The aim of this short report is to provide a technical comparison between the top two voice-over-internet-protocol (VOIP) applications for mobile phones, in order to help users decide whether it is feasible to use VOIP in place of regular GSM or CDMA cellular calls when not on WiFi, and which app would suit their needs best. The Skype and Viber mobile apps are available on the most popular mobile operating systems, and allow users to make calls using the internet instead of cellular networks. The apps have differing feature sets, but both apps offer voice calling, instant messaging (IM) and file sharing with other users. In addition, Skype offers video calling, while Viber offers integration with existing phone contacts (like Whatsapp) so there is no need for additional usernames (like Skype) or pins (like Blackberry messaging service) that have to be shared with other users first. Viber is only available on mobile platforms (IOS, Android, Windows phone, Blackberry), while Skype is available to Windows and Mac personal computers too. When using either of these (or other VOIP apps) over a WiFi internet connection, there are considerable cost savings to users compared to making calls over the cellular network. Most WiFi connections are either free, or the bandwidth is much cheaper than cellular bandwidth because of the bulk bandwidth usage over WiFi connections. When no WiFi connection is available, users would need to use cellular bandwidth to connect to the internet in order to use either of these two apps. However, cellular bandwidth is generally much more expensive than WiFi bandwidth, and without information about the amount of bandwidth consumed by these apps, it is impossible to determine whether this is a financially feasible or beneficial option to make calls. In addition to the bandwidth consumed while calling through apps like Skype and Viber, the apps also need to be continually connected to the internet in order to receive calls from other users, and without information about how much bandwidth is consumed during standby, it is impossible to determine whether it is worth keeping the app on constantly, which is necessary if it is to be used as an alternative to cellular voice calls. Other important considerations in comparing the apps include the performance of the apps in respect to battery drainage effects on the phone performance (CPU and RAM usage), although many consumers would agree that the affordability or cost-saving is the most important consideration.
METHODS
Skype and Viber apps are extensively tested on a mobile phone operating Android 4.0.1 (Samsung Galaxy S2). Bandwidth usage was recorded during standby and voice calls using the two apps. The Android OS has a native bandwidth logger which was used to compare the WiFi usage between the two apps, while the cellular service provider’s bandwidth logging / billing feature was used to record cellular bandwidth usage. Calls of varying length (1 min – 5 min) were made on the two apps using both WiFi and cellular bandwidth connections. The call length was consistent between the two apps, such that the bandwidth usage can be compared directly. Daily standby bandwidth usage was also recorded using the phone and service provider’s bandwidth loggers, and was based on a 16 hour day (07:00 – 23:00). The native battery monitor was used to log standby battery usage as a % of total battery drain in a 16 hour day. The battery usage during voice calling was also measured using the native battery monitor. For this the battery was fully charged, and then calls of equal length were made with each of the apps (Skype and Viber), after which the % of battery usage was recorded. It is important to note that this method of comparing battery drain does not provide absolute data on battery usage, but rather provides an accurate measure of the relative battery usage between the two apps, which is useful in determining which app performs best in this metric. The native CPU and RAM monitors were used to log the instantaneous demand on these phone resources by the apps, repeated at set time intervals.
RESULTS
Bandwidth usage
The average bandwidth usage for voice calls when using cellular bandwidth was very low at about 0.39MB/min. Consumers should compare the bandwidth cost of making voice calls with VOIP apps to using normal cellular voice calls on their specific network and call plan in order to determine whether it is a more affordable option. The Viber app used significantly less bandwidth than the Skype app for both voice calls and during standby.
Bandwidth usage between the apps during voice calls different significantly when using both WiFi (p=0.055, n=20) and cellular bandwidth (p=0.0001, n=20), with Skype using significantly more bandwidth than Viber in both cases (Figure 1, Table 1). The difference in bandwidth usage was greater when using cellular bandwidth, with Viber (0.36 ± 0.07 MB/min) using 30% less bandwidth than Skype (0.46 ± 0.05 MB/min). For a one hour call, this would equate to a difference of about 8 MB. Depending on what a user’s specific bandwidth plan and costs, this may amount to a material difference.
The real bandwidth usage during voice calls in these tests differed from that given by the respective app developers. For Skype, the actual usage in these tests (0.46 MB/min) was much less than that given on the website (between 1.76 – 4.10 MB/min), while for Viber the actual usage (0.36 MB/min) was slightly more than that given on the website (0.23 MB/min). It is possible that the bandwidth usage varies with differences in network and bandwidth quality or speed. Assuming that the developers tested bandwidth usage globally, it may account for these differences. However, in the tests described in this report, the standard deviation (a measure of differences between data points within the same group) was always very small.
There was also a significant difference (p=0.0101, n=20; Table 1) in the amount of bandwidth used during standby between the two apps. The Skype app uses approximately 0.77 ± 0.37 MB/16hour day or 23.87 MB/month and Viber uses approximately 0.05 ± 0.03 MB/16 hour day or 1.55 MB/month (Table 1). This is clearly a great difference, although whether or not it makes a material difference largely depends on the cost of bandwidth to the user. The standby bandwidth usage of Viber is less than that of Whatsapp, the popular messaging app (based on ad hoc testing, not reported here).
Phone resources
In general, both apps were very efficient in utilizing phone resources, and there is certainly no reason to not use either of these apps based on their effects on phone performance. Neither of the apps consumed measurable amounts of battery power during standby. The next most important consideration in my opinion is the amount of battery power consumed while making calls. In this regard, the Viber app used significantly less battery power than the Skype app (p=0.0007, n=8; Table 2). The actual values here (a percentage of battery power consumed during a 60 minute call) will vary from user to user (due to differences in phone configurations and battery capacities), but the relative difference is of meaning here (see Figure 2). To give an indication of how this compares to the battery usage during a cellular call (we are after all looking for a more affordable alternative to making cellular calls), the battery usage of the Viber app during calls was less than the battery usage during a cellular call on the Samsung Galaxy S2 (based on ad hoc testing, not reported here).
In contrast, the CPU usage of the Skype app was significantly less than that of the Viber app (p=0.0002, n=30; Table 2). This is somewhat confusing, since CPU usage is mainly responsible for battery usage. The RAM used by the Skype app during standby is also significantly less than that used by the Viber app (p>0.0001, n=20), while the size of the Skype app (21.74MB) is larger than the Viber app (15.54MB).
Figure 1. The mean bandwidth (data) usage (MB / minute) used by two VOIP apps for mobile phones during voice calls (bars represent means and whiskers represent standard deviation from the mean).
Figure 2. Comparison of the demand on phone resources by two VOIP apps for mobile phones (bars represent means and whiskers represent standard deviation from the mean, where relevant).
Table 1. Comparative means and standard deviations of bandwidth usage measured for two VOIP apps for mobile phones. The p-values were derived by t-tests (two-tailed, paired), with n indicating the number of data points.
Table 2. Comparative means and standard deviations of phone resource usage measured for two VOIP apps for mobile phones. The p-values were derived by t-tests (two-tailed, paired), with n indicating the number of data points.
CONCLUSIONS
Cellular calls vs. VOIP calls
Due to varying costs of cellular voice calls and cellular bandwidth (country and network specific), it is not possible to conclude whether it is universally more affordable to make calls through a VOIP app using cellular bandwidth than making regular cellular calls. However, by using the mean bandwidth usage (0.46 MB/min for Skype and 0.32MB/min for Viber) to calculate the cost of a VOIP call and comparing this with the costs for cellular voice calls charged by their network, users can determine for themselves which option is more affordable.
Skype vs. Viber
As far as the comparison between the two apps, the Viber app does trump the Skype app in most departments. The Viber app uses 30% less bandwidth for a voice call than the Skype app, and that can result in a meaningful difference in cost. When it comes to the standby bandwidth usage and battery usage during calls (both important factors if one is to use VOIP as an alternative to cellular calls), then Viber is by far a better choice, as it uses orders of magnitude fewer resources. The Skype app is of course still a great app in its own right if one wants to make video calls, and this is something the Viber app does not offer.
GLOSSERY
Bandwidth: equivalent data
Data: equivalent to bandwidth
Whatsapp: a popular texting app that uses cellular bandwidth instead of network-based sms / texting
VOIP: voice over internet protocol
Michael Turner
