ble data transmission by means of segmentation, ARQ (automatic retransmission request), in-sequence delivery, etc. The RLC layer has three functional modes: TM (transparent mode), UM (unacknowledged mode), and AM (acknowledged mode), which enable ser- vices with different speed and reliability for the upper layer. Chapter 9 ("User Plane Protocol Design for LTE System with Decode-Forward Type of Relay") examines the legacy UMTS RLC protocols and discusses its evo- lution for the future broadband network. Different operating schemes of RLC for © 2009 by Taylor & Francis Group, LLC Introduction the multihop relay-enhanced cell (REC) are investigated. The AM mode of RLC for the unicast traffic on the downlink user plane is studied in detail. Because the AM mode incorporates the ARQ functionality, it is by far the most important operation mode for reliable data transfer. The conclusion is that the two-hop RLC scheme performs poorly compared to the per-hop RLC scheme due to the asym- metric capacity on the two hops. This chapter concludes that even with the more efficient per-hop RLC scheme, a flow control mechanism on the BS (base station)- relay node (RN) hop has to be developed. However, the topic of flow control, as well as the impact of the HARQ/ARQ cross-layer interaction, would be left for our future study. Recently, there has been an increasing interest in using cellular networks for real-time (RT) packet-switched (PS) services such as voice over Internet protocol (VoIP). The reason behind the increased interest in VoIP is to use VoIP in all-IP networks instead of using circuit-switched (CS) speech. This would result in cost savings for operators because the CS-related part of the core network would not be needed anymore. In conventional cellular networks, RT services (e.g., voice) are carried over dedicated channels because of their delay sensitivity, while non- real-time (NRT) services (e.g., Web browsing) are typically transported over time-shared channels because of their burstiness and lower sensitivity to