Wireless Token Ring Protocol-Performance Comparison with IEEE 802.11 ∗ Mustafa Ergen, Duke Lee, Raja Sengupta, Pravin Varaiya {ergen, duke, sengupta, varaiya}@eecs.berkeley.edu Department of Electrical Engineering and Computer Science University of California, Berkeley Berkeley, CA, USA 94720 Abstract The paper presents the performance advantage of Wireless To- ken Ring Protocol (WTRP) versus IEEE 802.11 in DCF mode. WTRP is a medium access control (MAC) protocol and is de- signed to provide quality of service in WLANs. WTRP supports guaranteed QoS in terms of bounded latency and reserved band- width which are crucial constraints of the real time applications and unapplicable in a IEEE 802.11 network. WTRP is a dis- tributed MAC protocol and partial connection is enough for full connectivity. The stations take turn to transmit and are forced to suspend the transmission after having the medium for a specified amount of time. WTRP is robust against wireless medium imper- fections. The DCF mode of IEEE 802.11, also a distributed MAC protocol, is based on contention among stations and is not ho- mogeneous due to the existence of hidden terminals and random behavior. Consequently, QoS is not provided. 1 Introduction Wireless local area networking is introduced to provide wire- less connectivity to stations that require rapid deployment. In wireless networks, participating stations can join or leave the net- work at any moment in time. IEEE 802.11 protocol is introduced in 1997 with a medium access control (MAC) protocol and sev- eral physical layer signalling techniques [11]. IEEE 802.11 MAC provides to two different access mechanisms based on contention (Distributed Coordination Function (DCF)) and polling (Point Coordination Function (PCF)). Due to the existence of hidden terminals and partially connected network topology, contention among stations in a wireless network is not homogeneous. Some stations can suffer severe throughput degradation in access to the shared channel when load of the channel is high [7], which also results in unbounded medium access time for the stations and un- fair resource distribution per station [8], [9], [12]. This challenge * Research Supported by Office of Naval Research (Autonomous Agents Net- work project (fund # 23083) N00014001061), CALTRANS. is addressed as quality of service (QoS) in communication net- works. WTRP-Wireless Token Ring Protocol is a MAC protocol in- tended to provide QoS in terms of bounded delay and reserved bandwidth. WTRP is built based on a distributed approach. Its advantages are robustness against single node failure, and its sup- port for flexible topologies, in which nodes can be partially con- nected for full connectivity and not all nodes need to have a con- nection with a central controller. Current wireless distributed MAC protocols such as the IEEE 802.11 (DCF mode) [11] and the ETSI HIPERLAN [10] do not provide QoS guarantees that are required by some applications. In particular, medium is not shared fairly among stations and medium-access time can not be controlled. WTRP is an ongoing work of [1] and previously presented in [3], [4], [5]. The latest version [2] includes improvements in the packet frames in order to convey more information to perform robust and quick network creation. A new finite state machine is introduced that response faster to the wireless medium changes than [4]. WTRP was first deployed for the automated highway project of CALTRANS [6] and now is extended to home and local area networks [2]. The outline of the paper is as follows; We explain the MAC protocol of IEEE 802.11 in DCF mode and the MAC protocol of WTRP in Section 2 and 3 respectively. We present the per- formance results in Section 4 and conclude the paper in Section 5. 2 MAC Protocol of IEEE 802.11 IEEE 802.11 MAC protocol in DCF mode is based on Carrier- Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. The medium access mechanism has two important mod- ules: carrier sense and backoff. Following the Figure 6, sta- tion waiting in the idle state senses the medium before making any attempt to transmit. There are two different carrier sense mechanisms: Virtual carrier sense (VCS) and physical carrier sense (PCS). VCS is determined by the network allocation vec- tor (NAV) which is set according to time specified in the duration 1 Proceedings of the Eighth IEEE International Symposium on Computers and Communication (ISCC’03) 1530-1346/03 $17.00 © 2003 IEEE