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Teach me QOS

hi! i was wondering whats actually a QOS and whether we do need them? And if we really do need them, how do i set them in my 3Com Wireless Modem Router - 3CRWDR101A-75

BE - Best Effort Forwarding 0%
AF1x to AF4x - Assured Forwarding, provides delivery of packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. 0%
EF - Expedited Forwarding, is intended to provide low delay, low jitter and low loss delivery of packets. 0%

How many percent should i set them and the reason?

thank you,
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giltjr
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QOS is generally used when you have mixed traffic types and one or more of those types of traffic MUST have its data get through.  Generally this is implmented when you are using VOIP.  If a HTTP, FTP, Telnet, or other non-VOIP connection drops a packet and it must be resent, the end user may see a small response time delay.  However if a packet is droped in a phone conversation the phone converstaion is not clear and very difficult to understand the other party.

So, in a network where you have VOIP, you use QOS and set VOIP as the highest/most important traffic so that there are no packets dropped.

But remember, this means that traffic that is considered of less importance could have packets discarded under high (very high) network stress.

If you don't have VOIP, then you get down to what traffic you have on your network and if you really want to give one type of traffic priority over another.
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aboka

ASKER

hi! so you mean basically QOS is to ensure that the packet gets to the destination - when its corrupted, etc, it will re-send?

And what does this means and how do i set them -
BE - Best Effort Forwarding 0%
AF1x to AF4x - Assured Forwarding, provides delivery of packets in four independently forwarded AF classes. Within each AF class, an IP packet can be assigned one of three different levels of drop precedence. 0%
EF - Expedited Forwarding, is intended to provide low delay, low jitter and low loss delivery of packets. 0%

And how do XP assign the priorities, since it only have QOS and not the option?

thank you,
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giltjr
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ASKER

hi! sorry for late reply, i have exchange my 3com modem router for another model - without the QoS settings. So we couldn't really test it out. But anyway, i would like to ask, hows the priority is set? Is it assign by the request program?

thank you,
Typically it is set either by the NIC driver or the switch/router and it is based on the port.  Some NIC's do not support QOS settings, so the first switch/router the traffic hits within the network will do this.

It is not all that bad that the NIC's don't support it, because in some cases it will not make a difference.

Say you have a Desktop PC and a VOIP phone in your office and there is on LAN connection.  There are couple things you can do:

    Install a cheap switch to connect both the PC and the phone
    Run two connections to your office
    Some phones have a switch built into them and so you connect the link to you phone and your PC to your phone.

In all of these situations it does not matter if your PC can do QOS or not, because the traffic to/from would typically all be the same priority.

If you were doing VOIP on your PC, then it would make a difference, as your PC would need to prioritize outbound traffic.