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Multiwavelength Optical Networks in Software Printer Quick Response Code in Software Multiwavelength Optical Networks

Multiwavelength Optical Networks generate, create qr codes none in software projects GS1 DataBar Family for subsequent requ Software Quick Response Code ests. Simulation results show that this type of architecture can offer increased optical connection utilization and wavelength channel reuse only when the signaling round-trip time is shorter than the edge delay.25 This requirement in turn implies that this architecture is most suitable to networks with small diameters, such as metropolitan area networks.

. Contention Resolution in OBS Networks Contention in OBS n Software QR Code JIS X 0510 etworks occurs when multiple bursts compete for the same outbound link in a burst-switching node. The contention problem is much more serious in OBS than in OPS because of the long and highly variable burst sizes. Wavelength conversion and de ection routing are two techniques that have already been described and can minimize the blocking probability in packet switching, and we have already pointed out the advantage of wavelength conversion in OBS.

As these two techniques have been analyzed in depth earlier they are not repeated here. Instead, the discussion here focuses on additional schemes that may be useful to further reduce the blocking probability in OBS networks. In [Yoo+00] an offset scheme similar to the one described above for pJET is used to isolate classes of bursts, so that the low-priority traf c does not contend with the highpriority traf c.

They demonstrate the effectiveness of wavelength conversion combined with the offset scheme and FDL buffering in reducing blocking. One of the problems that is encountered in OBS networks is that when contention exists between two bursts, one of the bursts is completely dropped without examining the amount of overlap between the two bursts (which could potentially be minimal). In applications that are delay sensitive but loss tolerant, such as real-time voice or video, it may be worthwhile to lose a few packets that overlap with another burst rather than losing the entire burst and having to retransmit it at a later time.

Burst segmentation, a technique that was proposed in [Vokkarane+03], addresses this issue by dropping only the packets that overlap between two contending bursts. To achieve this, it is necessary to break up the burst into a number of segments, each consisting of a header and a payload (Figure 10.35).

Clearly, there are a number of ways to segment the burst. The segments, for example, may be of xed or variable size. Fixed-size segments allow for easy synchronization at the receiver.

However, variable-size segments can accommodate variable-size packets more ef ciently. Furthermore, the length of the segment is also important. If the segments are long, more information will be lost during contention, whereas if the segments are short, more overhead will be required (more segment headers for the burst).

When there is contention between two bursts, only the segments of one burst that overlap with the other are dropped. A decision has to be made whether to drop the tail-end segments of the original burst (the burst that arrives at the switch rst) or the head-end segments of the contending burst (the burst that arrives at the switch at a later. The edge delay is d e ned as the time it takes from the instant the rst bit of the rst packet of the burst enters the buffer in the source node until the entire burst is released into the network.. Optical Packet-Switched Networks Seg 1 Seg 2 Seg 3 Seg 4 Seg 5 Segment Guard Payload Bits Type Seg ID Segment Length Checksum Segment Header Figure 10.35 Segmentation of a burst. (From [Vokkarane+03, Figure 2]. Copyright c 2003 SPIE. Used by permission of The International Society for Optical Engineering.). time). Tail-end seg QR Code 2d barcode for None ment dropping has the advantage that the dropped (and retransmitted) segments will most likely reach the destination in-sequence compared to the case where the head-end segments are dropped. The advantage of head-end segment dropping is that it ensures that a burst that has reached a switching node with no contention will not be segmented.

The segmentation approach can be taken a step further by combining segmentation with de ection routing. To avoid dropping the segments of a burst that are contending with another burst, these segments can be de ected to another outgoing link. A cap placed on the burst hop count will avoid de ection routing problems such as looping, multiple de ections that waste bandwidth, and long nal routes that increase the total processing time thus rendering the initial offset time insuf cient.

A number of approaches for implementing segmentation with de ection routing were presented in [Vokkarane+03]. Simulation results compared a number of different policies for contention resolution that included both segmentation and de ection techniques. The ve different policies that were examined were 1.

Drop policy (DP): The entire contending burst is dropped. (Used as an upper bound for comparison purposes.) 2.

De ect and drop (DDP): De ect the entire burst to another port. If the port is not available, drop the burst. 3.

Segment and drop (SDP): Segment the original burst and drop its tail. 4. Segment, de ect, and drop (SDDP): Segment the original burst and de ect its tail if the alternate port is free.

If the port is not available, drop the tail. 5. De ect, segment, and drop (DSDP): De ect the contending burst to a free port if available, otherwise segment the original burst and drop its tail.

Figure 10.36, showing the low load case, indicates that policies with segmentation perform better than the policies with no segmentation and that the policies with de ection perform better than the policies with no de ection. (At low loads spare capacity is plentiful, which allows for the successful completion of de ection routing.

) Additional simulation results in [Vokkarane+03] demonstrate that at high loads de ection-routing.
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