Introduction

The requirements for a network administrator have always been diverse. Get the users connected, make sure there's enough bandwidth to run applications, keep the network secure, and so on. When problems come up on a traditional network, the solutions have often been fairly straightforward. If users aren't connected, you connect them. If there's not enough bandwidth, you buy new equipment. If network resources are vulnerable, you introduce appliances and applications that provide enhanced security. It's a pretty tight loop of problems and solutions.

In today's networking environment, things have changed. You can save money by putting telephony on the network. That means VoIP. You can increase productivity by allowing wireless access to the LAN. That means Wi-Fi. Things are great until you have to combine the two. That often leads to problems. Managing a network that includes VoIP and Wi-Fi means more than just learning the black magic that makes each technology work. It also means learning how each technology causes the other to get a little blacker anda little more magical.

The goal of this paper is to combine a deep knowledge of 802.11 protocols, quality of service (QoS) mechanisms and VoIP handsets with the feedback aggregated from training hundreds of IT professionals each year to identify potential problems and solutions when rolling out VoIP on a Wi-Fi network.

Basic VoIP Requirements

To understand how VoIP communications are affected by wireless LANs, one must first understand how VoIP works. Detailing every VoIP protocol and the network requirements of each is outside the scope of this paper, but some basic points must be explained.

Many VoIP handset manufacturers use the G.711 codec because G.711 provides superior quality at the price of relatively high bandwidth requirements. I say, "relatively high bandwidth," because even the G.711 codec only requires a 64 kilobits per second (kbps) data stream in each direction (uplink to the access point [AP] and downlink from the AP). When encapsulation overhead from RTP, UDP, IP and Wi-Fi headers is accounted for, the bandwidth requirement is pushed to 92 kbps. Since that fails to include control traffic, it's reasonable to estimate that 100 kbps in each direction - 200 kbps of total bandwidth - is necessary for each G.711 call. In addition to understanding basic bandwidth requirements, one must also understand three important problems that may affect call quality: delay, jitter, and packet loss. Delay is the amount of time it takes the sound Benjamin Miller, Global Knowledge Course Director, CWNE, CWNT from your voice to reach the ear of the other person. Maximum acceptable delay limits for VoIP are considered to be 150-200 milliseconds (ms), depending on call quality requirements. Jitter is the variation in delay between packets. The jitter buffer holds packets so that they are received at consistent intervals. Significant jitter may cause the jitter buffer to increase to the point that delay reaches unacceptable levels. Packet loss occurs when the maximum delay specified in the jitter buffer is exceeded. Packet loss above 5% is considered unacceptable when using the G.711 codec.

Ethernet to Wi-Fi

Once the fundamental principles of VoIP are understood, it becomes important to understand how wireless LANs differ from wired LANs. Perhaps the most important concept to understand is that going from an Ethernet (802.3) network to a Wi-Fi (802.11) network only affects the lowest two layers of the network. From layer three up, wired and wireless LANs are exactly identical. In the VoIP world, this means that IP, UDP and RTP are used in exactly the same manner on a Wi-Fi network as they would be on an Ethernet network. To solve wireless VoIP problems, we must look at how the intrinsic nature of an 802.11 physical layer and data link layer will affect delay, jitter, and packet loss for VoIP communications. I've identified eleven (11) 802.11-specific topics that should be understood in order to resolve problems on a wireless VoIP deployment.

WLAN Capacity

The worst-kept secret in networking is that the throughput of Wi-Fi networks never comes close to hitting the advertised 54 (Mbps) rate. A far better-kept secret is why this is happening and exactly how it affects applications that run on the network.

Whenever a packet of 802.11 data is sent, a series of Interframe Spaces (quiet periods) and Acknowledgments accompany that data. In addition, Wi-Fi networks have a random backoff sequence that allows a wireless AP and the stations that connect to it to share a wireless channel.

Related Courses

Voice over IP Foundations