 However, the compression ratio varies based on the transmitted data, for instance the transfer of already-compressed files (ZIP files, JPEG images, MP3 audio, MPEG video) leads to negligible on-line compression.When using an on-line compression standard, the data rate varies during operation, compressed files might be sent at approximately 50 kbit/s, uncompressed files at 160 kbit/s, and pure text at 320 kbit/s.In such situations, a small amount of memory in the modem, a buffer, is used to hold the data while it is being compressed and sent across the phone line, but in order to prevent overflow of the buffer, it sometimes becomes necessary to tell the computer to pause the datastream.