Requiring an ACK every four packets makes the system work like it has a packet size of 512 bytes, but in the case of an error, typically only requires 128 bytes to be re-sent. Moreover, it reduces the amount of data flowing in the reverse direction by four times. This is of little interest in the typical modem's full duplex operation, but is important in half duplex systems like Telebit models which have 19 kB speed in one direction and 75 bits/s in the return channel.