When using Scheduler A and B in this study, the parameterminAgeThd is set to
a static value of 30 ms. Since the use of RTTI reduces the transmission time of a radio block to 10 ms, setting this parameter to 30 ms makes a scheduled user wait until three other users have been scheduled until being scheduled again. 30 ms is therefore optimized for multiplexing four users on the same timeslot bin which results in a total of sixteen users on eight TSs. In a future study, it would be interesting to let this parameter be set dynamically depending on the level of multiplexing. When no multiplexing is present, the value should be set to zero and then increase by 10 ms for each added level of multiplexing. Obviously, there is a limit where the wait time starts affecting the VoIP performance too much.
The simulations in this study were to be run with a static number of users which means that no calls would have been initiated or terminated except at the very beginning and at the end of the simulations. This is obviously far from real life behavior and therefore a dynamic flow of user arrivals and departures would be preferred when running simulations of this kind. This would impact the VoIP capacity even more due to increased SIP signaling payload.
It could also be interesting to investigate how different user types could affect the system capacity. For example, it would be interesting to know what impact users only utilizing the presence service would have on the system. Since a session initiation has to be done for each sent presence message (if only utilizing the pres- ence service), the choice of flow release time would probably affect the performance a great deal.
Furthermore, the human hearing sense can compensate for occasional packet losses, but when several consecutive packets are lost the perceived speech quality is decreased. Therefore, it might be interesting to evaluate not only the overall packet loss rates, but also the number of consecutively lost packets.
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Appendix A
Abbreviations
AMR Adaptive Multi Rate
BSC Base Station Controller
BSS Base Station System
BTS Base Transceiver Station
CIR Carrier to Interference Ratio
CS Circuit Switched
GGSN Gateway GPRS Support Node
GMM/SM GPRS Mobility Management and Session Management
GPRS General Packet Radio Service
GSM Global Service for Mobile transmission
IMS IP Multimedia Subsystem
IRA Interference Rejection Algorithm
LA Location Area
MCS Modulation and Coding Scheme
MMTel MultiMedia Telephony
MS Mobile Station
MSC Mobile Services Switching Center
PS Packet Switched
PSTN Public Switched Telephone Network
QoS Quality of Service
SGSN Serving GPRS Support Node
SIP Session Initiation Protocol
SMS Short Message Service
TDMA Time Division Multiplex Access
TS Timeslot
UA SIP User Agent
VLR Visitor Location Register
VoIP Voice over IP
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