Telephony Basics Part 1


This is the first of a multipart series of articles on Telephony from my old blog that I’ve decided to recycle. It was originally posted January 10th, 2012.

My last post generated a lot of comments about what I would call traditional telephony or what VoIP (IPT) installers refer to as Analog, Public Telephone Switched Network (PTSN), or Plain Old Telephone System (POTS).  Three years ago I was handed a traditional PBX to manage with very little training or experience beyond the Cisco Unified Call Manager.  To me at the time, the phone frame (equivalent to a patch panel in network parlance) looked like a tangled mess of spaghetti that I would never understand.  Thankfully I had a very patient manager and a local telephone consultant that helped me understand how things worked.

Punch Down Blocks

66 block: This is a type of punch down block used to terminate cabling.  The permanent wires are generally placed in the outer columns and the jumpers in the inner columns.  Some 66 blocks however have the permanent in the left most and up to three jumpers.

110 block:  This is a type of punch down block used to terminate cabling. The permanent wires are terminated first and then a spacer block with conductors is placed on top where the jumpers are terminated.


Punch-Down Tool:  This is the device used to terminate a jumper or pair from a cable to a 66 or 110 block.  The tool generally can be used with different blades depending on the style of block you are using.

Punch-Down Blade: The blade on the left is used for 66-block terminations and the right side is for 110-block.

Spudger (aka Spludger, Pick): This tool is used to remove or adjust jumper cables in cabling blocks.  It is very useful to pick out small bits of wire left behind after removing a termination.

Building Cabling

Main Distribution Frame (MDF):  This is where the phone system’s connections are terminated into either 110 or 66 punch down blocks.  Jumper cables are used to connect these terminations to house cabling, the telephone company demarcation point, or to other intermediary distribution frames.

Intermediate Distribution Frame (IDF):  This is generally a closet or enclosure in a remote part of the building that feeds back to the MDF via large 25 to 100 pair cables.  In the IDF jumpers are run from blocks connected to the feeder cables to blocks connected to house cabling.

Other Jargon

Demarc (Demark, Demarcation Point):  This is the demarcation between what is the local telephone company’s responsibility and what is the building owner’s responsibility.  Usually this is an enclosure with some sort of terminating block inside of it.

Local Exchange Carrier (LEC):  This is the local telephone company that delivers the physical PTSN to the building demarc.  Alternatively in some areas there are Competitive LECs (CLEC) that provide services over the LEC’s cabling.

Foreign eXchange Office (FXO):  This is the type of port that usually is connected to PTSN coming from the phone company.  It doesn’t provide its own power or ground.

Foreign eXchange Station (FXS):  This type of port is generally found on PBXs and analog adapters for VoIP systems to “power” analog devices like fax machines and analog phones.  It provides ring and supervisory voltage to the line.