frame control portion 3 b reads each rlp frame out of the first through the fifth rlp frame memories 381 to 385 and sends it to the reproduction portion 39 the reproduction portion 39 successively receives the rlp frames read out of the frame memories 381 to 385 and reproduces or regenerates the ppp frames when the ppp frame includes the ip protocol frame the ip protocol frame is converted into the ip protocol packet by the reproduction portion 39 to be sent to the ip protocol interface 31 when the ppp frame includes the ppp link control frame the ppp link control frame is processed by the transmission frame control portion 3 b to be subjected to predetermined processing on the other hand consideration will be made about transmitting the ip protocol packet from the interworking controller 3 to the mobile station ms 1 in this event the ip protocol packet is given through the ip protocol interface 31 to the framing portion 32 and is framed into the ppp frame by the framing portion 32 each ppp frame is successively delivered to the first through the fifth ppp frame memories 331 to 335 to be stored therein under control of the reception frame control portion 3 a thus the ppp frame memories are loaded with the ppp frames which are made to correspond to the radio channels in number in general the radio interface 36 is slow in speed in comparison with the ip.