1 Robust Rate Control for Heterogeneous Network Access in Multi-Homed Environments Tansu Alpcan, Member, IEEE, Jatinder Pal Singh, Member, IEEE, and Tamer Bas ¸ar, Fellow, IEEE Abstract—We investigate a novel robust flow control framework for heterogeneous network access by devices with multi-homing capabilities. Towards this end, we develop an H ∞ -optimal control formulation for allocating rates to devices on multiple access networks with heterogeneous time-varying characteristics. H ∞ analysis and design allow for the coupling between different devices to be relaxed by treating the dynamics for each device as independent of the others. Thus, the distributed end-to-end rate control scheme proposed in this work relies on minimum information and achieves fair and robust rate allocation for the devices. An efficient utilization of the access networks is established through an equilibrium analysis in the static case. We perform measurement tests to collect traces of the available bandwidth on various WLANs and Ethernet. Through simulations, our approach is compared with AIMD and LQG schemes. In addition, the efficiency, fairness, and robustness of the H ∞ -optimal rate controller developed are demonstrated via simulations using the measured real world network characteristics. Its favorable characteristics and general nature indicate applicability of this framework to a variety of networked systems for flow control. Index Terms—Wireless communication, heterogeneous networks, multi-homing, rate control, H ∞ -optimal control. ✦ 1 I NTRODUCTION C ONTEMPORARY networks are heterogeneous in their at- tributes such as the supporting infrastructure, protocols, and offered data rates. The multitude and variety of existing and emerging wireless and wired networking technologies con- tinue to be the driving force towards convergence of networks. It is commonplace today to have electronic devices with multi- ple networking capabilities. Personal computing devices, e.g., laptops, UMPCs (ultra-mobile PCs), PDAs, smartphones, are typically equipped with several access systems ranging from different types of IEEE 802.11 wireless local area networks (WLAN) to Ethernet, GPRS, and UMTS. On end-user devices a variety of applications emerge with different bandwidth requirements for multimedia access, gam- ing, and collaboration. Our objective is efficient utilization of multiple networks by devices via rate control and optimal assignment of traffic flows to available networks. The function- ality we envision can be described in a hypothetical scenario as follows. Imagine a user in a corporate setting participating in a video conference call via a device having both Ethernet and WLAN (say IEEE 802.11g) connectivity. While engaged in the conference proceedings the user is retrieving relevant files from a remote server, and at the same time generating • T. Alpcan is with the Deutsche Telekom Laboratories, Technische Univer- sit¨ at Berlin, Germany. E-mail: see http://www.tansu.alpcan.org • J. P. Singh is with the Dept. of Electrical Engineering at Stanford University and Deutsche Telekom Laboratories, CA. E-mail: jatinder.singh@telekom.de • T. Bas ¸ar is with the Coordinated Science Laboratory, University of Illinois, Urbana, IL. E-mail: tbasar@control.csl.uiuc.edu Research supported by Deutsche Telekom AG. An earlier version of this paper has appeared in the Proceedings of WiOpt 2007, 5th Intl. Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks, April 16-20, Limassol, Cyprus. content by taking notes on her/his blog. Several traffic flows of different characteristics are hence created by the device, which dynamically monitors the networks at its disposal. The device then routes the flows via these networks and dynamically reassigns them to different networks based on the varying network characteristics like available bit rate (ABR) and delay. While the distribution of traffic flows amongst different networks can enable better network utilization than single network use at a time, the variability in network characteristics like ABR and delay makes the problem of flow control and assignment challenging. This and similar problems have been explored from different perspectives. A game theoretic framework for bandwidth allocation for elastic services in networks with fixed capacities has been addressed in [1]– [3]. Packet scheduling for utilization of multiple networks has been investigated in [4]. A solution for addressing the handoff, network selection, and autonomic computation for integration of heterogeneous wireless networks has been presented in [5], which, however, does not address efficient simultaneous use of heterogeneous networks and does not consider wireline settings. In [6], the authors have explored design of a network comprised of wide area and local area technologies where user devices select among the two technologies in a greedy fashion so as to maximize a utility function based on wireless link quality, network congestion, etc. In [7], an optimal rate control scheme has been investigated where the queue size at a bottleneck link is optimized using an H ∞ control formulation. As evidenced by the existing body of literature, multiple network access in multi-homed settings where devices simul- taneously utilize available networks is a problem of keen academic interest. A few of the recent efforts include the work by Thompson et. al. [8] where flow based multi-homing in res- idential environments with access to heterogeneous networks has been explored, Shakkottai et. al. [9] where multi-homing of IEEE 802.11 wireless devices to access multiple APs has