three different kinds of messages fig2 a is the u2018 no data u2019 case where the originator 202 sends a message command block mcb 206 to the recipient 204 the recipient 204 responds with a message response block mrb 208 sent back to the originator 202 fig2 b is the write case where the originator 222 sends data to the recipient 224 following the mcb 226 the originator 222 sends the first data message area 228 to the recipient 224 if data area message buffer space is limited at the recipient not all of the message data can be sent in one data area flow control is realized through the link acknowledge ack 230 sent by the recipient back to the originator 222 when buffer space becomes available the originator 222 responds by sending the next data area 232 to the recipient it should be understood that this acknowledgment process can be repeated many times depending on the number of data areas transferred after the last data area 232 is received the recipient 224 sends the mrb 234 back to the originator 222 fig2 c is the read case where the originator 242 receives data from the recipient 244 following the mcb 246 the recipient 244 sends the first data area 248 back to the originator 24 if data area buffer space is limited at the originator not all of the message data can.