02-08-2013, 03:11 PM
A Seminar Report On OPTICAL BURST SWITCHING
ABSTRACT
Optical Burst Switching is a promising hybrid approach between coarse grain optical circuit switching and fine grain optical packet switching. burst switching (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 burst switching 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 optical burst switching 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 optical burst switching 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 optical burst switched networks as well as assessment of the architectural and economic aspects of implementing optical burst switching.
Optical Burst Switching 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.
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 OSI Layer 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.
HOW OPTICAL BURST SWITCHING WORKS
Optical burst switching is based on the separation of the control plane and the data plane. In optical burst switching 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.
ROUTING
One requirement of the optical switch is that it needs to decide which input has to be connected
to which output. This decision is typically made in electronic routers by reading the header of the input packet. However, packet header cannot be read in the optical domain. Further, electronic switches store packets in their buffers and forward them on the appropriate outputs. But no such buffers can be made in an optical router as no Optical RAMS are available. Fibre delay lines (FDLs) try to emulate a RAM but these are expensive for commercial use and also may not scale well. Due to the above differences optical routers become very different from electronic ones.
Developing the optical network not only means increasing the bandwidth ,it also means that the network should be capable of providing some scalable quality of service(QoS).Though several concepts have been suggested in this regard the optical burst switching stands out to be the prominent one.
OPTICAL CORE ROUTERS
The general architecture of an optical core routeris shown in Fig. 5,
Fig. 5. A general architecture of optical routers.
which mainly consists of input FDLs (fiberdelay lines), an optical switching matrix, a switch control unit(SCU), and routing and signaling processors. Data channels reconnected to the optical switching matrix and control channels are terminated at the SCU. Channel mapping logically decouples the channels from physical fibers wavelengths. The (fixed) inputFDLs, if provided, are used to delay the arriving data bursts, thus allowing the SCU to have enough time to process the associated
BHPs. Data bursts still remain in the form of optical signals in the core routers. The optical buffers of FDLs are used to resolve
data burst contentions on outgoing DCGs (data channel groups).The use of electronic buffers instead of FDL optical buffers was considered in [7]. Note that there are J incoming DCGs andJ outgoing DCGs in Fig. 5. A typical example of the general architecture is a symmetric router with input and output fibers, where each fiber has one DCG of channels and
oneCCG (control channel group) of channels. Fig. 6. Block diagram of a no blocking (symmetric) optical switching matrix.
OPTICAL BURST SWITCHING
In Optical Burst Switching, a control packet is sent first, followed by a burst of data without waiting for an acknowledgment for the connection establishment, this is called a one way reservation protocol. The main feature of OBS, is to switch a whole burst of packet whose length can range from one to several packets to a session using one control packet, and resulting in a lower control overhead per data unit. OBS uses out of band signaling, and the control packet and
the data burst are loosely coupled in time.
COMPARISON AND ADVANTAGES:
Circuit switching is good for smooth traffic and QoS guarantee due to a fixed bandwidth reservation. One problem with this kind of routing is that if the traffic on that path is burst, the path still has to be kept reserved. A second problem is that if the same wavelength at which path reservation started in the initial routers in the path is not available in some subsequent router, then wavelength conversion is required which again compromises some of the benefits of an all optical transparent path. The advantage of Packet switching is that a packet containing a header (e.g. addresses) and a payload is sent without circuit set up (delay) and we have static sharing of the link wavelengths among packets with different sources and destinations. However, due to the store and forward mechanism, every node processes the header of the packet arriving to know where to route it, and this make the use of a buifer et every node necessary. OBS combines both advantages of optical circuit and packet switching. Unlike the circuit switched approach it does not need to dedicate a wavelength for each end-to-end connection due to the fast release of the wavelength on a link after the burst passes by it. Also unlike the packet switched approach, burst data does not need to be buffered or processed at the cross connect since the OBS mechanism is a cut through one.
OPTICAL BUFFER
In telecommunications, an optical buffer is a device that is capable of temporarily storing light. Just as in the case of a regular buffer, it is a storage medium that enables to compensate for a difference in time of occurrence of events. More specifically, an optical buffer serves to store data that was transmitted optically. As light cannot be frozen, an optical buffer is made of optical fibers, and is, in general, a lot bigger than a RAM chip of comparable capacity would be. A single fiber can serve as a buffer, however, in general, a set of more than one is used. A possibility, e.g., is to choose a certain length D for the smallest fiber, and then let the second, third... have lengths . Another typical example is to use a single loop, in which the data circulates for a variable number of times.
CONCLUSION
In this paper we have discussed a novel paradigm called the opticai burst switching (OBS) as an efficient way to resolve the problem of congestion that the Internet is suffering from. Bursty traffic, for example IP traffic over WDM network will be supported. Optical packet and circuit switching were discussed and compared to the new OBS switching technique. Next, different OBS variations were described in addition to the Just Enough Time protocol was investigated. OBS is a very promising switching technique that will most likely be adopted in the future.