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Multicluster, Mobile, Multimedia Radio Network
Mario Gerla and Jack Tzu-Chieh Tsai, UCLA
Vision paper for ARPA WAMIS, fully distributed heirarchical wireless network based on clusters, using TDMA within clusters and CDMA across them.
- Datagrams for "document" traffic, virtual circuits supported for continuous media
- Passive ACK doesnąt work since SS-CDMA is used; hence need e2e as well as link-to-link flow and congestion control.
- clustering provides framework for CDMA code separation, channel access, power control, etc.
- TDMA used within a cluster. "Cluster head" (leader) elected by "lowest-ID" algorithm, which is more stable than "highest-connectivity".
- Routing within a cluster is fully distributed; intercluster packets must go through gateways that know both CDMA codes for adjacent clusters
- "Distributed gateway": a single logical gateway implemented across a pair of machines in different clusters that can hear each other. Most useful in very weak connectivity situations, unneeded complexity otherwise.
- COdes can be reused in non-adjacent clusters by 4-coloring graph; since also need a code for control channel, system requires 9 orthogonal codes.
- Frame is divided into "Info" and "Control" phases. During control phase, neighbors exchange a lot of data (eg power gain lists, connectivity info, code assignment, etc) using broadcast.
- Size of control frame grows linearly with number of radios in network! Authors estimate 24 slots enough for a "typical" system whose properties seem to be pulled out of their butt.
- Fast-reservation for virtual circuits (to handle mobility as circuits move around): shortest-path assignments using Bellman-Ford algorithm. No discussion of when to use fast reservation vs. the standard "static" scheme used in low-mobility situations.
- E2E thruput is lower than link thruput by about factor of 4 (simulation); why??
- Combined CDMA and TDMA yields 80 percent improvement over TDMA alone. Not surprising.
- A few "blue sky" extensions to protocol proposed.
No need for wired infrastructure; the network sets up and deploys anywhere and can handle a variety of applications.
- Vision, not implementation paper; lots of stuff glossed over or not explained.
- Complexity of combined TDMA and CDMA seems formidable, yet they donąt address. How much of this have they tried to build?
- Hands waved violently regarding how network can be synchronized for the TDMA part.
- Scalability is claimed as a goal, but the control-phase length seems to preclude this.
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