[VoIP] More "Authentic" CNET

[Steph Kerman]

Mad Mark wrote:
>> Date: Wed, 5 Dec 2007 22:56:31 -0800
>> From: keelan at mail.grenander.com
>> To: voip at ckts.info
>> Subject: Re: [VoIP] More "Authentic" CNET
>>> Nice idea for sure, but I think it would be better to get some 
>>> switching hardware with E&M lead input/output. (I think it's E&M 
>>> that's used for bidirectional 2-wire trunks... Can anyone confirm?)
>> E&M would be nice for the switches that supported it, but for many 
>> switches, E&M trunking equipment is probably very difficult to come
>> by. It would make the hardware implementation much simpler, though.
> If only I knew the inns and outs of E&M and other trunking technologies.
E&M trunks were normally used on derived facilities: very long metallic 
trunks that required polar duplex (DX) signaling because of the 
conductor resistance and usually 4 wire transmission because of the VF 
loss, and also used many "carrier" (analog FDM or digital PCM) trunks.  
Prior to widespread use of T1 carrier typical metallic interoffice 
trunks used "loop reverse battery supervision".

Even many T1 trunks used loop signaling too.  The widely used DPO and 
DPT T1 channel cards are for loop/reverse battery signaling trunks.  
2600 Hz "E" signaling units were also used for signaling over many 
carrier derived trunks.  Many of these, particularly the 2 wire ones, 
used loop reverse battery signaling too.  Loop signaling was most 
commonly used on 1-way trunks.  E&M was more the rule for 2-way 4-wire 
toll trunks.

Loop/reverse battery refers to the methods of controlling seizure and 
release towards the terminating end and answer supervision back to 
towards the originating end.  An older loop method used in manual common 
battery practice was loop high/low where answer supervision was 
indicated by drawing or delivering greater current.  This method was 
also used in some PBX tie trunk circuits that might be found in some 
equipment collectors' systems.

SXS and most other electromechnical switching systems use loop 
supervision internally and are inherently compatible with loop 
supervision trunks without additional trunk equipment as such.  FXS and 
FXO channel interfaces inherently use loop supervision because they 
emulate a telephone set, which uses loop supervision.  So using E&M 
would be a complication all around.

Steph