# Evaluation of VoIP QoS Performance in Wireless Mesh Networks
## 2 VoIP Application and Scenarios
### VoIP Implementation Scenarios  
VoIP applications in WMNs are challenging due to the VoIP traffic sensitivity to delay, jitter and packet loss [13]. WMNs implementation varies based on each scenario requirements and as a result these factors are also affected differently. These factors are considered and measured in a technical context, but from the user's perspective the main measurement factor is his/her satisfaction with the voice call quality. Hence, the user's perceived VoIP quality is not directly measured in terms of delay, jitter, and packet loss, but by the MOS rating scale defined by ITU-T [25]. A VoIP application in any type of network requires the necessary components to be in place so that the calling party (call initiator) and the called party (call receiver) can have a voice conversation for as long as they desire. In order to establish a VoIP call between the two parties, they both have to be registered with a call registrar. The call registrar requires a special protocol such as session initiation protocol (SIP) or H.323 to register all VoIP nodes in a client and server architecture [26]. These protocols are not only used for call registration, but also for call establishment, maintenance, termination, authentication, authorization, proxying, forking, redirection, and as call gateways to external phone systems. However, VoIP protocols cannot function alone and they are used with other protocols such as real-time transport protocol (RTP) for transporting VoIP traffic; real time control protocol (RTCP) for transporting control messages to support RTP; real-time streaming protocol (RTSP) as a control protocol for providing client/server and multimedia services by the support of RTP and RTCP services; session description protocol (SDP) for multimedia services session management; user datagram protocol (UDP) for best effort delivery services; and other OSI layer protocols, such as IP and so on. Researchers have proposed solutions such as admission control, load aware mobility management, routing protocols, packet aggregation, gateway placement and priority schedulers, to name a few, which can contribute to the VoIP quality improvement to some extent [27]. These solutions have been studied in detail and each solution's limitation and contribution have been analyzed in [27; 28]; a summary thereof is presented in Table 1. The review of the literature in this domain shows that there is still a clear need for further research in order to propose feasible, practical, and low-cost approaches for successful VoIP implementation in WMNs [27]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Solution Name** & **Advantages** & **Limitations** \\ \hline IEEE 802.11s [1] & EDCA, differentiated service and access categories, own routing mechanism using HWMP & Not well supported and VoIP performance is not improved significantly \\ \hline \multirow{2}{*}{Admission Control QoS [29]} & Channel reservation for multimedia traffic & Only solves the problem of access to the medium \\ \hline \multirow{4}{*}{Routing Protocols [30]} & Better usage of resources with reactive routing and better performance with the proactive routing with the cost of using more resources & Reactive routing is slow, proactive routing is resource hungry. Slow routing results in excess delay and use of more resources consumes lots of device processing power and battery life \\ \hline \multirow{2}{*}{IEEE 802.11e [31]} & EDCA function and good for infrastructure wireless network & Not developed for WMNs \\ \hline \multirow{2}{*}{Codec compression [32]} & Choice of high quality codec can improve voice quality & Codec delay is only one small portion of the whole problem and high quality codec increases codec delay and generates large VoIP packets \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and limitations of existing QoS solutions for VoIP over WMNs. VoIP service configuration and quality also depends on the type of codec. There are a number of codecs available, each having its own features and use cases.