On Mon, May 2, 2011 at 9:38 AM, Tom Miller <[email protected]> wrote: > - our vendor claims our current data network can easily handle VOIP traffic > since it's a small amount of traffic
Telco TDM voice is 64 kilobit/sec per channel. VoIP is that or less (VoIP uses compression), often 50 Kbit/sec or less. What's critical is not so bandwidth but latency (packet delay) and jitter (packet delay variation). You want your voice traffic given higher priority than your data traffic. Otherwise a momentary data burst (e.g., large file transfer) can disrupt voice calls. Managed switches that implement QoS are thus a must. Preferably, one implements a separate VLAN for phones. You give that VLAN a higher priority. This also isolates traffic, which can help security and keeps problems on the data side from screwing up your voice side (or vice versa). We even use our phone system (Nortel BCM) as the DHCP server on the VoIP VLAN, so even if our regular servers were to all suddenly die, the phones would keep working. Or if the BCM does down, the PCs still get DHCP from the Windows server. The phones figure out which VLAN to use via LLDP MED (Link-Layer Discovery Protocol - Media Endpoint Discovery). The switch sends LLDP Ethernet frames and the phone reads them. On our HP ProCurve switches, this is done just by designating the VLAN as "voice" in the switch config. The phones then use tagged frames for their traffic. The daisy-chain port to the PC passes untagged frames, and the PC is none the wiser Once the phones are on the right VLAN, the DHCP server on the BCM gives them all the other DHCP options they need -- base IP config, and the Nortel-specific DHCP options to tell them to use the BCM as the call server. Be aware that this makes your phone system dependent on your Ethernet switches and their support infrastructure. Most people know this at some level, but there can be subtle ramifications. For example, if you had a big batteries for your phones but shorter-lived batteries for your Ethernet switches, you'll need to upgrade your the UPSes on your Ethernet switches to keep the same phone runtime. Or if you had a 4-hour support contract on the phone equipment but have next-day on the Ethernet switches. > - our vendor claims we can use the existing data jack for the phones, and > plug the desktop PCs/laptops into the phone as a sort of switch. I'm > thinking this would add another level of complexity: phone is broke and by > the way you can't get on the network now. Many (most?) VoIP phones include a built-in 3-port Ethernet switch. One is for the uplink to the infrastructure, one is to connect a PC, and one is internal to connect the phone. It's your call how you want to do this. You're right in that it makes the phone a point-of-failure for the PC. On the other hand, you may cut your main switch port count in half, and you may save significant wiring costs. YMMV. I know our Nortel phones say they can only handle one MAC on the PC side. So you can't, for example, plug another switch into the PC side, and then hang both a printer and a PC off the phone. > - the reason the vendor suggests the above is that the current voice drops > (cat5) terminate to phone patch panels (in most cases). Those cables would > need to be cut and re-terminated to switches. By "phone patch panels" I assume you mean 66, 110, or similar punch-down blocks[1]? If so, your vendor is on the right track. Such blocks are not suitable for Ethernet as-is. They're often not built to CAT5 spec, and even when they are, they're rarely terminated that way. So you'd have to re-terminate all the cables. See above about wiring costs. [1] http://en.wikipedia.org/wiki/Punch_down_block#Gallery -- Ben ~ Finally, powerful endpoint security that ISN'T a resource hog! ~ ~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/> ~ --- To manage subscriptions click here: http://lyris.sunbelt-software.com/read/my_forums/ or send an email to [email protected] with the body: unsubscribe ntsysadmin
