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Talked about TinyOS concepts a bit. I'm sure everyone has read the papers, but nobody dared to stop Prof. Bridges. We just waited him out, until he moved on interesting stuff, like routing.

Next, we had a (very) brief look into the history of CSMA staring from ALOHA. It was useful to structure the vision of the world, e.g. a reminder that Ethernet detects collisions, and that big benefit of CSMA is the lack of central coordinator (IIRC ALOHA did have a lead station, although only to sync slot clocks, but what's important here is that it did not do arbitration). Nobody mentioned Appletalk's CSMA/CA, heh.

Issues in Wireless:

• Hidden Terminal problem: triangle, two initiators cannot hear each other and collide at the receiver, cannot back off each other. In WiFi receiver has RTS.
• More losses: Principally, the transmitter in Ethernet can assume that once the message is out, it was delivered. In wireless, MAC ACKs are essential.
• Dynamic routing: Needed for multihop networks, but the change rate may be much greater than in wired networks.

The last bit is the worst. So, we talked the routing problem (including "count to infinity" for DV, etc.). Although "very well mined", the wireless problem seems intractable in general. Therefore, apparenly all successful systems work by introducing asymmetry: a base station of some kind. It either flattens the topology like in 802.11, or creates enough asymmetry to break loops. For sensors, Direction Diffusion and Collection Tree Protocol are notable. The only symmetric thing that flops around is "AODV": Ad-hoc, On-demand Distance-Vector.

Homework: Read Trickle paper (Trickle: A Self-Regulating Algorithm for Code Propagation and Maintenance in Wireless Sensor Networks). It's Levis and Culler again, applying the gossip model to motes. Bet Brian will love it.

Speaking of homework, we submitted our 1-paragraph ideas. Amusingly, it looked like nobody submitted by e-mail. When I printed out my ideas (I had two), I thought that maybe it's all right for me to use dead trees, I'm positively ancient for a student. No idea what everyone else's excuses were. Oh well.

Also, Matthew said that he has a class blog, and he was "too lazy" to reply to my message with a URL. *goggle eyes*

Tags: cs-591-005

Posted by: Pete Zaitcev at 05:57 PM | No Comments | Add Comment
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