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Seperate Cables to Phones or Not?

Bill H

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I would like to hear from any of you out there with PBXnSIP as well as any other IP PBX system installations.


The questions I have are: Using an in-house network with no remote locations connected.


How many of you require a seperate wire run to the IP Phone as opposed to "sharing" a wire with the computer?

What condition would exist where this is needed?

What advantage or disadvantages are there?


Is it necessary to have the IP Phones and PBX on a completly seperate network, switches, wires and all?

Or will it work Ok over an existing network?


Lets say there are 25 computers and 25 telephones in this example and we are only concerned with the internal communications.


Bill H

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Well, that is a long discussion.


The main topic is QoS. Most phones that have a switch built-in have no problems with the switching load, as it is done by a piece of hardware. In order to ensure QoS, you practically need VLANs. Many companies solve the problem by just putting all VoIP gear into one VLAN and leave the rest in the VLAN 0 - which is the regular LAN. Then if you tag the VoIP VLAN with a higher priority, you can be sure that all packets make it.


You can also use phones to perform the tagging work. As many PC do not natively support VLANs, it becomes the job of the phone to put the PC into a specific VLAN with a specific priority.


And once that you have different LANs (may they be VLAN or LAN), you need to start routing between them. That means, you need to set up a router (or a PC that can act as a router) to send the packets back and forth between the LANs. This might become a little bit tricky as well.


Also, usually you need to operate a DHCP server in each VLAN. If you run a PC with different VLANs attached, you possible can use the same DHCP server for the different LAN.

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I believe the Mitels have a function whereby you can specify a DHCP option which tells the phone to tag its frames with VLAN X. I know the 802.1Q VLAN is an option with Snoms, can this also be done using DHCP, or does it require a combination of DHCP and provisioning?

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Quality of Service is the main reason for doing this... and this all depends on the use of the network in question.


We have a client on a different (nonPBXnSIP) installation that handles about 30k external calls per month on G711 codec running softphones and IP phones on the same network as the rest of the companies data traffic... There is no problem with this configuration and we have no QoS or VLANs in place... The key here is that the 10/100 network is totally underutilised and no single or group of users overloads their network switch or link in any way...


Conversely to this, we have a demo client on PBXnSIP with a main office and a separate building 2km down the road with a wireless link between them... Before QoS was installed on the wireless link, they had problems making/recieving calls over the wireless link intermittently - this was 100% attributed to running out of bandwidth on the link and it was not related to phone calls but to people dumping large files over the network link overloading it... with QoS in place, at least the calls improved, but bandwidth remains a problem...


We also have a client who runs a media house - doing video and design work - where they regularly transmit 5GB+ size files over the GB network... This site has VLANs and complex network setups to handle this problem and it was decided up front to use a separate routed IP network for the VOIP from the start because there would be no chance calls would be able to compete with the existing traffic on the same network ... The only downside in this entire scenario is that softphones running on users machines had to use the data network, whilst hard IP phones used the dedicated network.


Summary - it all depends on the network use....


Hope this helps a little for explaining it in less technical terms...

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  • 5 months later...

Our experience is that using good managed switched Cisco/Linksys SRW224P with QOS enabled on all ports we've have great luck with phones with PC's attached to the phone switch port SNOM 320's for example. This setup has been 100% functional including when we had a Wireless 54G bridge with 20 computers and phones in a remote building 1/2 mile away. We always use some sort of SNMP management tool to monitor all ports on all switches and we closely monitor PBXnSIP via WMI/SNMP on windows boxes and the Switch port stats...


VLanning is simply not needed, however we are looking into a very large deployment and VLAN may be the only / best way to deploy and we'll advise as this progresses.


Out installation has experienced call counts in excess of 45,000 minutes per month, with average call lengths of 2 minutes meaning we were taking 22,500 calls in a given month M-F 8-5 pm.


Smart - Managed Switches with 802.1P/Q enabled can easily handle the small loads RTP adds to the LAN.

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