This is the first of a pair of technology tutorials on all-optical switching by Geoff Bennett, vice president of technology advocacy at Marconi PLC (Nasdaq/London: MONI).
This tutorial covers the all-optical switches themselves – the various types, how they differ from electronic switches, where they sit in networks, what functions they perform, how they're controlled, and what they can and can't do.
The second tutorial covers optical switching fabric. In particular, it shows how different sizes and types of switch require different methods of routing light through their cores.
Both of these tutorials are based on a presentation given by Bennett at Opticon 2001, Light Reading's annual conference held in San Jose, Calif., in August of this year. Bennett would like to acknowledge the help of Peter Duthie, senior technical specialist, Marconi Optical Components, in preparing this presentation.
As a rule, Light Reading doesn't accept editorial contributions from manufacturers, but Bennett's tutorials provide valuable vendor-neutral insight into issues that have often been muddied by marketing hype.
In Bennett's view, the key to understanding all-optical switches is to consider the following issues in order:
Applications Identifying the purpose of an all-optical switch pinpoints key requirements in terms of scale, functions, and performance.
Techniques This covers how traffic is directed through the switch (the control plane) and the way in which it's handled (on its own dedicated wavelength or multiplexed with other traffic).
Technologies Dealt with in the second tutorial, this covers the fabric that routes optical pulses from input ports to output ports.
Can we practically, think about an Optical Hybrid Switch which may contain "OBS" for short lived traffic or called best effort traffic and "OCS" for long-lived traffic or video conferencing etc.
If we give strict priority to OCS traffic and equal priority to OCS traffic what could be possible discussions?
One problem in OBS is that it is very hard to do traffic engineering. I am very curious about the routing issues in OBS. Will it provide a single path for each source destination pair and update the routing table periodically, or provide multiple path candidates in the routing table?
Other comments on OBS and wavelength routing are as follows:
1. OBS is said to be better than the wavelength routing because it is able to multiplexing the data, thus more efficiently utilising the bandwidth. However, OBS is very bad at dealling with the blocking, whilst wavelength routing can provide guaranteed service upto a certain level.
2. Therefore, it is very hard to say which is better actually. Current network status shows that there are far more sufficient network resources in the backbone. So it seems that bandwidth efficiency is not the biggest concern. From this point of view, dynamic wavelength routing seems more adoptable.
I am wonder that the header is still processed in the electronic domain. And so the processing time is still slow. Also OE-conversion is still needed in this processing.
So, does OPS need OE-conversion? or my concept is not correct?
All-optical swtiching really can break the bottlenecks of the speed of transmission, but I would like to know how the header can be processed optcally. (Through optical Logic gate with syncronization processes?) - I just start working on this topic, and would like to learn more - thank you
KEOPS is undoubtedly the fore father in optical packet switch research and tonnes of research papers published after it. But with ATM diminishing and IP become the dominant player, there is really no point talking about KEOPS, WASPNET, ... what we need is an optical packet switch for IP packets.
The problem with switching IP packets is that the packet size distribution is highly dynamic at any instant of time. TO buffer these packets, you will need the same dynamic mechanism in the buffers. And this is exactly the weak point of optics, the buffers are build up of dumb static optical delay line where the next buffer is fixed step increase of the previous buffer. In short we have a highly random packet size distribution at the input and a dumb buffer.
What we need is what we don't have, repeating the facts stated in previous posted messages,there are no intelligence and no dynamic buffers. Unlesss there is breakthrough in this area, there's no ground breaking solution.
At one time Alcatel were working on an SOA-based, broadcast & select switch.
SOAs are quite a fast way to gate a B&S design, probably fast enough for OPS.
There are some interesting drawbacks to using SOAs, however. These include both a significant noise component, and the fact that you introduce a polarisation dependency (which adds a few network design challenges).
I'm planning an update to the article now and will include a more detailed description of broadcast and select (or gate), with both SOA and EDFA gating described.
* no (sufficient) optical logic exists * no (sufficient) optical buffers exist
This gives 3 functionalities to be solved :
* how to solve contention (2 packets to same output at the same time) * how to analyze routing information (i.e. the headers) * how to get enough time to analyze it (cus we don't have buffers)
In OBS, a work-around is proposed :
* separate your header and data (see below) * aggregate your data in packet trains * process header electronically, data optically
By aggregating your packets in bigger packets you reduce your header processing requirements (you simply have less headers).
Processing will still be done electronically, your data will be kept optically.
How ? By sending your headers ahead in time (called the "offset") on a different channel. This offset will then be used to give the intermediate nodes time to process their information, thus reducing the offset each step. So you'll have to make it large enough.
Note that this not solves the problem of contention.
I know this a very late reaction, but still i hope some of you will find it useful.
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