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Technology Stocks : Ciena (CIEN) -- Ignore unavailable to you. Want to Upgrade?

To: Doug who wrote (1064)	12/9/1997 9:21:00 PM
From: Tim Bagwell	Read Replies (1) \| Respond to of 12623

Doug, you ask some tough questions but I'll see what I can do. a) The use of the terms WDM and DWDM have become somewhat blurred. It used to be that systems with 32 channels and more were called DWDM, however, now some 16 channel systems are called DWDM. It's really an ambiguous distinction that is up to the manufacturer to label. In my book, DWDM still refers to systems that have 32 or more channels because these press the technology to the limits. Channel loss will usually not be affected by the number of channels but crosstalk certainly will. As more channels are added channel spacing decreases. New amplifier technology that is out a few years will increase the available bandwidth further but for now systems are limited to about 30-40 nm of amplifier bandwidth. As you modulate the laser it produces sidebands. These sidebands can overlap and cause crosstalk if the channels are spaced too close. So for a given bit rate there is a limit to channel spacing before bit error rate begins to suffer dramatically. The splitters used are called multiplexers and demultiplexers. These are wavelegnth selective elements and therefore the technology used will also limit how close channels can be placed. However, there appear to be ways to circumvent some of these issues with clever network design. Consequently, we are now hearing of channel spacings as low as 0.4nm which seemed unlikely a year ago. But it gets expensive to play these games. One also has to worry about laser drift as channels get closer together. It forces the use of stabilized lasers adding further cost and complexity. b) Only electronic switching is used now. Optical switches are not fast enogh to do routing. They are, however, fast enough to do network reconfiguration where you might add another fiber to handle increased trafic on Mother's Day or if you have a fiber break (which happens more often than you might think). So all protocol handling is done by means of electronic switching usually at the terminal points of the link. c) There are different types of protocols. Some such as ATM or IP contain routing information in every packet. It has to be this way because packets get interleaved together from all sources. There is one twist however. There is a mode known as broadcast where data can be sent to many subscribers at once to avoid repetition. Just how the packets are managed is not clear to me. My understanding is that this is not used frequently. d) I'm not sure which mux/de-mux operation you are referring to. Mux/de-mux of optical channels is very straightforward. Routing of hardwire protocols is very complex and requires enourmous computing muscle because of the massive task of redirecting and reconnecting all the packets from the originator. Keep in mind this all refers to long haul networks which are more sophisticated. For metro or LAN's the design is usually simpler because the fiber runs are shorter. Hope that helps.