# communication 101 [closed]

I have a couple of really basic questions.

(1) When a device(Device 1) communicates to another device(Device 2), do the devices have to operate in the SAME frequency for the data transmission to take place? Or will similar frequencies do??

(2) If the devices operate at say, 10 Ghz, can another device(Device 3), having the same operating frequency listen in on the transmission between Devices 1 and 2?? Even though its not the recipient??

(3) Suppose device 1 operates at say 5 GHz. Devices 2 and 3 operate at 4 GHz. How can device 2 communicate with device 1 using radio waves? Should the device have a different frequency for uplink and downlink?? In the case that it is able to transmit to device 1, does device 3 also get the data being transmitted(since it has same frequency as that of device 2)?

• This seems to be a very big question. An important idea is: communication needs the transmitted signal to change. If the transmitted signal doesn't change at all, it is only able to say "I am transmitting", and nothing else. How much the transmitted signal needs to change depends on how much information needs to be sent (per second, or per unit time). That introduces the idea of bandwidth, roughly, the amount the transmitted signal will change by. Try to use a search engine to find and understand bandwidth, then try asking a less broad question. – gbulmer Sep 13 '14 at 13:29

First of all I think you are messing a little with two concepts: band and carrier frequency.

Band: describes you how wide will your transmitted signal in the frequency domain. e.g.: audio band is from 20Hz to 20kHz so it is 20000-20=19980Hz wide.

Carrier frequency: is the frequency at which the signal is transmitted in the air. The wifi router you have at home has a carrier frequency of 2.4GHz, and the data it transmits has a band of 22MHz.

If you look at a wifi power spectral density you will then see a sort of rectangle, 22MHz wide, centered on 2.4GHz.

1) The carrier frequency must be the same, i.e. if I transmit my data over 1MHz you can't expect to receive it over 2MHz. Band is also usually the same, or wider in the receiver: if I transmit some data that is 100KHz wide you can receive it if your receiver can accept a band of 100KHz or more.

2) Absolutely yes. If I am showing my friend a picture on my phone can my girlfriend see it if she's looking at it too? Well she probably could see it anyway but you got the point.

3) Devices 2 and 3 can not talk to 1. Different carriers = different languages. In order to get all the three talking together they must share the same carrier, or each one should have its private carrier with the other two, e.g.:

• 1MHz between 1 and 2
• 2MHz between 2 and 3
• 3MHz between 3 and 1

Sometimes devices use different carriers for uplink and downlink, as you sensed. That's true for wifi: the carrier is at 2.4GHz, the band is some 22MHz wide, but this band is divided in two slices: one for uplink and one for downlink. Assuming the slices are equal you can see all this as two bands 11MHz wide, one centered on a 2.4GHz-5.5MHz carrier, the other centered on 2.4GHz+5.5MHz.

(1) I assume you are talking about wireless transmissions. The frequencies will never be exactly the same, but have to be close to each other.

(2) Yes, this is always possible. Unless you encrypt it.

(3) You have to distinguish between full- and half-duplex systems. The former can communicate in both directions at the same time, whereas half-duplex means that you require two carriers which can be either wires or carrier frequencies.

When two devices try to communicate at the same frequency, there will be collisions and data loss. This problem can be addressed with "carrier sense (multiple access), collision detection and avoidance" (CSMA/CA).

• Maybe it might help to change "When two devices try to communicate at the same frequency" to "When two devices try to transmit at the same time on the same frequency". Also there are many ways to address this, for example spread spectrum techniques. – gbulmer Sep 13 '14 at 18:14