In BSC and CTC NJE transmissions, the record segments are first compressed. This compression results in string control bytes (SCBs) interspersed in the stream data with duplicate character strings compressed. The resulting format always begins with an initial SCB and may have additional SCBs interspersed.

The SCB defines how many blanks or repeating non-blank characters are to be inserted by the receiver (up to 31) or the length of strings without any duplicate characters (up to 63 characters). An SCB must appear at least once every 64 bytes, and describe up to 63 bytes of following data. A “stand-alone” SCB with a value of zero, sometimes referred to as an “EOR SCB”, is always placed at the end of each NJE record.

The SCB bit settings specify the compression parameters. SCBs are always used, whether or not the data is actually compressed; that is, even non-compressed data must be interspersed with SCBs that have the two high-order bits set (see Table 1).