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QoS is the measure of transmission quality and service availability of a network (or internetworks).

Service availability is a crucial foundation element of QoS. The network infrastructure must be designed to be highly available before you can successfully implement QoS. The target for High Availability is 99.999 % uptime, with only five minutes of downtime permitted per year. The transmission quality of the network is determined by the following factors:
•Loss—A relative measure of the number of packets that were not received compared to the total number of packets transmitted. Loss is typically a function of availability. If the network is Highly Available, then loss during periods of non-congestion would be essentially zero. During periods of congestion, however, QoS mechanisms can determine which packets are more suitable to be selectively dropped to alleviate the congestion.
•Delay—The finite amount of time it takes a packet to reach the receiving endpoint after being transmitted from the sending endpoint. In the case of voice, this is the amount of time it takes for a sound to travel from the speaker’s mouth to a listener’s ear.
•Delay variation (Jitter)—The difference in the end-to-end delay between packets. For example, if one packet requires 100 ms to traverse the network from the source endpoint to the destination endpoint and the following packet requires 125 ms to make the same trip, then the delay variation is 25 ms.
Each end station in a Voice over IP (VoIP) or Video over IP conversation uses a jitter buffer to smooth out changes in the arrival times of voice data packets. Although jitter buffers are dynamic and adaptive, they may not be able to compensate for instantaneous changes in arrival times of packets. This can lead to jitter buffer over-runs and under-runs, both of which result in an audible degradation of call quality.

Translation:

In packet tracer, I’m going to start simulation mode and then on PC 1, I’m going to open up a web browser and go to Cisco.com.

Now the first thing that PC needs to do is determine the IP address of Cisco.com.
So it’s going to send out a DNS request message for Cisco.com. So scrolling down here notice we’ve got a DNS query for Cisco.com. Capture forward; that gets sent to the router. When it gets sent across the serial link, notice the DSCP is set to zero. No quality of service is applied to this packet. So that will go to the DNS server.

DNS server will reply back. Packet will go back to the PC and now the PC can send TCP traffic to the router.
PC is going through the three way handshake.
Notice now we get HTTP traffic.
Notice on inbound, HTTP traffic, in this case we’ve got an HTTP request. Doesn’t have DSCP markings but on outbound, out of the router the DSCP is set.

So capture forward, notice how this packet has a little square here showing us that DSCP was set on ingress DHCP is also set on egress.

So capture forward, what you’ll notice is, we have this little square to indicate that quality of service value has been specified on the packet….