VoIP Question

L

LittleMe

2[H]4U
Joined
Feb 20, 2001
Messages
2,977
We're starting the process adding VoIP to our NEC system and we're getting ready to purchase the switches. We're going to put all our VoIP clients on a seperate network from our data. We've talked with another school district about their choices and found out they purchased Gigabit switches for their VoIP clients and didn't setup any QoS. Their reason being that, all the traffic is on a standalone network anyway and it being Gigabit should be enough to prevent flow problems.

My boss is looking at going with 3560G-48PS-E's and setting up QoS. I'm leaning towards going with 3560-48PS-E's and setting up QoS. Difference for us is around $4,000 per a switch.

So what I'm asking is, do we really need the Gigabit for VoIP, shouldn't 10/100 be just fine? Can anybody point me to some doc's about VoIP on Gigabit vs 10/100?

Thanks
 
That seems like a waste of money to buy a physically separate infrastructure just for the phnoes. And considering a single call will be at the most (using g711 including headers) around 87kbps, gig ports just for a phone is overkill. I would revisit your design and implement QoS on your existing data infrastructure to prioritize voice and signaling traffic and place the voice traffic in a separate vlan.

Her eis a good document showing the different bandwidth utilization for the different voice codecs:

http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html
 
Impulse25 said:
That seems like a waste of money to buy a physically separate infrastructure just for the phnoes. And considering a single call will be at the most (using g711 including headers) around 87kbps, gig ports just for a phone is overkill. I would revisit your design and implement QoS on your existing data infrastructure to prioritize voice and signaling traffic and place the voice traffic in a separate vlan.
Click to expand...

Agreed. With QoS and a little planning, your traditional 10/100 network will work just fine.
 
Are you planning on speaking in Hi-Def? :p

Take into consideration this little tidbit: a T1 consists of 24 56kbps channels. that means you can get 24 active phone calls on a traditional PBX, with perfectly fine voice quality.
(companies have operated on this for years, and still do)

essentially, you'd need at least 56k per phone, to maintain "traditional" quality.

(like everyone else said) You're current network with some good QoS would do it just fine.
 
Thanks for the link and information everybody. To followup a bit. The only reason for buying the switches is because we're actually adding a new location where this is going. It's a new site being built and connected to our old site via fiber. There will be two NEC boxes linked together across the fiber so we can keep extensions between the sites but only the new site will have VoIP clients. We have to buy switches for the new site. With the ports needed, it's either buy 5 3560G's for data and VoIP, or try and save and buy 3 3560G's for data and 2 3560's for VoIP.
 
The whole point (or at least one of the biggest reasons) for voip is cost savings due to using the same infrastructure for data as you do voice. Putting 2 total different data infrastructures in place is a total waste of money.

Most of our infrastructure is Gig uplink and 100 Mb clients with the end users PC plugged into the phone. Each chassis has about 250+ clients (Cisco 6509 chassis).

Actually G.711 takes about 87K accounting for voip overhead.
 
KaosDG said:
Take into consideration this little tidbit: a T1 consists of 24 56kbps channels. that means you can get 24 active phone calls on a traditional PBX, with perfectly fine voice quality.
(companies have operated on this for years, and still do)
Click to expand...
Actually when using circuits like that for voice you loose one channel for signaling purposes, so you get capped out at 23 on a T1.
 
moetop said:
I also thought the channels were 64K
Click to expand...

well, theres some confusion too on that... (I admit it confuses me)

a PRI (as i thought it was) is 23B + 1D channel, so 23 64k channels, with 1 channel to signal them all (lol LOTR)

and a t1 was 24 56k channels, with 8k per channel stolen for signalling?

no?

should I go back under the rock?

it's all a blur to me since i became management :(
 
There are two different signalling methiods, one is CCS, Common Channel Signalling, which uses one channel out of the 24 DS0's in a T1 to handle all signalling. Then there is CAS, which is Channel Associated Signalling (I think, not sure), which actually inserts signalling data into the data portion of each frame allowing the use of all 24 channels for voice calls.

Anyway, regarding the number of switches, do you know if your phones support gig as well? If they do, just get the three gig switches as the PCs will plug into the phone and the phone then into the network. If your phones don't support gig, then is gig to the desktop needed at this location? If not then I'd just go with three 10/100 switches.
 
Thanks Impulse25, I didn't know about CAS only CCS. I must admit I really don't know to much about this stuff.
 
We just got the phones in today and I saw them this afternoon. They do not support a gig, so there's another reason not to buy gig switches for them now. Gig will be used on the desktops though.
 
You must log in or register to reply here.
Back
Top