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Chapter 1 Optical Networking: Principles and Challenges 1 Course Information Email: [email protected] or [email protected] 研究室:機械大一樓 104室 電話:7232105-7047 Course Web Site: http://120.107.172.220/ http://deron.csie.ncue.edu.tw 2017/5/24 [email protected] 2 教師時間 Meeting Meeting 2017/5/24 [email protected] 3 評分標準 期中考 30% 期末考 30% 論文報告2篇 20% 程式設計1題 20% 4 Outlines 1.1 Introduction 1.2 Telecom Network Overview 1.3 Telecom Business Models 1.4 Roles of Three Fields in Optical Networking 1.5 Cross-Layer Design 1.6 TE vs . NE . vs . NP 1.7 What is an Optical Network? 1.8 Optical Networking: Need + Promise = Challenge! 1.9 xDM vs . xDMA 1.10 Wavelength-Division Multiplexing (WDM) 1.11 WDM Networking Evolution 1.12 WDM Network Constructions 1.13 WDM Economics 1.14 Sample Research Problems 1.15 Road Map - Organization of the Book 5 1.1 Introduction We need to be ready with the appropriate technologies and engineering solutions to meet the growing bandwidth needs of our information society. Optical networking using wavelengthdivision multiplexing (WDM) 6 1.2 Telecom Network Overview Access network Metropolitan-area network Backbone network backbone regional Local 7 The access network Enable the end-users to get connected to the rest of the network infrastructure. spans a distance of a few kilometers (<20 km) Current solution for access are dial-up modems, higherspeed lines (such as T1/E1), digital subscriber line (DSL), and cable modem. However, the access network continues to be a bottleneck, and users require (and are demanding) higher bandwidth to be delivered to their machines. Passive optical networks (PONS) based on inexpensive, proven, and ubiquitous Ethernet technology (and referred to as EPON) seem an attractive proposition for this market segment. PON technology in general, and EPON in particular, will be studied in Chapter 5. 8 metro-area network The metro-area network typically spans a metropolitan region, covering distances 20~200 km. Given the deep-rooted legacy of SONET/SDH ring networks SONET = Synchronous Optical Network; SDH = Synchronous Digital Hierarchy multi-wavelength versions of these rings are being deployed for our metro networks. Important characteristics of optical metro networks will be discussed in Chapter 6. 9 Backbone network The backbone network spans long distances, e.g., each link could be a few hundreds to a few thousands of kilometers in length (>200km). set up to provide nationwide or global coverage. Most telecom backbone networks are deployed today as an interconnection of "stacked" SONET/SDH rings, the fibers support multiple wavelengths using WDM transmission equipment; however, by "tying“ together several wavelengths on different fiber segments, one can create logical rings, and these rings can "meet" one another at some junction nodes, Backbone network will be discussed in Chapters 17 and 18. 10 1.3 Telecom Business Models Skip 11 1.4 skip 12 1.5 Cross-Layer Design The need for tight coupling between network architectures and device capabilities. Without a sound knowledge of device capabilities and limitations, one can produce architectures which may be unrealizable; conversely, research on new optical devices, conducted without the concept of a useful system, can lead to sophisticated technology with limited or no usefulness. 13 14 1.6 (TE) vs. (NE) vs. (NP) Traffic Engineering (TE) vs. Network Engineering (NE) vs. Network Planning (NP) TE: "Put the traffic where the bandwidth is." NE: "Put the bandwidth where the traffic is." NP: "Put the bandwidth where the traffic is forecasted to be.“ 15 Traffic Engineering (TE) Since the goal of TE is to "put the traffic where the bandwidth is," TE is essentially a "routing problem," where the traffic to be routed could be packets, packet flows, or bandwidth chunks (i.e., circuits). Routing and assigning appropriate bandwidth to packet flows and circuits is also referred to as bandwidth provisioning, or provisioning for short. Thus, TE is an "online,“ dynamic problem whose decision-making time is very quick, perhaps on the order of milliseconds. The typical performance metric used to evaluate a TE algorithm is "blocking probability" (by (implicitly) assuming that the network is operating at "steady-state") volume of control overhead, convergence time (to reduce routing instability), etc. 16 Network Engineering (NE) As a network continues its operation, and as traffic on it builds up (perhaps asymmetrically), certain parts of the network may become more congested due to increasing traffic, and these parts may need "help" in the form of additional capacity to relieve the congestion. The decision-making time is perhaps on the order of weeks or months. Thus, a typical performance metric for a NE problem could be "exhaustion probability“ which determines when a current network, given a traffic-growth pattern, will run into capacity exhaust. This (NE) is a very realistic problem in our operational networks; and, unfortunately, it has been underestimated in the academic research literature. 17 Network Planning NP The NP description is almost the same as that of NE, except that an additional phrase "forecasted to be" is appended. NP corresponds to the planning (i.e., design) of a network from scratch, with a decision-making timescale of perhaps a few years. A sample NP problem is the following: Given a set of traffic demands between various pairs of nodes (which is also called a "traffic matrix"), design the network for minimum cost, i.e., determine how much capacity to put on each link of the network, as well as routing of traffic through the network links. (Note that the typical performance/optimization metric 18 for a NP problem is "cost.") Network Planning NP As an example, the cost could be the sum of the (bandwidth) cost of all the links. In the brief NP problem description (above), no statement was made about the connectivity between network nodes (i.e., "network topology"). By default, the network topology (or graph) may be given. But an additional dimension to the NP problem would be to also determine the topology (while achieving minimum cost). This "dual" problem can be stated as follows: Given the traffic demands, and the maximum cost (including perhaps the topology and capacity of each link), determine how to establish the demands so that the network throughput (in terms of carried demands) is maximized 19 Summary of TW, NW, NP 20 1.7 What is an Optical Network? 21 Optical network The links require “transmission equipment,” while the nodes require “switching equipment.” An optical amplifier can simultaneously amplify all of the signals on multiple wavelength channels (perhaps as high as 160) on a single fiber link, independent of how many of these wavelengths are currently carrying live traffic. However, many attempts at developing all-optical switches have indicated that optical switching is still in its infancy. 22 Optical network Thus, an optical network is not necessarily all-optical: the transmission is certainly optical, but the switching could be optical, or electrical, or hybrid Also, an optical is not necessarily packet-switched. It could switch circuits (Chapters 7-12, 15-16), or sub-wavelength-granularity bandwidth pipes (Chapters 1314), or "bursts," where a burst is a collection of packets (Chapter 18). 23 PMO and FMO As an example, consider that two users located at the two coasts of the USA, need to exchange some large files. Under present mode of operation (PMO) of today's data networks, a simlple "traceroute" from Davis to Boston indicates that the file transfers may encounter 20 router hops, at each of which there exists the possibility of buffering (and hence delay), as well as loss (due to buffer overflow). In future mode of operation (FMO) of data networks, by exploiting the underlying support from optical-networking technologies, one should be able to "dial up" a fat bandwidth pipe (of appropriate capacity and duration) to complete the task. It is not necessary that all such applications need to accomplished in "one hop.“ 20 hops down to 3 or 1 (not necessary) 24 Separated control network plane Note that signaling in IP networks is "in-band," so (short) control packets may have to contend with (long) data packets for transmission bandwidth. As data entities that need to get transferred over our networks get longer, control packets are expected to encounter more contention for bandwidth. Thus, one can create a separated control network by setting aside a wavelength (or a sub-wavelength granularity bandwidth chunk using a traffic-grooming principle) on each link for this purpose, so that control packets have their own dedicated network. 25 1.8 Optical Networking: Need + Promise = Challenge! Life in our increasingly information-dependent society requires that we have access to information at our finger tips when we need it, where we need it, and in whatever format we need it. Internet and ATM networks - unfortunately, don't have the capacity to support the foreseeable bandwidth demands. 26 Fiber Fiber optic technology huge bandwidth (nearly 50 terabits per second (Tbps), low signal attenuation(衰減) (as low as 0.2 dB/km), immunity to electromagnetic interference, high security of signal because of no electromagnetic radiation, so difficult to eavesdrop, no crosstalk and interferences between fibers in the same cable, low signal distortion(扭曲), low power requirement, low material usage, small space requirement, and low cost. high electrical resistance, so safe to use near highvoltage equipment or between areas with different earth potentials. 28 Solving Problem Our challenge now is to turn the promise of optical fiber technology to reality to meet our information networking demands of the foreseeable future. Solving Problem Network lag. Not enough bandwidth today Exponential Growth in user traffic. 29 opto-electronic bandwidth mismatch Given that a single-mode fiber's potential bandwidth is nearly 50 Tbps, which is nearly 3-4 orders of magnitude higher than electronic data rates of a few gigabits per second (Gbps), every effort should be made to tap into this huge opto-electronic bandwidth mismatch. 30 Solution in Optical Network In an optical communication network, this concurrency may be provided according to either wavelength or frequency [wavelength-division multiplexing (WDM)], time slots [time-division multiplexing (TDM)], or wave shape [spread spectrum, code-division multiplexing (CDM)]. 31 Why not TDM or CDM? Optical TDM and CDM are somewhat futuristic technologies today. Under (optical) TDM, each end-user should be able to synchronize to within one time slot. The optical TDM bit rate is the aggregate rate over all TDM channels in the system, while the optical CDM chip rate may be much each higher than user's data rate. 32 Why not TDM or CDM? Both the TDM bit rate and the CDM chip rate may be much higher than electronic processing speed, i.e., some part of an end user's network interface must operate at a rate higher than electronic speed. Thus, TDM and CDM are relatively less attractive than WDM, since WDM — unlike TDM or CDM — has no such requirement. 33 1.9 xDM vs. xDMA We have introduced the term xDM where x = {W, T, C} for wavelength, time, and code, respectively. Sometimes, any one of these techniques may be employed for multiuser communication in a multiple access environment, e.g., for broadcast communication in a local-area network (LAN) Thus, a local optical network that employs wavelength-division multiplexing is referred to as a wavelength-division multiple access (WDMA) network; and TDMA and CDMA networks are defined similarly. 34 Basic Concept WDM is the ability to combine Multiple sources of data using Multiple wavelengths (colors) of light on One strand of fiber cable Source 1 Source 2 Source 3 Source 4 Its Analog Transmission Attenuation Dispersion Nonlinearity Reflectance Transmitted data waveform Waveform after 1000 km Fiber Types ... Multi-mode fiber allows multiple modes of light to propagate along its length at various angles. Typically: 62.5/125um, 50/125um Single-mode fiber allows a single mode of light to propagate along its core efficiently. Typically: 8/125um, 8.3/125um, 9/125um 1.10 WDM Wavelength-division multiplexing (WDM) is an approach that can exploit the huge opto-electronic bandwidth mismatch by requiring that each end-user's equipment operate only at electronic rate, but multiple WDM channels from different endusers may be multiplexed on the same fiber. Under WDM, the optical transmission spectrum is carved up into a number of non-overlapping wavelength (or frequency) bands, with each wavelength supporting a single communication channel operating at whatever rate one desires, e.g., peak electronic speed. WDM devices are easier to implement since, generally, all components in a WDM device need to operate only at electronic speed; as a result, several WDM devices are 38 available in the marketplace today, and more are emerging. 39 ITU recommended Bands E: 1360-1460 nm S: 1440-1530 nm C: 1530-1565 nm L: 1565-1625 nm U: 1625-1675 nm Light Spectrum Approximate Attenuation of Single Mode fiber cable Infrared Visible 700 900 1100 “O” Band ~ 1270-1350 nm “E” Band ~ 1370 - 1440 nm “S” Band ~ 1470 - 1500 nm “C” Band ~ 1530 - 1565 nm “L” Band ~ 1570 - 1610 nm 1300 1500 1700 nm 1.10.1 ITU Wavelength Grid There is a strong need for the standardization of WDM systems so that WDM components and equipments from difference vendors can inter-operate with one another. Thus, industry standards for wavelengths have been developed under the leadership of the International Telecommunications Union (ITU) [ITU02b]. A standard set of wavelengths, called the ITU grid, has been defined to coincide with the 1550-nm low-loss region of the fiber. Specifically, this grid is anchored at a frequency of 193.1 THz (which corresponds to a wavelength of 1552.52 nm). There is a 100-GHz grid, which means that spacing between adjacent channels is 100 GHz, which corresponds approximately to 0.8-nm wavelength channel spacing around the anchor frequency. 42 WDM-routed networks Optical signal and wavelength: Resonate/suppress WDM and TDM TDM Time Division Multiplexing WDM Wavelength Division Multiplexing ITU wavelength grid 45 For denser packing of channels, a 50-GHz grid has also been defined around the same reference frequency of 193.1 THz [ITU02b]. The 50-GHz grid is obtained by adding a channel exactly half way between two adjacent channels of the 100-GHz grid. Continuing this process, a 25-GHz grid can also be defined, and it can support 600 wavelengths [ITUOZb]. 46 WDM Thus, by allowing multiple WDM channels to coexist on a single fiber, one can tap into the huge fiber bandwidth, with the corresponding challenges being the design and development of appropriate network architectures, protocols, and algorithms. WDM devices are easier to implement since, generally, all components in a WDM device need to operate only at electronic speed; as a result, several WDM devices are available in the marketplace today, and more are emerging. 47 Development of WDM Since 1990 Several Conference: Country: ICC: IEEE International Conference on Communications OFC: Optical Fiber Communications U.S., Japan, Europe WDM: backbone, global coverage. 48 1.10.2 A sample WDM Networking Problem End-users in a fiber-based WDM backbone network may communicate with one another via all-optical (WDM) channels, which are referred to as lightpaths. A lightpath may span multiple fiber links, e.g., to provide a "circuit-switched" interconnection between two nodes which may have a heavy traffic flow between them and which may be located "far" from each other in the physical fiber network topology. Each intermediate node in the lightpath essentially provides an all-optical bypass facility to support the lightpath. 49 WDM network Complete graph, N nodes, N(N-1)links. The number of links is increased with the number of nodes. Technological constraints dictate that the number of WDM channels that can be supported in a fiber be limited to W. RWA Problem (routing and wavelength assignment): given a set of lightpaths that need to be established on the network, and given a constraint on the number of wavelengths, determine the routes over which these lightpaths should be set up and also determine the wavelengths that should be assigned to these lightpaths so that the maximum number of lightpaths may be established. . Lightpaths that cannot be set up due to constraints on routes and wavelengths are said to be blocked, so the corresponding network optimization problem is to minimize this blocking probability. 50 wavelength-continuity constraint In this regard, note that, normally, a lightpath operates on the same wavelength across all fiber links that it traverses, in which case the lightpath is said to satisfy the wavelengthcontinuity constraint. Thus, two lightpaths that share a common fiber link should not be assigned the same wavelength. 51 wavelength converter facility However, if a switching/routing node is also equipped with a wavelength converter facility, then the wavelength-continuity constraints disappear, and a lightpath may switch between different wavelengths on its route from its origin to its termination. RWA problem: Routing and Wavelength Assignment (RWA) problem 52 1.11 WDM Networking Evolution Point-to-Point WDM Systems When the demand exceeds the capacity in existing fibers, WDM is turning out to be a more costeffective alternative compared to laying more fibers. installation/burial of additional fibers and terminating equipment (the "multifiber" solution); a four-channel "WDM solution" where a WDM multiplexer (mux) combines four independent data streams, each on a unique wavelength, and sends them on a fiber; and a demultiplexer (demux) at the fiber's receiving end separates out these data streams; and OC-192, a "higher-electronic-speed" solution. 53 Four channels of point-to-point WDM 54 The analysis in [MePD95] shows that, for distances lower than 50 km for the transmission link, the "multi-fiber" solution is the least expensive; but for distances longer than 50 km, the "WDM" solution's cost is the least with the cost of the "higherelectronic-speed" solution not that far behind. WDM mux/demux in point-to-point links is now available in product form from several vendors such as IBM, Pirelli, and AT&T [Gree96]. Among these products, the maximum number of channels is 160 today, but this number is expected to increase soon. 55 1.11.2 Wavelength Add/Drop Multiplexer (WADM) or Optical Add/Drop Multiplexer (OADM Bar state cross state 56 WADM Architecture: States: DEMUX A set of 2x2 switches (one switch per wavelength) MUX Bar state: If all of the 2 x 2 switches are in the "bar" state, then all of the wavelengths flow through the WADM "undisturbed." Cross state: electronic control (not shown in Fig. 1.3), then the signal on the corresponding wavelength is "dropped" locally, and a new data stream can be "added" on to the same wavelength at this WADM location. More than one wavelength can be "dropped and added" if the WADM interface has the necessary hardware and processing capability. 57 1.11.3 Fiber interconnection Device passive star (see Fig. 1.11), passive router (see Fig. 1.12), and active switch (see Fig. 1.13). 58 passive star (see Fig. 1.11), The passive star is a "broadcast" device, so a signal that is inserted on a given wavelength from an input fiber port will have its power equally divided among (and appear on the same wavelength on) all output ports. "collision" will occur when two or more signals from the input fibers are simultaneously launched into the star on the same wavelength. Assuming as many wavelengths as there are fiber ports, an N x N passive star can route N simultaneous connections through itself. 59 Passive Star 60 passive router (see Fig. 1.12), A passive router can separately route each of several wavelengths incident on an input fiber to the same wavelength on separate output fibers this device allows wavelength reuse, i.e., the same wavelength may be spatially reused to carry multiple connections through the router. The routing matrix is "fixed" and cannot be changed. Such routers are commercially available, and are also known as Latin routers, waveguide grating routers (WGRs), wavelength routers (WRs), etc. Again, assuming as many wavelengths as there are fiber ports, a N x N passive router can route N2 simultaneous connections through itself (compared to only N for the passive star); however, it lacks the broadcast capability of the star. 61 Passive Router 62 active switch (see Fig. 1.13). The active switch also allows wavelength reuse, and it can support N2 simultaneous connections through itself (like the passive router). But the active star has a further enhancement over the passive router in that its "routing matrix" can be reconfigured on demand, under electronic control. However the "active switch" needs to be powered and is not as fault-tolerant as the passive star and the passive router which don't need to be powered. The active switch is also referred to as a wavelength-routing switch (WRS), wavelength selective crossconnect (WSXC), or just crossconnect (XC) for short. (We will refer to it as a WRS in this book.) 63 Active Switch 64 Wavelength Convertible Switch The active switch can be enhanced with an additional capability, viz., a wavelength may be converted to another wavelength just before it enters the mux stage before the output fiber (see Fig. 1.6). A switch equipped with such a wavelengthconversion facility is more capable than a WRS, and it is referred to as a wavelength-convertible switch, wavelength interchanging crossconnect (WIXC), etc 65 1.11.4 Development of WDM networks The first generation of WDM: point-to-point physical links include design and development of WDM lasers and optical amplifiers (OAMP) [Liu02]. The second generation of WDM is capable of establishing connection-oriented end-to-end lightpaths in the optical layer by introducing optical add/drop elements (WADM or OADM) and optical cross-connects (OXC). The ring and mesh topologies can be implemented using these OADMs and OXCs. The lightpaths are operated and managed based on a virtual topology over the physical fiber topology, and the virtual topology can be reconfigured dynamically in response to traffic changes. The technical issues of second-generation WDM include the development of OADM and OXC, wavelength conversion, routing and wavelength assignment (RWA), interoperability among WDM networks, network control and management. 66 3rd generation The third generation of WDM is expected to support a connectionless optical network. The key issues include the development of optical access network (such as passive optical network (PON)), and optical switching technologies, generically referred to as Optical "X" Switching (OXS), where X = P (for packet), B (for burst), L (for label), F (for flow), C (for cluster or circuit), etc. Sorne of these techniques, namely, OPS and OBS will be discussed in Chapters 17 and 18. 67 68 1.12 WDM Network Construction 1.12.1 Broadcast-and-Select (Local) Optical WDM Network A local WDM optical network may be constructed by connecting network nodes via two-way fibers to a passive star, The information streams from multiple sources are optically combined by the star and the signal power of each stream is equally split and forwarded to all of the nodes on their receive fibers. A node's receiver, using an optical filter, is tuned to only one of the wavelengths; hence it can receive the information stream. the passive-star can support “multicast” services. 69 70 Passive-Star-Based Optical WDM LAN vs. Centralized, nonblocking-Switch-Based LAN Passive Star WDM has following advantages: In the space-division-switch solution, the "switching intelligence" is centralized. However, the passive star relegates the switching functions to the end nodes If a node is down, the rest of the network can still function. Hence, the passive-star solution enjoys the fault-tolerance ad-vantage of any distributed switching solution, relative to the centralized-switch architecture, where the entire network goes down if the switch is down. 71 Passive Star WDM has following advantages it allows multicasting "for free." There are some processing requirements with respect to appropriately coordinating the nodal transmitters and receivers. Centralized coordination for supporting multicasting in a switch (also referred to as a "copy" facility) is expected to require more processing. can be potentially much cheaper since it is purely glass with very little electronics. 72 1.12.2 Wavelength-Routed (Wide-Area) Optical Network The network consists of a photonic switching fabric, comprising "active switches" connected by fiber links to form an arbitrary physical topology. Each end-user is connected to an active switch via a fiber link. The combination of an end-user and its corresponding switch is referred to as a network node. Each node (at its access station) is equipped with a set of transmitters and receivers, both of which may be wavelength tunable. A transmitter at a node sends data into the network and a receiver receives data from the network. 73 74