X2 (protocol)
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X2 was a 56 kbit/s modem protocol developed by U.S. Robotics, and competed in a format war with Lucent and Rockwell's joint K56flex protocol until the adoption of the V.90 standard.
The protocol used V.34+ to upload at 33.6 kbit/s, and downloaded under PCM at 56 kbit/s.
X2 was found to consistently have a lower top end speed, yet overall performance was faster than K56flex. The incompatibilities experienced between variant Lucent and Rockwell chipsets in K56flex were also avoided as there was only one X2 standard.
| Telephone network modem standards |  |
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|---|---|---|---|
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ITU V-Series | V.92 | K56flex | X2 | MNP | Hayes command set |
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