11.6 SNMP on Wireless Networks

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Although they share standard elements and mechanisms, wired and wireless networks have significant differences. In addition to the conventional wired network, wireless networks have the following unique issues:

• Secondary hierarchy. The wireless network environment is hierarchical, with portable devices associated to a given access point. Standard network management products designed for wired networks often do not represent this critically important tiered topology network structure and are more likely to represent the wireless network as a flat topology.

• Roaming. The wireless network environment supports dynamic cell connection or roaming, which is the process of changing the network connection of a portable device from one access point to another as the user changes location.

• Persistence of mobile units. In an environment comprised of portable devices, the devices’ persistence also becomes a factor. Unlike desktop systems or other network components that operate continuously or are powered on/off daily, portable devices typically are turned on and off frequently throughout the day, making it difficult to monitor them.

• SNMP agents. The ability of devices on wired and wireless networks to host an SNMP agent also impacts network management functionality. Desktop or notebook computers have adequate amounts of memory and processing power to support an SNMP agent operating as a background task handling requests from the management station. With PDAs and other handheld devices, however, memory and processing power are limited resources, making it difficult to provide agents for these devices.

11.6.1 Performance of SNMP

Managing a wireless network is a significantly more difficult task than managing a wired network. One reason is the unpredictable behavior of the wireless channel due to fading, jamming, and atmospheric conditions. This variability of signal quality can reduce the efficiency of the management operation. The bandwidth of wireless links is another issue. Bandwidth will always be limited due to the properties of the physical medium and regulatory limits on the use of radio spectrum, making it necessary for network protocols to utilize the available bandwidth efficiently.

Using mobile agents to facilitate management offers several benefits, including reduced network traffic and efficient utilization of processing resources. On the other hand, the use of mobile agents consumes considerable resources from the agent host, which can pose a problem for mobile terminals like PDAs. And the response time in notifying the network manager depends on the wireless link quality. Response time can vary from several seconds to even several minutes depending on the link condition. Management functionality is unusable in situations with bad link quality and with system load at 20% of the whole capacity of the system.

Unusable means not being able to update the whole MIB tree using SNMP’s get-next operation. Accordingly, a management station can manage several times the number of devices when the signal strength is good than when it is bad with the same system load. The response time increases in an exponential way as the load of the system grows.

Using SNMP’s get-bulk request, on the other hand, provides the means to retrieve much more data about a network entity—as much data as is possible to carry in the response message. This is especially useful when channel conditions are bad because the number of round-trip time-outs is reduced, which is important to maintain performance in a wireless environment. With traffic reduced between manager and agent, the probability of UDP packet loss is also much less.

While measurements obtained through the get-next command are good for short transactions, such as retrieving individual objects when the query and response messages fit into a single UDP packet, if the MIB contains a considerable amount of tables, the retrieval process will involve a high number of protocol data unit (PDU) exchanges over the network, which in the case of bad link quality, could make management performance worse. But using a single get operation makes it much easier to adapt channel conditions for the current transaction. The user can specify relevant options like time-out for the response message or a maximum number of retransmission with respect to link quality.

The SNMP agent itself could provide an approximate indication of the link condition. For instance, information about the number of retransmissions performed by the manager/agent application or maximum delay time on receiving a response measured by the manager could be useful for determining wireless LAN performance.

11.6.2 Platform Support for Wireless

IT departments tend to favor a single management console that integrates wireless management with existing wired infrastructure. Yet with so much change in the wireless industry and the rapid pace of standards development, many established network management platform vendors do not yet support wireless networks. Instead they rely on wireless vendors or third parties to provide management applications that can be incorporated into their management platform to manage the wireless component of the network. With its simplicity, SNMP provides management capabilities from a central point of control that spans wired and wireless environments.

There are NMSs that have been specifically designed for the wireless environment, enabling administrators to navigate, examine, and manage the wireless network Such wireless management systems may even offer seamless integration into larger corporate enterprise network management platforms, such as Hewlett-Packard’s OpenView Network Node Manager, to provide a single management environment. This integration offers IT managers the tools to install, configure, centrally monitor, and manage the entire enterprise, wired and wireless. This approach also helps IT departments avoid the additional expense of purchasing, training, and maintaining a new management tool. A good wireless NMS should offer the following basic features:

• Automated discovery of new access points and remote configuration capabilities;

• Access point configuration to include a group management capability, a configuration database, and standard SNMP support;

• Configuration of the access control list (ACL) to regulate what devices are allowed to associate with an access point;

• Threshold and event notification, whereby designated events trigger alerts before a system problem or failure occurs (there should also be real-time updates on network events);

• Exception-based management to allow key parameters in the wireless LAN to be monitored for values that exceed a predefined threshold, so network operators or support personnel can be notified;

• Single-source report generation for inventory, configuration, and usage;

• Data trending to track and trend network traffic and usage and allow for proactive analysis of network utilization;

• Mapping to provide a concise network representation, including a tree view showing all devices in various levels of detail (there should be a facility for creating custom maps and importing device-specific images);

• Drill-down and component-zooming capabilities to allow information to be expanded as required;

• Intelligent agent technology to monitor and coordinate tracking information from multiple data sources, thereby reducing the amount of management traffic on the network;

• Remote firmware and software revision control to permit the configuration of system components from a central location without having to tie up staff resources with on-site service calls;

• Wireless proxy agent to support non-SNMP mobile devices such as PDAs.

Some vendors offer graphical management systems for their wireless products. Cisco’s Wireless Manager is one example. It is a UNIX-based solution that can be run locally from a server console—or remotely via remote login (rlogin), Telnet, or remote shell (rsh)—from a UNIX workstation, or a Windows computer.

Wireless devices can even be managed through a Web browser. Although the Web management interface may not support remote configuration of any securitysensitive parameters, it gives IT staff more flexibility to monitor network performance from any location that has Internet access. To implement Web management, the browser proxy is configured for direct Internet connection. The administrator then enters the IP address of the target access point in the browser window. The administrator is prompted for a user name and password. Once connected, a navigation tree appears on the left side, providing access to the management features.

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