Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 152

The nature of VoLTE traffic is different from normal IP/data traffic. The IP packets size of a VoLTE traffic is small and is predictable, whereas the IP packets of normal data services are larger than voice packets, busty in nature, and may be downloaded or uploaded quickly. After normal IP packets transfer, radio resources allocated to the UE may be released by the eNodeB. The allocation and releasing of radio resources between UEs and the eNodeB for normal IP traffic is dynamic in nature and involves signaling overhead over the LTE air interface. Unlike normal IP traffic, the data rate of voice packets of a VoLTE call is low and remains the same throughout the conversation. Thus, the same radio resources may be allocated semi‐statically for the entire duration of the session, and no additional radio resources are required to be allocated dynamically by the eNodeB. Because of this, a semi‐static allocation of radio resources reduces the signaling overhead between the UE and eNodeB. In addition to this, the UE is also not required to monitor the downlink Physical Downlink Control Channel (PDCCH) for additional resources. During a voice call, only one party talks at a time while the other party listens.

To support all the above aspects of a VoLTE call over an IMS, the following functionalities are required to be made available and enabled in the E‐UTRAN (UE and eNodeB) for rich user experience and an optimum allocation and usages of radio resources over the LTE air interface.

 Semi‐Persistent Scheduling, for allocation of semi‐static radio resources to a UE, thus reducing signaling and scheduling overhead between UE and eNodeB.

 Discontinuous Reception (DRX), to lengthen the battery life of UE that does not require to monitor the PDCCH continuously.

 Robust header Compression, for reducing the IP Header size for voice traffic.

 Transmission Time Interval (TTI) bundling, enabling a UE, located at the cell edge, to transmit in UL in four subframes in a row. TTI bundling increases the possibility of receiving of time‐sensitive voice traffic packets successfully from UE to eNodeB. TTI bundling avoids the retransmission requirements that would, otherwise, involve signaling overhead and cause a delay in receiving voice packets from UE to eNodeB, resulting in poor user experience.

Support of all of the above functionalities is implemented at the LTE/E‐UTRAN air interface Layer 2 Medium Access Control (MAC) protocol. For more information on these functionalities toward a VoLTE call, refer to TS 36.321 [93]. The related MAC layer parameters for a VoLTE call are configured by the RRC layer of the E‐UTRAN. Apart from the MAC Layer, the Radio Link Control (RLC) layer also facilitates acknowledged mode (AM), for SIP signaling, an unacknowledged mode (UM), transfer functions for voice traffic.