e time start the forwarding of user data. The source eNode B forwards PDCP service data units (SDUs) (i.e., IP packets that could not be successfully sent in the source cell) toward the target eNode B. Note that the L2 protocols including RLC/MAC are reset in the target cell (i.e., no HARQ/ARQ status information is preserved); the header compression engine in the PDCP layer is also reset. For more details on the handover procedure, see 3GPP TS 36.300 [14]. When the UE arrives at the target cell, it accesses the cell via the RACH. However, in order to reduce the interruption time due to potential collisions on the RACH, it is possible to use a dedicated preamble for the access. The term preamble refers to the signature sequence that is sent by the UE on the RACH slot, and it is used to identify the access attempt. There are 64 pre- ambles for contention-based access, and a separated set of preambles is used in a contention-free, dedicated manner-for example, for handover access (or for © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology access to regain uplink time synchronization). The target eNode B can reserve a dedicated preamble for the particular handover instance of the UE and can signal this preamble to the UE via the transparent container in the handover command. Because the preamble is dedicated, no other UEs can use it at the same time, which ensures that the access attempt will be contention free. With the preceding handover scheme, an interruption time in the range of 15 ms can be ensured. Finally, it is worth mentioning that LTE provides an efficient recovery mecha- nism for handover failure cases, when the handover could not have been com- manded by the network due to the loss of the radio link. Although such radio link losses should be rare events in a well-planned network, their occurrence cannot be completely ruled out, especially due to the potential harsh interference conditions on the cell edge. If the UE loses the radio link, it reselects to a sui