i think it has to do with Quality of Service:

http://www.netcraftsmen.net/component/content/article/68-network-infrastructure/811-qos-for-ip-telephony-ip-videoconferencing-and-microsoft-unified-communications.html

http://www.netcraftsmen.net/resources/archived-articles/506.html

I read that article as saying that the network traffic from the PBX comes from  a Tlan, Elan or Clan and that traffic needs to use QOS. Sure voice traffic should be prioritized over data with QOS but I am asking what the difference is between a Tlan, Elan and Clan.

If I had to guess I would guess that the Tlan is the actual voice traffic and the Elan is the call setup and control. Maybe Elan and Clan are two terms for the same thing?

I found some good information on another site.

http://www.tek-tips.com/viewthread.cfm?qid=1612427

Crowtalks (TechnicalUser) 17 Jul 10 19:04  
It's recommended that you keep your TLAN separated from the customer's CLAN, by a router.

That way, when you are running screwy IP phone trouble in the future (and you will) and the customer's IT department guys try to blame your system, you can plug an IP phone into your TLAN and prove that it works fine on your LAN and that they must have issues in their LAN which is probably sharing traffic between phones and other devices.

That's the main reason for the distinction between TLAN and CLAN
NCSS  NCTS  NCTE
 
 

peterdonnelly (IS/IT--Management) 18 Jul 10 5:51  
My understanding is as follows:-

ELAN is the nortel private LAN.  No customer equipment needs to be able to see this LAN /VLAN.  The ELAN can be but doesn't need to be routable.

TLAN - is another nortel LAN but this LAN needs to be routable so that the IP phones can talk to the SIG Server / Call Server / Callpilot.  

CLAN - This is the customer LAN.  Hosts and whtever else.

Personally, I like to have another network/vlan for the ip phones thenselves.