In contrast with non-SNA use of SCBs, for SNA buffers, SCBs are used to compress/compact the entire buffer; that is, the entire buffer is treated as data as far as the SNA transport subsystem is concerned.

The SCB bit settings specify the compression and compaction parameters, but the encoded bits are different for SNA than for non-SNA transmissions. SCBs are always used, whether or not the data is actually compressed; that is, even uncompressed data must be interspersed with SCBs that have the two high-order bits set.

The one-byte SCB consists of a 2-bit description field, followed by a 6-bit count field. This count is the number of characters that are described by this SCB. Therefore, one SCB may replace up to 63 characters, or identify up to 63 intervening uncompressed characters before the next SCB. In all cases, a count of 0 is a reserved value. The SCB format is shown in Table 1.

The only restriction on the placement of SCBs is that an SCB must appear at least once every 64 bytes, and describe up to 63 bytes of following data. This restriction applies to non-SNA use of SCBs as well.