SNA data transmission requires the use of compression. to reduce the length of records for transmission by removing blanks and duplicate characters. Compaction (reducing the length of records by representing certain 8-bit characters with only 4 bits) is optional and is controlled by NJE and SNA protocols. (See Session Initialization in Link Protocols for additional information on compaction and compression.)

The SNA string control byte (SCB) defines the beginning and end of compressed and compacted data. An SCB must begin each RU following the exchange of FM headers. See String Control Byte (SCB) for additional information for the SCB.