How to approach problematic areas using simulators and more

It’s no secret that mobile devices are taking over the world. Today, it’s possible to complete basically any operation, anywhere, within a few seconds – all thanks to smartphones. This market is constantly growing, and slowly outrunning laptops, not to mention other stationary devices.

With the growing number of mobile devices on the market, the problem of how to keep up with the needs of users and provide them with high-quality software is also increasing. To meet these demands, we need a specific approach. That’s why testing for mobile apps is a completely different topic than web application testing.

To automate or not to automate? That is the question!

I think testing mobile applications is a good candidate for automation. It often allows you to provide high test coverage. However, it can be time-consuming and not very profitable due to the specifics of a project.

For over a year, together with my QA team, we tested specific, short advertisements – each of them unique. There were at least 5 of them a day. The space for automation was also limited and covered only basic elements such as detecting html5_interaction or playback of subsequent video elements and detecting whether the game ‘install’ button was clicked.

Test coverage

Our tests covered a wide range of devices: iPhones, the entire range of Android phones, and Amazon devices.

The test coverage included:

Division by system:

  • Android from 5.1.1 to the newest ( it’s now 10.0)
  • iOS from 9.3.5 to the newest ( it’s now 13)

Division by type of devices:

  • Amazon (in this case one device was enough, e.g. Fire HD 8 Tablet)
  • Low-spec devices – e.g. iPhone 5s or Samsung J3
  • High-spec devices – e.g. iPhone 8+ or Samsung Galaxy J7
  • Wide aspect ratios – e.g. Samsung Galaxy S8+ or Google Pixel 2
  • Old iPad and New iPad – e.g. iPad 2 Air old and the new generation
  • Android Tablet – e.g. Samsung Tab S3
  • iPhone X’s – this type of device generates a lot of visual issues so had a separate test case as a type of device

What tests were carried out and what were the most common problem areas?

Testing for mobile devices is not the same as testing desktop applications, not only regarding the number of devices but also the methods of testing them and the focus areas.

Testing for mobile apps problem 1: Scaling

We focused on scaling and loading the ad. When the company logo or inscriptions were covered, the issue had high priority. The phone’s notch, for example on the iPhone X or wide aspect ratio devices such as Samsung Galaxy s8 +, was a big problem (e.g. the notch covering half of the name of the advertised place).

Testing for mobile apps problem 2: iPads

Tests on iPads generated a lot of errors, due to the fact that they have the ability to rotate 360 ​​degrees. For this reason, there were often problems with some missing parts of the images that were not fully displayed, lack of screen adjustment. This sometimes even resulted in the video stopping or the entire advertisement jamming. The problems were so frequent that iPad fixes on the dev side “ruined” the functionalities of other devices or were simply not feasible. After taking all of the conditions under consideration, especially the challenging time frames for our tests, we decided to lower the priority of iPad fixes.

Testing for mobile apps problem 3: Functional side

Functional tests were performed in various combinations. The most problematic area, it turned out, was “the background issue”. Going back to the app after putting it in the background made some of the mechanisms in the ad fail. Another thing was that functions were failing to shut down after switching them to the background – for example, the music from the video remained on. This was the most common thing that happened with the videos.

Testing for mobile apps problem 4: Open store

Going to the store or opening dedicated links were also very important. It was quite a challenge to check the ad specific availability in a given country because when an item is not available in your country, the Android (Google Play) store will simply display this information. However, it’s not as easy with the App Store. In Apple’s case, you will receive a blank page and no information about what’s happened. Which obviously is not what we want for our users to experience.

Testing for mobile apps problem 5: Performance

Performance tests were carried out, using one of the tools for testing mobile apps that I recommend – Charles Proxy – something which I will elaborate on later in the article. It helped to simulate the slowdown of the internet up to 512 kbps, but we most often used 3G which was enough to induce the performance problems we were analyzing.

Tools for testing mobile apps: Charles Proxy

So what is Charles Proxy? According to their website

“Charles is an HTTP proxy / HTTP monitor / Reverse Proxy that enables a developer to view all of the HTTP and SSL / HTTPS traffic between their machine and the Internet. This includes requests, responses and the HTTP headers (which contain the cookies and caching information).

For me, Charles Proxy helps to monitor requests or to exchange the body of the request. For example, Xcode only has the simulator for iOS 9.3 but our tests had to be performed on iOS 9.3.5. So I had to rewrite the rule but to do that all I needed was to configure the file of the app and simply change the value in the body of the request

Note: To use simulators for iOS 9 you must have Xcode installed below version 11.1 because already in this version support system version for simulators is from 10.3.1 +

Charles Proxy: How to set it up?

To set up Charles Proxy in a way so it can read traffic between machines, all you need are a few, short steps:

  1. To Download Charles Proxy you need to go to: If you want to try it out first, there’s a free trial for 30 days.
  2. After installing and opening the app click on the tab Help -> SSL Proxying -> Instal Charles Certificate on a Mobile Device or Remote Browser
Testing for mobile apps - Charles Proxy - Certificate

Then we have information about the name and port of the proxy server, there will also be some info about the installation of the certificate from the site on your phone.

Where should you fill this data? It depends on the OS:

Android: Settings -> WiFi -> Manage network settings -> Show advanced options -> Proxy -> Manual -> Enter the Server Name and Port and click Save (example on Samsung Galaxy J3)

iOS: Settings -> Wifi (hold the given network) -> Configure Proxy -> Manually -> Enter Server and Port -> Click “Save” (iPhone 5s example) After entering and saving, go to to download the certificate.

3. The last thing you need to do is enable our SSL proxying to the wild card

Now you can see your traffic. Also, the one thing I mentioned earlier was, namely: slowdown the internet to check the performance of our ad or app – you can find this feature on Throttle Settings in the Proxy tab.


Note: For Android 7+ you need to add the XML file (or ask the developer for it) to your application along with the configuration file that allows you to monitor the connection. You can also find more information on how to do this in the documentation:

The basic test coverage for mobile devices

Buying every possible device to test the hell out of every application is quite expensive and requires you to always be up to date with new devices. If you have a tight budget and you’re not so great with time either, you have to consider which devices and systems are the most important for the tests. To decide the priorities, think about what systems are the most popular and then simply – test them. In the case of iOS, most users update to the latest version. Over time, Apple ceases the support of old versions of applications. Interesting fact: at this point on iOS 10.0.2 (e.g. iPhone 6s) there is no application that would allow us to record the screen.

In the table below you can see the usage of all iOS OS version:

Testing for mobile apps Adoptation trends

As far as Android is concerned, it is not as obvious as with the OS versions. There are still devices with Android 3 or 4 that people still use on a daily basis. On the plus side, versions of Android usually aren’t too different from each other. When there’s a bug it’s rarely found only in one version, usually, it occurs on most of the other systems too.

When we can’t use physical devices for our tests, we can use tools for testing mobile apps such as iOS Simulator or Android Emulator. From my experience, Apple’s Xcode Simulator is a very useful tool. In contrast to Apple, testing Android apps this way is way harder, and I would opt for using physical devices whenever possible. Why? I’ll explain in a second.


Now, let me tell you about the simulators and emulators that are available with Xcode for iOS and Android Studio for Android.

Simulator for iOS

IOS simulators have many options and can really reflect the real devices. Often, the bugs you can find on the simulator match the ones on the physical device. The simulator for IOS, just like a physical device, includes a silent button, sound buttons, locks, and home button. We have many types of devices to choose from and almost every OS version that has ever been released is available.

Tip: To use Charles Proxy for reading traffic with our iOS simulator we need to enable ‘macOS Proxy’ and Install Certificate on iOS simulator.

So if you don’t want to invest a lot of money in buying all types of devices, Xcode simulators will cover most of the basic tests.

Below there is an example of options in a simulator and other types of devices supported in Xcode:

Android emulator

Unfortunately, Emulators in the Android Studio generate a lot more issues than the iOS Simulator. In my experience, a lot of issues found on emulators do not occur on a physical device, so in the case of Android, it is better to buy a device or use device farms.

If you want to try it for yourself – to set up a new emulator you need to choose a new hardware type and OS version.

In the Android emulator, there are a lot of options to use, e.g.: the health of the battery, location, and type of network. It never harms to try all these out and see how they work for you. And if you don’t fall in love with it, just like I didn’t, check out Best device farms for ios and android


When it comes to testing whole systems, or even a few applications, which for financial reasons and time frames are not profitable to automate, you have to consider what physical phone resources you have, what your base test coverage may be and what tests the crucial ones for the application are. When you figure these things out, testing will be automatically much more effective and cost-effective.

Thanks for reading the whole article! I hope it provided a helpful dose of knowledge and helps you to find your feet in an era of the growing popularity of mobile devices and their testing.

Check out some other articles on testing on our Blog:

Introduction to OSI model and TCP/IP for Testers

Most applications out there run on the HTTP protocol, so having a solid understanding of this protocol will make your testing work much more manageable. We explored this in a previous post: What is HTTP protocol – introduction to HTTP for Testers. But there’s more to networks than just HTTP. In this post, we are going to dive deeper into networks by exploring the OSI model.

My main goal in this article is to show you the OSI model and explain how data flows in a network. Then I will go through the differences between the OSI model and TCP/IP. At the end of the article, I will also mention a few protocols used in networks.

But before we get into the details, I should explain some basic terminology.


LAN (Local Area Network) and WLAN (Wireless Local Area Network) 

Networking basics LAN WLAN

LAN is a local network that consists of a group of computers and devices connected via a single physical network (cables). It is limited to a specific geographic area/location.

An excellent example of this kind of network would be a library, office, or home. I don’t think most of us use a LAN in our homes these days, because a LAN connects devices via cables.  Nowadays, our devices are connected wirelessly via WIFI, so we’re talking about WLAN.

WAN (Wide Area Network)

WAN combines numerous sites and covers large geographic regions (connecting physically distant locations). The best example of this is the internet itself – that is, thousands of local networks (LAN / WLAN) connected. 

Another example would be connecting three company offices in different cities. Each office has its LAN. By combining them, we could create the company’s own internal network – WAN.

Networking basics WAN

Differences between IP and MAC address

You have probably already heard of and know something about what an IP is. However, you may not have met the concept of a MAC address. So, let me explain in a few words what an IP is, and then a MAC address, to illustrate the key differences between them.

IP (internet protocol) 

We use IP for communication between different networks (to address and transport data from one network to another). It performs the role of routing, i.e., searches for the fastest route to pass a data packet. An IP address is a logical address – this means that it is allocated depending on which network the device has been connected to. If a device is in two networks, it will have two IP addresses.

MAC address (Media Access Control)

MAC is a physical address with a unique identifier burned out on the network card. It identifies specific devices and is assigned by the manufacturer. MAC addresses are used for communication within one network, e.g., in a home network, if you want to connect a computer to a printer or other devices, it will use MAC addresses to do that.

Key differences to remember



Logical address

Physical address

Identifies connection with a device in the network

Identifies device in the network

Assigned by the network administrator or ISP (internet service provider)

Assigned by the manufacturer

Used in WAN communication

Used in LAN/WLAN communication

OSI model

The OSI model has never been directly implemented as it’s mostly a reference architecture on how data should flow from one application to another through a network. TCP/IP is used, and these days it’s the most popular. After the OSI model, I will say more about TCP/IP. But it’s good to start with the OSI because it’s easier to understand some of the concepts. 

Networking basics OSI model

The OSI model consists of 7 layers divided into two groups:

  • Host layers (happening on the computer side. Responsible for accurate data delivery between devices)
  • Media layers (happening on the network side. Responsible for making sure that the data has arrived at its destination)

7. Application layer

In this layer, the user directly interacts with applications. Here is decided which interfaces are used to interact with the network through the corresponding protocols in this layer. 

Examples of such applications are chrome or Gmail:

  • Chrome uses the HTTP / HTTPS protocol
  • Gmail uses email protocols like SMTP, IMAP.

The applications themselves are not in the application layer – in this layer, there are only the protocols or services that the applications use.

6. Presentation layer

The task of this layer is proper data representation, compression/decompression, encryption/decryption. This ensures that the data sent from the X system application layer can be read by the Y system application layer.

5. Session layer

This layer is responsible for creating, managing, and then closing sessions between two applications that want to communicate with each other. 

4. Transport layer

The task of this layer is to make sure that the data has arrived safely from the sender to the recipient. When it sends data, it breaks it into segments. When it accepts data, it puts it back into a stream of data.

Networking basics Transport Layer

In this layer  two protocols are used: TCP and UDP (later on in the article I’ll be saying more about these)

3. Network layer

Provides addressing and routing services. It defines which routes connect individual computers and decides how much information to send using one connection or another. Data transferred through this layer are called packets.

Places two addresses in the packet sent:

  • Source address
  • Destination address

This layer is based on IP (internet protocol).

2. Data-link layer

This layer deals with packing data into frames and sending them to the physical layer. It also oversees the quality of the information provided by the physical layer. It recognizes errors related to losing packages and damaging frames and deals with their repair. 

1. Physical layer

This is the physical aspect of the network. This applies to cables, network cards, WIFI, etc. It is only used to send logical zeros and ones (bits). It determines how fast the data flows. When this layer receives frames from the data link layer, it changes them to a bitstream.

Encapsulation and decapsulation of data

Encapsulation and decapsulation of data

Encapsulation adds pieces of information to data sent over the network. This occurs when we send data. At each layer, some information is added to our data. We combine the address of the sender and recipient, the encryption method, data format, how the data will be divided, sent, etc.

Decapsulation occurs when we receive information. It consists of removing pieces of information collected from the network. At each layer, some info disappears. In the end, the user only gets what he should receive without the IP, MAC address, etc.

Differences between the OSI model and TCP/IP

The TCP/IP model has a similar organization of layers to the OSI model. However, TCP/IP is not as rigorously divided and better reflects the actual structure of the Internet.

Networking basics Differences between OSI model and TCP:IP

In TCP/IP, there are only four layers:

  • Application layer
  • Transport layer
  • Internet layer
  • Network interface layer

The OSI model makes a clear distinction between layers and some concepts. In TCP/IP, it is harder to make this clear distinction and explain these concepts. Now you can see why I introduced to you the OSI model before the TCP/IP.

The TCP/IP application layer contains three layers from the OSI model:

  • Application layer
  • Presentation layer
  • Session layer 

The working of the application layer in the TCP/IP is a combination of these three layers from the OSI model. In this layer, we have various protocols such as HTTP, DNS, SMTP, FTP. 

The transport and internet layers in TCP/IP work, as I described in the OSI model. But in the next section, I will be revealing more details on how the transport layer protocols (TCP and UDP) work.

The network interface layer in TCP/IP is a combination of two layers form the OSI model (data link and physical layer). I’m not going to go into the details of this layer. But in the OSI model, I described the critical functions of these two last layers. Here in TCP/IP, these functions are realized in one layer.

Protocols in the TCP/IP model

Internet layer protocols

ARP (Address Resolution Protocol)

Used to identify the MAC address. If the device knows the IP address of the target device, then ARP sends a request to all of the devices in the LAN to search for the MAC address of the device with the given IP. Then the device with that IP sends an ARP response with its MAC address. 

This information will be saved in the ARP table. In windows or macOS, open terminal and type arp -a. Then you should see the ARP table.

In the image below, you can see how this process works when an ARP request matches the IP of the device.

Networking basics

The RARP protocol performs the reverse operation.

IP (Internet protocol)

I explained at the beginning of this article what IP is. But I want to make clear that the IP in the TCP/IP model is in the internet layer. It is also good to add that IP has two versions.

  • IPv4
  • IPv6

The second one has been introduced because IPv4 addresses are ending. IPv6 is more efficient, has better routing, and is safer. 

ICMP (Internet Control Message Protocol)

This acts as a tool for solving problems. The ICMP reports any communication errors between hosts. ICMP messages can help to diagnose a problem. For example, if the router or host is overloaded, ICMP can send a message to slow down the transfer rate.

ICMP is used in the ping program, which allows the diagnosis of network connections. Ping lets you check if there is a connection between the hosts. It also allows you to measure the number of packets lost and delays in their transmission.

In the terminal, type ping After ping, you need to provide the host. You can choose any website. I’m going to check my connection with the scalac site. To exit ping, use CTRL + C.

Ping sends an ICMP packet to the host provided. In my case, I sent 17 packets and received back 17 packets. In this short connection, I didn’t lose any packets. The program also counts the time gap between sending and receiving packets. In the end, the program summarizes the connection and shows us the minimal/ average / maximum time gap between sending and receiving packets.

Transport layer protocols

TCP (Transmission Control Protocol)

TCP is a highly reliable and connection-oriented protocol. It applies the 3-way handshake principle. Before it sends any data, it will first establish a connection.

Networking basics - Transmission Control Protocol

This rule consists of three steps, made to establish a connection.

  1. SYN – The device sends a message to the server, “I want to connect with you.”
  2. SYN / ACK – When the server receives the message, it will reply that it is ready for communication.
  3. ACK – The device sends confirmation of receiving the response from the server and that it is ready for communication.

The high reliability of TCP is due to the device, making sure that the data sent has been received by the server. Then the server makes sure that the data sent to you have been collected by you. If the server sends 10 data packets, and for some reason, you do not receive one of them, and you do not confirm the receipt, this server will try to send the lost package again. 

TCP also provides data delivery in order. Each sent packet is numbered. Although packets may still arrive out of order, TCP will arrange them in order before sending them to the application.

To summarize the advantages of TCP:

  • Set up a connection before sending any data
  • Data delivery acknowledgment
  • Retransmission of lost data
  • Deliver data in order

UDP (User Datagram Protocol)

UDP sends data and doesn’t care if the device has received it or not. It also doesn’t care if some packets are lost. But the significant advantage of the User Datagram Protocol is that the packet sizes are smaller than TCP (about 60% lighter). 

Networking basics connection

UDP is an economical version of TCP. 

  • Connectionless and unreliable.
  • No data retransmission
  • No data delivery acknowledgment
  • Data can arrive out of order

You may ask the question, then why use UDP? It’s such an unreliable protocol!

In some cases, UDP is better because TCP has significant overheads (data retransmission, delivery acknowledgment, etc.) UDP is often used to transmit data in real-time: video streaming or audio such as Skype calls.

Application layer protocols

Network management protocols

DNS (Domain Name Services) – Changes the domain name to an IP address. The domain name is used because it’s human-friendly. It’s easier to remember a domain name ( than an IP address ( When you type any website address into a browser, then the browser sends a request to the DNS for the IP address of that domain.

Networking basics management protocols

If you type into a browser IP, then you should see the Google page because this is Google’s IP address. I can get it directly by requesting the DNS in the terminal. Type in terminal: nslookup 

NTP (Network Time Protocol) – This is an uncomplicated and straightforward protocol. It is used for automatic time synchronization in devices connected to a network. Imagine now manually synchronizing time for 10 or 50 devices. This would be ineffective.

Some devices, procedures, or safety mechanisms require accurate time synchronization for proper operation. Also, thanks to the NTP, finding the causes of any network or device errors is easier. Because using the logs, we will be able to find out what the order of events was that caused the failures.

SNMP (Simple Network Management Protocol) – This is used for monitoring, management of updates, and diagnostics of networks and network devices.

Remote authentication protocols

SSH (Secure Shell) – This allows you to remotely log in to the terminal in network devices and administer them (e.,g. router, firewall, a remote server). SSH is secure because communication is encrypted. SSH uses the TCP protocol.

File transfer protocols

FTP (File Transfer Protocol) –  The purpose of this protocol is to display a list of files/folders, adding, deleting, or downloading them from the server. A good example is sending website files to a server. To do this, you need to use an FTP client with which you can authenticate yourself and get access to the FTP server. A popular FTP client is FileZilla. FTP uses TCP. 

A significant flaw of FTP is the lack of data encryption. Therefore, to ensure secure authentication and transfer of files, it is worth using FTPs (FTP Secure and FTP-SSL) or SFTP (SSH File Transfer Protocol). They work in the same way as FTP but extend its functionality by encrypting the transmitted data.

Email protocols

SMTP (Simple Mail Transfer Protocol) and IMAP (Internet Access Message Protocol) are two protocols used in sending and receiving emails. SMTP’s task is to send email messages from a client to an email server or between email servers. IMAP is used to manage and retrieve email messages from an email server.


This image shows an example when a sender ( and a recipient ( have different email service providers.

  1. In the beginning, the email message is sent to the sender’s email server (Gmail)
  2. Then the Gmail email server sends an email message to the recipient’s email server  (WP)
  3. Finally,  IMAP retrieves the email message from the wp email server to our client.

When the sender and recipient have the same email service provider (Gmail), step 2 will be skipped.

Browser protocols

HTTP/HTTPS – I have written a separate article on HTTP. You can read it here: What is HTTP protocol – introduction to HTTP for Testers. I explain there exactly how HTTP works. HTTPS extends HTTP functionality with data encryption protocols.

VoIP protocols (Voice over IP)

SIP (Session Initiation Protocol) – This performs an administrative function (using TCP). It is used only to set and close an audio or video connection.

RTP (Real-Time Transport Protocol) – This is used to transfer data during audio or video calls (using UDP).

For example, let’s say you want to call someone on Skype. SIP will be used to establish the connection. When the connection is established, the RTP springs into action and transmits the data. When you end the conversation, SIP will close the connection.


You have come to know many new concepts today. You now know how data flows in networks. They go through a rather complicated process. All of the topics I have touched are so extensive; they could easily have a separate article for themselves. However, I have tried to present them to you at a fairly general level, easy to understand. Without going too deeply into the more technical aspects.

If you think I have managed to explain things understandably and interestingly, please share this article on social media. And if you have any questions, also feel free to ask them in the comments below.

If you’re testing web apps then you need to know what the purpose of HTTP protocol is and how it works. Actually, in my first project I lacked this knowledge. However, since then I’ve realised how important it is and that’s the reason I want to share it with you. In this article I will show you the basics of HTTP protocol. For example, how the BE (back-end) and FE (front-end) communicate, where you can see the data and how you can use this knowledge to add extra information to your bug tickets.

Hopefully, with a little practice, you’ll be able to determine where the error lies (BE or FE). Which is something your developers will thank you for. So without further ado – let’s get started!

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