[NTHU计算机网络基础]Wireless 802.11

IEEE 802.11 is designed for a limited geographical area(hoems, offices, campuses, stations)the signals propagating through spaceknown as Wi-Fi additional features:(1)power management(无线通讯情况下电源管理很重要,不送封包的时候进入睡眠状态)(2)security mechanisms(别人很容易受到来自各处的封包,所以需要加密,需要计算,需要耗电) physical layer: defined two radio-based physical layer standard(抗干扰)(1)one using the frequency hopping跳频(2)second using direct sequence Hideen node problem: A B C D双方虽然听不到对方的讯号(在各自涵盖范围之外),但同时传送给相同的对象会造成冲撞。(但是无法support collision detection)two frames from A and C may collide with each other at B。 Exposed node problem双方虽然听得到对方的讯号,但是同时可传送给不同的对象而不发生collision。 解决办法:CSMA/CA(Carrier Sense Multiple Access with Collision Avoidance)(根本做不到Collison Detection,而且无法完全避免)(1)Sender and receiver exchange control frames with each other before the sender actually transmits any data.(2)This exchange informs all nearby nodes taht a transmission is about to begin(3)Sender transmits a Request to Send(RTS) frame to the receiver(广播,存储了duration field)  The RTS frame incldue a field taht indicates how long the sender wants to hold the medium  Length of the data frame to transmitted(4)Receiver replies with a Clear to Send(CTS) frame(广播,存储了duration field)  This frame echoes this length field back to the sender   Any node that sees the CTS frame  it is close to the receiver, therefore  cannot transmit for the period of time specified in the CTS time if two or more nodes detects an idle link and try to transmit an RTS frame at the same time  Their RTS will collide with each otherso the senders realize teh collision has happened when they do not receive the CTS frame after a period of timeEach sender waits a random amount of time before trying again.The amount of time is defined by the same exponential backoff algorithm used on the Ethernet.(collision次数越多,等待发送的时间越长) we can use CSMA protocol but the performance is not good enough802.11 using ACK frame in CSMA/CAReceiver sends an ACK frame to the sender after successfully receiving a frame 802.11 Frame Types(1)1Control Frames: RTS, CTS, ACK, Poll(2)Management Frames: Probe Request, Beacon, Authentication