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A phone can indicate that its traffic is important by marking traffic at layer three.

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Now at layer three, we would be using an IP version four header or IP version six header.

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In this course, we'll concentrate on IP version four.

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In an IP version four header, there's a field called type of service.

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It consists of eight binary values.

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Now.

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In the old days, we used to only look at the most significant three bits.

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So these three bits of the header were read by routers and networking devices to determine how important

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the traffic is.

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Now, in the same way as cause, the values are from zero all the way up to seven, so 012 all the way

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up to seven.

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Voice once again would be marked with a cost value of five.

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So a phone as an example, would mark its traffic at layer two and then market at layer three to indicate

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how important the traffic is.

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The problem with this mechanism is we only have seven classes.

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Class zero would be used for best effort.

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In other words, you're not going to get a decent quality of service and often six or seven or both

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are used for network protocols.

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So you are limited in the number of classes that you can use for traffic.

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So voice may be using five, video may be using four.

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What are you going to use for ERP or business traffic versus HTTP traffic versus other traffic on your

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network?

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You're running out of classes and in addition, the classes weren't standardized, so any traffic could

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end up in any class.

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So this was adjusted to use six binary bits and this is known as DSP or differentiated services code

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points.

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It's the same field in the IP version for header.

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It's just the interpretation of the bits rather than using three bits.

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Six bits in the header are read and marked to determine how important the traffic is.

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This is known as IP precedence.

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So when only three bits are used, it's known as IP precedence.

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When six bits are used, it's known as differentiated services code points or DHCP.

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Now DHCP is backward compatible to IP precedence.

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These values in DHCP are known as class selector values or C values.

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So you may see someone talk about this being IP precedence five, but they may also mention this as

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class selector five.

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It depends whether we're using the old naming convention of IP precedents or or the newer DHCP descriptions.