Chapter 10 Optical Networks

1. Chapter 10 Optical Networks • 10.1 Network Architecture and Topologies • 10.1.1 Wide-Area Networks • 10.1.2 Metropolitan-Area Networks • 10.1.3 Local-Area Networks • 10.2 Network Protocols and Layers • 10.2.1 Evolution of Protocols • 10.2.2 Evolution of WDM Networks • 10.2.3 Network Planes • 10.3 Wavelength-Routing Networks • 10.3.1 Wavelength Switching and Its Limitations • 10.3.2 Architecture of Optical Cross-Connects • 10.3.3 Switching Technologies for Cross-Connects

2. Chapter 10 Optical Networks • 10.4 Packet-Switched Networks • 10.4.1 Optical Label Swapping • 10.4.2 Techniques for Label Coding • 10.4.3 Contention Resolution • 10.5 Other Routing Techniques • 10.5.1 Optical Burst Switching • 10.5.2 Photonic Slot Routing • 10.5.3 High-speed TDM Networks • 10.6 Distribution and Access Networks • 10.6.1 Broadcast-and-Select Networks • 10.6.2 Passive Optical Networks

3. 10.1 Network Architecture and Topologies • Networks can be classified into three groups: Local area networks (LANs), Metropolitan-area networks (MANs), and Wide-area networks (WANs). • An alternative classification used by the telephone industry refers to LANs as access networks, MANs as metro networks, and WANs as transport networks. • Figure 10.1 shows an example of a WAN covering a large part of the United States. Such networks are also called mesh networks.

4. Figure 10.1: An example of a wide-area mesh network designed with hub topology. 10.1.1 Wide-Area Networks

5. 10.1.1 Wide-Area Networks • Hubs or nodes connect any two nodes by creating a “virtual circuit” between them. This is referred to as circuit switching. An alternative scheme, used for the Internet, is known as packet switching. • In the mesh-network architecture of Figure 10.1, only some nodes are connected directly through point-to-point links. • The creation of a virtual circuit between two arbitrary nodes requires switching at one or more intermediate nodes. Such networks are called multihop networks.

6. 10.1.1 Wide-Area Networks • In all-optical WDM network, a WDM signal passes through intermediate nodes without being converted to the electrical domain. • An optical add-drop multiplexer is used at the destination node to add or drop channels at specific wavelengths. Such a network is referred to as being “transparent.” • Transparent WDM networks do not require demuxing and O/E conversion of all WDM channels. As a result, they are not limited by the electronic-speed bottleneck and may also help in reducing the cost of installing and maintaining a network.

7. 10.1.2 Metropolitan-Area Networks • Figure 10.2 shows the architecture of a MAN schematically. The topology of choice for MANs is a ring that connects to the WAN at one or two egress nodes. • This ring employs up to four fibers to provide protection against network failures. • Two of the fibers are used to route the data in the clockwise and counter-clockwise directions. • The other two fibers are protection fibers, deployed when a point-to-point link fails.

8. 10.1.2 Metropolitan-Area Networks • Figure 10.2: Schematic of a MAN with a ring topology. It is connected to a WAN at egress nodes (EN) and to multiple LANs at access nodes (AN). ADM stands for add-drop multiplexer.

9. 10.1.2 Metropolitan-Area Networks • A network is called self-healing if the fiber is switched automatically. The central ring in Figure 10.2 is called a feeder ring as it provides access to multiple LANs at access nodes. • The advantage of a WAN in the form of regional rings is that such a configuration provides protection against failures. • The use of protection fibers in each ring ensures that an alternate path between any two nodes can be found if a single point-to-point link fails.

10. 10.1.3 Local-Area Networks • Many applications require LANs in which a large number of users within a local area are interconnected in such a way that any user can access the network randomly to transmit data to any other user. • System architecture plays an important role for LANs. Three commonly used topologies are shown in Figure 10.3 and are known as the bus, ring, and star topologies.

11. 10.1.3 Local-Area Networks • Figure 10.3: Schematic illustration of the (a) bus, (b) ring, and (c) star topologies employed for local-area networks.

12. 10.1.3 Local-Area Networks • In the case of bus topology, all users communicate with each other by tapping into a central optical fiber (the bus) that transports all data in one direction. • The bus topology is often employed for cable-television (CATV) networks. • In the case of ring topology, the network functions by passing a token (a predefined bit sequence) around the ring. Each node monitors this token and accepts the datum if it contains its own address.

13. 10.1.3 Local-Area Networks • In the case of star topology, all nodes are connected to a star coupler at a central location. • The star coupler receives the signal power transmitted by each node and distributes it equally to all nodes such that all nodes receive the entire traffic. • This type of loss is known as distribution loss and is much smaller for the star topology compared with the bus topology.

14. 10.1.3 Local-Area Networks • Most CATV networks employ the bus topology. A single optical fiber carries multiple video channels on the same optical wavelength through a technique known as subcarrier multiplexing (SCM). • A problem with bus topology is that distribution losses increase exponentially with the number of taps and limit the number of subscribers that can be served by a single optical bus.

15. 10.1.3 Local-Area Networks • Even when fiber losses are neglected, the power available at the N-th tap is given by where PTis the transmitted power, Cfis the fraction of power coupled out at each tap, and d accounts for the insertion loss, assumed to be the same at each tap. • Optical amplifiers can be used to boost the optical power along the bus periodically to solve the distribution-loss problem.

16. 10.2 Network Protocols and Layers • The complexity of designing and maintaining a network is handled through specific protocols and a layered architecture in which network functions are divided among several layers. • Each layer performs a specific function and provides a specific service to the layer above it. • We focus on several network protocols and discuss how their use in core networks has evolved with the advent of WDM technology.

17. 10.2.1 Evolution of Protocols • The open-systems-interconnection (OSI) reference model divides any network into seven layers from the standpoint of functionality. Such a scheme is known as the OSI seven-layer model. • Each layer performs services for the layer on top of it and makes requests to the layer lying below it.

18. 10.2.1 Evolution of Protocols • The lowest layer in the OSI reference model constitutes the physical layer. Its role is to provide an “optical pipe” with a certain amount of bandwidth to the data link layer located above it. • The second layer is responsible for creating a bit stream through multiplexing and framing that is transmitted over the physical layer. • The third layer is known as the network layer. Its function is to create a virtual circuit between any two nodes of the network and provide end-to-end routing between them.

19. 10.2.1 Evolution of Protocols • The fourth layer, known as the transport layer, is responsible for the error-free delivery of data between any two nodes across the entire network. • The last three layers are known as the session, presentation, and application layers. They provide higher-order services and are not relevant in the context of this chapter. • In the case of circuit-switched SDH networks, the combination of bottom three layers implements the SONET protocol.

20. 10.2.1 Evolution of Protocols • For SDH networks, the physical layer consists of fibers, amplifiers, transmitters, receivers, and other network elements. • The data link layer multiplexes various 64-kb/s audio channels into a bit stream at the desired bit rate through electrical TDM. • The network layer provides switching at intermediate nodes to establish a connection between any two nodes of the network.

21. 10.2.1 Evolution of Protocols • The operation mode for SONET/SDH networks was modified when the transmission of both audio and computer data was required over the same physical layer. • The ATM protocol employs 53-byte packets (with a 5-byte header) that are transmitted over the network. • As shown in Figure 10.4(a), the ATM protocol was built on top of the SONET in the sense that the SONET infrastructure became the physical layer of the ATM network.

22. 10.2.1 Evolution of Protocols • Figure 10.4: Layered architecture for SDH networks: (a) ATM over SONET, (b) IP over SONET, and (c) IP over ATM.

23. 10.2.1 Evolution of Protocols • As seen in Figure 10.4(b), IP traffic can be carried directly over SONET using the layer model. It can also be transported over the ATM protocol. • In the latter case, the network design becomes quite complicated, as shown schematically in Figure 10.4(c), because of the nesting of three separate protocols.

24. 10.2.2 Evolution of WDM Networks • With the advent of WDM systems, ITU has introduced a new layer known as the optical layer. This layer sits at the bottom of the layer stack and may be thought of as a part of the physical layer. • A lightpath transmits data between these two nodes at the bit rate at which individual channels of a WDM system operate; it is also called a clear channel.

25. 10.2.2 Evolution of WDM Networks • Figure 10.5: Optical layer associated with a fiber-optic WDM link. It may consist of up to three sections depending on the role of each network element.

26. 10.2.2 Evolution of WDM Networks • As shown in Figure 10.5, the optical layer consists of three sublayers or sections, known as the optical transmission section, optical multiplex section, and optical channel section. • Not all sections need to be present at every component along the fiber-optic link. For example, only the transmission section is required at the location of each amplifier, where all WDM channels are amplified without any channel switching. • Figure 10.6(a) shows the layered approach for WDM networks in which IP traffic is routed over ATM switches through the SONET protocol.

27. 10.2.2 Evolution of WDM Networks • Figure 10.6: Evolution of WDM networks through MPLS and GMPLS schemes: (a) IP over ATM; (b) IP over SDH; (c) IP over WDM.

28. 10.2.2 Evolution of WDM Networks • It is possible to eliminate the ATM layer through a switching scheme known as multiprotocol label switching (MPLS). • MPLS deals with multiple protocols through a label attached to each packet and provides a unified scheme that can transport both the ATM and IP packets across a packet-switched network. • At the next stage of the evolution, IP traffic is planned to be routed directly over WDM networks.

29. 10.2.3 Network Planes • In the seven-layer OSI model, functions performed by a network were grouped into different layers. • An alternative scheme divides the operation of a network into three separate planes called (1). transport plane, (2). control plane, and (3). management plane. • The transport plane focuses on the transport of data across a network. It should provide the bidirectional flow of information among various nodes, while maintaining signal quality.

30. 10.2.3 Network Planes • In the case of WDM networks, the transport plane not only provides transmission among nodes but it also performs all-optical routing, detects faults, and monitors signal quality. • The routing function is performed by an OXC that can switch WDM channels at each node in a controlled fashion. • In an automatic-switched optical network, all optical switching is performed in the transport plane to set up lightpaths in a dynamic fashion.

31. 10.2.3 Network Planes • The role of a control plane is to control electronically how optical switching is performed in the transport plane. • This plane supports the setup and removal of connections between any two nodes of the network. It also provides protection and restoration services in case of a failure. • Figure 10.7 shows how the control plane interfaces and directs the traffic being transported over the transport plane of a WDM network.

32. 10.2.3 Network Planes • Figure 10.7: Optical transport and control planes associated with a WDM network.

33. 10.2.3 Network Planes • The role of the management plane is to reconfigure WDM channels so that the bandwidth is utilized in an efficient fashion and to monitor network performance. • The ITU has also recommended a telecommunications management network reference model. • It consists of 4 layers devoted to the management of business, service, network elements, and network performance.

34. 10.3 Wavelength-Routing Networks • In an automatic-switched optical network, OXCs are used at the intermediate nodes to set up lightpaths in a fast and flexible manner. • Since individual channels in the core network operate at a bit rate of 10 Gb/s or more, it is important that traffic be aggregated appropriately to avoid wasting the bandwidth. • Figure 10.8 illustrates a simple six-node WDM network in which two wavelengths l1and l2 are used to establish multiple lightpaths among its 6 nodes through OXCs.

35. 10.3.1 Wavelength Switching and Its Limitations • Figure 10.8: Schematic of a six-node network. Wavelength-routing switches (WRS) are used to establish lightpaths among various nodes using only two wavelengths. The dashed and dotted lines show the paths taken by the l1 and l2 channels, respectively.

36. 10.3.1 Wavelength Switching and Its Limitations • Figure 10.8 shows two wavelength-continuous lightpaths. Nodes A and C are connected through l1, whereas a lightpath at l2connects nodes A and F. • The design of a wavelength-routing network is simplified considerably if wavelength converters are employed within each OXC. • Such OXC devices change the carrier wavelength of a channel without affecting the data carried by it.

37. 10.3.1 Wavelength Switching and Its Limitations • In the case of fixed-alternate routing, each wavelength router in the network contains a routing table. • This routing table assigns a priority number to various potential routes. The router tries the first route with the highest priority. • This kind of routing scheme is simple to implement in the control plane and it can be used to recover from a link failure.

38. 10.3.1 Wavelength Switching and Its Limitations • In the adaptive routing scheme, the lightpath connecting two network nodes is chosen dynamically, depending on the state of the network at the time decision is being made. • This approach requires an algorithm that computes the “cost” of each potential lightpath in terms of some specific design objectives.

39. 10.3.2 Architecture of Optical Cross-Connects • The architecture of OXCs depends on several factors and requires a tradeoff between the cost of optical hardware and the ease of network management. • For example, one can utilize wavelength conversion at every network node to eliminate wavelength blocking completely, but the hardware cost then increases considerably.

40. 10.3.2 Architecture of Optical Cross-Connects • Figure 10.9 shows five architectures for OXCs. • The design shown in part (a) is the traditional approach in which WDM signals reaching the node over different input fibers are first demultiplexed into individual channels (operating typically at a bit rate of 10 Gb/s) and then converted into the electric domain using a set of receivers. • All electrical bit streams enter a digital cross-connect (dashed box) that routes them to different transmitters, as dictated by the control software.

41. 10.3.2 Architecture of Optical Cross-Connects • Figure 10.9: (a) An OXC with electronic switching; (b) an OXC with wavelength conversion at each node; (c) an OXC with shared conversion; (d) an OXC with partial conversion; (e) a wavelength-selective OXC with no conversion.

42. 10.3.2 Architecture of Optical Cross-Connects • The output of transmitters is then multiplexed to form WDM signals that are transported over output fibers. Full wavelength conversion is possible if tunable transmitters are employed at each node. • Considerable effort was being directed toward developing OXCs that avoid conversion of optical signals to the electric domain at each node.

43. 10.3.2 Architecture of Optical Cross-Connects • Figure 10.9(b) shows an OXC that provides the same functionality without requiring electronic conversion. • In this device, demultiplexed optical channels are fed into a photonic cross-connect (solid box) consisting of a bank of directional switches that direct each channel to a different port, as dictated by the control software. • Because all signals remain in the optical domain, channel wavelength can only be changed by employing wavelength converters－devices that produce a copy of the signal at one wavelength to another wavelength.

44. 10.3.2 Architecture of Optical Cross-Connects • The number of wavelength converters required in Figure 10.9(b) equals MN when the node is designed to handle traffic on M fibers, each carrying N wavelengths. • To reduce the hardware cost, several solutions are possible. • In one approach, wavelength converters are employed only at a few intermediate nodes. • In another, wavelength converters are shared in a loop-back configuration as shown in part (c).

45. 10.3.2 Architecture of Optical Cross-Connects • The option shown in part (d) employs only a small number of wavelength converters at each node (partial conversion). • In all cases, limited wavelength conversion introduces some probability of wavelength blocking. • This probability can be reduced considerably if wavelength converters are made tunable such that they accept input signals over the entire WDM-signal bandwidth and can also produce output in the entire range.

46. 10.3.2 Architecture of Optical Cross-Connects • The final design shown in Figure 10.9(e) is the most economical as it eliminates all wavelength converters. Such a device is referred to as the wavelength-selective OXC. • It is designed to distribute all input signals at a specific wavelength to a separate switching unit. Each unit consists of a M x M switching fabric that can be configured to route the signals at a fixed wavelength in any desirable fashion. • Extra input and output ports can be added to allow the dropping or adding of a local channel at that wavelength.

47. 10.3.2 Architecture of Optical Cross-Connects • Wavelength-selective OXCs are transparent to both the format and bit rate of the WDM signal. They are also cheaper and help to reduce overall cost. • The constraint that every lightpath must use the same wavelength across all point-to-point links eventually limits the capacity of the network. • In general, short lightpaths with only a few hops experience little or no wavelength blocking.

48. 10.3.2 Architecture of Optical Cross-Connects • The number of wavelength converters required to eliminate wavelength blocking depends on the algorithm used for assigning wavelengths and routing channels across the network. • In the case of a static network (permanent lightpaths), even 5% wavelength conversion was found to eliminate wavelength blocking in a 24-node network. • In the case of dynamic networks (traffic-dependent lightpaths), the number of converters needed at a node changes with time in a random fashion.

49. 10.3.3 Switching Technologies for Cross-Connects • The MEMS technology has attracted the most attention for constructing OXCs as it can provide relatively compact devices. It is used to fabricate microscopic mirrors that can be rotated by applying an electric signal. • The MEMS switches are divided into two broad categories, referred to as two-dimensional (2D) and three-dimensional (3D) configurations, depending on the geometry used to interconnect the input and output fibers. • Figure 10.10 shows schematically the configuration of a 2-dimensional OXC in which a 2D array of free-rotating MEMS mirrors is used to switch light from any input fiber to any output fiber.

50. 10.3.3 Switching Technologies for Cross-Connects • Figure 10.10: Schematic of a two-dimensional MEMS-based OXC. Microscopic mirrors are rotated to connect input and output fibers in an arbitrary fashion.