D4 Framing Standard

In telecommunication, a D-4 is a framing standard for traditional time-division multiplexing, which standard describes user channels multiplexed onto a trunk that has been segmented (framed) into 24 bytes of 8 bits each.

Note: The multiplexing function is performed in the D-4 framing structure by interleaving bits of consecutive bytes as they are presented from individual circuits into each D-4 frame.

Within Telecommunications operations, a D4 Bank also refers to a type of multiplexer which mux's 24 DS0 signals into a single DS1.

Source: from Federal Standard 1037C

A minor correction from above, there is no actual D4 framing format. The D4 Framing format that is referenced above is actually more of a marketable term, adopted by "techno Jargon" to simplify what is actually called Superframe Framing Format, an embedded 12 bit framing code. The D4 is (as stated above) a 48-channel (dual 24 channel) channel bank chassis, which can multiplex two banks of 24 DS0 channels into 2 T1's or a single T1C.

The Superfame Format was originally introduced with the D2 Channel Bank. The D2 Channel bank was a 96 channel based chassis which could provide four 24 channel T1s, two T1C's or a DS2. The T1 Extended Superframe (ESF) Format (a 24 bit code composed of 6 framing bits, 6 CRC Error Checking Bits and 12 Signaling Bits)was introduced with the D5 Channel Bank, a true 24-channel channel bank chassis, which could also be software optioned to use the Superframe framing format.

So, why is the T1 Superframe framing format commonly called D4 Framing? There are may iteration of the answer but the most common by me over the years was that the abbreviations for Superframe (SF) was in conflict with the sill commonly used technology of the day, Single Frequency Signaling (SF Signaling). SF Signaling was the method of Inband circuit control before the advent of the Extended Superframe (ESF) framing format. The ESF T1 framing format allowed for the signaling information to be carried out of band and not as an Inband tone. Consequently, the D4 channel bank was in deployment and so the Superframe format became associated with the D4 Channel Bank, or D4 Framing.

So, is it wrong to say D4 Framing, not really, but understanding what it means is important.

This information can be found in the original AT&T Publications.