Random Access Channel
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Random Access Channel (RACH) is used in mobile phones or other wireless device on a TDMA-based network when it needs to get the attention of a base station in order to initially synchronize its transmission with the base station.
Random Access Channel is a shared channel that is used by Wireless Access terminals to Access the Access Network(TDMA/FDMA,and CDMA based network) especially for initial Access and Bursty data transmission.
A key feature of a Random Access Channel is that messages are not scheduled (compared to for example a "Dedicated Channel" in UMTS, that is assigned exclusively to one user at a time). There is no certainty, that only a single device makes a connection attempt at one time, and collisions can result.
web cites of further description
http://www.umtsworld.com/technology/cell_access.htm
