On analog telephone lines, voice communications and signaling share the same pair of wires, called the local loop. These copper wires connect the telephone set all the way to the phone company central office. Audible tones called DTMF or touch-tones are used to dial phone numbers and respond to voice commands. Lifting the handset or pushing a talk button completes a circuit that tells the telephone company switching system that your phone is in use. Ringing is a different electrical signal from the telco office to the handset. Caller ID information is a digitized signal that tells you who is calling.
This type of signaling is called in-band or common channel signaling. One shortcoming of this methodology is that sharing the line usually means you can signal or talk, but not generally at the same time. What may be less obvious is that in-band signaling is a slow process. It's not so much how fast you can push those buttons to dial a call. It's how fast a calling system can dial numbers. Each tone has to persist long enough to be properly decoded at the other end. The process of connecting and disconnecting calls, called call setup / teardown, creates wait time that can add up over thousands and thousands of calls.
What about digital trunking? Substituting a T1 line for 24 analog phone circuits creates a more efficient connection path between your PBX system and your supplier's central office. It's also likely to save you money if you have more than 6 to 8 outside lines. But talk and signaling still share the same lines. On T1 lines it's called CAS or Channel Associated Signaling because the digital line is make up of individual channels. T1 CAS is another implementation of in-band signaling.
ISDN, Integrated Services Digital Network, changes all that. An ISDN PRI trunk, also called a T1 PRI, separates talk and signaling. All of the signaling is conveyed over one dedicated delta channel. The other 23 bearer channels carry one telephone conversation each. This methodology is called CCS or Clear Channel Signaling. It's also called out of band signaling. Advantages of ISDN signaling include faster call setup / teardown, marginally better voice quality since the talk channels can use all of their bits for digitized voice signals, and ability to support features such as ANI or or Automatic Number Identification or Caller-ID.
The telephone carriers went to clear channel signaling decades ago to improve the speed and efficiency of their networks. They use a protocol called SS7 for Signaling System Seven. SS7 connects all the local (Class 5) and long distance (Class 4 and above) switches through separate switching links. Very large call centers may find it advantageous to by SS7 switching services to connect to the public telephone network. Such services not only include call control, but may also provide access to national line information and calling name databases.
The new competitor to traditional switching for call centers is SIP or Session Initiation Protocol. SIP is used in VoIP telephone systems to handle call control. The VoIP equivalent to the ISDN trunk is the SIP trunk. SIP trunks are not channelized. Everything is transmitted in streams of packets. A SIP trunk can carry dozens of telephone conversations and their associated SIP signaling simultaneously. The service provider handles the conversion from SIP to SS7 or ISDN to terminate calls to the public telephone network.
What type of telephone trunking and signaling is best for your contact or call center? Let our telecom technology experts review your situation, make recommendations and get you the best pricing for your call volume.
