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SEMINAR ON OPTICAL BURST SWITCHING (OBS)
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...
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.
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.
Optical burst switching 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 optical burst switching 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 network and 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.
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).
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.
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
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.
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 â€œ 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
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 . 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 , , 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 , 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.
Fig. 4. An example of the data burst format at layers 2 and 1.
An example of the data burst format at layers 2 and 1. (PT), payload length (PL), number of packets (NOP), and the Offset of padding. PT is an option indicating the type of data Packets in the data burst. PL indicates the length of the payload in bytes. NOP specifies the number of packets in the payload. The offset indicates the first byte of padding. Padding may be required if a minimum burst length is imposed. In Fig. 4, the Synchronization pattern in layer 1 is used to synchronize the Optical receiver at the egress edge router. The guard band at the Beginning (preamble) and end (postamble) of a data burst help To overcome the uncertainty of data burst arrival and data burst Duration due to clock drifts between nodes, the delay variation In different wavelengths, mismatch between data burst arrival Time and slotted optical switching matrix configuration time, And no deterministic optical matrix configuration times. Other Optical layer information (OLI) such as performance monitoring And forward error correction could also be included.
Like the packet header in conventional packet-switched networks,the BHP contains the necessary routing information to be used by core routers to route the associated data burst hop by hop to its destination edge router. Apart from the routing information carried by the conventional packet header, e.g., in IPv4, IPv6, or MPLS-like , the BHP contains OBS specific information
as its payload which includes burst offset-time, data burst duration/length, data channel carrying the burst, the bit rate at which the data burst is sent, and QoS, among others. Various layer 1 (L1) and layer 2 (L2) technologies can be used for the
Control channels. One example is Packet over SONET . Except for the separate transmission of headers and payloads
And being switched in different domains, there is no fundamental difference between packet switching and the OBS.
However, in the OBS, a burst header must explicitly reserve the Switching resources in advance at each hop along the path for
its burst payload, while in store-and-forward packet switching, the reservation of switching resources is made implicitly, i.e., when a packet is sent out from an electronic buffer. The link utilization of the OBS network will largely depend
on the number of channels dedicated to transmitting BHPs (as well as other control packets) and the guards in each data burst.
Consider a WDM link having channels with control channels and data channels, . Suppose the data channel rate is Gb/s and the control channel rate is Gb/s. The maximum link utilization. For, and, . As a data burst can be sent out on a data channel only if its BHP can be sent out on a control channel, there is a minimum requirement for the average Data burst length in order to prevent congestion on control channels . Since we will often deal with time domain issues in the OBS, it is convenient to use time duration instead of bytes
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 . 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.
Fig. 6. Block diagram of a nonblocking (symmetric) optical switching matrix.
Various optical switching matrices, e.g., the broadcast-andselecttype switch described in  and the switching fabrics proposed in â€œ, could be used in Fig. 5. Advanced optical technologies, implementation complexity, cost, and switch performance
(e.g., burst loss ratio) will certainly have impact on the design of the optical switching matrix. Here we consider an ideal no blocking optical switching matrix with output queuing. Block diagram of an no blocking optical switching matrix is given in Fig. 6 where the spatial switch is able to switch data burst from any incoming wavelength/channel to any FDL
as long as it does not overlap with other data bursts. Each optical buffer has WDM FDLs with th FDL being able to delay time, , and it is assumed that .Note that an FDL in Fig. 6 has wavelengths. By default there is always an FDL with zero delay time, denoted by 0 with An example of the optical switching matrix is shown in Fig. 7 where.
The function of the SCU in Fig. 5 is similar to a conventional electronic router. The routing processor runs routing and other Control protocols for the whole OBS network. It creates and maintains a routing table and computes the forwarding table for the SCU. Forwarding can be connectionless or connection-oriented (prior path establishment through signaling). After
Forwarding table lookup, the SCU decides on which outgoing and CCG to forward each arriving data burst and its BHP.
If there are free data and control channels available from these groups, either when the data burst arrives to the optical switching Matrix or after some delay in an FDL buffer, the SCU will then select the FDL of the optical buffer and configure the optical Switching matrix to let the data burst pass through. Otherwise, the data burst is dropped. In arranging the transfer of a data burst andits corresponding BHP in the optical switching matrix and SCU,respectively, the SCU tries to resynchronize the data burst and
the BHP by keeping the offset time as close as possible to .If a data burst enters the optical switching matrix before itsBHP has been processed (this phenomenon is called early burst arrivals), the burst is simply dropped. This is because data bursts are optical analog signals. If no path is set up when a data burst enters the optical switching matrix, it is lost. Since a BHP and its data burst are switched in the SCU and the optical switching matrix, respectively, the delay introduced by the Input FDL should be properly engineered such that under the normal traffic condition data bursts are rarely dropped due to Early arrivals
Optical Switching Techniques 1. Optical Circuit Switching
A circuit switched network has to have a dedicated wavelength path for the duration of its connection. In order for a circuit switched network to operate, a circuit is defined from the start of the connection to the end This circuit is then reserved for this connection only, but becomes available once the connection is terminated
Referring to figure I, if a connection between points A and B is required, then a circuit is setÃ‚Â¬up via SI, S3, S4 and S5. Other routes are possible allowing for resilience, and it should be noted that the links between the switches might consist of more than one circuit to allow multiple circuits to be set up
CallAaccept sinnalsignalAH.Ã‚Â«|JI f-.iQnnl
Circuit Switching Signaling
2. Optical Packet SwitchingPacket switching, in computer networking and telecommunications, is the now-dominant communications paradigm in which packets (units of information carriage) are routed between nodes over data links shared with other traffic. This contrasts with the other principal paradigm, circuit switching, which sets up a dedicated connection between the two nodes for their exclusive use for the duration of the communication. Packet switching is used to optimize the use of the bandwidth available in a network, to minimize the transmission latency (i.e. the time it takes for data to pass across the network), and to increase robustness of communication.
Packet switching works by sending the packets of information along the appropriate route.
The router decides the appropriate route when the packet arrives. In Packet switching
each packet (a piece of data) contains a additional information in it (header), rather like
the address on an envelope, and each switch in the network (usually called routers) looks
at this information and directs it onward accordingly. As an example imagine information
being sent from point C in Figure 3, and its destination is D. A packet of information
leaves C and is directed by Rl onto R3, R3 then directs the packet to R4 and then onto D.
However, it may not always occur like this. Perhaps during the transfer the link between
111 and R3 experiences a slow connection or is lost, Rl would then start sending the
packets to R2, R2 would then send it to R5, and so on.
A main feature of packet switching is store and forward. Meaning that a packet needs to
be completely assembled and received by a source and each intermediate node before it
can be forwarded. This will let the packet experience a delay proportional to Lp ,the length of a
packet at each node and will make necessary the existence of a buffer at each intermediate
node of the network, with a size of at least Smax. Here Smax is the maximum value of Lp.
Ftgurc3. Packet switching Network
Another method, which tries to absorb the advantages of both the above methods, is Optical Burst switching, which is described in the next section.
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. Meaning that they are separated at the source by an offset time, which is larger than the total processing tiine of the control packet along the path. In consequence this eliminates the need for the data burst to be bufTered at any subsequent intermediate node just to wail for the control packet to get process
OPTICAL BURST SWITCHED NETWORK
OPTICAL BURST SWITCHING -NETWORK
Optical burst Switching nodes
In OBS, the wavelength of a link used by the burst will be released as soon as the burst passes through the link, either automatically according to the reservation made or by an explicit release packet. This means that bursts from different sources to different destinations can effectively utilize the bandwidth of the same wavelength on a link in time-shared statistical multiplexed manner. In case the control packet fails to reserve the wavelength at an intermediate node, the burst is not rerouted, it is dropped. OBS protocols are not all the same; some of them support a reliable burst transmission, which has a negative acknowledgment that is sent back to the source node, which retransmits the control packet and the burst after that. Other OBS protocols are not reliable and don't have such negative acknowledgment
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.
pai allium Bandwidth Utilization Latency (set-up) Optical Buffer Overhead
(per unit data) adaptively (truffle & fault)
Circuit Low High Not required Low Low
Packet High Low Required High High
Burst High Low Not required Low High
In optical burst switching, offset time is the time between the burst header/control packet. The offset time used in one-way reservation schemes allows the network time to schedule the burst and setup resources prior to burst arrival is sent into the network. The offset time can be varied to allow the network time to configure based on the information carried in the burst header packet. By varying the offset time, different levels of quality of service can be provided.
Latency in a packet-switched network is measured either one-way (the time from the source sending a packet to the destination receiving it), or round-trip (the one-way latency from source to destination plus the one-way latency from the destination back to the source). Round-trip latency is more often quoted, because it can be measured from a single point. Note that round trip latency excludes the amount of time that a destination system spends processing the packet. Many software platforms provide a service called ping that can be used to measure round-trip latency. Ping performs no packet processing; it merely sends a response back when it receives a packet (i.e. performs a no-op), thus it is a relatively accurate way of measuring latency.
Where precision is important, one-way latency for a link can be more strictly defined as the time from the start of packet transmission to the start of packet reception. The time from the start of packet reception to the end of packet reception is measured separately and called "transmission delay". This definition of latency is independent of the link's throughput and the size of the packet, and is the absolute minimum delay possible with that link.
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.
VARIATIONS OF OPTICAL BURST SWITCHING
There are three variations of burst switching: Tell-and-go (TAG), in-band-terminator (113 T) and reserve-a-fixed-duration (RFD). In all three variations, bandwidth is reserved at the burst level using one a way process, and the most important point is that bursts are cut through intermediate nodes, instead of being stored and forwarded.
1. Tell-and-go (TAG)
In TAG, the source sends the control packet on a separate control channel to reserve bandwidth and set the switches along the path for a data burst that can be sent on the data channel without receiving an acknowledgment first. This means that the oil'set time T between the control and the burst packet is much smaller than the circuit set up time. After the burst is sent, another control signal is sent to release the bandwidth.
2. In-band-Terminator (IBT)
In IBT, every burst has a header like in packet switching and also a special delimiter or terminator indicating the end of the burst. IBT is not exactly like packet switching that has a store and forward mechanism, instead, IBT uses virtual cut through. Specifically, a source and the intermediate node can send the head of a burst even before the tail of the burst is received, This means that the burst will encounter less delay and a smaller buffer size is needed at a node, except for one case when the entire burst has to wait at a node because the wavelength at the link is not available.
3. Reserve-a-fixed~duration (RFD)
RFD is somehow similar to TAG, in the sense that the control packet is sent first to reserve bandwidth and set the switches, followed by the data burst after a time offset T. However, in RFD, the bandwidth is reserved for a duration specified by the control packet which , like a header of variable length packet, contains the burst length. However, this means that the burst will have a limited maximum size
4. Just Enough Time Protocol (JET) JET is a RFD scheme. The source node having a burst of data to transmit, sends at the beginning a control packet on a signaling channel which has a dedicated wavelength other than for the data to the destination node. At each node on the way, the control packet is processed in order to establish an all optical path for the data burst. Each node of the path chooses a convenient wavelength on the outgoing link, reserves bandwidth on that link and sets up the optical switch, this is all based on the information carried by the control packet. During that time, the data burst wait for a time offset T, at the source node in the electronic domain. In JET the intermediate network nodes work as follows. The incoming data from end-stations is buffered according to its destination. After some time the data is ready for dispatch as an optical burst. A control signal (the burst header) is then sent to the next downstream node and some time later 'T offset (launch)' the burst is transmitted on the wavelength specified in the header. T offset is the time delay between a header and its respective data. T is sufficient for the intermediate nodes to fulfill the arrival of a burst header on the control channel of a link which signals a node to attempt to reserve a wavelength/time-slot for the soon-to-arrive data to be switched to an output link closer to the destination. Full wavelength translation capability at each link is needed so that any burst can be routed to any free wavelength on the output link; therefore the wavelength of a burst has local significance only. The downstream node then sends a new header to the next downstream node. At each hop T offset is reduced by the processing time (per-hop-offset or Tpro) at each node; therefore for a burst to travel n hops, Toffset(launch) >.n * Tpro. The advance notice provided by the header suffices that when the data-burst arrives at an intermediate node, that node is already set to route the signal from input to output channel to output channel.
OPTICAL LABEL ADDRESS DECODER
SATELLITES IN ATM
CIRCUIT SWITCHING -DISADVANTAGES
Â¢ Inefficient utilization of resources.
Â¢ Dependence on speed/protocol (Opaque).
Â¢ Speed limitations imposed by available electronic processing capabilities.
Â¢ Faces two technological bottlenecks: Processing speed and buffering.
Â¢ Noise accumulation
Â¢ No/limited QoS management.
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.
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Optical Networks1 by Rajiv Ramaswami & Kumar N.Sivarajan
MICRO ELECTRO MECHANICAL SYSTEMS
HOW OPTICAL BURST SWITCHING WORKS
TRANSMISSION OF DATA BURST
OPTICAL CORE ROUTING
o OPTICAL SWITCHING MATRIX
OPTICAL SWITCHING TECHNIQUES
o OPTICAL CIRCUIT SWITCHING
o OPTICAL PACKET SWITCHING
o OPTICAL BURST SWITCHING
COMPARISON AND ADVANTAGES
VARIATIONS OF OPTICAL BURST SWITCHING
o TAG(TELL AND GO)
o IBT(IN BAND TERMINATOR)
o RFD(RESERVE FIXED DURATION)
o JET(JUST ENOUGH TIME)