VOIP IP Phone LAN

[robmurphy]

Please not description below is for an office in the UK.

At present in the office we have about 20 Linksys SPA921/941 phones on a hosted PBX. The phones connect to a Cisco Catalyst 2960 48 port switch.

At present the switch carries voice traffic only. The switch did have 2 port base VLANs setup so that the phones were equally spilt across 2 DSL routers. The split was done as 20 phones on G711 is to much for the uplink. all phones active would be about 1.7 Mbs uplink. It was also spilt so we could test DSL connections from different providers. What this did mean was that we had to re-patch cables to the switch when someone moved.

The 2 VLANS connect to different subnets, one 192.168.x.x and one with 30 public IP address on it. The default VLAN is used to connect to the switch for management, and connects to another 192.168.x.x subnet

To resolve the re-patching problems I have changed some of the ports from access mode to trunk mode, and made that port a member of both VLANs. The VLAN tagging is 802.1q. The phones have been set to use the VLAN ID, and the ID on the phone set to the appropriate VLAN ID.

I have not used the cisco voice VLAN feature, mainly as I do not fully understand it, and that appears to only use 1 VLAN ID. I need at least 2, and will be adding 2 more in the near future, as we have 2 other DSL circuits to test.
This appears to be working fine.

What I would like to know is are there any settings regarding the ports I should set.

I would also like to know what the switch will do if a device not setup for VLANs (like an IP Phone with the VLAN ID set to off or a Win 7 laptop/Desktop) is connected to one of the trunk ports?

From what I have seen with the VLAN ID set to off on the phone it still connects, and gets frames from the VLAN with the highest ID.

What I would like ideally is that when the trunk port is connected to a non trunk port that it gets connected to the default VLAN on the switch. That way the phone could reach the management network. DHCP and tftp could then set the config of the phone, including setting the VLAN ID up.

Rob.

 

[Crusty]

If your device doesn't have a VLAN set on it then the packets will be untagged and will fall onto the native VLAN on the port. So you need to make your trunk ports use your default VLAN as the native VLAN on the trunk port so when a non vlan device connects it can use the default VLAN.

I'm not sure how the 2960 does it, but with my ProCurve 2810 switches I just set my default VLAN to 'untagged' on the trunk port, and the rest of the VLANS to 'tagged' on the same port.

 

[drebo]

Ack, your network is way screwed up. You need to do some major work on it.

First off, 20 phones is too much for a hosted PBX. Your estimate of 1.7 Mbps is off. For 20 phones, you're actually looking at closer 2.5 Mbps, after all signalling is taken into account. You should be using g.729 at the very least.

Second, the Cisco Voice VLAN feature allows a phone to talk to the switch and automatically negotiate the VLAN on which the phone itself will talk. Any device then plugged in to the built-in switch on the phone will use the regular access VLAN configured on the switchport. Voice VLANs are configured per port, so you can have as many as you want (limited, obviously, to one per port). Voice VLANs are far more preferable to trunk mode ports, even if you have to manually configure the VLAN on the phone.

Untagged traffic entering a trunk port will be assigned to the native VLAN of that trunk port.

You should really be using a layer 3 switch for your setup, as convoluted as you've made it.

It could be vastly simplified, though, if you were to get someone in there with a bit of experience with VOIP and network engineering.

 

[robmurphy]

I'd like to find someone with more experience in VoIP than me, but they are not that common. I've been working in VoIP over 5 years now. I've dealt with many other engineers, but VoIP experience is not that common here. I'm the only engineer here with VoIP experience.

The company policy is to use the same as it sells to its customers. 20 users on a hosted PBX is fine, that is why the phones are spilt across 2 DSL links. The DSL links have 1.6 and 1.7 Mbs uplink.

We are at present using G711 as that is what most of the Hosted PBX systems like. Most of them are very wary of G729. We may move to this later. The SIP signalling does not actually use that much bandwidth, and also most of it occurs before the media is sent in both directions. For example apart from message to indicate answer the last messages sent when originating a call are the status 180 and status 183 messages. With a good Hosted PBX provider the RTP for the ringing phase of the call should only start after these messages. That only leaves the call clearing messages. There are also some background SIP messages these and RTCP may add some more overhead but not that much.

I think the figures you are using for bandwidth are using 10ms periodicity for the Codec. I have set the usual 20ms for the phones. Depending on how much of the ethernet frame is allowed for the bandwidth used for G711 at 20 ms periodicity is between 80 Kbs and 87.2 Kbs, see: http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml#topic1

If you use 10 ms periodicity then you get about 100 to 110 Kbs for G711. As said we are using 20 ms. The defaults for the SPA series phones is actually 30 ms periodicity. So even if the users are not that well split, say 8/12 gives 1046 Mbs for the 12 user DSL and about 700 Kbs for the 8 users. This also assumes that the DSL line uses about the same amount of header information as Ethernet does.

We also use the office connection to test different DSL suppliers as stated before. This means each port on the switch will need to be able to deal with phones with at least 4 different VLAN IDs on them. One of the requirements of the work is to have the office so that any of the SPA phones we use can be connected into any of the Voice LAN ports. People move desk, or areas re-organised on a frequent basis.

I did some more research on the Cisco Voice VLAN feature and it would appear it will not work with the older SPA phones. The phones used are SPA921/SPA941. As far as I understand it Cisco has a protocol (not sure if this is done by CDP) that tells the VoIP phone which VLAN to use. The Firmware level required by that feature is well past what is available for the SPA921/941. The latest and last firmware for these phones is 5.1.8.

We do not have an L3 switch, and I need to meet the requirements with what we have available. A 48 port L3 switch is not exactly cheap, and I'm not sure what advantage it would have. There is no L3 switch needed by the phones. With the hosted VoIP in the UK the RTP is always to/from the hosted PBX over the DSL link even for local extension to extension calls. What advantage for the phone network would the L3 switch give? The phones L3 routing is done by the DSL router.

It would be nice to see what some else suggests using the equipment available e.g. SPA921/941 phones and a Cisco Catalyst 2960 48 port switch. The requirements are:

- Be able to split the phones across different DSL links.

- Be able to move the phones to different links in the office without re-patching, and have the phone connect to its associated DSL link.

- It would be helpful if when an SPA921/941 was connected with the VLAN feature turned off that the Phone connected to a different VLAN, this could be the default, or another VLAN.

Just saying get different phones and a 48 port L3 switch is not an answer. The phones will change as the company progresses. A different switch may be used. But for the present the phones still need to work, be split across multiple DSL links, and be movable without repatching.

Please also note that ALL the VoIP projects I have worked on used VLANs. There does seem to be a dislike of using VLANs on this forum. As things are in the UK with VoIP and converged voice/data LANs VLANs are used all the time.

Rob.