Special Report: Next Generation Internet Applications

Optical Packet Switching Optical Packet Switching (OPS) is an emerging switching technology. Striving for an all-optical switching solution, researchers have experimented with two forms of optical packet switches. First, slotted optical packet switches provide the ability to optically switch fixed length packets. This enables higher speed switching because less direction information has to be determined prior to switched transmission. Slotted optical packet switches also permit the implementation of fiber delay lines, which adds the benefit of optical buffering to the switch. Figure 17 depicts the flow of a slotted optical packet switch. First, the input signal is pre-amplified as necessary (?). Next, the packet header is reviewed and optional packet realignment occurs (@). The packet is then sent through a delay line (A) to allow the header processing unit time to pre-configure the optical switch (B and C). The optical packet switch allocates the appropriate optical input to the appropriate optical output (D), a new header is attached (E), and the optical packet is amplified prior to retransmission. The process repeats and the switch is reconfigured each time. Currently, header processing and switch configuration is done in the electrical domain. Research is underway to append an optical header, based on advanced modulation techniques, to effectively provide all-optical table-referenced routing and switching functionality. Optical Packet Switch Preamplification of optical input signal. Packet header review and optional packet alignment. Delay on packet to allow for (4). Header processing unit reads packet header. Header processing unit pre-configures optical packet switch Contention resolution provided by packet switch. Optical inputs sent to appropriate optical outputs. 7 New header written.

Optional power re-amplification. Unslotted optical packet switches perform switching on variable length packets. The architecture of unslotted optical packet switches is simpler than slotted optical packet switches because there is no pre- configuration of the switch itself nor any segmentation and reassembly of the header information. Unfortunately, because the switch is not preconfigured, contention problems arise. This results in higher packet loss.

2.4.2 Optical Burst Switching Optical Burst Switching (OBS) is a more efficient switching scheme than OPS. The reservation of bandwidth is unidirectional, thereby eliminating the necessity of timely response messages. Aggregating packets in bursts of data allows for less processing overhead and increases the overall speed of the network. By utilizing bursts and unidirectional end-to-end bandwidth reservation techniques, OBS networks eliminate the need to process packets at intermediate network nodes and establish a direct network segment from source to destination. The current trend is for the development of OBS to continue to grow. Once optical buffering techniques mature, OPS should become more prevalent then OBS.

There are three different techniques to achieve OBS. They vary primarily in terms of bandwidth release.[21] In-Band terminator (IBT) optical burst switching operates by detecting an IBT, or empty data set, inserted after the optical data burst, to release reserved bandwidth. Figure 18 depicts the typical IBT OBS frame. Figure 18: IBT Optical Burst Switch Frame First, address information is sent in the form of a header or control packet (?), followed by burst data ending with the IBT (@). After some processing delay, the necessary network bandwidth is reserved (A) and the burst data is switched from input to output (B and C). Once the IBT is detected, the network bandwidth is released (D).[21] The process repeats for multiple bursts. Figure 19 illustrates the flow of IBT OBS. The second technique for optical burst switching is tell-and-go (TAG). TAG OBS operates the same as IBT OBS except a release packet replaces the in-band terminator. The switch releases the reserved bandwidth when it receives the release packet. An alternative to the use of release packets is refresh packets. Refresh packets are sent periodically to indicate that the connection is still active. If no release packets are received within a timeout period then the bandwidth would be released.[21] The third technique for optical burst switching is reserve-a-fixed-duration (RFD). RFD OBS operates by including, within the control packet, the duration of the necessary bandwidth reservation. This method eliminates the overhead associated with the IBT and release packet of the previous two techniques. RFD provides a more efficient allocation of bandwidth and buffers.[21] RFD is currently the focus direction of most OBS research.

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