| 单词 | CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) |
| 释义 | CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) A medium access control (MAC) protocol used in some bus networks, CSMA/CA includes a priority scheme to guarantee the transmission privileges of high-priority stations. CSMA/CA requires a delay in network activity after each completed transmission.That delay is proportionate to the priority level of each device, with high-priority nodes programmed for short delays and low-priority nodes programmed for relatively long delays. As collisions still may occur, they are managed either through collision detection or through retransmission after receipt of a negative acknowledgment (NAK). CSMA/CA is more expensive to implement than CSMA/CD (CSMA with Collision Detection) because it requires that additional programmed logic be embedded in each device or network interface card (NIC). CSMA/CA does, however, offer the advantage of improved access control, which serves to reduce collisions and, thereby, improve the overall performance of the network. CSMA/CA is half-duplex (HDX) in nature. Wireless LANs (WLANs), as standardized in IEEE 802.11, employ CSMA/CA. The 802.11 standard uses a positive acknowledgement ( ACK) mechanism, which requires that the transmitting station first check the medium to determine its availability.The transmitter sends a short request to send (RTS) packet that contains the source and destination network addresses, as well as the duration of the subject transmission. If the shared medium is available, the destination station responds with a clear to send (CTS) packet. All devices on the network recognize and honor this acknowledged claim to the shared network resources. If the source station does not receive an ACK packet from the destination station, it retransmits RTS packets until access is granted. See also 802.11, bus, CSMA/CD, HDX, and MAC. |
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