 V.42bis added a string compression algorithm called BTLZ (British Telecom Lempel Ziv), in which frequently occurring strings of characters are replaced by code words.The "dictionary" of strings is dynamically maintained during transmission and keeps track of changes in the data; new strings are added and old strings are deleted.As the data may not always be compressible (for example encrypted or pre-compressed data) the modem has the ability to switch between compressed and uncompressed modes of operation.