Optical Cross-connects OXCs COMPONENTS

202 OPTICAL FIBERS AND COMPONENTS Mirror Inside Outside x y axis Figure 8.27 The gimbaled mirror. Input wavelengths Output wavelengths MEMS array MEMS array Figure 8.28 The 3D MEMS architecture. fibers, and each micro-mirror is dedicated to an output fiber. Switching of the incoming light on an input fiber i to an output fiber j is achieved by appropriately tilting their dedicated micro-mirrors so that the incoming light is reflected to the output fiber. The operation of this MEMS architecture requires a sophisticated control mechanism that it can tilt the angle of the micro-mirrors and maintain them in position for the duration of the connection. The number of micro-mirrors increases with the square root of the number of input and output ports. This permits scaling of the optical switching fabric to thousands of ports with a low insertion loss of about 3 dB. Finally, the 1D MEMS optical switch fabric uses a single MEMS array. In this architec- ture, a dispersive optics element is used to separate the input DWDM optical signal into the individual wavelengths. Each wavelength strikes an individual micro-mirror which directs it to the desired output where it is combined with the other wavelengths via a dis- persive element. There is one micro-mirror per wavelength, which makes the architecture scale linearly with the number of wavelengths. Semiconductor optical amplifier SOA A semiconductor optical amplifier SOA is a pn-junction that acts as an amplifier and also as an on-off switch. A pn-junction, as shown in Figure 8.29, consists of a p-type PROBLEMS 203 p -type n -type Current Optical signal Figure 8.29 A pn-junction. Polymer waveguides Polymer waveguides SOAs l 1 l 2 Figure 8.30 A 2 × 2 SOA switch. and an n-type semiconductor material. A p-type semiconductor is doped with impurity atoms so that it has an excessive concentration of mobile electron vacancies, known as holes . The n-type semiconductor has an excess concentration of electrons. When p-type and n-type semiconductors are placed in contact with each other, the holes move from the p-type semiconductor to the n-type semiconductor and the electrons move from the n-type semiconductor to the p-type semiconductor. This creates a region with negative charge in the p-type semiconductor, and a region with positive charge in the n-type semiconductor. These two regions are known as the depletion area. When charge is applied, the width of the depletion region is reduced and current flows from the p-type semiconductor to the n-type semiconductor causing the depletion area to become an active region. An optical signal is amplified if it passes through the depletion area when it is active. When no charge is applied to the pn-junction, the optical light that passes through the depletion area is absorbed. SOAs can be used to build optical switches. A 2 × 2 switch is shown in Figure 8.30. The waveguides act as splitters or couplers, and are made of polymeric material. The incoming wavelength λ 1 is split into two optical signals, and each signal is directed to a different SOA see Figure 8.30. One SOA amplifies the optical signal and permits it to go through, and the other SOA stops it. Accordingly, wavelength λ 1 leaves the 2 × 2 switch from either the upper or the lower output port. The switching time of an SOA switch is currently about 100 psec. PROBLEMS 1. Let us assume that in a WDM point-to-point link each wavelength is used to transmit SONET SDH frames at the rate of OC-48STM-16 i.e., 2488 Mbps. Calculate the total capacity of the link for W = 1, 16, 32, 128, 512, 1024. Repeat these calculations assuming that the rate 204 OPTICAL FIBERS AND COMPONENTS of transmission over a single wavelength is: OC-192STM-64 9953 Mbps, OC-768STM- 256 39,813 Mbps. 2. Browse the Internet to find out the maximum number of wavelengths which is currently com- mercially available in a WDM point-to-point link. 3. What are the differences between a single-mode and a multi-mode fiber? The standard for the 1-Gbps Ethernet makes use of both single-mode and multi-mode fibers. Browse the Internet to find out how each of these fiber modes are used. 4. Explain what is attenuation and dispersion. 5. Explain the terms transparent switch and opaque switch. 6. Draw a three-stage Clos network. What are the main differences between a Banyan network and a Clos network? Hint: for information, check the literature on ATM switch architectures. 7. Use the 2D MEMS OADM shown in Figure 8.26 to design an OADM that serves a single fiber with 64 wavelengths. Each 2D MEMS is assumed to have 32 × 32 ports. 9 Wavelength Routing Optical Networks Wavelength routing optical networks have been successfully commercialized and standards bodies, such as the IETF, OIF, and ITU-T, are currently active in the development of the standards. A wavelength routing optical network consists of optical cross-connects OXCs interconnected with WDM fibers. Transmission of data over this optical network is done using optical circuit-switching connections, known as lightpaths. In this chapter, we explore different aspects of the wavelength routing optical networks. We first start with a description of the main features of a wavelength routing network and introduce the ever important concept of a lightpath and the concept of traffic grooming, which permits multiple users to share the same lightpath. We also present protection and restoration schemes used to provide carrier grade reliability. Information on a lightpath is typically transmitted using SONETSDH framing. Ether- net frames can also be transmitted over an optical network. In the future, it is expected that information will be transmitted over the optical network using the new ITU-T G.709 standard, part of which is described in this chapter. G. 709, also known as the digi- tal wrapper , permits the transmission of IP packets, Ethernet frames, ATM cells, and SONETSDH data over a synchronous frame structure. The rest of the chapter is dedicated to the control plane for wavelength routing networks. We present different types of control plane architectures, and then describe the generalized MPLS GMPLS architecture and the OIF user network interface UNI. GMPLS is an extension of MPLS, and was designed to apply MPLS label-switching tech- niques to time-division multiplexing TDM networks and wavelength routing networks, in addition to packet-switching networks. The OIF UNI specifies signaling procedures for clients to automatically create and delete a connection over a wavelength routing network. The UNI signaling has been implemented by extending the label distribution protocols, LDP and RSVP.

9.1 WAVELENGTH ROUTING NETWORKS

A wavelength routing or routed network consists of OXCs interconnected by WDM fibers. An OXC is an N × N optical switch, with N input fibers and N output fibers see Section 8.3.5. Each fiber carries W wavelengths. The OXC can optically switch all of the incoming wavelengths of its input fibers to the outgoing wavelengths of its output Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2 206 WAVELENGTH ROUTING OPTICAL NETWORKS fibers. For instance, it can switch the optical signal on incoming wavelength λ i of input fiber k to the outgoing wavelength λ i of output fiber m. If output fiber m’s wavelength λ i is in use, and if the OXC is equipped with converters, then the OXC can also switch the optical signal of input fiber k’s incoming wavelength λ i to another one of output fiber m ’s outgoing wavelength λ j . In addition to switching individual wavelengths, an OXC can switch a set of contiguous wavelengths known as a waveband as a single unit. That is, it can switch a set of contiguous wavelengths of an input fiber to a set of contiguous wavelengths of an output fiber. This can be a desirable OXC feature, because it can reduce the distortion of the individual wavelengths. In addition, an OXC might not have the capability to separate incoming wavelengths that are tightly spaced. In this case, it can still switch them using waveband switching. Finally, an OXC can also switch an entire fiber. That is, it can switch all of the W wavelengths of an input fiber to an output fiber. There are several technologies available for building an OXC, such as multistage interconnection networks of 3-dB couplers, MEMS, SOA, micro-bubbles and holograms see Section 8.3.5. New technologies are expected to emerge in the future. An OXC can be used as an optical adddrop multiplexer OADM. That is, it can termi- nate the signal on a number of wavelengths and insert new signals into these wavelengths. The remaining wavelengths are switched through the OXC transparently. An example of an OADM is shown in Figure 9.1. One of its output fibers is fed into a SONETSDH DCS. Typically, a SONETSDH frame is used to transmit data on each wavelength. The DCS converts the incoming W optical signals into the electrical domain, and extracts the SONETSDH frame from each wavelength. It can then switch time slots from a frame of one wavelength to the frame of another, terminate virtual tributaries, and add new ones. The resulting W new streams of SONETSDH frames are transmitted to the OXC, each over a different wavelength, and then are switched to various output fibers.

9.1.1 Lightpaths

An important feature of a wavelength routing network is that it is a circuit-switching network. That is, in order for a user to transmit data to a destination user, a connection has to be first set up. This connection is a circuit-switching connection and is established by using a wavelength on each hop along the connection’s path. For example, let us consider that two IP routers router A and router B are connected via a three-node wavelength DCS Input WDM fibers Output WDM fibers Switch fabric . . . . . . . . . Figure 9.1 An OXC with an attached SONETSDH DCS. WAVELENGTH ROUTING NETWORKS 207 a A three-node wavelength routing network b A lightpath between Routers A and B OXC 1 OXC 2 OXC 3 l 1 ,..,l W l 1 ,..,l W Router A Router B l 1 ,..,l W l 1 ,..,l W l 1 l 1 OXC 1 OXC 2 OXC 3 Router A Router B l 1 l 1 Figure 9.2 A lightpath. routing network see Figure 9.2. The links from router A to OXC 1, from OXC 1 to OXC 2, from OXC 2 to OXC 3, and from OXC to router B, are assumed to be a single fiber carrying W wavelengths, which are referred to as λ 1 , λ 2 , . . . , λ W . Data is transmitted unidirectionally, from router A to router B. To transmit data in the opposite direction i.e. from router B to router A, another set of fibers would need to be used. Assume that IP router A wants to transmit data to IP router B. Using a signaling protocol, A requests the establishment of a connection to B. The connection between Routers A and B is established by allocating the same wavelength say wavelength λ 1 on all of the links along the path from A to B i.e., links A to OXC 1, OXC 1 to OXC 2, OXC 2 to OXC 3, and OXC 3 to B. Also, each OXC is instructed to switch λ 1 through its switch fabric transparently. As a result, an optical path is formed between Routers A and B, over which data is transmitted optically from A to B. This optical path is called a lightpath , and it connects Routers A and B in a unidirectional way from A to B. In order for B to communicate with A, a separate lightpath has to be established in the opposite way over a different set of fibers that are set up to transmit in the opposite direction. When establishing a lightpath over a wavelength routing network, the same wavelength has to be used on every hop along the path. This is known as the wavelength continuity constraint . The required wavelength might not be available at the outgoing fiber of an OXC, through which the lightpath has to be routed. In this case, the establishment of the lightpath will be blocked, and a notification message will be sent back to the user requesting the lightpath. To decrease the possibility that a lightpath is blocked, the OXC can be equipped with converters. A converter can transform the optical signal transmitted over a wavelength to another wavelength. In an OXC, for each output fiber with W wavelengths, there might be c converters, where 0 ≤ c ≤ W . When c = 0, we say that there is no conversion; when 0 c W , we say that there is partial conversion; and when c = W , we say that there is full conversion. Converters are still expensive, and so they may be deployed in certain strategic OXCs in a wavelength routing network. A common assumption made in the literature is that a converter can transform a signal on a wavelength λ to any wavelength. However, currently, it can only transform it to another wavelength which is within a few nm from wavelength λ. An example of different lightpaths established over a wavelength routing network is shown in Figure 9.3. The optical network consists of OXCs 1, 2, and 3. Only OXC 3 is assumed to be equipped with converters at least two, for the sake of this example. IP Routers A and B are attached to OXC 1 at two different input ports; IP router C is attached to OXC 2; and IP router D is attached to OXC 3. The following lightpaths 208 WAVELENGTH ROUTING OPTICAL NETWORKS O E Router B Router A Router D Router C E O OXC 3 l 3 O E E O OXC 2 OXC 1 l 1 l 1 l 1 l 1 l 1 l 1 l 2 l 3 Figure 9.3 An example of different lightpaths. have been established: from router A to router C over OXCs 1 and 2; from router B to router D over OXCs 1 and 3; and from router C to router D over OXCs 2 and 3. The wavelengths allocated to each lightpath are indicated in Figure 9.3. Wavelength λ 1 is used for the lightpath from router A to router C on all of the hops; that is, from A to OXC 1, then from OXC 1 to OXC 2, and finally from OXC 2 to C. The lightpath from router B to router D uses λ 1 on the hops from B to OXC 1 and from OXC 1 to OXC 3, and λ 2 on the hop from OXC 3 to D. Finally, the lightpath from router C to router D uses λ 3 on the hops from C to OXC 2 and then from OXC 2 to OXC 3, and λ 1 on the hop from OXC 3 to D. As mentioned above, the transmission on a lightpath is unidirectional. In Figure 9.3, only the lightpaths from Routers A to C, B to D, and C to D are shown. For bidirectional communication between two routers, a separate lightpath has to be set up in the opposite direction through the same OXCs. For instance, for bidirectional communication between Routers A and C, another lightpath has to be set up from router C to router A via OXCs 2 and 1 using separate fiber links. Finally, note that the data transmission within the wavelength routing network occurs entirely in the optical domain. In Figure 9.3, the dotted lines along the IP routers signify the boundary between the electrical domain [E] and the optical domain [O]. Lightpaths can be either static e.g. in an ATM PVC connection or dynamic e.g. in an ATM SVC connection. Static lightpaths are established using network management procedures, and generally remain up for a long time. Virtual private networks VPNs can also be set up using static lightpaths. Dynamic lightpaths are established in real-time using signaling protocols, such as IETF’s GMPLS and the user network interface UNI proposed by the Optical Internetworking Forum OIF. These protocols are discussed in detail in Sections 9.5 and 9.6 below.

9.1.2 Traffic Grooming

A lightpath is used exclusively by a single client. Quite often, the bandwidth that a client requires is a lot less than the wavelength’s bandwidth, which means that part of WAVELENGTH ROUTING NETWORKS 209 the lightpath’s bandwidth goes unused. To resolve this, the bandwidth of a lightpath is divided into subrate units, so that it can carry traffic streams transmitted at lower rates. A client can request one or more of these subrate units. This technique, known as traffic grooming , allows the bandwidth of a lightpath to be shared by many clients. Compared to using an entire lightpath, traffic grooming improves wavelength utilization and client cost-savings. As an example, let us consider the six-node optical network see Figure 9.4. Infor- mation is transmitted over the optical network using SONETSDH framing with a trans- mission rate of OC-48STM-16 2.488 Gbps. A lightpath, indicated by a dotted line, has been established from OXC 1 to OXC 3 through OXC 2 using wavelength λ 1 . The subrate unit is an OC-3STM-1 155 Mbps, which means that 16 subrate units of OC-3STM-1 are available on the lightpath. A user, attached to OXC 1, who wants to transmit data to another user, attached to OXC 3, can request any integer number of OC-3STM-1 subrate units up to a total of 16. If the traffic between these two OXCs exceeds 2.488 Gbps, then more lightpaths can be established. A lightpath can be seen as a tunnel between the originating and terminating OXCs. That is, the data streams transmitted on the lightpath between OXCs 1 and 3, can only originate at OXC 1 and terminate at OXC 3. No data can be added to the lightpath or dropped from the lightpath at OXC 2. As explained in the previous section, if no conversion is available, then the same wavelength has to be allocated to a lightpath on all hops. However, the wavelength continuity constraint is not necessary if conversion is available. For example, the lightpath between OXCs 1 and 3 uses the same wavelength because it was assumed that OXC 2 is not equipped with converters. However, if OXC 2 is equipped with converters, then the lightpath can be established by using any wavelength on the links OXC 1 to 2, and OXC 2 to 3. Finally, a data stream might traverse more than one lightpath in order to reach its destination. For example, assume that a user attached to OXC 1 requests four subrate OC- 3STM-1 units to transmit an OC-12STM-4 622 Mbps data stream to a user attached to OXC 4. In this case, a new lightpath has to be established between OXC 1 and 4, possibly over OXCs 6 and 5. Assume that a lightpath between OXCs 3 and 4 already exists. This lightpath shown in Figure 9.4 by a dotted line is routed through OXC 5 and uses wavelength λ 2 . In this case, the OC-12STM-4 data stream will be routed to OXC 3 over the lightpath from OXC 1 to 3, and then to OXC 4 over the lightpath from OXC 3 to 4. This solution assumes that there is available capacity on both lightpaths to carry the 622-Mbps data stream. Also, it assumes that OXC 3 has a SONETSDH DCS OXC 1 l 1 l 1 OXC 5 OXC 6 OXC 4 OXC 3 OXC 2 l 2 l 2 Figure 9.4 An example of traffic grooming. 210 WAVELENGTH ROUTING OPTICAL NETWORKS