What is a trunk or line?

Hi Mwenya,

i have the exact question in my mind also. to my understanding, the trunk (american terminology) is used to connect the switches in the exchanges and is typically an E1 (lowest) in the telco world. However, my question is, in the Erlang B formula, it does not define what the trunk dimension is. Is it an E1, an E3 ??

The Erlang B formula basically says given the desired grade of service and the amount of traffic, we can work out the number of trunks required but what is this trunk size physically ?

Do you have any thoughts on this or have anyone enlightened you on it ?

thanks.

Sometimes ‘trunk’ and ‘line’ is interchangeable and when you said that, there is a reference point which is normally omitted for convenience. For example, a ‘trunk’ in a PABX is actually a ‘line’ with respect to the telco side. And a ‘trunk’ of the telco means the link between telcos.

A little more on line terms…T1 aka Bell1 aka Digital Signal 0 or DS0 is an American standard, with 24 64k channels and a rate of 1.544k. E1-aka CEPT1 is the European standard with 30 64k channels and a rate of 2.048k, and J1 24 64K channels is the Japanese standard, same rate as US, but not compatible..line coding and framing is different.

Tre’

Hello Mwenya…

I would direct anyone looking for the skinny on Erlang to look for a white paper on the subject. Perhaps I’m just lazy… Also couldn’t hurt to look up poisson(sp) to compare.

Anyway, the way line vs. trunk works, in my opinion, is that a trunk ties switches together, whether it’s a switch in a network or a PBX at a company’s premises is really unimportant to the definition of a trunk. Trunks are normally not rich in features, and are almost always brought in (In the US) by legacy, T-carrier facilities.

A line on the other hand, connects end users, (stations) to a switch, and most always has a concentrating device between the switch and the customer. Centrex service is provisioned on lines… and there is a trunk to line ratio (such as, three T1s (72 trunks) go in, 96 lines come out). Or something like that.
Hope this helps some.

Lines connect users to networks, trunks interconnect switches.

Trevor, T1 is carrier, DS1 is the service provided, DS0 is the 64k timeslot.

Thank you for bringing that mistake to my attention Ezra.

I have been told that Erlang is always between 0 and 1. However, after reading the whitepapers, this is does not appear to be the case. Does anybody have any idea where this confusion can come from ? Is there eg another parameter related with Erlang that is always between 0 and 1 ?

An Erlang is merely one hour of talk time, or 36CCS, or 3600 call seconds.

If you have two trunks and each of them carries 30 minutes worth of traffic in an hour, you had one Erlang of traffic that hour.

One trunk can not possibly carry greater than 1 Erlang of traffic, could that be the 0 and 1 thing you refer to?

Also, Grade of Service must be between 0 and 1. Either 0% of calls get blocked or almost 100% of calls get blocked.