Author Archives: Tom

WiFi (2.4Ghz) Link Calculation

While searching around on how to calculate the maximum distance of a WiFi connection, I came across this very useful link budget calculator.

So far we’re still searching for a way to calculate the area of a fresnel zone which falls below ground. Based on a 1km link the zone starts at the antenna (8m high), extends to the user (1m high), and the fresnel zone has a radius at its centre point of 5.6m. This zone will intersect the ground at 371.181m and 990.905m from the antenna – thanks to Rory McCann for calculating this.

We’re still trying to work out the area of the Freshnel which travels underground. After doing this, we only then need to calculate the loss in dB. Any ideas?

View ‘dd’ progress

When reading/writing a disk image using Linux tool ‘dd’ launch as follows:
dd if=/dev/sdb of=filename.img & pid=$!
this will run ‘dd’ in the background.

To see the progress:
kill -USR1 $pid
(it will not kill the dd)

Notes:

  1. If you are doing it as a non-root user you’ll need to prefix those commands with sudo
  2. If you do that do ‘sudo ls‘ first, otherwise the ‘sudo dd‘ will fail waiting on the password which it’ll never get as its backgrounded.

Thanks to Cian.

Exporting physical Linux hard disk to a Virtual Machine

Shutdown and remove the hard disk, then connect to a separate PC with VirtualBox (virtualbox-ose) installed.

  • Take a ‘dd’ copy of the hard disk, eg: dd if=/dev/sdb of=filename.img
    (warning this may take a while and requires enough free disk space to hold a copy of the entire hard disk being cloned)
  • After the image has been successfully created, convert it to an image suitable for VMWare or VirtualBox using the command:
    vboxmanage convertfromraw filename.img -format VMDK filename.vmdk
  • This can be imported into VirtualBox by creating a new (Linux/Other Linux) Virtual Machine and “Use existing hard drive”. Click the browse button to go to the Virtual Media Manager, click ‘Add’ and select the VMDK file created in the previous step.

The virtual machine should now be ready to run in VirtualBox.
To import into VMWare, create a new virtual machine and virtual hard disk. Overwrite the new Virtual Hard disk by copying the VMDK file created earlier, into its place.