and : Competing Yet Complementary Technologies for The Metro Network
Introduction The next generation of equipment has been well accepted by service providers and is being usefully deployed into networks today to carry continually increasing network traffic. Compared to traditional equipment, NGADM is characterized by simple architecture, scalability, high capacity add/drop, multiple ring terminations, multiservices, and multiple fabrics, including STS and VT switching capabilities. At the same time, equipment is increasingly showing its viability in today s metro networks with tremendous available bandwidth over the existing fiber plant, along with a variety of service interfaces and superior optical transmission capabilities. A new generation of equipment is also being rolled out that provides features of both and. Some are systems with integrated units that provide grooming, add/drop and protection of a wavelength. Others are NGADMs with built in capability that provide optical multiplexing, amplification and transmission. Although a higher capacity technology such as brings about many benefits, this technology can also incur higher initial cost. In quite a few situations, we need to evaluate the benefits gained from and against the economical impact that each inherits. Important questions that need to be reviewed for each type of system include: why is needed; when is it needed; under what circumstances is more economical than a architecture; what are the factors that need to be taken into account for the deployment of technology; and when is grooming beneficial? The Scenarios of the Study This paper investigates an example metro network from some real life IOF applications to answer some of the questions previously mentioned. In this example network, three network architectures are discussed (see Figure 1): PMO a ring architecture with cross-connects and local rings Overlay a ring architecture that uses optimized overlaid rings to avoid ports at the pass-through nodes in the network an architecture in which equipment is deployed for transport with aggregation as needed for each wavelength The main objective of this study is to investigate the crossover points at which the economical advantages of the approaches and the approach exchange positions. The factors that directly influence the crossover point in question are the network traffic volume, distances between sites in the network, circuit types (OC-3, OC-12, OC-48 or Gigabit Ethernet), density of interface cards and shelf itself, diversity of traffic and the equipment cost. We will also discuss other impacts on selecting a core versus a core architecture in a carrier s metro network. For your convenience, a list of acronyms can be found at the end of this document. 1
60 km Ring 60 km Ring 105 km Ring Present Mode of Operation Route 1 Endpoints Route 2 Endpoints Route 3 Endpoints Route 4 Endpoints Overlay 105 km Ring Figure 1: The Three Main Scenarios Analysis of Crossover Points Four types of traffic demands have been tested in the models: OC-3, OC-12, OC-48 and GigE one type of demand at a time. Each end-to-end demand is called a circuit. For example, two end-to-end OC-48 demands in route one are defined as two (OC-48) circuits. In the modeling, each type of traffic demand has been routed evenly into the four routes so that each route gets the same amount of demands or number of circuits. The physical span distances are set to be uniformly in the initial comparisons. Then, some sensitivities have been conducted to evaluate the impact of distance change on the solutions. Protection is provided at both the tributary and network levels. Tributary interface cards are 1+1 protected for the solutions as well as the solution, except in the GigE case where no protection is provided on the interface card. provides BLSR protection and provides a BLSR-like shared path protection. designs are all OC-192 rings and the mux/demux has a capacity of 80 unprotected or 40 protected wavelengths. For designs, an abundant supply of fibers is assumed to be available and fiber spans between sites can be connected together to allow an optical bypass of a site by a ring. For design, limited fiber is assumed and equipment is required at each site to enable an optical bypass or termination as might be needed. This makes the model more generic. 2
In the PMO scenario, as illustrated in Figure 1, two four-node topological rings exist in this eightnode network, one on the left and one on the right. The traffic demands or circuits carried in routes one and four could only be routed through the two NEs in the upper middle of the network. The optimal routing for the other two routes is just direct connection between the two nodes on the far left and/or the far right. See Figure 2 for an example of the traffic routes. 60 km Ring 60 km Ring Route 1 Endpoints Route 2 Endpoints Route 3 Endpoints Route 4 Endpoints Figure 2: Traffic Routing in PMO Let s first assume four OC-48s are terminated at each of the four terminating sites (two OC-48 circuits for each route). A simple design shows two OC-192 rings, one on each topological ring, and 16 OC-192 ports on the line and cross-connect sides of the NEs in the network. If eight OC-48s are terminated at each terminating site (four OC-48 circuits for each route), then four OC-192 rings, two on each topological ring, and 32 OC-192 ports are required. Another solution for the same problem is to overlay the rings to bypass the cross-connect point in the network. In this solution, only one topological ring is formed. 3
Route 1 Endpoints Route 2 Endpoints Route 3 Endpoints Route 4 Endpoints 105 km Ring Figure 3: Traffic Routing in the Overlay Scenario Figure 3 shows the optimal routing of the demands for this overlay solution. Given four OC-48 demands terminated at each end site, two OC-192 rings still exist (with longer links than PMO, though) but only 12 OC-192 line side ports in a six-node network. If eight OC-48s are terminated at each terminating site (four OC-48 circuits for each route), then four overlaid OC-192 rings and 24 OC-192 ports are required. The third approach is a deployment in which core equipment is deployed in seven locations and transponders (4:1 muxponder, OC-48 to 10 Gbps in this example in Figure 4) are installed in the four terminating nodes. equipment is deployed at all nodes, including the three nodes without circuit terminations in the model, to enable reconfigurable optical bypass as needed. 4:1 Muxponders 4:1 Muxponders Figure 4: Wavelength Traffic in the Core Scenario 4
In the case of eight OC-48 terminations per node (four circuits per route), the ring has four total wavelengths in any particular span. The thin lines represent protection wavelengths and the thicker lines represent working wavelengths. Assuming the system can handle 40 protected wavelengths in a ring, a single such system can accommodate up to 20 protected 4:1 transponders or 80 protected OC-48 terminations at each terminating node. Traffic volume, in Gbps, is a function of the number of circuits in all four routes. For example, traffic volume is said to be 16 x 2.5 Gbps = 40 Gbps when 16 OC-48 circuits exist (four in each route). Each OC-48 circuit carries 2.5 Gbps of traffic in the network. For OC-3 circuits, Figure 5 shows the economic trend of each solution as the number of circuits in the network increases. As indicated in the chart, the solution has a higher initial cost than the two solutions. However, when the number of OC-3 circuits in the network reaches 52, or the traffic volume reaches 108 x 0.15552 Gbps = 16.8 Gbps (each OC-3 carries 155.52 Mbps or 0.15552 Gbps of information), becomes less expensive than the PMO. And when the traffic volume reaches 132 x 0.15552 Gbps = 20.5 Gbps, it becomes less expensive than the overlay solution. 100.0 90.0 80.0 70.0 Normalized Cost 60.0 50.0 40.0 30.0 20.0 10.0 0.0 PMO Overlay 4 20 36 52 68 84 100 116 132 148 164 180 196 212 228 244 260 276 292 308 Number of Circuits Figure 5: Versus for OC-3 Circuits 5
100.0 90.0 80.0 70.0 Normalized Cost 60.0 50.0 40.0 30.0 20.0 10.0 0.0 PMO Overlay 4 20 36 52 68 84 100 116 132 148 164 180 196 212 228 244 260 276 292 308 Number of Circuits Figure 6: Versus for OC-12 Circuits 100.0 90.0 80.0 70.0 Normalized Cost 60.0 50.0 40.0 30.0 20.0 10.0 0.0 PMO Overlay 4 20 36 52 68 84 100 Number of Circuits Figure 7: Versus for OC-48 Circuits 6
For OC-12 demands, the crossover point is at traffic volume of 68 x 0.622 Gbps = 42.3 Gbps for PMO and 132 x 0.622 = 82.1 Gbps for overlay (Each OC-12 delivers 0.622 Gbps of information). In the OC-48 case (see Figure 7), at 24 x 2.5 Gbps = 60 Gbps becomes more competitive than the PMO and at 28 x 2.5 Gbps = 70 Gbps overtakes the Overlay. The demand for GigE transport in metro networks is growing at a very fast pace. We have modeled GigE in our study with an eight-port GigE card that multiplexes eight GigE signals into a 10 Gbps wavelength on the system. Figure 8 is a pictorial description of the comparison among the three scenarios for GigE circuits. 100.0 90.0 80.0 70.0 Normalized Cost 60.0 50.0 40.0 30.0 20.0 10.0 0.0 4 20 36 52 68 84 100 116 132 148 Number of Circuits PMO Overlay Figure 8: Versus for GigE Circuits 7
The crossover points for the GigE model are 32 x 1.25 Gbps = 40 Gbps for PMO and 52 x 1.25 Gbps = 65 Gbps for overlay. Sufficient numbers of circuits are tested in each of the models so that the general trends of the economics can be clearly seen in the displays. The crossover points are sensitive to price and unit interface density. Improvements in one of these factors can shift the crossover point towards or towards. Although the density and relative pricing of the equipment may influence when the periodical hikes are happening in the graphs, the overall crossing and separation of the curves is eventually inevitable as the traffic volume escalates. However, a decision still needs to be made for a particular network with a forecast of growth: or? Table 1 is a summary of the crossover points for all four demand types we have modeled in the study. Note that at any traffic volume that is larger than or equal to a crossover point, the approach becomes more economic. Circuit Type PMO (Gbps) Overlay (Gbps) OC-3 16.8 20.5 OC-12 42.3 82.1 OC-48 60.0 70.0 GigE 40.0 65.0 Table 1: Summary of Crossover Points in Gbps In all cases, the overlay tends to delay the time when the curves switch positions. At the crossover points, the total numbers of OC-192 rings in the overlay network are 4, 10, 8 and 8 for the OC-3, OC-12, OC-48 and GigE models, respectively suggesting that, when 4 to 10 OC-192 rings exist in this example IOF overlay network design, a solution should be considered. Please note that this situation is ideal where only high speed (OC-3 and above) TDM and GigE service interfaces are involved, and the traffic pattern is perfect to carry end-to-end wavelengths in the most efficient way. In situations where low speed services, such as DS1 and DS3 are included, the crossover points will be delayed because of the efficiency of solutions in handling such services. While in other situations in which such data services as FICON/ESCON are mandatory, would be more favorable than. For more complicated network and traffic distribution, Fujitsu is conducting research using more sophisticated modeling tools and design skills, and additional papers will be published in the future. 8
Sensitivity to Span Distance and Fiber Cost solutions in the model use overlaid OC-192 rings as the demands grow. Additional fibers are used by each overlaid ring, which requires a lot of fibers (one pair per ring). We assume that the wideband OC-192 optics have an 80 km budget without dispersion compensation or additional amplification. We then examined the impact of the physical span distance, from, 20 km, 25 km to 30 km, on the crossover points of the and the overlay scenarios. The physical spans are between two physically adjacent nodes regardless of whether they are terminating or pass-through (without terminating any demands). From Figure 1 and Figure 3 and previous discussions, we learned that overlay has a longer ring distance than the PMO, although reduces the number of network ports and therefore the overall cost. In the four OC-48 termination example, two rings are required in both the PMO and the overlay scenarios. However, the total distance of a ring is only 60 km (4 x 15) in PMO while it is 105 km in overlay. The maximum link (between two terminating nodes) distance is 30 km for PMO and 45 km for overlay. We then applied a simple cost weight of $300 per single fiber mile in the models and necessary cost weights to the dispersion and amplification efforts. When the physical span distance increases, the fiber cost increases more drastically in than because uses a pair of fibers for every ring carrying up to 10 Gbps traffic, while uses one ring to carry up to 40 wavelengths of 10 Gbps traffic per wavelength. See Figure 9 for the crossover points relative to span distance. Crossover Point in Gbps 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 20 km 25 km 30 km Span Distance OC-3 OC-12 OC-48 GigE Figure 9: Crossover Point Sensitivity to Span Distance 9
A major drop occurs at the 30 km span distance because that is where the total link distance is over 80 km in overlay and the cost of amplification and dispersion compensation kicks in. At this point the number of OC-192 rings in overlay is 2, 4, 4 and 4 for OC-3, OC-12, OC-48 and GigE, respectively. The economics of is very much improved at this critical point. Note that does not reach a point where additional equipment is required until the spans are well above 100 km. The additional equipment at nodes for optical bypass enables even longer spans in the model. A Point-to-Point Scenario An interesting alternative for the scenarios establishes point-to-point connectivity with diversely routed protection as shown in Figure 10. Working routes are shown with thicker lines. Figure 10: A Point-to-Point Scenario A simple count of OC-192 line ports shows a total of 16 when each terminating site has eight OC-48s (four OC-48s per route). This count is eight ports less than the overlay. Table 2 gives the crossover points, including this new point-to-point case. Circuit Type PMO (Gbps) Overlay (Gbps) Point-to-Point (Gbps) OC-3 16.8 20.5 40.4 OC-12 42.3 82.1 No OC-48 60.0 70.0 130.0 GigE 40.0 65.0 85.0 Table 2: Summary of Crossover Points in Gbps, Including Point-to-Point 10
The point-to-point solution seems to delay the crossover points significantly in all cases. Especially in the OC-12 case, we could not find a crossover point after inputting 324 total OC-12 circuits (equivalent to 48 point-to-point OC-192 pipes), into the model. While performing well in the short span settings depicted in Figure 10, the OC-192 pipes really traverse a long distance from end-point-to-end-point, either for the working paths or for the protection paths. In the distance sensitivity test, when each physical span increases to 21 km, the protection path of circuit Route 1 (refer to the route indices in Figure 1) and the working path of Route 3 are 84 km, exceeding the dispersion and db loss budgets (80 km). When the span distance increases to 27 km or higher, both the working and protection paths of Routes 1 and 3 exhaust their reach. And, the protection paths of Routes 2 and 4 are reaching 81 km. The following are the sensitivities of distance on the crossover points between the point-to-point and technologies. We observed a sharper drop of almost all crossover points than the case of overlay. Note that the OC-12 model does not have a crossover point in the number of circuits we were able to input in the model. We assume it is at a very large number or infinity in the sensitivity test in Figure 11. Crossover Point in Gbps 200.0 180.0 160.0 140.0 120.0 100.0 80.0 60.0 OC-3 OC-12 OC-48 GigE 40.0 20.0 0.0 20 km 25 km 30 km Span Distance Figure 11: Crossover Point Sensitivity to Span Distance: versus Point-to-Point 11
Conclusions As expected, the scenarios have a low initial cost. When the traffic volume is low, a architecture is more economical than the architecture. Our modeling indicates that when designing a overlay network with OC-3, OC-12, OC-48 and Gigabit Ethernet demands and when the design requires less than four to ten OC-192 rings, a network is a better choice. As the traffic volume grows, will eventually prevail and become the choice of the network technology. Timing of this crossover is sensitive to such things as the span distances, pricing and interface density. The differences between demand types are mainly caused by the design efficiency of the two technologies interface cards in terms of density and cost. Our study also shows span distances usually trigger the extra requirement for regenerators, amplifiers and DCMs in the routes. Long span distance tends to favor a architecture because of the efficient use of fibers and better optical bypass capabilities at intermediate nodes. Additionally, higher fiber cost (more than $300/fiber mile modeled in this study) and situations in which fiber constraints are enforced will lead to more consideration for than because saves a tremendous amount of fiber in the network. Different alternatives and their economic impact in designing the same network is an interesting study. We investigated three different approaches in designing a network. overlay seems to always be better than PMO. point-to-point performs even better. These results may not apply in all situations; however, the implication is that, in large network designs, the most optimized network may not necessarily be one single architecture. One part of the network may adopt rings while another part implements pointto-point. Most often, the core part of the network will justify a architecture. 12
Acronym BLSR DCM GigE IOF NE NGADM PMO STS TDM VT Descriptor Bidirectional Line Switched Ring Dispersion Compensation Module Dense Wavelength Division Multiplexing Gigabit Ethernet Interoffice Facility Network Element Next-Generation Add/Drop Multiplexer Present Mode of Operation Synchronous Optical Network Synchronous Transport Signal Time Division Multiplexing Virtual Tributary 13