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.
Chapter Summary
This chapter presented the seamless mobile communications services provided by operators to their subscribers through the interworking and interoperations of mobile communications networks operated by them. Interoperations among the networks are important to offer communication services to roaming users. Interworking and interoperations between two network elements or different networks are established by introducing new logical interfaces that connect enhanced or upgraded network elements. Core network elements, e.g. MSC, SGSN, and MME, are upgraded to support additional interworking functions in an LTE/EPS network.
This chapter also presented the UE mode of operations that enables a user to make a voice call over a legacy as well as an LTE/EPS network. UE mode of operations is important to achieve the interworking of mobile communications networks. The interworking features, i.e. CSFB and SRVCC, may be deployed as the separately licensed feature on top of an existing network. We then presented the routing methods, either through the home network or visited network, available to route the roaming user data to an external packet network. We closed this chapter with the functions that are required to be performed from the E‐UTRAN point of view to support VoLTE calls over IMS.
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