Codec Freedom -- Specifications -- Connection Profiles - Cellular CSD

Cellular phone calls are tightly bound to the GSM codec. It is usually not possible for application code to intervene with this codec, but most cellular networks do support a data call, known as Carrier Switched Data. Like voice calls, this reserves a repeating slot for the connection, leading both to reliable latency times and charges by the minute.

CSD connections act like modem connections. They call the "modem" on a given phone number, and once connected they can pass all data. It is an 8-bit clean channel, so when asked for codes that need escaping, nothing is returned.

Connection Profile

CSD provides a modem connection, which is 8-bit clean and does not require filling of the line. It is not predestined to be used for bit-insertion mode, because the available bandwidth calls for compression mechanisms that deliver bursty frames.

For packet mode, the following profile defines the behaviour of a suitable escaping discipline: