{/* banner prompt: Of course! Here are a few creative and engaging prompts for a banner image, designed to capture the essence of the blog post.

### Main Prompt (Cinematic & Evocative)

This prompt focuses on the "resurrection" theme of bringing old hardware back to life with modern, efficient software.

**Prompt:**
> A cinematic, dramatic shot of a dusty, forgotten 90s-era beige PC tower sitting on a workbench in a dimly lit workshop. A brilliant, glowing blue USB cable connects a modern Android smartphone to the PC's front port, acting as a lifeline. Ethereal, glowing data streams, shaped subtly like the Alpine Linux mountain logo, emanate from the PC's vents, illuminating the scene and casting long shadows. The old machine hums with new purpose, transformed into a powerful network gateway. Moody, high-contrast lighting, tech noir aesthetic, hyper-realistic, 8k. --ar 16:9

---

### Alternative Prompts

Here are two other options with different artistic styles.

#### Option 2 (Technical & Stylized)

This prompt emphasizes the technical, blueprint-like nature of the project.

**Prompt:**
> A stylized, futuristic blueprint schematic on a dark grid background, illustrating the post's network topology. At the top, a sleek icon of an Android phone connects via a glowing USB data line to a central, semi-transparent block representing an old computer. The iconic Alpine Linux mountain logo glows brightly from within this central block. From the block, a solid ethernet data stream flows out to a home router icon. The entire image has a clean, technical, and cyberpunk feel, with digital annotations and connection labels. Widescreen, vector illustration style, highly detailed. --ar 16:9

#### Option 3 (Minimalist & Symbolic)

This prompt focuses on the core components in a clean, modern, and symbolic way.

**Prompt:**
> A minimalist composition on a clean, dark grey background. In the center, a simple, iconic representation of an old PC case is given new life. A single, vibrant cyan USB cable plugs into it from one side, originating from an abstract smartphone shape. From the other side, a stable, glowing ethernet cable extends outwards. The Alpine Linux mountain symbol is subtly but clearly etched onto the side of the PC case, glowing with a soft white light. The image conveys elegance, efficiency, and the concept of "powerful simplicity." Professional product shot style, soft studio lighting. --ar 16:9 */}

# Building a 4G/5G Backup Mobile Gateway with Alpine Linux on Old Hardware

*How to repurpose an outdated PC into a reliable backup internet gateway using Android USB tethering*

---

## The Problem

Internet goes down. It always does at the worst possible time. However, even though that only happens a couple of times a year, an always-on 4G/5G backup isn't strictly necessary. It would be nice to have a plug-and-play solution, like using your phone (which probably has plenty of data) as a rescue for those few hours of downtime.

If you have a spare Android phone and an old PC gathering dust, you can build a proper backup gateway that kicks in when your main connection fails, with no expensive hardware required.

This guide walks through the entire process, including the real-world gotchas encountered along the way.


## Hardware Requirements

- An old/low-powered PC (sub 1GHz, even under 512MB RAM is workable)
- One or two ethernet NICs (two is ideal, one works fine)
- A USB port for the Android phone
- An Android phone with a mobile data plan
- A USB stick (2GB+ is plenty) to install from

For this project, I used a rugged industrial PC made by **Advantech Australia**. It's equipped with an **Intel Atom D510 CPU** and **2GB of memory**. Its industrial build and dual NIC cards make it a perfect "warhorse" for a dedicated gateway role. 

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/2026/04/repurpose-your-old-pcs-how-to-install-alpine-linux-on-them/2026-04-20-19-42-33.png)

The machine used in this guide had two onboard NICs, an 8GB internal flash module (DOM, or Disk on Module), and a larger hard drive set aside for backups. Very modest specs, perfectly adequate for a router role. Please execute me for the dust collected on this machine.


## Why Alpine Linux?

For low-powered hardware doing a single job, Alpine Linux is the right tool:

- Boots in under 5 seconds
- Idles at ~30–50MB RAM
- Tiny base install (~130MB on disk)
- Uses OpenRC (no systemd overhead)
- The `rndis_host` kernel module handles Android USB tethering natively, with no extra drivers needed
- Actively maintained, security-focused

Other options like Debian minimal or OpenWrt on x86 are also viable, but Alpine wins on resource footprint for truly constrained hardware.


## Network Topology

With two NICs the setup is clean, with no double-NAT:

```
Android Phone (USB tethering)
        |
        | usb0
  [Alpine Linux PC]
   eth0  |  eth1
    |         |
Your Router  (spare / unused)
 (WAN port)
```

With a single NIC, the same approach works: `eth0` connects to your router's WAN port, `usb0` is the phone. Your router handles the LAN side as normal. The Alpine box just NATs between `usb0` and `eth0`.

> **Note on double-NAT:** With a single NIC and a typical home router, you'll have three layers of NAT (phone → Alpine → router). For general internet use this is completely fine. Port forwarding gets complicated, but for a backup scenario it's a non-issue.


## Step 1: Creating the Install USB

Download the Alpine Linux "standard" x86 ISO from [alpinelinux.org](https://alpinelinux.org/downloads/).

Write it to a USB stick using `dd`:

```bash
dd if=alpine-standard-x.x.x-x86.iso of=/dev/sdX bs=4M status=progress
```

Replace `/dev/sdX` with your USB stick's device. **Double-check this**, because `dd` will happily wipe whatever you point it at.

> **Important:** Use `dd` directly, or Rufus in **DD mode** if on Windows. Do not let Rufus create a FAT32 partition layout, because old BIOSes can choke on it. Alpine ISOs are hybrid MBR images and work correctly when written raw.


## Step 2: BIOS Boot Gotchas (American Megatrends)

Old AMI BIOS boards have a few quirks that can stop USB boot from working even when USB is set as first priority:

- **Plug the USB in before powering on.** USB drives often only appear in the boot device list if they're present at POST time.
- **Check for two separate boot priority menus.** AMI BIOS often has a top-level device-type priority (Removable → HDD → CD) *and* a separate submenu for ordering within each type. The USB stick needs to be first in *both* places.
- **Enable Legacy USB Support**, usually found under Advanced or Integrated Peripherals.
- **Use a rear USB 2.0 port.** Front panel ports and USB 3.0 ports are often ignored by old BIOSes during boot enumeration.
- **Use a USB 2.0 flash drive** if possible. USB 3.0 sticks can fail to enumerate on old BIOSes.
- **Try the one-time boot menu first**: on most AMI boards, hit **F8** or **F11** at POST. This bypasses the boot order entirely and lets you pick the USB stick directly, which is useful for confirming the stick itself is bootable before adjusting BIOS settings.

In my case, I was finally able to have it boot into the Alpine Linux installation interface after plugging the USB drive into the back (rear) of the machine.

---

## Step 3: Identifying Your Disks

Once booted into the Alpine live environment, identify your drives before installing:

```sh
cat /proc/partitions
```

Example output:

```
8    0  312571224  sda     ← ~150GB (Windows drive)
8   16    7880544  sdb     ← ~7.5GB (target install drive)
8   32    3910656  sdc     ← ~3.7GB (Alpine USB stick)
```

To confirm the nature of a drive, particularly whether it's a USB device or an internal flash module:

```sh
dmesg | grep sdb
```

If the output references `sd X:X:X:X` (SCSI enumeration) rather than `usb-storage`, the drive is connected internally, likely a DOM (Disk on Module), common on older appliance-grade hardware. These are perfectly fine for Alpine and handle the low write workload of a router OS well.

> **Tip:** A drive formatted as FAT32 with write cache disabled and a single partition spanning the whole disk is a typical signature of a factory-installed DOM or embedded flash device.

---

## Step 4: Installing Alpine

Run the setup script:

```sh
setup-alpine
```

Work through the prompts:

- **Keyboard layout:** your preference
- **Hostname:** something useful, e.g. `mobile-gateway`
- **Network interface:** configure `eth0` for DHCP during install; skip `eth1` for now
- **Root password:** set something strong
- **Timezone:** your local timezone
- **NTP client:** choose **chrony** (see note below)
- **Mirror:** pick a geographically close one or `r` for auto-detect
- **SSH:** `openssh` is fine
- **Disk mode:** type `sys`, which is a standard permanent install to disk
- **Which disk:** type the device name of your target drive (e.g. `sdb`)

The installer will wipe the selected disk and install Alpine. When finished, remove the USB stick and reboot.

### Why chrony for NTP?

For a machine that's offline frequently or has intermittent connectivity, exactly the backup router use case, chrony is the best choice. It handles time jumps gracefully when connectivity comes and goes, and compensates for clock drift between connections. `busybox-ntpd` is too basic for this, and `openntpd` handles irregular connectivity less gracefully than chrony.

---

## Step 5: Post-Install (Making It Boot)

If the machine boots back into another OS after install, the BIOS is still preferring the wrong drive. Go back into BIOS and find the **Hard Disk Drive Priority** submenu and set your Alpine drive (the 8GB/DOM one) as first in that list.

Use the one-time boot menu (F8/F11) to test first: select your Alpine drive directly to confirm it boots before making the BIOS change permanent.

---

## Step 6: Configuring USB Tethering

Plug in the Android phone and enable **USB Tethering** from the phone's settings (usually under Settings → Network → Hotspot & Tethering).

The Alpine kernel will load `rndis_host` automatically and a new interface will appear, typically `usb0`:

```sh
ip link show
```

Bring it up and get an IP via DHCP from the phone:

```sh
udhcpc -i usb0
```

Test connectivity:

```sh
ping -c 3 1.1.1.1 -I usb0
```

---

## Step 7: NAT and IP Forwarding

Install iptables if not already present:

```sh
apk add iptables
```

Enable IP forwarding:

```sh
echo 1 > /proc/sys/net/ipv4/ip_forward
```

Set up NAT (assuming `usb0` = WAN, `eth0` = LAN out to router):

```sh
iptables -t nat -A POSTROUTING -o usb0 -j MASQUERADE
iptables -A FORWARD -i eth0 -o usb0 -j ACCEPT
iptables -A FORWARD -i usb0 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
```

Make IP forwarding persistent by adding to `/etc/sysctl.conf`:

```
net.ipv4.ip_forward = 1
```

Save and persist iptables rules across reboots:

```sh
rc-update add iptables
/etc/init.d/iptables save
```

---

## Step 8: Hotplug Script (Plug In and It Just Works)

Rather than running commands manually every time, Alpine's `mdev` can detect when the phone is plugged in via USB and automatically bring up the tethering interface and routing.

Create `/etc/mdev/usb-tether.sh`:

```sh
#!/bin/sh
# Called by mdev when a USB network interface appears

IFACE="usb0"

if [ "$ACTION" = "add" ] && ip link show "$IFACE" > /dev/null 2>&1; then
    udhcpc -i "$IFACE" -q
    iptables -t nat -A POSTROUTING -o "$IFACE" -j MASQUERADE
    iptables -A FORWARD -i eth0 -o "$IFACE" -j ACCEPT
    iptables -A FORWARD -i "$IFACE" -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
fi
```

Make it executable:

```sh
chmod +x /etc/mdev/usb-tether.sh
```

This gives you plug-in activation: connect the phone, USB tethering enables on the phone, and the gateway comes up automatically. Unplug to deactivate.

---

## Second NIC: What to Do With It

If your machine has two NICs, you have more flexibility:

- **eth0**: connect to your router's WAN port (LAN out)
- **eth1**: leave unconfigured unless you need a second downstream device connected directly

There's no need to bridge or assign an IP to `eth1` for a basic single-router setup. Leave it unconfigured and use it later if needed.

---

## Summary

{/* | Component | Choice | Reason |
|-----------|--------|--------|
| OS | Alpine Linux | Minimal footprint, fast boot, good hardware support |
| NTP | chrony | Handles intermittent connectivity gracefully |
| Tethering | Android USB (rndis_host) | Native kernel support, no extra drivers |
| Install mode | sys | Standard permanent disk install |
| Activation | mdev hotplug | Plug-in and it just works | */}

The end result is a machine that sits quietly on the shelf, draws minimal power, and becomes a functional mobile gateway the moment you plug a phone into it: no GUI, no fuss, no expensive hardware.

---

*Built and tested on an old low-powered PC with AMI BIOS, dual onboard NICs, and an internal DOM flash drive. Alpine Linux 3.x.*

e8479c4c8a0735d56539ded3d444abb51329c469dedde23e34a1f7099446d78a

A step-by-step guide to installing Alpine Linux on old, low-powered hardware and configuring it as a mobile backup gateway using Android USB tethering. Covers BIOS quirks, disk identification, NAT/iptables setup, and hotplug automation with mdev.

Turn an old, dusty PC into a reliable 4G/5G backup internet gateway using Alpine Linux and Android USB tethering, with no expensive hardware required.

Repurpose Your Old PCs: How To Install Alpine Linux on Them

{/* banner prompt: Here is a creative and engaging prompt for a banner image, based on the blog post:

---

**Prompt:**

A cinematic, digital art representation of network Quality of Service (QoS) fixing a poor VoIP call. On the left side of the image, a chaotic and fragmented sound wave, glowing with angry red and orange static, is bombarded by a storm of jumbled data packets representing video streaming and file sharing. In the center, this broken signal flows into a sleek, futuristic router icon that acts as an intelligent filter, visibly organizing the data streams. Emerging from the right side of the router is a perfect, serene, crystal-clear sine wave, glowing in a calm, brilliant blue, traveling smoothly through a clean, open digital space. The background is a dark, high-tech grid, emphasizing the contrast between the digital chaos and the resulting clarity. --ar 16:9

Additional instructions: If you use an router, use an abstraction router not a real one */}

## Background

To establish a direct connection between the office and the cloud, I reconfigured my client's network settings in the office.

Below is the previous network diagram:

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/trak-network-pre.svg)

Internet is provided by NBN with a DSL connection (on 50/30 plan), and the speed is not very ideal. As you can see from the diagram above, computers and other devices in the office are connected to a Meraki router/security appliance (MX64W), which is then connected to the ISP-provided modem/router. All phones are connected to a PoE switch, which is connected to the modem/router. With this configuration, we have difficulties monitoring and managing the entire network securely and remotely as the WAN-facing router simply doesn't have the capabilities to do so.

The new network diagram:

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/trak-network-post.svg)

With the modem set to bridge mode, the new configuration lets the Meraki router manage the entire network, which is much more secure and easier to manage. Through a generic and a PoE switch, there are approximately 30 devices in total, all connected to the Meraki router.

However, after the new configuration, the sound quality of the VOIP calls dropped to a very low level. The calls were crackling and static noise could be heard in the background. The staff were not happy about this issue. In the old configuration, the network traffic was handled by two different networking devices, and VoIP traffic was not interfered with by other network traffic. Now all the inbound/outbound traffic is handled by a single Meraki router.

## Solution

### ChatGPT

Actually, it is a good time to test the current AI's ability in assisting IT professionals to solve their problems. A simple query with ChatGPT:

```
What is the best way to fix VOIP poor sound quality and calls are cracking?
```

Below is the response from ChatGPT:

#### Troubleshooting Poor VoIP Call Quality

If your VoIP calls have cracking sounds or poor quality after changing your network environment, it is likely due to network settings or hardware issues. Here’s how to identify and fix the problem:


##### **Common Causes**
1. **Bandwidth Issues**
   - Insufficient upload/download speed can lead to packet loss or jitter.
   - VoIP calls require stable bandwidth.

2. **Quality of Service (QoS) Settings**
   - VoIP traffic may not be prioritized, causing delays or interruptions.
   - Heavy internet usage (e.g., streaming) can degrade call quality.

3. **Network Equipment**
   - Lower-quality switches, routers, or cables might cause issues.
   - If phones are powered via PoE, insufficient power can lead to problems.

4. **Jitter or Latency**
   - VoIP calls require low latency ( &lt;150ms) and minimal jitter.

5. **Packet Loss**
   - Even minor packet loss can cause cracking sounds in calls.


##### **How to Fix It**
###### 1. **Check Network Bandwidth**
   - Run a speed test to ensure sufficient upload/download speeds.
   - Reserve at least **100kbps per call** in both directions.

###### 2. **Configure QoS**
   - Log into your router's settings and enable QoS.
   - Prioritize traffic from VoIP device IPs or specific VoIP ports (e.g., SIP, RTP).

###### 3. **Upgrade or Optimize Hardware**
   - **Switch**: Use a managed PoE switch with better power output and QoS support.
   - **Router**: Ensure it’s VoIP-compatible and supports QoS.
   - **Cables**: Use CAT5e or CAT6 cables for better signal quality.

###### 4. **Adjust VoIP Codec Settings**
   - Use a low-bandwidth codec like **G.729**.
   - Use **G.711** for higher audio quality if bandwidth allows.

###### 5. **Minimize Interference**
   - Avoid heavy data usage during calls.
   - Create a **separate VLAN for VoIP traffic** to isolate it from other network traffic.

###### 6. **Test Power Supply to VoIP Devices**
   - Check the switch's PoE budget.
   - Upgrade to a higher-wattage PoE switch if needed.

###### 7. **Monitor and Troubleshoot the Network**
   - Use tools like **PingPlotter**, **Wireshark**, or your VoIP provider’s diagnostics to check for jitter, latency, or packet loss.

###### 8. **Contact Your VoIP Provider**
   - They may need to adjust settings like the jitter buffer or perform further troubleshooting.

##### **Recommended Equipment for Your Business Setup**
For a setup with 8 phones, a boardroom, and workstations:
1. **Managed PoE Switch**: 
   - Consider **Cisco Catalyst** or **Ubiquiti UniFi** switches.
2. **Router**:
   - Use a business-class router like the **Meraki MX series** or similar devices with strong QoS support.


##### **Network Configuration**
1. **Create VLANs**:
   - Separate VoIP traffic from general office traffic for better isolation and performance.
   
2. **Enable QoS**:
   - Prioritize traffic on the VoIP VLAN or by device IP/MAC addresses.
   
3. **Test the Network**:
   - Use tools like **Wireshark** or **PingPlotter** to identify and resolve jitter, latency, or packet loss issues.

For the suggestions above, we are already operating at the maximum speed available from NBN at this office location, so increasing bandwidth is not an option.

So to fix the issue, what we can do are: 1) we need to configure the Meraki router to prioritize the VoIP traffic, and 2) we need to create VLANs to separate the VoIP traffic from the general office traffic if the first step does not help solve the issue.

Let's get started with the first step: configuring the Meraki router to prioritize the VoIP traffic.

After logging into the Meraki Dashboard, from the left-hand side pane, go to "Security & SD-WAN" -> Configure -> SD-WAN & Traffic Shaping to access the "SD-WAN & Traffic Shaping" page.

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/2024-11-28-16-47-10.png)

Now scroll down to the "Traffic Shaping rules" section.

We set up rules for traffic shaping, and the VoIP traffic will be prioritized so that it will not be affected by other traffic. We also want to limit the bandwidth usage of file sharing, social web & photo sharing, and all video & music streaming traffic so that they won't affect the VoIP service.

Below is the screenshot of the new "Traffic Shaping rules":

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/2024-11-28-16-47-15.png)

After setting up the above rules, the sound quality of the VoIP calls instantly improved. No more complaints from the staff. The ticket was closed.

To further improve the network performance, we also created a new VLAN for VoIP traffic:

From "Security & SD-WAN" -> Configure -> "Addressing & VLANs", we can create a new VLAN for the VoIP traffic.

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/2024-11-30-11-05-36.png)

![](https://storage.googleapis.com/www-tyolab-com-blog-assets/images/blog/client-network/2024-11-30-11-04-38.png)

If you do not have a Meraki router, you can still use the same method to prioritize VoIP traffic as long as your router supports the QoS feature. Most routers on the market support QoS. If not, consider using embedded platforms like OpenWRT, DD-WRT, Tomato, etc., as long as they support your router.

If you are looking for a business-class router that supports QoS, check out the [Meraki MX series](https://store.tyo.com.au/shop/meraki/sd-wan/). The MX series is a great choice for small and medium-sized businesses that have little to no IT support or a small IT team and require robust security, advanced QoS features, and simplified cloud-based management to optimize network performance and ensure seamless connectivity.

An extra note, the VoIP service provider requires port forwarding to be enabled for each phone device on the router to allow the VoIP traffic to go through it, which can be easily done with the Meraki Dashboard too.


<ContactFooter />


c0ff9b42e3e70b4737c79373cf64f2227decdd497e4b5100634b8eae830c2521

Analyzing the issue and providing a solution for fixing poor VOIP sound quality and call crackling.

Fixing Poor VOIP Sound Quality and Call Crackling. Through analyzing the issue and providing a solution for fixing poor VOIP sound quality and call crackling.

Tag: Router

Tag: Router

Repurpose Your Old PCs: How To Install Alpine Linux on Them

Fixing Poor VOIP Sound Quality and Call Crackling