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 is addressed as quality of service (QoS) in communication net- works. The paper presents the performance advantage of Wireless To- WTRP-Wireless Token Ring Protocol is a MAC protocol in- ken Ring Protocol (WTRP) versus IEEE 802.11 in DCF mode. tended to provide QoS in terms of bounded delay and reserved WTRP is a medium access control (MAC) protocol and is de- bandwidth. WTRP is built based on a distributed approach. Its signed to provide quality of service in WLANs. WTRP supports advantages are robustness against single node failure, and its sup- guaranteed QoS in terms of bounded latency and reserved band- port for flexible topologies, in which nodes can be partially con- width which are crucial constraints of the real time applications nected for full connectivity and not all nodes need to have a con- and unapplicable in a IEEE 802.11 network. WTRP is a dis- nection with a central controller. Current wireless distributed tributed MAC protocol and partial connection is enough for full MAC protocols such as the IEEE 802.11 (DCF mode) [11] and connectivity. The stations take turn to transmit and are forced to the ETSI HIPERLAN [10] do not provide QoS guarantees that suspend the transmission after having the medium for a specified are required by some applications. In particular, medium is not amount of time. WTRP is robust against wireless medium imper- shared fairly among stations and medium-access time can not be fections. The DCF mode of IEEE 802.11, also a distributed MAC controlled. protocol, is based on contention among stations and is not ho- WTRP is an ongoing work of [1] and previously presented mogeneous due to the existence of hidden terminals and random in [3], [4], [5]. The latest version [2] includes improvements in behavior. Consequently, QoS is not provided. 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 1 Introduction 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]. Wireless local area networking is introduced to provide wire- The outline of the paper is as follows; We explain the MAC less connectivity to stations that require rapid deployment. In protocol of IEEE 802.11 in DCF mode and the MAC protocol wireless networks, participating stations can join or leave the net- of WTRP in Section 2 and 3 respectively. We present the per- work at any moment in time. IEEE 802.11 protocol is introduced formance results in Section 4 and conclude the paper in Section in 1997 with a medium access control (MAC) protocol and sev- 5. eral physical layer signalling techniques [11]. IEEE 802.11 MAC provides to two different access mechanisms based on contention 2 MAC Protocol of IEEE 802.11 (Distributed Coordination Function (DCF)) and polling (Point Coordination Function (PCF)). Due to the existence of hidden IEEE 802.11 MAC protocol in DCF mode is based on Carrier- terminals and partially connected network topology, contention Sense Multiple Access with Collision Avoidance (CSMA/CA) among stations in a wireless network is not homogeneous. Some scheme. The medium access mechanism has two important mod- stations can suffer severe throughput degradation in access to the ules: carrier sense and backoff. Following the Figure 6, sta- shared channel when load of the channel is high [7], which also tion waiting in the idle state senses the medium before making results in unbounded medium access time for the stations and un- any attempt to transmit. There are two different carrier sense fair resource distribution per station [8], [9], [12]. This challenge mechanisms: Virtual carrier sense (VCS) and physical carrier ∗Research Supported by Office of Naval Research (Autonomous Agents Net- sense (PCS). VCS is determined by the network allocation vec- work project (fund # 23083) N00014001061), CALTRANS. 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 Post-Tx backoff successful Busy during backoff PCS Busy Idle for Idle During Tx VCS IFS time Backoff Wait Just Transmitted Pre-Tx backoff Ack or CTS successful Medium not busy All other transmitted frames during Tx attempt whether successful or not Finish Tx Sequence Tx Still in sequence & Retry and last step successful Figure 1. Main Flow for IEEE 802.11 DCF MAC Protocol S S random new random I I backoff backoff F F field of packets [11]. This gives station to understand the time, Station 1 (7 slots) CTS ACK (10 slots) NAV S S NAV D S D Station defers Station 2 I RTS I DATA I the channel is occupied, due to an ongoing transmission. On the NAV F F F other hand, PCS is a notification mechanism from physical layer S S S random remaining backoff backoff Station 3 (9 slots) (2 slots) ACK to MAC layer saying that there is no signal detected. By com- NAV NAV D Station defers, but keeps backoff counter (=2) D S Station 4 I I DATA I Station sets NAV upon receiving RTS bining VCS and PCS, MAC implements “collision avoidance” F F F S S S mechanism of CSMA/CA [11],[12]. The station before initiating S I any transmission first checks VCS and then senses the medium Station 5 F ACK Station sets NAV upon receiving RTS S Station sets NAV upon receiving Station 6 CTS, this station is hidden to for a DIFS time by PCS. DATA station 1 time If a station finds the medium busy, it waits until the carrier sense mechanism notifies the station that the medium is idle. Figure 3. Timing Diagram for IEEE 802.11 Next, the station goes to “backoff” state from “PCS & VCS” state and selects a backoff interval uniformly out of a contention window [11]. Contention window doubles in every unsuccess- RTS/CTS mechanism is disabled when the data size is below the ful transmission, consequently the station waits longer in back- RTS threshold, specified in the standard [11]. off. If the station senses a transmission while it counts down in “backoff” state, it suspends the transmission and goes back to 3 MAC Protocol of WTRP carrier sense state and waits until the medium becomes idle and then starts counting down from where it stopped. After back- Main flow of WTRP as shown in Figure 6 is designed to off, the station transmits the packet in “Tx” state. The station compete with wireless medium facts. WTRP implements sev- waits for ACK or CTS frame to make sure that the transmission eral modules to cope with “mobility”, “interference and colli- is successful in “Sequence & Retry” state. In case of unsuccess- sion avoidance”, and “guaranteed transmission”. The WTRP con- ful transmission, it doubles its contention window and increments structs a ring wherein the transmission proceeds in one direction its retry counter. When the retry count reaches maximum value, along the ring. Each station has a successor and a predecessor station gives up transmitting that packet. An illustration of timing which is enough for the ring to be fully connected. An illustration diagram is shown in Figure 3. of timing diagram is shown in Figure 51. Each station is given a A station reserves the medium by sending a Request-To-Sent certain time called token holding time (THT). After receiving the (RTS) frame. The stations receiving the RTS frame stop trans- token frame, station is allowed to transmit packet up to a THT and mitting except the station to whom the RTS is destined. Destined passes the token to its successor. Assume that there are N stations station sends CTS frame to acknowledge the transmitting station in a ring. We define Tn to be the time wherein station Sn trans- that it is ready to receive. RTS and CTS frames contains dura- mits between it gets and it releases the token. Time that takes for tion fields in which the other stations learn how long the medium one rotation of token is bounded by Maximum Token Rotation will be busy and set their NAVs accordingly. As it can be seen from Figure 3, stations wait SIFS time between packets [12]. The 1PROP stands for propagation time of a signal in the medium. Proceedings of the Eighth IEEE International Symposium on Computers and Communication (ISCC’03) 1530-1346/03 $17.00 © 2003 IEEE FC RA DA SA NoN GenSeq Seq 1666244bytes Figure 2. Token Frame Solicit_Successor Set_Predecessor Set_Predecessor Station A Br. S S C B Time S Station A C Time NS SA SB S S A A C A Set_Successor Set_Predecessor Set_Successor B Station B S S S A B C B S S Station B A C NS Br. SC SB NS C C Station C Station C SC SC (a) Joining (b) Exiting Figure 4. Management Procedures Token Token ber updated, it assumes that the ring owner is unreachable and it PROP PROP T S o o o T S 1 2 N 1 Time elects itself to be the ring owner. Station 1 Station N Connectivity manager resident on each node tracks transmis- PROP PROP sions from its own ring and those from other nearby rings.