Adaptive QoS Control by Toggling Voice Traffic between Circuit and Packet Cellular Networks JaeWon Kang, Badri Nath DATAMAN Lab., Department of Computer Science Rutgers University, Piscataway, NJ 08854-8019 {jwkang, badri}@cs.rutgers.edu Abstract— As cellular packet data services become widely deployed by the rollout of the networks such as General Packet Radio Service (GPRS) and 3G cellular networks, packet-switched voice service such as voice over IP (VoIP) can soon be expected to be offered as an alternative to circuit-switched voice service. While circuit-switched voice offers better quality, packet-switched voice offers better resource utilization due to its multiplexing and compression techniques that can be used in packet radio princi- ple. Therefore, the capability of having voice service over either circuit-switched network or packet-switched network provides a tradeoff between voice quality and bandwidth utilization. This paper proposes three schemes that allow users to toggle between circuit-switched voice and packet-switched voice based on the desired tradeoff. The basic idea is to allow ongoing voice traffic to alternate between circuit-switched network and packet- switched network using a 3-way calling mechanism in the SIP- enabled GSM/GPRS cellular network. We also show how these toggle schemes can be used depending on the cell load. Detailed performances of the proposed schemes are evaluated using simulation with realistic parameters of a cellular environ- ment. The result shows our schemes can alleviate cell overloading as well as provide the smooth transition of an ongoing call between circuit-switched network and packet-switched network. Our schemes can be easily deployed in any cellular mobile network where circuit-switched network and packet-switched network coexist. I. I NTRODUCTION Carrier grade voice is still the killer application in cellular networks and provides significant revenue to the carriers [1]. On the other hand, Internet Telephony (or IP Telephony) has been gaining popularity in the packet-switched networks, e.g. the Internet. As cellular carriers start supporting the packet data service such as General Packet Radio Services (GPRS) [2], [3], we believe VoIP service will soon be available over cellular packet networks as well. We also believe even though IP-based solution is gaining popularity in cellular networks, the circuit-switched network will not disappear soon due to its wide deployment. In the GSM/GPRS cellular network shown in Fig. 1, when a GPRS mobile station is in Class-A mode of operation, it supports simultaneous operation of GSM and other GPRS services [4]. Therefore, if VoIP service is deployed in the GPRS network, mobile users will have an option to make a call either through the circuit- switched network (GSM network) which provides better call quality at the expense of bandwidth, or through the packet- switched network (GPRS network) which is more bandwidth- efficient. IP phone Data Network PSTN phone Network Voice Circuit Wireless Network GSM Packet GPRS Wireless Network MS Fig. 1. GSM/GPRS Cellular Network Architecture From a bandwidth point of view, VoIP offers significant advantages over the circuit-switched connection (here, GSM call). Therefore, the capability of toggling the voice traffic between these two networks can be used as a tool to handle dynamically changing load in a cell. Many bandwidth adap- tation schemes have been introduced in wireless networks to deal with the problem of handling limited wireless bandwidth. Most of these schemes propose how to allocate or adjust wireless bandwidth efficiently for new or existing connections when load on the system changes dynamically. In [5], when a handoff occurs to a cell where there is not enough bandwidth, a channel currently being used is divided into two subchannels. One subchannel serves the existing call and the other serves the handoff call. [6] proposes two bandwidth adaptation schemes, one for revenue and anti-adaptation, and the other for revenue and fairness. In their schemes, each call is classified into a certain class and can take several units of bandwidth predefined in that class. However, most of the bandwidth adaptation schemes proposed so far are very difficult to deploy since they require wireless bandwidth to be split and merged at will. But, as cellular carriers start deploying the packet data service while maintaining the existing circuit-switched network due to its wide deployment, the bandwidth adaptation by alternating between these two networks becomes easy to deploy from a bandwidth-management point of view. This paper proposes three bandwidth adaptive toggle schemes which provide a seamless transition, which can be triggered either by the user or the base station, of an ongoing voice call between GSM and GPRS networks. The bandwidth adaptation in our schemes is conducted by the base station’s toggling the voice traffic between GSM and GPRS networks using a 3-way calling mechanism based on the criteria such as Normalized Mean Opinion Score (MOS), call hiccups, and call duration in a cell. Although we propose our schemes based on the GSM/GPRS cellular network, they can be easily deployed in any cellular network where circuit-switched network and GLOBECOM 2003 - 3498 - 0-7803-7974-8/03/$17.00 © 2003 IEEE