Telos NX12 Benutzerhandbuch

122 |  appendix 3

Recall that each ISDN BRI has two possible B channels. It is possible to order a 

line with one or both of the B channels enabled – and each may be enabled for 

voice and/or data use. Phone terminology for the class of service is CSV for Circuit 

Switched Voice and CSD for Circuit Switched Data. (In contrast to PSD, Packet 

Switched Data, which is possible, but irrelevant to our needs.)
CSD is not required for Telos Nx12 connections. Even though you are sending a 

stream of data, the phone network knows to translate this back to analog at the far 

end due to the call setup message sent on the D channel when the call is made. If 

this ISDN BRI circuit will also be used with an audio codec such as the Zephyr or 

Zephyr Xstream then CSD should be ordered as it is required for audio codec calls.
CSV is for interworking with voice phone service and allows ISDN to interwork 

with analog phone lines and phones. 
You will be ordering one or more ISDN BRI 2B+D circuits with CSV (required) 

and CSD (optional) on each B channel. Euro ISDN normally provides both CSV 

and CSD which is acceptable.
NT1s

The ISDN standard specifies two reference points, the “U” and the “S” interfaces. 

The U is the single-pair bare copper from the Telco CO. A device called a “Net-

work Termination, Type 1” converts this to the two-pair S interface. 
In Europe, Asia, South America, and most of the world the NT1 is always pro-

vided by the phone company, and only the S interface may be on user equipment. 

If you plan on taking your TWOx12 to Canada or North America you will need 

to rent or purchase an external NT1 for use while in those countries. One rental 

source is Silver Lake. See their website at www.silverlakeaudio.com .
Hunt Groups

Just as with analog lines, ISDN BRI lines can be placed in a “hunt group” where 

a single number can be given to callers and the calls will “roll over” from busy 

channels to idle channels. This is sometimes referred to as ISG (incoming service 

grouping) or rollover lines. This would usually be a requirement for an on-air phone 

system. You may choose to exclude 1 or 2 channels from the hunt group (ie the 

Hot line and the Warm line). Be sure to specify to your Telco how you expect this 

to work.
Line Deactivation

The Euro ISDN standard allows ISDN BRI lines to be deactivated by the Telecom 

when not in use. It is impossible to distinguish a deactivated line from a line 

which is defective and out of service. If  your application is critical you may wish to 

request that the Telecom disable line deactivation.

FAXABLE ISDN ORDER FORM (for Euro ETS 300)

The following form may be used to place orders for ISDN lines in countries using 

the ETS 300 protocol. These should give the Telecom all of the information they 

need.