable OADM (ROADM). ROADMs can add/drop traffic onto/from different wavelengths at different time. They provide desirable flexibility and enable fast provisioning of dynamic traffic. While fixed OADMs are commonplace today, the Reconfigurable OADMs are in the early technology stages. OADMs have been attracting research interest from both academe and industry. 33
Tuning Constraint For reconfigurable OADMs, there are several different architectures. One architecture employs switches to achieve add/drop functionality, and is called switching-based architecture. Another ROADM architecture uses tunable devices, e.g., fiber Bragg grating (FBG) or thin film filter (TFF), to selectively add/drop a designated wavelength, and it is called tuning-based architecture. The tuning-based architecture is more cost-effective than the switching-based one. In the tuning-based architecture, a ROADM adds/drops different wavelengths by tuning the tuning head to the corresponding wavelengths. If a ROADM is not adding/dropping, the tuning head can stay or park on any free wavelength, or between any two adjacent wavelengths. 34
Tuning Constraint To set up a connection, ROADMs at the source and destination nodes must be tuned to the same free wavelength. As the tuning process is virtually continuous, the tuning head will pass through all the wavelengths between the current position and chosen wavelength. However, if the tuning process passes through some working wavelengths, it will cause a service interruption on the traffic carried by those wavelengths, which is unacceptable and should be avoided. Consider the case: the current position of the tuning head of ROADM 1 is on wavelength λ1 ROADM 2 is on wavelengths λ4, a connection needs to be set up between these two ROADMs. there is traffic on wavelength λ3 bypassing these two ROADMs. 35
Tuning Constraint Since the tuning heads of the ROADMs are on the different sides of wavelength λ3, the two ROADMs cannot tune to the same wavelength without crossing wavelength λ3 and the connection has to be blocked. This imposes another constraint on setting up a connection, and we call it the tuning constraint. Since the tuning head of a ROADM is not allowed to cross working wavelengths, it can only tune within a certain tuning range. Note that the tuning range varies dynamically as new connections are set up and existing connections are terminated. 36
Problem Statement A network node can have multiple ROADMs in order to add/drop multiple wavelengths simultaneously. Two or more ROADMs at a node may be connected serially, so adding/dropping at one ROADM may also affect the tuning range of the other ROADMs at the same node. When setting up a connection, we need to determine which ROADMs should be used at the source and destination nodes, the wavelength and direction, if the ring is bidirectional. The chosen wavelength must be in the tuning range of both the ROADMs. This is called direction, wavelength and ROADM assignment (DWRA) sub-problem. 37
Problem Statement Since the connection may affect the tuning range of the bypassed ROADMs, it should be decided: to which positions the tuning heads of those ROADMs should be tuned before the connection is established. This decision will determine the tuning ranges of the ROADMs. It has significant impact on the establishment of future connections. This is called the tuning-head positioning (TP) sub-problem. 38
Given: Overall Problem Statement a WDM ring network using tuning-based node architecture with the tuning constraint, dynamic traffic, which arrives one by one. Determine: direction, wavelength and ROADM assignment (DWRA) for the connection, and tuning-head positioning (TP) for the affected ROADMs. Objective: minimize the overall connection blocking probability. 39