EE 228 a - Lecture 5 - Spring 2006 WiFi Models

This lecture describes WiFi models via an example of QoS over IEEE 802.11 network. First we will give an overview of 802.11 protocol. Then a model proposed by Bianchi [1] will be presented to compute the packet delay of 802.11 DCF (Distributed Coordination Function). We extend this model to 802.11e EDCF (Extended Distributed Coordination Function) to study the QoS for voice it can provide. I. Overview of IEEE 802.11 Fig. 1 IEEE 802.11 Infrastructure Network Fig 1 shows typical data traffic in an 802.11 Infrastructure network. A number of wireless hosts/clients communicate with an AP (Access Point), which is normally attached to a wired network (e.g. Ethernet). In 802.11b, except that the physical header of each packet is transmitted at the same data rate, each station can choose one of the four rates for the MAC layer payload from 11Mbps, 5.5Mbps, 2Mbps and 1Mbps, depending on the wireless channel condition (a lower rate is chosen for a weaker channel to guarantee the probability of error). There are ”downlink” traffic from the AP, and ”uplink” traffic to the AP. For VoIP (Voice over IP), downlink and uplink traffic are symmetric. In this lecture, we will focus on deriving the packet delay and providing QoS in such networks. There are two types of functions in 802.11 MAC: Point Coordination Function (PCF) and Distributed Coordination Function (DCF). PCF is a polling protocol, which enables the clients to transmit data one by one. This function, however, is not robust, and is not implemented in most products. PCF is simple to analyze because of its similarity to TDMA. The commonly implemented DCF is based on CSMA/CA (Carrier Sensing Multiple Access/Collision Avoidance). Each station (including the AP and clients) randomly chooses a time slot to transmit a packet. If packet is collided (when another station happens to choose the same slot), it will double its Backoff-Window and try again later. DCF is more difficult to analyze due to its random nature. In 802.11b with rate 11Mbps, pure data transmission typically gets a throughput of 6Mbps; and for VoIP with 64kbps/direction, 12 connections can be supported (about 1.5Mbps in total). The remaining bandwidth is spent in collisions, backoffs and packet headers. Fig 2 shows how DCF works. II. Analyzing VoIP in 802.11 DCF We wish to support n VoIP connections (Fig 3). That is, we hope to send V1, V2, ..., Vn in 20 ms. Given a data rate of the network, there is a maximum n feasible, which is the ”Call Capacity”. Note that the bottleneck is at the AP, since AP needs to send n flows. Our approach to find the Call Capacity is shown in Fig 4. After assuming a number of connections, we derive the delay and check whether the delay requirement is met at the AP. If not, this number is beyond the Call Capacity. Bianchi’s Model