How important is QoS on a local network? Everything is set up using 100Mbit switches, and I will be using analog lines to connect to the outside world. No more than 40 phones, but only 4 right now. I guess my question is, if someone decides to transfer a 2GB DVD image off our file server onto their workstation, will that impact any calls going on at that time?
This answer depend on many factor but there is two way I think to see QoS:
- Cross your finger
- Try to manage it
it really depends on your network and what you will be doing. If you have 40 users doing nothing but email/web/word/excel/CRM then you probably won’t have a problem. As you say if they start swapping DVD images, that might slow things down.
Remember though that switches don’t saturate the link with other people’s traffic- so as long as the guy who wants the DVD isn’t the one who needs to use the phone (and the image isn’t coming off the * server) it will still be fine, as port a->b does not affect port c->d traffic.
Where you might run into problems is with Uplinks- when you connect many switches together. If the links between them are only 100Mbit, then that creates a bottleneck. Since you only have 4 users at the moment it shouldn’t be a problem though.
I recomment get a ~16-24 port switch, unmanaged for now. Once you grow beyond that- get a good ~48 port managed switch with QoS and all the trimmings, as you will then be able to afford it 
What I will recommend for your situation and for anyone’s general purpose is if you can upgrade to a gigabit network you should absolutely do it. And to maximize throughput for your voice traffic there’s nothing better than prioritizing your voice traffic over data and if possible put your voice traffic on a seperate VLAN so it does not get choked up by other data traffic. In this case there will be no need for finger crossing.
This in your case will place your voice traffic ahead of any other traffic such that DVD image downoads will not matter as long as it is (1) Data traffic has a lower priority and (2) Data traffic is on a different VLAN which does not see your voice VLAN and therefore cannot send/receive or broadcast packets or any kind of traffic to the voice VLAN.
Bottom line is with voice integrated into your LAN, QoS is a big issue because delays, jitters, latency, cadence, etc is not tolerable (by voice traffic) at all, defering those issues to the data side of the network is advisable since it is not noticed by the user.
Thanks for all the help. I’m stuck with 10/100 for the network, as all the wiring was inherited when we moved into the building. Our switches have gigabit uplinks though. And I’m starting to replace all of our dumb switches with smart ones. The only thing I see as an issue with VLANs is the desk will have one ethernet wire running to it and the phone will just daisy chain from that, so I can’t separate the network like that, but I should be able to do QoS on the switches.
On a side note, has anyone had any experience with these switches? I use the dumb versions of them and they seem to get the job done well enough.
Of course you can seperate them if your VLAN is well setup. The cable at the desk should go to the phone first, then the phone feeds to the PC (not vice-versa) so that the phone gets the best possible bandwidth, besides most PCs don’t have daisy-chain ports for phones but phones do have them. What you need to do is configure a MAC based VLAN i.e. put the MAC addresses of all your IP phones in a seperate prioritized VLAN and the PC MAC addresses in a different (less priority) VLAN. Doing this, it wouldn’t matter where anything connects to the network, the switch would discover the individual MAC addresses and put the devices in the appropriate VLAN. The other more cumbersome way is to assign the different ports on a switch to the different VLANs, the thing with this method is you will have to keep good documentation on what port belogs to what VLAN, which port a phone should go and which is for a PC. Moreover daisy chaining does not work well with this method because it will assume both daisy-chained devices on the same port belong to the same VLAN.
Depending on the phone, you maybe able to set the VLAN tag on the phone itself and then set a different default VLAN tag on the switch port. Then any traffic from the phone will include the VLAN tag and the switch will send that traffic to the switch ports associated with the phone system VLAN. The traffic from the PC will not have a VLAN tag, so the default VLAN tag of the switch port the PC is connected to would be used. This will seperate your phone traffic from the PC traffic at the switch port.