# Frequency Hoping spread spectrum question

1. Aug 27, 2006

### dervast

Hi to everyone.. I am just reading an artical abouts FHSS... as it describes the frequency hoping adds security to the system cause u must know the algorithm for the frequency changes:)
Unfortunately the article doesnt mention if this algorithm is transmitted at the start of the conversasion betwwen the Base station and the mobile terminal . If that is true then i should only attend the transmission from the beggining so i should have the algorithm for the frequency changes. Do u think i am correct or not?
Thx a lot

2. Aug 27, 2006

### Averagesupernova

I'm sure it varies from system to system. I imagine it could be similar to the 'enigma' used by Germany in WW2.

3. Aug 27, 2006

### dervast

What do u mean it varies from system to system? I can find here only 2 different variations
a)the algorithm is known from before (a construction parameter) so if i want to steal data from a wireless link i wii always know in which frequencies i would find data
b)The algorithm is always transmitted at the start of the conversation .. so if i want to steal data i only have to start capturing frames at the begining of conversation so i can capture the algorithm being used too

4. Aug 27, 2006

### NoTime

c) The algorithm is only updated when the mobile is connected to the base station.

5. Aug 28, 2006

### dervast

c)so it must be transmitted back to the mobile station... Right? Do u know if that is the working solution?

6. Aug 28, 2006

### NoTime

Not transmitted. Physical link mobile/base.

For some things. Don't know if it's all cases.

7. Aug 28, 2006

### chroot

Staff Emeritus
There is no single implementation of FHSS. Some simple implementations use channel sequences that are fixed in both the transmitter and receiver. More advanced implementations use cryptographic protocols and a form of secret-sharing to configure the transmitter and receiver before data transmission begins.

- Warren

8. Aug 28, 2006

### dervast

"Some simple implementations use channel sequences that are fixed in both the transmitter and receiver. " If they are fixed then the implementation provides no security.. Do u think that i am correct?

9. Aug 28, 2006

### chroot

Staff Emeritus
Not all FHSS implementations are intended to be secure -- some are just intended to be immune to interference.

- Warren

10. Aug 28, 2006

### Staff: Mentor

Very little security against a determined hacker, true. They are mainly used for tolerance of noise, not for security. You use encryption techniques as chroot said for the FH sequence, as well as encrypting the data, if you are serious about keeping you out. Er, sorry, I meant keeping out hackers.

11. Aug 28, 2006

### turbo

By the way, Heddy Lamar and George Antheil invented this technology. Ms Lamar had been married to a weapons manufacturer who also designed control systems, and apparently she was a quick study. The mechanical system they designed to control frequency-hopping (intended to prevent jamming of guided torpedoes) was pretty much ignored by the Navy brass until Sylvania picked it up over a decade later, using electronics, not coded mechanical rolls.

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