Getting Jittery
There are four major reasons for implementing QoS:
• Latency, the amount of time it takes for a packet to get from one place to another, may be caused by bandwidth saturation, lack of resources (CPU or RAM) on a network device, distance or type of connection. You can reduce latency only so much--there's no way to send a message and receive a reply via a satellite link in less than 500 ms, for example.
• Jitter refers to the variance in latency. Unless two nodes are on the same switch, latency will vary greatly from packet to packet. When network bandwidth is saturated, jitter increases. For applications such as file downloads or Web browsing, this is not usually a big deal. However, streaming video and VoIP suffer greatly from high jitter. QoS can be used to help even out jitter by giving streaming traffic a higher bandwidth priority. Another solution is to increase buffer size.
• Random packet loss, which occurs when networks or devices are oversaturated, causes clipping in streaming media, reset and dropped connections, and other transaction difficulties. Worse, dropped packets need to be retransmitted, compounding the problem. QoS methods can limit the amount of bandwidth a protocol or connection uses, thus preventing or limiting oversaturation.
• Controlling bandwidth use is the final reason for QoS. Traffic like FTP and streaming video can siphon off bandwidth like a napkin sponging up grease from a pepperoni pizza. Peer-to-peer file sharing has caused headaches on most college campuses, with some campus networks experiencing 100 percent saturation from P2P traffic alone. Enterprise networks may find that remote-control software is slowing the responsiveness of the Web servers. If fast Web server responsiveness is important to the company's image, this is an issue.
Being able to control bandwidth is especially critical when there isn't much to go around. QoS can give priority to business-critical applications, like Citrix, network-management tasks or database queries, while leaving the leftover for less important activities.
Never underestimate the damage a Web browser can do to a network: Most QoS devices will let you specify minimum and maximum rates per protocol, while advanced products let you specify minimum and maximum rates per session. You could say that every individual streaming video session will get a minimum of 100 Kbps, for example, but all the streaming video sessions combined can't use more than 1 Mbps. You can also use QoS to give certain users preferred status.
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