Wireless

Recent articles, Higher Learning, Higher Speed: Campuses Graduate to 802.11n and Beyond 802.11n: Enterprise WLAN Trends For 2010, describe some of the trends driving the adoption of 802.11n wireless networking in higher education and enterprise campuses.

Moving to a wireless access network offer many benefits, including: flexibility, mobility, energy savings and reduced cabling. However, managing performance in a wireless environment is challenging since wireless bandwidth is a shared, limited, resource that can easily become congested. Bandwidth management is further complicated by the rapidly changing traffic patterns as users move from one part of the network to another.

The sFlow standard is currently supported by most switch vendors and is widely used to provide network-wide visibility. However, sFlow is not limited to monitoring switches. Deploying sFlow capable wireless access points extends monitoring into the wireless network, delivering the visibility needed for effective bandwidth management. The diagram at the top of the page shows how sFlow is used to centrally monitor the performance of the entire wireless infrastructure. When combined with sFlow from switches, sFlow delivers end-to-end visibility into the performance of the entire wired and wireless network.

The first task in managing wireless performance is rapidly identifying areas of the network experiencing performance problems.  Each wireless access point uses sFlow's scalable "counter push" mechanism to export utilization and error statistics, allowing a central sFlow analyzer to rapidly pinpoint overloaded access points (see Link utilization).

(chart created using sFlowTrend)

The trend chart above shows a sharp increase in transmission failure and retry counts, indicating a severe performance problem. The next step is to find the source of this congestion. Each wireless access point uses sFlow's packet sampling mechanism to export packet headers, allowing the sFlow analyzer to identify sources of traffic.

(chart created using sFlowTrend)

The chart above shows the top connections and protocols making use of the wireless access point. Looking at the top connections chart, it is clear that the increased load (shown in red) is due to an afpovertcp connection between dchp0 and dhcp6. The traffic associated with this connection is peaking at nearly 30M bits/s, resulting in poor performance. Using the information from the chart to install a rate limit in the wireless access point provides a short term fix, restoring network service.

Further investigation reveals that a laptop is using the wireless network to backup its entire hard drive. A longer term solution uses traffic measurements to develop traffic shaping policies that balance the requirements of different traffic classes. In this case, creating a low priority class for backup traffic helps prevent future quality of service problems.

In this example, sFlow was used to manually identify and manage traffic. However, the real-time, network-wide visibility that sFlow provides makes it possible to automate performance management, ensuring fair access to all network users (see Network edge).