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Seminar 2011 Optical Burst Switching

SEMINAR

ON

OPTICAL BURST SWITCHING (OBS)

CONTENTS

ABSTRACT

INTRODUCTION

MICROELECTRO MECHANICAL SYSTEMS

NETWORK SWITCH

HOW OPTICAL BURST SWITCHING WORKS?

TECHNOLOGY OVERVIEW

TRANSMISSION OF DATA BURST

ROUTING

OPTICAL CORE ROUTING

  • OPTICAL SWITCHING MATRIX

OPTICAL SWITCHING TECHNIQUES

  • OPTICAL CIRCUIT SWITCHING
  • OPTICAL PACKET SWITCHING
  • OPTICAL BURST SWITCHING

COMPARISON AND ADVANTAGES

VARIATIONS OF OPTICAL BURST SWITCHING

  • TAG(TELL AND GO)
  • IBT(IN BAND TERMINATOR)
  • RFD(RESERVE FIXED DURATION)
  • JET(JUST ENOUGH TIME)

APPLICATIONS

DISADVANTAGES

CONCLUSION

REFERENCES

...

ABSTRACT

OpticalBurstSwitching is a promising hybrid approach between coarse grain optical circuit switching and fine grain optical packet switching. burstswitching (OBS) is proposed as a way to streamline both protocol and hardware in building the future generation Optical Internet. Byleveraging the attractive properties of optical communications and at the same time, taking into account its limitations, OBS combines the best of optical circuit switching and packet/cell switching. In this paper, the general concept of OBS protocols and in particular, those...

INTRODUCTION

optical burstswitching OBS is a switching concept which lies between optical circuit switching and optical packet switching. Firstly, a dynamic optical network is provided by the interconnection of optical cross connects. These optical cross connects (OXC) usually consist switches based on 2D or 3D Micro electro Mechanical mirrorsMEMS which reflect light coming into the switch at an incoming port to a particular outgoing port. The granularity of this type of switching is at a fibre, waveband (a band of wavelengths) or at a wavelength level. The finest granularity offered by an OXC is at a wavelength level. Therefore this type of switching is appropriate for provisioning light paths from one node to another for different clients/ services e.g. SDH (Synchronous Digital Hierarchy) circuits.

At present opticalburstswitching is an area that is attracting a lot of attention and is a potential method by which future optical networks may use the available optical resources more effectively. However, several issues still need to be addressed before opticalburstswitching can enter service in a real optical network. In particular, the technological demands and restrictions of electronic and optical components have to be considered with regard to an application in opticalburst switched networks as well as assessment of the architectural and economic aspects of implementing opticalburstswitching.

OpticalBurstSwitching operates at the sub-wavelength level and is designed to better improve the utilisation of wavelenghts by rapid setup and teardown of the wavelength/lightpath for incoming bursts. In OBS, incoming traffic from clients at the edge of the network are aggregated at the ingress of the network according to a particular parameter (commonly destination). These packets can also be aggregated according to quality of service (QoS). Therefore at the OBS edge router, different queues represent the various destinations of class of service. Therefore based on the assembly/aggregation algorithm, packets are assembled into bursts using either a time based or threshold based aggregation algorithm. In some implementations, Aggregation is based on a Hybrid of Timer and Threshold. From the aggregation of packets, a burst is created and this is the granularity that is handled in OBS.

Also important about OBS is the fact that the required electrical processing is decoupled from the Optical process. Therefore the burst header generated at the edge of the network is sent on a separate control channel which could be a separate control wavelength. At each switch the control channel is converted to the electrical domain for the electrical processing of the header information. The header information precedes the burst by a set amount known as an offset time. Therefore giving enough time for the switch resources to be made available prior to the arrival of the burst.

Opticalburstswitching has many flavours determined by the current available technologies such as the switching speed of available core optical switches. Most optical cross connects have switching times or the order of milliseconds but require tens of milliseconds to set up the switch and perform switching. Therefore, OBS utilising this type of switching cannot rely on the one way signalling concept as defined by Just-In-Time (JIT) and Just-Enough-Time (JET).

The initial phase of introducing opticalburstswitching would be: after burstification process, based on a forwarding table bursts of a particular destination are mapped to a wavelength. As the burst requests a path across the network, the request is sent on the control channel, at each switch, if it is possible to switch for the wavelength, the path is set up and an acknowledge signal is sent back to the ingress. The burst is then transmitted. Under this concept, the burst is held electronically at the edge and the bandwidth and path is guaranteed prior to transmission. This reduces the amount of bursts dropped. The effects of dropping bursts can be detrimental to a network as each burst is an amalgamation of IP packets which could be carrying keepalive messages between IP routers. If lost, the IP router would be forced to retransmit and reconverge.

Under the GMPLS control plane, forwarding tables are used to map the bursts and the MPLS (Multiprotocol Label Switching) base 'PATH' and 'RESV' signals are used for requesting a path and confirming it is set up. This is a two way signalling process which can be inefficient in terms of network utilisation. However for increasingly bursty traffic, the conventional OBS is the preferred choice.

Under this conventional OBS, a one way signalling concept as mentioned previously is used. The idea is to hold the burst at the edge for an offset period while the control header traverses across the network setting up the switches, the burst follows immediately without confirmation of burst setup. There is an increased likelihood for bursts to be dropped but contention resolution mechanisms can be used to ensure alternative resources are made available to the burst if the switch is blocked ( being used by another burst for the incoming or outgoing switch port). An example contention resolution solution is deflection routing, where blocked bursts are routed to alternative port until the required port becomes available. This requires optical buffering which is implemented mainly by fibre delay lines.

One way signalling makes more efficient use of the networkand the burst probability of blocking can be reduced by increasing the offset time, thereby increasing the likely hood of switch resources being available for burst.

OPTICALFIBRE

STRUCTURE OF OPTICAL FIBRE

Microelectromechanical Systems (MEMS)

It is the technology of the very small, and merges at the nanoscale into "Nanoelectromechanical Systems" (NEMS) and Nanotechnology. In Europe, MEMS are often referred to as Micro Systems Technology (MST). It should not be confused with the hypothetical vision of Molecular nanotechnology or Molecular Electronics. These devices generally range in size from a micrometer (a millionth of a meter) to a millimeter (thousandth of a meter). At these size scales, a human's intuitive sense of physics do not always hold true. Due to MEMS' large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass. They are fabricated using modified silicon fabrication technology (used to make electronics), molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing very small devices. MEMS sometimes go by the names micromechanics, micro machines, or micro system technology (MST).

NETWORK SWITCH:

A network switch (or just switch) is a networking device that performs transparent bridging (connection of multiple network segments with forwarding based on MAC addresses) at full wire speed in hardware. The use of specially designed hardware also makes it possible to have large numbers of ports (unlike a PC based bridge which is very limited by expansion slot count).

A switch can connect Ethernet, Token Ring, Fibre Channel or other types of packet switched network segments together to form a heterogeneous network operating at OSILayer 2 (though there may be complications caused by the different MTUs of the standards).

As a frame comes into a switch, the switch saves the originating MAC address and the originating (hardware) port in the switch's MAC address table. This table often uses content-addressable memory, so it is sometimes called the "CAM table". The switch then selectively transmits the frame from specific ports based on the frame's destination MAC address and previous entries in the MAC address table. If the destination MAC address is unknown, for instance, a broadcast address or (for simpler switches) a multicast address, the switch simply transmits the frame out of all of the connected interfaces except the incoming port. If the destination MAC address is known, the frame is forwarded only to the corresponding port in the MAC address table. If the destination port is the same as the originating port, the frame is filtered out and not forwarded.

Switches, unlike hubs, use microsegmentation to create collision domains, one per connected segment. This way, only the NICs which are directly connected via a point-to-point link, or directly connected hubs are contending for the medium. If the switch and the equipment (other than a hub) it connects to support full-duplex then the collision domain is eliminated entirely.

HOW OPTICAL BURST SWITCHING WORKS?

Opticalburstswitching is based on the separation of the control plane and the data plane. In opticalburstswitching data packets are aggregated into much larger bursts before transmission through the network. This allows amortization of the switching overhead across multiple packets.

The burst is preceded in time by a control packet, which is sent on a separate control wavelength and requests resource allocation at each switch. When the control packet arrives at a core cross-connect (or switch) capacity is reserved in the cross-connect for the burst. If the required capacity can be reserved the burst can pass through the cross connect.

TECHNOLOGYOVERVIEW
WDM is a method of transmitting data from different sources over the same fiber-optic link at the same time; each data channel is carried on its own unique wavelength. The result is a link with an aggregate bandwidth that increases with the number of wavelengths employed. In this way, WDM technology can maximize the use of the available fiber-optic infrastructure – what would normally require two or more fiber links will now require only one.

WDM technologies primarily differ in the number of available channels. Coarse wave division multiplexing (CWDM) combines as many as 16 wavelengths onto a single fiber; dense wave division multiplexing (DWDM) combines as many as 64 wavelengths onto a single fiber.

With DWDM technology, the wavelengths are closer together than CWDM, meaning that transponders are generally more complex and expensive than CWDM. However, with DWDM, the advantage is a much higher density of wavelengths, and also longer distance. DWDM is emerging as a preferred solution for providing scalable and efficient optical networking technologies of the future.

The key objective of the hardware-based OBS protocol implementation is to dynamically manage commercially available WDM switches. An OBS network comprises OBS network controllers and clients with OBS network interface cards (NICs). OBS network controllers direct the optical data bursts received from a source-client OBS NIC to a destination-client OBS NIC.

Advances in Xilinx FPGA technology have made it possible for the MCNC-RDI to build a NIC that implements the JIT signaling protocol for an OBS network. The OBS NIC uses DWDM technology to transmit and receive data optically on specific wavelengths and is capable of handling data rates as high as 1.25 Gbps. The NIC card can be tuned dynamically to as many as eight different DWDM wavelengths.

In the JIT protocol, a control packet reserves a wavelength channel in the network for a period of time L equal to the burst length, starting at the expected arrival time R (this can be adjusted by the number of hops that a burst needs to travel and the processing time at each intermediate node).

If the reservation is successful, the control packet adjusts the offset time for the next hop and forwards it on. If the reservation is not successful, the burst will be blocked and the packet will be discarded. Because JIT is a one-way reservation protocol, buffering does not occur at the node level, thus reducing any latency. Implementation of JIT with an efficient scheduling algorithm can further decrease the probability of burst loss.

in optical packet-type WDM networks, the basic data block to be transferred is a super packet, called burst, which is a collectionof data packets having the same network egress address and some common attributes, like QoS requirements. A blockdiagram of an optical burst-switched (OBS) network is shown in

Fig. 1,

Figer 1 An optical burst-switched network.

Which consists of optical core routers and electronic edge routers connected by WDM links. Packets are assembled into bursts at network ingress, which are then routed through the OBS network and disassembled back into packets at network Egress to be forwarded to their next hops (e.g., conventionally routers). Edge routers provide burst assembly/disassembly Functions and legacy interfaces (e.g., gigabit Ethernet, packet Over SONET (PoS), IP/ATM, etc.). A core router is mainly composed of an optical switching matrix and a switch control unit (SCU). A burst consists of a burst header and a burst payload. The Burst payload is also called data burst in this paper. For the Optical burst switching (OBS) considered here, a data burst (Payload) and its header are transmitted separately on different Wavelengths/channels with the burst header slightly ahead in Time (see Fig. 2), and arswitched in optical and electronic domains, respectively, at each core router they traverse. The burst header contains all the necessary routing information to beused by the switch control unit (SCU) at each hop to configure the optical switching matrix to switch the data burst optically(see Fig. 3). The separate transmission and switching of data bursts and their headers will help to facilitate the electronic processing of headers and lower the opt electronic processing capacity required at core routers. Further, it can provide

ingress-to-egress transparent optical paths for transporting data Bursts.

Fig. 2. TRANSMISSION OF DATA BURST AND THEIR HESDERS(BHP) ON A WDM LINK


Fig. 3. Illustration of burst transmission in an OBS network.

. As the burst header is sent in the form of a packet, it is Called burst header packet (BHP) hereafter. Similar to packet Switching, both connectionless and connection-oriented burst Forwarding could be used in the OBS.Throughout the paper, we use channel to represent a certain unidirectional transmission capacity (in bits per second) between Two adjacent routers. A channel may consist of one wavelength Or a portion of a wavelength, in case of time-division or Code-division multiplexing. Channels carrying data bursts are called data channels, and channels carrying BHPs and other Control packets are called control channels (see Fig. 3). Control Packets are used to exchange routing and network information. A channel group is a set of channels with a common type and Node adjacency. A WDM link in Fig. 1 represents a total transmission Capacity between two routers, which usually consists of a data channel group (DCG) and a control channel group (CCG) In each direction. The channels of a DCG as well as its corresponding CCG could be physically carried on the same fiber or On different fibers. In the following, we use channel and wavelength Interchangeably. An example of the transmission of bursts on a WDM link is shown in Fig. 2, where the WDM link has one DCG composed Of two channels and one CCG composed of only one channel. There is an offset time between a data burst and its BHP. The Initial value of the burst offset-time is set by ingress Edge router, which may be the same for all bursts or may be different From burst to burst. The function of the burst offset-time Depends on the design of optical core routers. For optical core Routers using input FDLs (fiber delay lines) to delay the arrivals of data bursts to the optical switching matrix, thus allowing the SCU to have sufficient time to process their BHPs, the main Function of the offset time is to resolve BHP contentions on outgoing CCGs of optical core routers [7]. For optical core routers Without input FDLs, the offset time should also allow the SCU At each hop along the path to have enough time to process the BHP before its associated data burst arrives. In the latter case, The burst offset-time would be proportional to the number of Hops the burst will traverse in the OBS network [6], [8], and is Much larger than the offset time in the former case. In both cases, the traffic condition in the network should be taken into account In choosing the offset time. The burst offset-time could also be adjusted to support QoS [12], and may play an important role in Traffic scheduling/management for optical core routers without Buffer or with buffer of very limited storage capacity. To simplify the design of the SCU, in particular, the channel scheduling, optical core routers with input FDLs are considered

In this paper. To have the burst offset-time well under control Within the OBS network, at each hop the burst traverses, the core Router tries to “resynchronize” each BHP and its associated data burst by keeping the offset time as close as possible to, but

No less than. The typical value of is zero, meaning a BHP should be sent out no later than its associated data burst. Due to the input FDLs at core routers, it is not always necessary To restrict to nonnegative values, as a BHP may be behind The data burst at one node but could catch up at the next node. An example of the data burst format is shown in Fig. 4. Each Packet is delineated within the actual payload by a frame header (H). The header of the actual payload includes payload type Fig. 4.