# Interconnecting Haptic Interfaces with High Update Rates through the Internet +
## 5 Simulation Results
### Packet Arrival Deviation  
Packet Arrival Deviation (PAD) is a metric proposed by the authors for the performance evaluation of real-time haptic protocols. It is similar to jitter, but it can offer more precise picture of the real-time network delay conditions than jitter, since it can take into account both the receiver end for the time variation of received packets (packet reception jitter at the receiver end), as well as the ACK packets reception time variation at the sender (packet reception jitter at the sender end). Moreover, it takes also into account the changes in the sending rate of the source focusing on the fluctuations of the Internet bandwidth. Table 1 does not include the PAD, as it is only presently proposed by the authors. The upper limit of PAD should be equal to jitter's, which is 2 ms.  
Figure 14 displays the standard deviation of the packet arrival for different delays. The Internet bandwidth is 20 Mbps. The protocols with the higher standard deviation are the TCP, SCTP and the DCCP protocols. UDP and RTP and ETP protocols present very small standard deviation of packet arrival, lower than 2.5 ms. This difference between TCP, SCTP, DCCP and the UDP, ETP, RTP is the way they send their packets. The first group sends its packets in burst inside a congestion window (CWND), while the second group sends its packets with an almost steady inter packet gap.  
Figures 15 and 16 show the Packet Arrival Deviation when the bandwidth of the network changes from 1 to 20 Mbps. SCTP and DCCP and TCP have been excluded from Figures 15 and 16 for 1 Mbps as they showed unacceptable values of PAD. The conclusions of this graph are similar to those of Figure 14. TCP shows the highest PAD as it uses the congestion control windows. UDP and RTP show very little PAD because they do not enforce any congestion control algorithm. For network bandwidth higher than 10 Mbps, there is no network congestion and all protocols show the same behavior.  
Figure 14: Packet arrival deviation for internet bandwidth 20 Mbps.  
Figure 15: Arrival deviation for internet delay 40 ms.  
Summarizing, not all transport protocols should be used for transferring haptic data over the Internet. Protocols that include TCP-like congestion control algorithms, such as the TCP, the SCTP and the DCCP, should be avoided in heavily congested networks. Timely delivery protocols such as the UDP, the ETP and the RTP are more suitable for real-time haptic data transferring. The UDP protocol is being used both by the ETP and the RTP protocol. When the haptic payload is 64 bytes, the efficiency of UDP is 88.88%. This is very crucial, as haptics demand very high sending rate. When the network is congested, the packet loss is rather high (Figure 4). Figures 7 and 9 illustrate network behavior for 20 and 5 Mbps bandwidth with 40 ms delay. In these network conditions the UDP and the RTP protocols have steady throughput. The UDP protocol consumes lower bandwidth, as it is more efficient. The ETP protocol interacts very well when the network conditions are sufficient. UDP and RTP retain lower jitter and PAD from the other protocols in all network conditions. The ETP outperforms the UDP and the RTP when the network is heavily congested (Figures 11 and 12). ETP in order to avoid congestion, it decreases its sending rate by increasing the inter packet gap. Lower packet loss, jitter and PAD are the outcome of this reduction. The RTP and the UDP have better performance in most network conditions. The ETP protocol applies congestion control algorithms and should be used in heavily congested networks.