Architecture and Methodology for Mobile-Executed Handoff in Cellular ATM Networks

Overview/Main Points

• Idea: set up static rooted tree of ATM switches. Each path from root to a leaf is assigned a virtual conn. number (VCN); root is connected to wired network.
• At call setup, call is assigned a pair of VCN's (one in each direction) corresponding to the leaf (BS) at which the call was attached.
• Standard ATM switch routing (lookup table) translates between VCN's and port numbers in the switch.
• Root determines (from VCN) which BS is serving the mobile, so it can assign the appropriate VCN to route returning packets back down the tree. (So it's really a pair of overlapping half-duplex trees.)
• To handoff: mobile moves to a new BS and starts transmitting packets w/the new VCN of that path. Root recognizes this as a handoff event, and now assigns appropriate new VCN to returning packets.
• Special hardware (VCN monitor/translator) required at root to do this, which has a "last-known-location" register and location comparator to detect handoffs.
• To do mobile-to-mobile calls: roots of connection trees can be connected together. Network call processor must become involved for this one. Expect this to be rare since trees cover large geographic area.
• Base station overloading: it can happen. Statistical multiplexing suggests that this should be rare. Authors derive the probability of overloading and expected overload period as a function of number of users in total tree and number of users each BS can handle, using binomial distribution to assign users to BS's. (Assumption: cellular link is bottleneck; wired infrastructure has bandwidth to spare.)
• Efficiency (utilization of switch resources): for large M (no. of calls supported by a single BS), traffic tends toward being evenly balanced, as you would expect. For a given overload probability, you trade off utilization efficiency vs. size of the tree (area covered).
• Authors also (in appendix) derive a dynamic model that accounts for call arrivals, completions, and handoffs. Under heavy traffic, its result converges to that of the simple model.
• Authors show how their scheme can be implemented in ATM cells using the modified-polling MAC scheme (each BS periodically polls its mobiles and passes a token among them for transmission; mobile requests to join BS's polling sequence during a specific segment of the frame). Authors show how the info required by their system fits into the signalling field of the frame.
• Future work: nonuniform prob. of assignment of calls to BS's; choosing handoff cell site based on chan quality; call dropping in case of severe overload.

Relevance

Flaws

• What cellular wireless systems currently use ATM?
• Trees are a non-optimal routing topology, though authors do derive the expected congestion.
• Tries to address everything from mathematical characterization of routing tree behavior to frame format for the implementation; maybe should have been separate papers.