An Overview of DSS1 ------------------- by Cruise-CTRL Integrated Services Digital Network - what a buzzword. Back in the mid to late eighties, that's all we heard about. The new all-digital telecommunications package that would allow for rates of up to 64 Kbit/sec. And it's here, and getting more and more common every day. There are two primary signaling systems involved in ISDN: SS7 and DSS1. SS7, or Signaling System 7, is a well-known entity - as a matter of fact, SS7 is not limited to ISDN - it's an independent protocol used for things other than ISDN, too. But DSS1, or Digital Subscriber Signaling System 1 (they seem to have forgotten an S here - typical) is limited to ISDN. DSS1 handles signaling between the end nodes (users, the local loop, whatever you want to call it) and the local telco switches. It's on the ISDN customer's premises and handles subscriber switching. There have been a lot of compatibility problems with DSS1 - when the first ISDN sites came out several years ago, every vendor had their own protocol, and nobody could talk to each other. Here is where National ISDN 1 steps in. This is a fairly new, standardized ISDN protocol, and it was designed to handle all this compatibility mess. The old sites that were put in before this still have problems talking to others. A typical residential ISDN subscriber has 2B + 1D channels - that is, two 64 Kbit/sec B channels for data and voice transfer, and a D (delta) channel which handles switching. The D line is DSS1 and, before its acronym was coined, it was pretty much known as just that - the "D-channel protocol." Basically, DSS1 carries pertinent switching information (the subscriber's phone number) in what's called a message. There is separate signaling between the local loop and trunks (between switches), and this keeps endusers away from trunk signaling equipment (the old world of the bluebox). The trunk signaling is done by SS7. On a local loop, a caller on a regular analog phone (using a Terminal Adaptor, or TA) could make a call , and the DTMF signals would be sent to the user's PBX. There, the DTMF tones would be converted to a DSS1 setup message, which has a 16 bit address field. The user's central office switch would then convert the DSS1 message to an SS7 ISDN User Part message. From there, the SS7 signal would travel through the network to the receiving party's CO. The CO would convert the SS7 signal to (you guessed it) a DSS1 message. The ISDN equipped PBX on the called party's end would then, if necessary, convert the DSS1 message to DTMF tones, and the phone would ring. If the recipient's phone was an ISDN set, the DSS1 message would go straight to it, rather than having to do an extra DTMF conversion. Also, if there was no PBX on the site, but just a single ISDN phone on the local loop, the DSS1 signal from the CO would go straight to the phone. And if the call was made to a node on the same CO, SS7 wouldn't be used at all - the DSS1 signal would travel from one node on the CO to the other node, working just like a regular same-CO phone call would, not using trunk lines at all. Another tidbit that mi ght be useful: the Bellcore National ISDN informational hotline number is (800) 992-4736.