RSVP Extensions For UNI Signaling

APPENDIX: SIMULATION PROJECT: CALCULATION OF CALL BLOCKING 239 the print out by hand and verify that the logic of your program is correct. That is, it advances correctly from event to event, and for each event the data structures are correctly maintained. This is a boring exercise, but it is the only way that you can make sure that your simulation works correctly Simulation Experiments Now that your simulation is ready, use it to carry out the following experiments. Set W = 5, all arrival rates γ i = γ , i = 1, 2, 3, 4, and make the destination probabilities p ij , for each OXC i equaprobable. Run your simulation program for different values of γ for the following configurations of converters: • No converters: c i = 0, i = 1, 2, 3, 4 • Full conversion: c i = W, i = 1, 2, 3, 4 • Partial conversion 1: c i = 2, i = 1, 2, 3, 4 • Partial conversion 2: c i = i + 1, i = 1, 2, 3, 4 10 Optical Burst Switching In a wavelength routing network, a connection has to be set up before data can be transmitted. The resources remain allocated to this connection even when there is no traffic transmitted. In view of this, connection utilization can be low when the traffic is bursty. In this chapter, we examine a different optical networking scheme, which is better suited for the transmission of bursty traffic. Because the data is transmitted in bursts, this scheme is known as optical burst switching OBS. OBS has not as yet been standardized, but it is regarded as a viable solution to the problem of transmitting bursty traffic over an optical network. In an OBS network, the user data is collected at the edge of the network, sorted per destination address, and transmitted across the network in variable size bursts. Prior to transmitting a burst, a control packet is sent into the network in order to set up a bufferless optical connection all of the way to the destination. After a delay, the data burst is transmitted optically without knowing whether the connection has been successfully established all of the way to the destination node. The connection is set up uniquely for the transmission of a single burst, and is torn down after the burst has been transmitted. That is, for each burst that has to be transmitted through the network, a new connection has to be set up. OBS was preceded by an earlier scheme known as optical packet switching OPS. This scheme was also known as optical ATM, since it copied many features of the ATM technology in the optical domain. An optical packet network consists of optical packet switches interconnected with WDM fibers. The switches are either adjacent or connected by lightpaths. Several different designs of optical packet switches have been proposed in the literature. The user data is transmitted in optical packets, which are switched within each optical packet switch entirely in the optical domain. Thus, the user data remains as an optical signal for the entire path from source to destination, and no optical-to- electrical and electrical-to-optical conversions are required. As will be seen, an optical packet switch makes use of optical memories to store optical packets destined to go out of the same output port. Since optical memories do not exist yet, fiber delay lines FDLs are used in their place. FDLs are useful in prototypes, but they are not suit- able for commercial switches. In view of this, optical packet switching has not been commercialized. In this chapter, we first examine some of the features of optical packet switching and give an example of an optical packet switch. Subsequently, we discuss the main fea- tures of OBS, and describe the Jumpstart signaling protocol, a proof-of-concept protocol developed to demonstrate the viability of OBS. Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2 242 OPTICAL BURST SWITCHING

10.1 OPTICAL PACKET SWITCHING

A WDM optical packet network consists of optical packet switches interconnected by fiber links. An optical packet switch switches incoming optical packets to their desired output ports. As will be seen, the switching of the packets is done in the optical domain. Two switches are interconnected by one or more fibers, each running W different wavelengths. Also, it is possible that two switches be interconnected by lightpaths. In this section, we examine some of the features of optical packet switches, and give an example of a switch. Optical packet switches operate in a slotted manner and switch fixed-size packets. Unlike ATM, the size of a fixed packet is not limited to 53 bytes. In fact, the packet size can be variable, but the time it takes to transmit it is fixed. Variable packet sizes can be transmitted within a time slot of a specified duration by varying the transmission rate. An optical packet switch consists of input interfaces, a switching fabric, output inter- faces, and a control unit. When a packet arrives at an optical packet switch, it is first processed by the input interface. The header and the payload of the packet are separated, as shown in Figure 10.1. The header is sent to the control unit, where it is processed after it has been converted to the electrical domain. The payload remains as an optical signal and is switched to the destination output interface through the switch fabric. At the output interface, it is combined with the header and then transmitted out. The separation of the header from its payload is necessitated by the fact that it is not technically feasible at this moment to process the header optically. In view of this, the header has to be converted to the electrical domain so that it can be processed by the CPU. The header can either be transmitted on the same wavelength as the payload, or placed in an electrical subcarrier above the baseband frequencies occupied by the packet payload, and then transmitted both optically in the same time slot. An alternative solution is to transmit the payload and the header in separate wavelengths, but in the same time slot. Also, there should be a gap between the header and the payload, so that there is time to process the header prior to the arrival of the payload. An optical packet switch and in general any packet switch requires buffering. If the switch is non-blocking which is typically the case, then these buffers have to be placed at the output ports as shown in Figure 10.2. Since each fiber consists of multiple wavelengths, it is possible that multiple packets might arrive at the same time, each carried on the same wavelength i, but originating from differing input fibers. Let us assume that the destination of these packets is the same output fiber. If there are no converters present, then only one packet will be allowed to be transmitted out on the ith wavelength, and the rest will have to be buffered. On the other hand, if full conversion is available, up to W Optical switch Payload Payload Payload Optical packet wavelength i output port j Hdr CPU Re-combined wavelength i output port j Hdr Hdr Figure 10.1 The header and payload are separated. OPTICAL PACKET SWITCHING 243 Optical Switch Input fibers Output fibers . . . . . . . Figure 10.2 Contention in an optical packet switch. packets where W is the total number of wavelengths can be simultaneously transmitted. If more than W packets arrive at the same time, then the additional packets will have to be buffered. Optical buffers are technologically infeasible. One solution is to convert the optical packets into the electrical domain and buffer them into electrical memories. However, this approach will significantly slow down the operation of the switch. Currently, optical buffers are implemented by fiber delay lines FDL. An FDL can delay a packet for a specified amount of time, which is related to the length of the delay line. A buffer for N packets with a FIFO discipline can be implemented using N delay lines of different length. Delay line i can delay a packet for i timeslots. Since there are W wavelengths, each delay line can delay up to W packets. Fiber delay lines require lengthy pieces of fiber and so cannot be commercialized. The lack of optical buffering is the Achilles’ heel of optical packet switching. An alternative solution is to use deflection routing. When there is a conflict between two packets, one is routed to the correct output port, and the other is routed i.e., deflected to an alternative output port. The alternate path for routing deflected packets to their destination might be longer. Also, deflection routing imposes an additional load on the links used for the deflected packets, which has to be taken into account when planning the network and the routes that deflected packets will follow. Finally, only a limited number of packets can be deflected at any time.

10.1.1 A Space Switch

In this section, we describe an example of an optical packet switch that uses a space switch fabric. The switch consists of N incoming and N outgoing fiber links, with W wavelengths running on each fiber link see Figure 10.3. The switch is slotted, and the slot is long enough so that an optical packet can be transmitted and propagated from an input port to an output optical buffer. The switch fabric consists of three parts: packet encoder, space switch, and packet buffer . The packet encoder works as follows. For each incoming fiber link, there is an optical demultiplexer which separates the incoming optical signal to W different wave- lengths. Each optical signal is fed to a different tunable wavelength converter which converts its wavelength to a wavelength on which it will be transmitted out of the switch. The space switch fabric can switch a packet to any of the N output optical buffers. The output of a tunable wavelength converter is fed to a decoupler which distributes the same signal to N different output fibers, one per output buffer. The signal on each of 244 OPTICAL BURST SWITCHING 1 1 W 1 N d 1 N N 1 W d d d 1 d 0T dT N d dT 0T . . . . . . . . . . . . Space switch Packet buffer Packet encoder i Demux Mux Convert Decoupler Coupler Gate FDL i 1 N d 1 d d d Figure 10.3 An architecture with a space switch fabric. these output fibers goes through another decoupler which distributes it to d + 1 different output fibers, and each output fiber is connected through an optical gate to one of the FDLs of the destination output buffer. The packet buffer consists of couplers and output buffers, which are implemented in FDLs. Specifically, an output buffer consists of d + 1 FDLs, numbered from 0 to d. FDL i delays a packet for a fixed delay equal to i slots. FDL 0 provides zero delay, and a packet arriving at this FDL is simply transmitted immediately out of the output port. Each FDL can delay packets on each of the W wavelengths. For instance, at the beginning of a slot FDL 1 can accept W optical packets – one per wavelength – and delay them for one slot. FDL 2 can accept W optical packets at the beginning of each time slot and delay them for two slots. That is, at slot t, it can accept W packets again, one per wavelength and delay them for two slots, in which case, these packets will exit at the beginning of slot t + 2. However, at the beginning of slot t + 1, it can also accept another batch of W packets. Thus, a maximum of 2 W packets can be in transit within FDL 2. The same goes for FDL 3 through d. The information regarding which wavelength a tunable wavelength converter should convert the wavelength of an incoming packet and the decision as to which FDL of the destination output buffer the packet will be switched to is provided by the control unit, which has knowledge of the state of the entire switch. OPTICAL BURST SWITCHING OBS 245

10.2 OPTICAL BURST SWITCHING OBS

OBS was designed to efficiently support the transmission of bursty traffic over an optical network. OBS was based on the ATM block transfer ABT, an ITU-T standard for burst switching in ATM networks see Section 4.6.3. OBS is still been developed and it has not as yet been standardized. An OBS network consists of OBS nodes interconnected with WDM fiber in a mesh topology. An OBS node is an OXC see Section 8.3.5. It consists of amplifiers, multi- plexersdemultiplexers, a switch fabric, and an electronic control unit see Figure 10.4. The OBS node can switch an optical signal on wavelength λ i of an input fiber to the same wavelength of an output fiber. If it is equipped with converters, it can switch the optical signal of the incoming wavelength λ i to another free wavelength of the same output fiber, should wavelength λ i of the output fiber be in use. Assume that full conversion applies, and that each converter can convert an optical signal to any other wavelength. Unlike wavelength routing networks, where a connection can remain active for a long time, the switch fabric of an OBS node demands an extremely short configuration time. The OBS network is accessed by OBS end devices, which are IP routers, ATM switches, or frame relay switches, equipped with an OBS interface. Devices which produce ana- logue signals, such as radar, can also be attached to the OBS network. Each OBS end device is connected to an ingress OBS node. An OBS end device collects traffic from various electrical networks, such as ATM, IP and frame relay, and it then transmits it to destination OBS end devices optically through the OBS network. The collected data is sorted based on a destination OBS end device address and is assembled into larger size units, called bursts. As shown in Figure 10.5, in order to transmit a burst, the end device first transmits a control packet, and after a Control unit Input WDM fibers Output WDM fibers Switch fabric . . . . . . Figure 10.4 Reference OBS node. Control packet Client network End- device End- device Burst Control packet Burst A B Figure 10.5 A control packet is transmitted prior to transmitting a burst. 246 OPTICAL BURST SWITCHING delay, known as the offset, it transmits its burst. The control packet contains information such as the burst length and the burst destination address. It is basically a request to set up a connection i.e., a lightpath, end-to-end. After the transmission is completed, the connection is torn down. As in wavelength routing networks, two adjacent OBS nodes can be linked by one or more optical fibers, each carrying W wavelengths. Therefore, up to W bursts per fiber can be simultaneously transmitted out. An end device might also have the ability to transmit W bursts to its ingress OBS node simultaneously, or it might have only one wavelength available on which to transmit one burst at a time. Control packets can be transmitted either optically on a designated signaling wave- length or electrically over a packet-switching network, such as an IP or ATM network. In either case, the control packet can only be electronically processed by each OBS node. This means that if it is transmitted in the optical domain, it will have to be converted back to the electrical domain. As can be seen, there exists a separation of control and data, both in time and physical space. This is one of the main features of OBS. It facilitates efficient electronic control while it allows for a great flexibility in the format and transmission rate of the user data since the bursts are transmitted entirely as optical signals which remain transparent throughout the network. The time it takes for the connection to be set up depends on the end-to-end propagation delay of the control packet, the sum of all of the processing delays of the control packet at all of the intermediate OBS nodes, and configuration delays. Depending upon the propagation delay between adjacent OBS nodes, the configuration at each node might overlap to some extent. The time it takes for a burst to reach the destination end device is equal to the end-to-end propagation delay, since it is transmitted as an optical signal that traverses the OBS switches without any processing or buffering delays. In view of this, the transmission of a burst is delayed by an offset so that it always arrives at an OBS node, after the control unit of the node had the chance to process the control packet associated with the burst and configure the OXC. Let t proc be the time it takes to process a control packet at a node, t conf be the time it takes to configure an OXC, and N the total number of OBS nodes along the path of the burst. Then, a good upper bound for the offset can be obtained using the expression: Nt proc + t conf . 10.2.1 Connection Setup Schemes To set up a connection, there are two options: on-the-fly connection setup and confirmed connection setup . In the on-the-fly connection setup scheme, the burst is transmitted after an offset without any knowledge of whether the connection has been successfully established end-to-end. In the confirmed connection setup scheme, a burst is transmitted after the end device receives a confirmation from the OBS network that the connection has been established. This scheme is also known as Tell and Wait TAW. An example of the on-the-fly connection setup scheme is shown in Figure 10.6. End- devices A and B are connected via two OBS nodes. The vertical line under each device in Figure 10.6 is a time line and it shows the actions taken by the device. End device A transmits a control packet to its ingress OBS node. The control packet is processed by the control unit of the node and if the connection can be accepted it is forwarded to the next node. This processing time is shown by a vertical shaded box. The control packet is received by the next OBS node, it is processed, and assuming that the node can accept OPTICAL BURST SWITCHING OBS 247 Time Offset Control packet Burst B A Figure 10.6 The on-the-fly connection setup scheme. Burst Time Control packet B A Figure 10.7 The confirmed connection setup scheme. the connection, it is forwarded to the destination end device node. In the mean time, after an offset delay, end device A starts transmitting the burst which is propagated through the two OBS nodes to the end device B. As we can see in this example, burst transmission begins before the control packet has reached the destination. In this scheme, a burst might be lost if the control packet cannot reserve resources at an OBS node along the burst’s path. The OBS architecture is not concerned with retransmissions, as this is left to the upper networking layers. Also, it is important that the offset is calculated correctly. If it is too short, then the burst might arrive at a node prior to the control packet, and it will be lost. If it is too long, then this will reduce the throughput of the end device. An example of the confirmed connection setup scheme is shown in Figure 10.7. End device A transmits a control packet which is propagated and processed at each node along 248 OPTICAL BURST SWITCHING the path as in the previous scheme. However, the transmission of the burst does not start until A receives a confirmation that the connection has been established. In this case, there is no burst loss and the offset can be seen as being the time it takes to establish the connection and return a confirmation message to the transmitting end device. In the rest of this section, we will assume the on-the-fly connection setup scheme, unless otherwise stated.

10.2.2 Reservation and Release of Resources in an OXC

In configuring an OXC, two schemes are available: the immediate setup scheme and the delayed setup scheme . In the immediate setup scheme, the control unit configures the OXC to switch the burst to the output port immediately after it has processed the control packet. In the delayed setup scheme, the control unit calculates the time of arrival t arrival of the burst at the node, and it then waits to configures the OXC at t arrival . Two different schemes also exist as to when the control unit will instruct the OXC to release the resources allocated for switching the burst. These schemes are the timed release scheme and the explicit release scheme. In the timed release scheme, the control unit calculates when the burst will completely go through the OXC, and when this time occurs it instructs the OXC to release the allocated resources. This requires knowledge of the burst duration. An alternative scheme is the explicit release scheme, where the transmitting end device sends a message to inform the OBS nodes along the path of the burst that it has finished its transmission. A control unit instructs the OXC to release the connection when it receives this message. Combining the two setup schemes with the two release schemes gives the following four different mechanisms: 1. Immediate setup with explicit release 2. Immediate setup with timed release 3. Delayed setup with timed release 4. Delayed setup with explicit release In Figure 10.8, we show the case of immediate setup with timed release, and the case of immediate setup with explicit release. These two schemes have been used in the Just-In- Time JIT OBS architecture see Section 10.3. The total amount of time an OXC remains configured for the transmission of the burst is shown by a vertical white box along the time line associated with the switch. The timed release scheme does not require a release message, which has to be processed before the OXC is instructed to release the resources allocated to the burst. In view of this, the resources might be released quicker than in the explicit release scheme. The timed release scheme is more complicated to implement than the explicit release scheme. However, in the explicit release scheme, the duration of burst resource allocation exceeds the duration of the actual burst transmission. The delayed setup scheme can be combined with timed release or with explicit release. Figure 10.9 shows an example of the delayed setup scheme with timed release, also known as the Just-Enough-Time JET scheme. The total amount of time an OXC remains configured for the transmission of the burst is shown by a vertical white box along the time line associated with the switch. The timed setup with timed release is the most efficient of all of the four combinations.