在UCL通信系2005年考卷有一题请前辈们指点

isiminmin

In the GSM standard, the allowed data rate is 9.6 kbit/s and the speech transmission rate is 13 kbit/s. Relate these figures to the effective
transmission rate allocated to each user in GSM.

The voice codec used in GSM mobile handsets specifies a full rate output
stream of 13 kbit/s. Explain why the highest rate for a data transmission
in a current GSM network is 9.6 kbit/s

请各位前辈指点。。。。