The question of which measurement approach best meets your needs is answered by determining a combination of factors: on which layer your application is operating, its physical architecture and who needs the data.
You may have a full-blown, multitiered, centrally located e-commerce site, where client experience must be measured in relation to promotional and merchandizing efforts. It's complex, but it's all fed from a central switch, making it possible to set a passive probe on a span port to see all the application traffic.
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Or your application may be required to deliver 256 Kbps of consistent bandwidth for streaming media. You'll benefit from agents on both sides of the link to test that the network is being held steady.
Data needs are usually determined on a parochial basis. If the network folks are buying a measurement tool, they're looking for monitors that search out network faults and clearly prove the network is running well. So, SNMP data collection as well as proprietary switch and router MIBs using agentless remote data collection will be the right choice. Application awareness is limited to knowing the application is running on the server, so a download of a Web page using a robotic active transaction will do.
At the other end of the spectrum, business owners want to measure what traffic is driving certain revenues--a feat that still is not really possible with performance software. What is possible for this group is monitoring key transactions in real time to measure the customer's experience. For this sort of deep application/transactional dive server, agents with specific transactional capability, or probes with the same deep knowledge of your transaction type, are in order. Both are available, but they require professional services and a top-down team effort.
Performance-management offerings continue to evolve with more and more data collection available. The mountain of measurements provided will be useful to your organization. Just be sure you know what to measure before you start.
Bruce Boardman, executive editor of Network Computing, tests and writes about network and systems management. Write to him at [email protected].