Packet Race condition May 21, 2001

ISS Amateur Radio Status: May 21, 2001

By Miles Mann WF1F,

MAREX-MG (Manned Amateur Radio Experiment, North American Division)

This weeks ISS packet tip. The ISS Packet Race Condition:

Last week I received an email from an amateur radio station, that was surprised to see me respond to this ISS Packet Chat room Ping. The sending station did NOT see his chat room ping ECHO back from ISS. He asked the question, why did others see this packet repeat back from ISS, when he did not see his packet repeat from ISS.

The answer is a little complex but can be summed in the words "Race Condition"

This packet condition I am going to talk about, only applies to a packet which was correctly decoded and then retransmitted back to earth.

First a little history on ISS packet. The existing TNC has suffered a possible battery failure. This caused the TNC settings to go back to the default mode. The ISS crew has been too busy to connect up a computer to reprogram the correct TNC settings. Dave Larsen (MIREX) has provided me with a partial set of parameters which are his best guess on what the current TNC settings on ISS have defaulted to (MIREX is managing the TNC ISS project, thanks Dave)

Key ISS TNC settings

Slot 1 [10 milliseconds]

Persist OFF

TXD 15 [150 milliseconds]

Axdelay 0

Settings used on Mir’s

Slot 10 [100 milliseconds]

Persist 63 [counter range 63-255]

TXD 150 [150 milliseconds]

Axdelay 150 [150 milliseconds]

I am going to invent a few word here to help explain the Race Condition:

RecieverTurnAroundTime = The total time it takes your radio, to switch from a Transmit mode to Full receive mode and start passing audio signals to your TNC.

Comparison: MIR and ISS Settings.

On the Mir system we had the ability to remotely adjust the TNC settings (we liked to fiddle to find the most optimum for a space based TNC). The Minimum time to issue the command to transmit was 100 milliseconds (slot = 10). The minimum time from setting the transmitter on, until actually sending packet data was minimum of 300 milliseconds. This combination gave Earth based transmitters over 400 milliseconds to get ready to start receiving data from Mir.

(Slot + TXD + AXDELAY = ReciverTurnAroundTime)

The ISS TNC settings have not been optimized due to the temporary battery problem. We hope to either have the crew run the setup program in the future or replace the TNC with a new TNC with the default settings BURNED into the ROM.

The ISS Minimum time to issue the command to transmit is 10 milliseconds (slot = 1). The minimum time from setting the transmitter on, until actually sending packet data is a minimum of 150 milliseconds. This combination gave Earth based transmitters over 160 milliseconds to get ready to start receiving data from ISS.

(Slot + TXD + AXDELAY = ReciverTurnAroundTime)

Note: The estimated 160 millisecond ReciverTurnAroundTime for ISS, appears to be padded by other timers in the TNC. Most of the actual ReciverTurnAroundTime from ISS are much longer, which indicates there are other factors which reduce the number of times the "Short – ReciverTurnAroundTime" are observed.

There are 3 primary reasons you may not see your Return message coming back from ISS, (Assuming your message was re-transmitted from ISS).

The Transmit time delay on ISS may be too short for some Earth based transmitters to recover fast enough to be in Receive mode before ISS starts sending a packet. When the delay is High, your RX has a better chance of seeing the packet. When the delay is Low, your RX may not be ready fast enough to hear the full packet which will cause a CRC error and prevent the packet from being decoded. Try setting you TNC PASSALL to Enabled. This will allow damaged packets to be viewed.

2. Your receiver may be too slow. The time it takes your receiver to switch from Transmit to Receive may be too long to allow your TNC to hear the first part of the packet message. I do not have any specification on VHF equipment, however I have reviewed a few documents for HF equipment and it seems that most good HF equipment switches from Transmit to RX in 3-10 milliseconds (which is good).

You then must allow a little more time for the AGC to fully wake up the receiver. And you need a few extra milliseconds for your TNC to wake up (this is where the AXDelay values helps)

(Maybe there is an expert in this field that can shed more light on this topic, how long does it take a VHF receive to switch from Transmit to full RX?)

My old DJ-580 2-meter HT, has a slow recovery on the Receiver. There is a Repeater in my town that has a courtesy beep right after you stop transmitting. When I transmit with the DJ-580, I do not hear the beep because my RX is too slow. When other stations Transmit, then I can hear the beep, my TM-733 always hears the beep.

3. Slow Earth TNC. Some TNC's wait for the squelch to break, others are constantly

monitoring the audio stream looking for packet data. The TNC's that monitor the

audio stream for packet data are usually a little faster than the TNC's designed

to wait for the Squelch to open. My Kantronics TNC's support both modes, Squelch and Internal.

CD = Software (listen to audio for a packet)

CD = Internal (wait for squelch)

(note: I had an old kpc-3, which would go deaf progressively deaf after running

for several days and a power cycle would cure the problem. I have not seen this bug on any newer Kantronics equipment.)

Read your TNC manual and try different modes to faster turn around times.

Temporary Solution:

Set up a backup radio and TNC configured to Receive only. Then after a pass review both logs or compare logs of a local station. Set your TNC to PASS all data. If you are using a Squelch, set it to is lowest setting. Make sure you are sending the correct audio level into your TNC.

Problem Frequency:

I think this packet race condition is real, but is not a serious issue. I only hear what appears to be a race condition problem once per orbit. If the ISS crew has time in the future to reload the TNC, the new settings should eliminate the packet race condition for all but the slowest Earth Receivers.

ISS Voice Tips:

Over the past few weeks, Susan and Jim have been talking to West-Coast USA Amateur radio stations. The orbits of ISS change and repeat over an 8 week cycle. For the month of June, the orbits of ISS favor Australia and the Pacific Rim part of the world.

The ISS crew can only use the Amateur Radio station, during their OFF-Times, when all other important work has been completed. Voice contacts are purely Random. However your best times to find the crews are during the evening hours on ISS (ISS crews are in UTC time. Best times between 1500 – 2200 UTC).

Pacific and European Voice contacts:

Note, under the current radio plans, the ISS crews change frequencies which favor different parts of the world, the frequencies TX 145.200 RX 145.800 are for Amateur Radio stations in Europe, for the USA TX on 144.490 and RX on 145.800.

Good luck all, suggest you get your tape recorders ready and start listening to the ISS channels. Please observer the proper calling procedures.

Wait for ISS Crew to call CQ or QRZ.

Send only your call sign and wait for crew to acknowledge.

Listen closely for the call of the station she is talking to.

If you do not hear your call sign, do not transmit again until you hear the ISS say CQ or QRZ

Please be courteous.

73 Miles WF1F MAREX-MG

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Until we meet again

DOSVIDANIYA Miles WF1F

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