To support real-time applications such as Voice-over-IP within an 802.11 Wireless LAN, efficient handoff mechanisms are required when a mobile client moves from one Access Point to another. In this paper, we present the design, implementation and performance results of MAC layer (layer-2) handoff algorithms which implement make-before-break mechanisms at the MAC layer unlike current algorithms that are based on break-before-make. As a baseline, we first present an algorithm that uses a single radio card on the client (as is done traditionally), but optimizes MAC layer handoff by periodically probing in the background for Access Points on other channels even when it is already associated with an Access Point and actively sending and receiving data. We then present two novel algorithms that use two radio cards on the client so that when one card is involved in communicating data, the other card can probe for neighboring Access Points. Of these multiple radio algorithms, the first one uses dedicated data and control cards at the client thereby implementing a soft version of make-before-break. The second algorithm uses two cards which can both perform control and data forwarding functions thereby implementing a strict version of make-before-break. Experimental results from a prototype implementation show that the make-before-brake algorithms lead to a significant reduction in MAC layer handoff overhead; the algorithm for the strict version of the make-before-break mechanism leads to sub 10 millisecond hand-off latency.

This paper presents the design and implementation of a low latency layer-3 handoff scheme for 802.11 wireless mobiles roaming between access points that are connected to different IP subnets. Called LIHP (link-layer initiated handoff protocol), the scheme uses link-layer frames (e.g., ARP and 802.11 management frames) as triggers to send route updates to the mobiles and to an access gateway, which serves as the gateway to the Internet for the mobiles. This approach allows for IP packet routing to and from a mobile to begin immediately after the mobile establishes a link-layer association with a new access point. Furthermore, the design does not require any modification to the mobiles. Each mobile is assigned a single IP address and maintains its IP address as it moves from one access point to another, thus preserving TCP connections across hand-offs. The access points (called access routers in LIHP) and the access gateway cooperate to track each mobile and forward packets using IP tunnelling. Experimental results from a prototype implementation show that the scheme can achieve layer-3 handoff latency on the order of a few milliseconds

BabyBondConnect (BBC) is a software package for trading goods with combined-value in a call market. The software was developed at Caltech under the direction of Dr. John O. Ledyard.The poster session will explain in detail the problem of trading combined-value goods, the mechanism used by BBC, and the technical specification of the software package. The poster will suggest further development of the BBC software package, the role software engineering and experimental economics can play in understanding traditional markets and electronic markets, and how new market mechanisms can be developed and tested.