Mobile Connectivity Protocols and Throughput Measurements in the Ricochet MicroCellular Data Network (MCDN) System Mike Ritter, Robert J. Friday, Rodrigo Garces, Will San Filippo, and Cuong-Thinh Nguyen, Arty Srivastava Metricom, Inc. 333 West Julian Street San Jose, CA 95110 {mwritter, friday, rgarces, will, ctnguyen, asrivastava}@metricom.com ABSTRACT We describe the protocols implemented in the Ricochet MCDN system to provide continuous connectivity to mobile users traveling up to 70 mph. These protocols are general in nature for any frequency-hopping microcell-based system, particularly those that follow the FCC part 15.247 rules [9] and operate in unlicensed spectrum. We also present throughput measurements as a function of velocity and describe a model to predict those numbers based upon the protocols implemented. The MCDN system is a mesh-based system of microcells that are connected wirelessly to an interspersed mesh of wired access points (WAPs) that cover approximately 12 square miles on average [7]. The average microcell density is approximately 5-6 per square mile, with 3-8 overlapping cells at each point. Since the system is entirely packet-based, we have instantaneous hand-off between microcells as there are no complicated cellular-type negotiations for circuits required as all of the information needed to route the packet through the system is included in the header; however, when traveling through the mesh of microcells at a high rate of speed, the mobile unit must acquire new microcells fast enough to ensure continuous connectivity. The system must also know how to route packets to the mobile unit as it drops old cells and acquires new ones, as well as being able to contact a moving mobile unit. This paper discusses the acquisition, registration, and routing protocols that make this possible and reviews performance data of typical mobile users. Keywords Mobility, Wireless Networks, Wireless Protocols, Wireless Routing, MCDN System Architecture. 1. INTRODUCTION A user of the MCDN system has a radio modem, also known as a subscriber device or mobile unit, attached to or embedded into some computing device that requires network connectivity. This computing device (or in some implementations, the modem) must support PPP in order to negotiate its way onto the network. The radio modem typically appears to the user’s computing device as a modem supporting the AT command set. This mobile unit can appear at any place in the MCDN system and can be mobile at speeds up to 70 MPH while sending and receiving packets. The goal of the MCDN mobile architecture is to make this possible. There has been much work done lately on ad-hoc routing for mobile nodes [1], [3], [4]. This total solution is not required for the MCDN architecture as the infrastructure, which consists of microcells either wired or wirelessly connected to a high-speed backbone, is fixed in place, as will the vast majority of commercial deployments of any microcell system. Only the subscriber devices themselves change places on a near real-time basis. This allows us to split the routing system into three different links: 1) between the gateways and the wired microcells over the wired backbone, 2) the wireless links between the mesh of fixed microcells and wired microcells, and 3) the links between the microcells and the mobile units. The wired backbone uses standard IP routing by encapsulating the wireless packets in UDP and forwarding them to a gateway. The wireless mesh uses a proprietary Bellman-Ford type of routing system based on throughput estimates when moving packets towards the wired backbone, and a proprietary loose-source routing system when moving packets towards the mobile units. The link between the mesh of microcells and the mobile units is a pure broadcast system at the data link level based upon acquisition tables kept in the microcells. This architectural separation allows the wireless routing system to scale independent of the number of subscriber devices, thus making it feasible to support hundreds of millions of mobile units in its current incarnation. Another method of supporting mobility in IP is the Mobile IP specification [6]. This specification was not optimized for the high degree of mobility required by the MCDN system, and the overhead of maintaining and configuring all of the microcells and mobile units with fixed IP addresses and IP subnets is undesirable. However, the architecture upon which Mobile IP is based, that of a care-of-address server, is used in the network in the form of MCDN gateways which funnel all of the user’s traffic Permission to make digital or hard copies of part or all of this work or personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or a fee. ACM SIGMOBILE 7/01 Rome, Italy © 2001 ACM ISBN 1-58113-422-3/01/07…$5.OO 322to the Internet or an intranet. When a user’s device connects to the network it typically dials the radio modem with an ATDT command using a logical name or phone number. Upon receiving that command, the radio modem asks the Name Service to resolve that logical name into an MCDN path (in an analogous manner to DNS.) The key to any addressing architecture is the ability to name and route between entities in the system. The MCDN Name Service provides this by mapping any pair of names of entities to a list of addresses that can be used to route packets between the two entities. This route is referred to as an “MCDN Path.” The MCDN Path consists of a list of addresses of waypoints between the two entities. A waypoint can be, for example, an IP address, a phone number, a geographic area describing a list of microcells, or a microcell identifier. The routing system ensures that the packet visits each waypoint and thereby delivers the packet to its final destination. Once that resolution is done, the mobile unit can append the MCDN path to any packet, send it to any microcell, and the packet will be automatically routed to its destination, the correct logical device. When the user’s computing device is attempting to negotiate PPP to connect to the network, the radio modem establishes a virtual connection, analogous to TCP, to the
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