关于控制帧的疑惑

lizx30340

Mattbew S.Gast 的802.11 Wireless Networks: The Definitive Guide 中第四章控制帧中有这么一段话：
ToDS and FromDS bits
Control frames arbitrate access to the wireless medium and thus can only originate from wireless stations. The distribution system does not send or receive control frames, so these bits are always 0.

既然分布式系统即不发射也不接收控制帧，如果两个通过不同的AP接入到网络的工作站相通讯，ACK帧怎么从接收端发送到发送端？