Distributed allocation of time slots for real-time traffic in a wireless multi-hop network 1 Emma Carlson, Holger Karl, Adam Wolisz TU Berlin, Einsteinufer 25, 10587 Berlin, Germany carlson karl wolisz @tkn.tu-berlin.de fon: +49.30.31429243, fax: +49.30.31423818 Christian Prehofer DoCoMo Euro-Labs, prehofer@docomolab-euro.com Abstract: This paper examines a new mechanism for distributed resource reservation that offers support for ap- plications with QoS requirements in a wireless multi-hop network. This mechanism is based on IEEE 802.11 DCF and includes end-to-end reservations of time slots and dis- tribution of reservation information to mobile nodes un- aware of the reservation, piggy-backed on existing medium access control messages. We investigate average packet delay, packet delay dis- tribution, packet loss rates and throughput of our scheme compared to IEEE 802.11 DCF using simulations. Our scheme results in a better support for real-time transmis- sions than DCF. 1. Introduction Multi-hopping has been widely advocated as a means to improve the operation of wireless communication sys- tems. But with respect to real-time applications that have stringent Quality-of-Service (QoS) requirements for their transmissions, there are still some unsolved is- sues (e.g., [3]). These applications require, among other things, a low and stable time delay of packets when transmitted over a multi-hop network as packet delay jit- ter reduces the user-experienced quality. A major source of delay variation is the employed wireless transmission system. The distributed Wireless Local Area Network (WLAN) standard IEEE 802.11 is well known, usually used with a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) based, distributed medium access control scheme, the Dis- tributed Coordination Function (DCF) [2]. This standard works well for normal data that has no stringent demands on the time delay, but for real-time applications, the DCF has the disadvantage that it introduces, at every hop of a multi-hopping path, uncertainties as to when a packet can be transmitted. These medium access uncertainties add up to variations in the total delay; also, the absolute delay of a packet can be large due to repeated medium access procedures. Hence, to support real-time appli- cations that are sensitive to both the absolute value and to variations of the packet delay, medium access must be efficient and its uncertainties must be removed. One way of doing this is to allow stations to reserve future re- sources, i.e. periodic time slots, for their real-time packet transmissions, akin to the case of connection setup in 1 This work has been partially supported by DoCoMo Euro-Labs. wired networks. This way, real-time transmissions can obtain preference over plain data transmissions and can actually receive guaranteed delay (as far as this is possi- ble in wireless networks). To perform resource reservation in a centrally con- trolled network, it suffices to somehow reserve the time slots along the multi-hop path and all possibly conflict- ing nodes would know about the reservation. Setting up such a reserved multi-hop “virtual connection” in a system with distributed medium access control is more difficult, but can still be done by means of reservation messages. But as nodes move around (or are switched on), they can move into the region of an existing reser- vation without being aware of it – a case quite differ- ent from connections in wired networks. Hence, the re- served “virtual connection” has to be maintained as well, requiring mechanisms that allow mobile nodes to learn about existing reservations. In addition, mobility entails breakage of reserved paths, which also requires main- tenance. This maintenance has to be light-weight and should extend existing medium access procedures – a need for connection management mechanisms that is ab- sent in wired networks and in this form not yet discussed for wireless networks either. This paper presents such an extension to IEEE 802.11 that enables a simple end-to-end set-up of periodically reserved time slots over a multi-hopping path and that provides maintenance means to inform oblivious nodes of a reserved transmission with only a small error rate. The protocol is called Distributed end-to-end Allocation of time slots for REal-time traffic (DARE). This paper is organized as follows: Previous work in the area is described in Section 2, followed by a brief summary of the IEEE 802.11 DCF in Section 3 and a description of our protocol extensions in Section 4. Sec- tion 5 describes the evaluation model and in Section 6, the first evaluation results of the DARE mechanism are described. Section 7 concludes this paper and gives sug- gestions for further work. 2. Related work Quality-of-service support for wireless networks with distributed control has been discussed by several authors, e.g. [4, 13, 8, 6, 5]; only some of this work is discussed here. In [11, 12], transmission of jamming bursts is used. A station that wants to transmit starts its access proce- In Proc. European Wireless 2004, Barcelona, Spain, February 2004