CDMA2000 1X (IS-2000), also known as 1x and 1xRTT, is the core CDMA2000 wireless air interface standard. The designation "1x", meaning 1 times Radio Transmission Technology, indicates the same radio frequency (RF) bandwidth as IS-95: a duplex pair of 1.25 MHz radio channels. 1xRTT almost doubles the capacity of IS-95 by adding 64 more traffic channels to the forward link, orthogonal to (in quadrature with) the original set of 64. The 1X standard supports packet data speeds of up to 153 kbit/s with real world data transmission averaging 80–100 kbit/s in most commercial applications. IMT-2000 also made changes to the data link layer for greater use of data services, including medium and link access control protocols and QoS. The IS-95 data link layer only provided "best efforts delivery" for data and circuit switched channel for voice (i.e., a voice frame once every 20 ms).
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