TAILIEUCHUNG - Next Generation Mobile Systems 3G and Beyond phần 4

thông tin liên lạc trung bình. Trái ngược với giao thức CSMA / CD được sử dụng trong Ethernet, nơi phát hiện va chạm có thể dễ dàng nhận ra, giao thức CSMA / CA (được phát triển cho một mạng không dây ) làm cho một nỗ lực để tránh va chạm, bởi vì người nhận không dây có khó khăn trong việc phát hiện va chạm. | 100 WIRELESS LAN EVOLUTION communication medium. In contrast to the CSMA CD protocol used in Ethernet where collision detection can be easily realized the CSMA CA protocol developed for an wireless network makes an effort to avoid collisions because the wireless receiver has difficulty with collision detection. The receiver uses the following features and functions Adaptive collision window CW based random backoff time to reduce the probability of collisions Different interframe space IFS to prioritize different types of transmissions Acknowledgement frame to realize the stop and wait ARQ Request to send RTS and clear to send CTS handshaking to solve the hidden terminal problem Network Allocation Vectors NAV to realize virtual carrier sense As in other random access protocols the random backoff time in CSMA CA works to avoid collisions between transmissions from different STAs. The random backoff time can be calculated from this equation Backofftime Random Slottime In this equation Random 0 CW CWmin CW CWmax and Slottime is the value of the corresponding PHY characteristic. Suggested values are CWmin 31 and CWmax 255. If it is the current packet s first transmission CW is set to CWmin. After each collision of this packet the collision avoidance mechanism doubles CW until it reaches CWmax. CWnew CWold 1 PF - 1 In this equation PF is equal to 2. This is referred to as the exponential backoff algorithm. The offered load to the channel is high when experiencing a collision so increasing the CW to increase the backoff time of each colliding STA helps decrease the collision probability. The IFS is a time interval after a busy state of the channel. This interval plays an important role in CSMA CA for collision avoidance and prioritized transmissions. The IFS requires an STA to wait for a period of time after it senses the idle state of the channel. Then the STA waits for a random backoff time before transmitting its frame. There are four basic types of IFS

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