Channelized vs Unchannelized T1's

- and the unchannelized myth -

In theory, there really is no such thing as an "unchannelized T1".  All T1's have the same structure, which includes 24 embedded DS0's.  It is a standard, and all T1's are channelized.  

But the term is here and is not going away, so let's define it.  

Transmission End - the sender of data across a T1 sends the data in a serial stream, and by the way, the T1 is a serial transmission standard.  However, the data - just before being placed on the T1 circuit - is broken up into frames, and each frame represents one DS0 (64 kbps) of data.  Now, the sender may actually be sending 24 separate DS0 data streams, or he may be sending a full T1 data stream, or he may be sending several fractional T1 streams (groups of DS0's combined, such as 256 kbps (4 DS0's).

Receiving End - the receiver, of course, will get whatever the sender transmitted.  No matter what, though, he will be receving one T1 of data, multiplexed into 24 DS0's.  His equipment must be configured to combine or not combine the channels, depending on what the sender sent.  So, Sender & Receiver need to have their equipment configurations matched.

The concept of "unchannelized" simply means the sender and receiver have configured their equipment to use all 24 DS0's as one serial stream of data.  The sender places a 1.536 Mbps stream of data onto the T1, breaking it up into 24 time-division multiplexed DS0's and adding 8 kbps of framing bits for synchronization.  The receiver takes the 1.544 Mpbs stream, removes the 8 kbps framing bits, and concatenates the 24 DS0's into a single 1.536.  So the T1 is channelized but the way the data is used is not - it is unchannelized after reception.

If the T1 was considered channelized - then both sender and receiver are using sub-rate (less than T1 rate) data streams.  This could be 24 separate DS0's as in the case with PBX voice circuits, or they may be using 12 DS0's for 12 voice channels, and the other 12 as one combine data stream.

Therefore, all T1's are channelized, but when the sender & receiver combine all 24 channels into one 1.536 data stream, then it is called unchannelized - even though the T1 itself is still channelized.  They could have avoided all the confusion by simply calling it T1 unchannelization or T1 channel concatenation - that way it would be clear that it is a process performed on a channelized T1 circuit.