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Table Of Contents  The TCP/IP Guide
 9  Networking Fundamentals
      9  Network Performance Issues and Concepts

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Simplex, Full-Duplex and Half-Duplex Operation
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Quality of Service (QoS)

I mentioned in my discussion of common network performance measurements that there were many different aspects to network performance. I also introduced the concept of latency, which measures how long it takes for data to travel across a network. Latency is one important part of a larger issue in networking that is sometimes called quality of service or QoS.

The inherent nature of most networking technologies is that they are more concerned with pumping data from one place to another as fast as possible than they are with how the data is sent. For example, the Internet is designed on top of the Internet Protocol, a packet-switching technology that is designed to get packets from point “A” to point “B” in whatever way is most effective, without the user necessarily having any ability to know what route will be taken. In fact, some packets in the same data stream may be sent along different routes. Packets may be stored for a while before being forwarded to their destination, or even dropped and retransmitted.

For most applications, such as simple file or message transfers, this is perfectly fine. However, there are applications where this sort of service is simply of “too low quality”. In these cases, the nature of how the data is delivered is more important than merely how fast it is, and there is a need for technologies or protocols that offer “quality of service”. This general term can encompass a number of related features; common ones include the following:

  • Bandwidth Reservation: The ability to reserve a portion of bandwidth in a network or interface for a period of time, so that two devices can count on having that bandwidth for a particular operation. This is used for multimedia applications where data must be streamed in real-time and packet rerouting and retransmission would result in problems. This is also called resource reservation.

  • Latency Management: A feature that limits the latency in any data transfer between two devices to a known value.

  • Traffic Prioritization: In conventional networks, “all packets are created equal”. A useful QoS feature is the ability to handle packets so that more important connections receive priority over less important one.

  • Traffic Shaping: This refers to the use of buffers and limits that restrict traffic across a connection to be within a pre-determined maximum.

  • Network Congestion Avoidance: This QoS feature refers to monitoring particular connections in a network, and rerouting data when a particular part of the network is becoming congested.

So, in essence, quality of service in the networking context is analogous to quality of service in the “real world”. It is the difference between getting take-out and sit-down service at a nice French restaurant—both cure the hunger pangs, but they meet very different needs. Some applications, especially multimedia one such as voice, music and video, are time-dependent and require a constant flow of information more than raw bandwidth; for these uses, a burger and fries in a paper bag just won’t cut the mustard. J

Key Concept: The generic term quality of service describe the characteristics of how data is transmitted between devices, rather than just how quickly it is sent. Quality of service features seek to provide more predictable streams of data rather than simply faster ones. Examples of such features include bandwidth reservation, latency minimums, traffic prioritization and shaping, and congestion limitation. Quality of service is more important for specialty applications such as multimedia than for routine applications such as those that transfer files or messages.

To support quality of service requirements, many newer technologies have been developed or enhanced to add quality of service features to them. This includes the ability to support isochronous transmissions, where devices can reserve a specific amount of bandwidth over time to support applications that must send data in real time. One technology that has received a lot of attention for its quality of service features is Asynchronous Transfer Mode (ATM). ATM is designed to support traffic management features that are not generally available on networks not created to provide quality of service features (such as Ethernet)

Note: “Quality of service” has become a big buzzword, lately. By itself, this term conveys about as much useful information about what the technology offers as being told that it is “high performance”. You have to dig past the marketingspeak and find out exactly what QoS features are being offered.


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