Antenna testing

How to test the OLPC XO antennas.

Why test them?

• they may have been damaged by impact, misuse, or electrostatic discharge,
• to classify laptops for repair,
• to verify a repair,
• to survey a site,
• to find parts of a site that have low coverage.

A single broken antenna degrades the performance of the laptop. Although there are two antennas (XO-1, XO-1.5, XO-1.75), they are used for different purposes. Performance loss may include:

• reduced operating range from an access point,
• fewer networks visible when scanning,
• inability to access any network,
• inability to collaborate in the absence of a School Server, using Sugar activities, despite being able to access the internet,
• inability to collaborate despite being able to join an ad-hoc network.

Sugar activity collaboration in the absence of a School Server uses broadcast network packets. These packets are confined to one particular antenna due to the diversity function only handling unicast packets. So if this antenna is broken, internet resources can be available yet activity collaboration fail.

The antennas are connected, by cables inside the laptop, to the wireless card. The wireless card provides wireless networking with other OLPC XO, other computers, or access points.

Sugar

• identify a nearby access point and locate it on the Neighbourhood View,
• wait for a few minutes to ensure a scan occurs,
• the signal level is shown as a liquid fill level for the circular access point icon.

GNOME

• click to open the network menu, and identify a nearby access point,
• the signal level is shown graphically on the menu item,
• connect to an access point,
• the signal level is shown graphically on the panel.

Linux

• connect to an access point,
• use Browse, Firefox or Epiphany to visit a web page,
• open a Terminal and prepare a command:

iwconfig eth0

• while transferring data, press enter in the Terminal window,
• read the output of iwconfig for the signal and noise values.

Open Firmware

Open Firmware contains an antenna testing facility. It requires a minimum version:

• XO-1, requires Q2F14 or later,
• XO-1.5, requires Q3C10 or later,
• XO-1.75, requires Q4D25 or later,
• XO-4, requires Q7B07 or later.

Testing against an access point

You will need:

• an open wireless access point, with or without an IP network behind it,
• an OLPC XO, the test target.

Start the XO and get the Ok prompt, then type:

essid NETWORK
test-antenna

Where NETWORK is wireless access point name.

An updating display of signal strength is given. It looks roughly like this:

Associate with: X
keys: (0,1,2,3,d,a,s,q)
now rssi -33 snr 57 nf -90 avg rssi -33 snr 57 nf -90 ant d rx 1 tx 1

During the test the following values are displayed ten times a second:

• now rssi - the latest beacon received signal strength indicator, in dB, calculated as the signal to noise ratio plus the noise floor,
• now snr - the latest beacon signal to noise ratio, in dB,
• now nf - the latest receiver noise floor, in dBm,
• avg rssi - the averaged received signal strength indicator,
• avg snr - the averaged signal to noise ratio, in dB,
• avg nf - the averaged noise floor, in dBm,
• ant - the antenna mode requested by the host; 1, 2, or d for diversity,
• rx - the antenna being used for receive; 1, 2, or d for diversity,
• tx - the antenna being used for transmit; 1, 2, or r for same as the receive antenna,

The rx, tx or ant values show the antenna allocation. The wireless card generally uses both antennas.

Watch the avg rssi value. Move the laptop in relation to the access point, raise or lower the antennas, enclose them with your hand, and look at the result. Press enter to keep a result on screen. You should see:

• rssi will increase as the laptop moves closer to the access point,
• rssi will change slightly between raised or lowered antennas.
• rssi will decrease as the antennas are enclosed by a hand,

During the test there are keyboard keys with special meaning:

• key s to perform a network scan,
• key a to deassociate and reassociate with access point, (needed for some access points that deauthenticate the client after a period of inactivity),
• key 1, 2 or d to switch between using antenna 1, antenna 2, or both,
• key q to quit.

The test can be used to indicate the performance of antennas, transmission coax, termination, socket, antenna switch, and receiver. But there are sources of unreliability in the test:

• the radio noise environment,
• the precise position of the antenna or laptop, such as in a null,

These sources of unreliability make comparisons difficult, but if you see substantially different values greater than 20 dB between the two antennas, then one may be damaged.

The test also does not test transmit by the laptop, only receive. (Although associating to the access point does require successful transmission, the test displays received signal strength).

If you identify an antenna as damaged, schedule the laptop for a detailed repair. Look carefully at the coax cable leading from the wireless card to the axle of the antenna. If the cable is twisted, or broken, then replace the antenna. Check that the connector is latched into the socket on the wireless card. The top surface of the connector should be parallel to the card. The antenna itself is not serviceable.

Antenna configuration

The wireless card may be instructed to change the antenna preference using the d, 1, and 2.

• d - select diversity mode, the default, which is supposed to encourage the firmware to choose which antenna is the best to use, but on XO-1.5, XO-1.75 and XO-4 may choose the right-hand antenna instead,
• 1 - select the right-hand antenna,
• 2 - select the left-hand antenna,

The 0 and 3 keys are for testing; they intentionally violate the firmware specification:

• 0 - send an invalid antenna selection command with value zero; causes the firmware to report invalid values for a while, then selects the left-hand antenna,
• 3 - send an invalid antenna selection command with value three; causes the firmware on XO-1.5, XO-1.75, and XO-4 with 8686 modules to report diversity mode at baseband processor register level for a short time.

(An XO-4 may have either a dual-antenna 8686 or a split-antenna 8787. See how to identify wireless LAN device.)

Setting up a test laptop

You might do this if an access point is unavailable. You will need:

• an OLPC XO of known provenance, the test host,
• an OLPC XO, the test target.

Start the test host XO and get the Ok prompt, then type:

ok select /wlan:force
ok d# 11 " X" adhoc-start
ok test-antenna

X is the network name. You may choose any name according to your site convention.

The values are not updated until another node joins the network.

Start the test target XO and get to the Ok prompt, then type:

ok essid X
ok test-antenna

Both sets of values should begin to update.

During the test, certain messages may be emitted, and these have special meanings:

• Event: Link Sensed - the other laptop is connected ... or more exactly, the number of nodes in the network has increased from one to more than one, occurs within two seconds,
• Event: HWAC - adhoc BCN lost - the other laptop is disconnected ... or more exactly, the number of nodes in the network has fallen to one, occurs within about ten seconds.

The values displayed by each laptop relate to the receive path of that laptop. If there are more than two laptops associated with an adhoc network, the values shown will include receive performance of transmissions from both laptops.

Setting up a test access point with a laptop

This creates a network "xxAP" with no DHCP service:

ok select /wlan:force
ok do-ap

References

Link Budget

The link budget is the total gains and losses of a radio link. The link budget must be sufficient for the OLPC XO to use a wireless internet.

For an OLPC XO using an access point, this is the sum of:

• for the access point:
  • the transmit power,
  • the loss in the transmission line (the coax and connectors between the transmitter to the antenna, normally very short),
  • the gain of the antenna,
• for the environment:
  • the loss due to distance,
  • the loss due to obstructions,
  • the loss due to noise, (from other radio transmitters),
• for the laptop:
  • the gain of the antenna,
  • the loss in the transmission line (the coax and connectors between the antenna to the receiver),
  • the sensitivity of the receiver.

For two OLPC XO using an adhoc network, the link budget is the sum of:

• for the first laptop:
  • the transmit power of the wireless card,
  • the loss in the transmission line (the coax and connectors between the transmitter to the antenna),
  • the gain of the antenna,
• for the environment:
  • the loss due to distance,
  • the loss due to obstructions,
  • the loss due to noise, (from other radio transmitters),
• for the second laptop:
  • the gain of the antenna,
  • the loss in the transmission line (the coax and connectors between the antenna to the receiver),
  • the sensitivity of the receiver.

See

• http://en.wikipedia.org/wiki/Link_budget