[Warwraith]

Do area jammers require an electronic warfare skill to use? Also do they stack? is  If I have two rank 4 area Jamey's would the noise start at 8 and drop 2 for each meter?

[martinchaen]

According to the rulebook, it's just an on/off device, with a wireless bonus of excluding devices you designate; so no, I don't see how they require EWar to use. I believe there is a Matrix Action that uses the EWar skill, but that is separate from the device.

Similarly, I do not believe they stack, but I could be wrong. The text of the jammer states that it " generates noise equal to its Device Rating"; as such, two jammers in close proximity would still just generate Noise Rating of 4, but you could spread that noise over a greater area and create overlapping zones for less reduction with more jammers.

[EtherealGyre]

So how do they work exactly?

How do I know if it works to jam something? Like say a wireless grenade.

[Michael Chandra]

If Noise > Device Rating, then Wireless Bonuses shut off. It says Wireless Functionality but it's in the Wireless Bonuses section, so Aaron claims it's just Wireless Bonuses. Furthermore, Matrix Actions get penalized, so Deckers will have a hard time hacking you.

[martinchaen]

Well, that depends on your GM, I guess; he could tell you, or not. "You see a grenade coming at you" doesn't really tell you if it's live or not, so it might be wise to get the hell out of dodge.

That's more of a narrative thing, and I presume you're talking about game mechanics.

If you look at page 421 under Wireless Bonuses, you'll see the following:
"If there is a Noise Rating from a situation that is greater than the item’s Device Rating, not including distance, the item temporarily loses its wireless functionality (see Noise, p. 230)."

There's some contention about whether a wireless grenade is actually wireless, because the way the above text is written it can be interpreted as the grenade simply losing it's Wireless Bonus, which is that you can detonate it even if you don't have a DNI. Some people take that to mean a grenade affected by noise greater than it's device rating can thus still be detonated if you have a DNI. I don't agree with this interpretation, but that's just me.

Basically, if you get a Rating 6 Area Jammer, any device (with the exception of those you designate, if you use the Jammer's wireless bonus) within 5m of you will be affected by a Noise Rating of 6. Anything from 6 to 10m of you will be affected by a Noise Rating of 5, and so on and so forth.

On the same page at the very top you'll find a list of device types and common Device Ratings. Basically, you can probably make things interesting for the opposing Decker by running up to him with a Rating 6 Area Jammer strapped to your chest.

[EDIT]
Ninja's by Michael

[Psikerlord]

I think generally speaking noise stacks, looking at the noise table modifiers. So two jammers will stack. In any event a rating 6 area jammer ought to protect you against the usual rating 1-2 wirless grenade link. Beware of rigger decks boosting noise cancellation. Or bring more jammers.

[martinchaen]

Psikerlord That can't be right, on the simple grounds that if a street sam spends 12k on jammers (10x R6 jammers at 1200 nuyen each) he'll be blanketing the area around himself with a total Noise Rating of 6*10=60, which makes no sense from a rule perspective.

The rules for jammers state that they "generate noise equal to their rating". To me, that reads like a constant Noise Rating of 6 not an incremental one; if they had written it as "generates noise equal to their rating which is cumulative with other sources of noise (including other jammers)", then I would have been prepared to accept such an interpretation.

Otherwise, it'd be far too easy to completely blow even riggers out of the sky with noise (grenade shaped jamming devices, anyone? TOSS THE DWARF!)

[Mirikon]

Jammers don't stack. The highest rating applies. Having overlapping jamming fields, however, can extend the area covered, much like how a network of cell towers increases the coverage area.

[DigitalZombie]

When I was in Afghanistan, we used our jammers to overlap, but you wouldnt know if you sat on an RCIED. Unless you elected to turn the jammer off, in which case you yourself likely wouldnt notice it either, but the rest of the patrol would definitely notice the huge explosion. 
I wonder if it will be possible, in a future sourcebook, to do the exact opposite. Like having a grenade that is set to detonate immediately, but it continuously gets a wireless signal saying, dont detonate yet. Using a jammer in that case would be very unhealthy, if you are close enough that is.   

[martinchaen]

Hehe, hooah, DigitalZombie. Thank you for your service; I was up in the mountains most of my tours for OEF:A, and I'm just glad I didn't have to deal too much with fucking IEDs.

As for the positive signal wireless grenade, I'd say it's probably just easier to use motion sensor grenades, if a little more risky

[Mirikon]

Hmm. Like a dead man's switch on the grenades? Could be possible, if you programmed it right, but it would be a direct line from the grenade to you in the matrix, leaving you open to hacking. Would probably be easier with a specialized detonator and explosives, since less reprogramming. The matrix problem still remains, as does the issue of interference. The problem with those kinds of setups is that they go off when either you or the device are cut off from the matrix or out of mutual signal range. Outdoors in downtown Seattle that isn't an issue, but indoors, going through areas with possible wireless shielding or increased noise, you may have it going off long before you would like to. In some ways, it is the inverse of the dangers of using a wireless signal to detonate. Very risky.

[Psikerlord]

hmm actually would a rating 6 jammer block a rating 4 jammer - so they dont add together, just rating 6 shuts down 4.

what about two rating 6 jammers.

Assuming they stack at all, which they probably shouldnt, for balance reasons, but then other sources of noise stack, why shouldnt jammers (balance reasons aside - even though, ultimately, balance is the only reason you need).

[Michael Chandra]

Jamming isn't a wireless bonus (or even wireless functionality if you don't follow Catalyst's RAI). Only excluding specific items is a wireless bonus, so at worst you lose that option.

Different sources of noise stack with each other, you don't face the same noise factor twice.

By the way, let's not forget the ultimate Missions approach: Buy 8 Alphaware Datajacks at chargen for 0.64 essence and 9.6k nuyen, get a Rating 6 Jammer afterwards, and buy (at chargen) a rating 6 Commlink that you slave your datajacks and other gear to. You're in 2+ Noise in the CZ, the Jammer provides 6 noise (and you don't need its wireless bonus), so any hacker faces 8+ Noise against you. At best they'll scrub+jack 3, keeping them at a -5 to hack your 6x2 dice. Have fun! And you got 8 Noise Reduction yourself so even without the wireless bonus of the jammer you keep your wireless bonuses on ALL your gear at any noise below 5. So only if the CZ Noise goes to 5+ you'd have to switch the wireless bonus of your jammer on (and slave it to your commlink) to be able to access all your wireless bonuses.

[DigitalZombie]

@martinchaen HAh great! I was in Helmand province with the Danish forces. Working as an electronic warfare operator ( isnt it sad, my complete function and purpose has been  degraded to a single skill in shadowrun  )

@Mirikon yeah something along those lines. But I was more thinking on having the "dont blow yet" transmitter deployed together with the grenade, like duct taped together, yet still wireless. I wouldnt use that setup as a runner as its an easy way to get notoriety But have the runners chase a demolition badguy, that utilises different setups for his explosives. Giving the runners a hard choice between ECM on or off.

@Psikerlord I dont think noise can cancel noise in SR. I believe its possible in theory to create a mirrored sinus curve to cancel the opposing one, just like it would be possible to cancel out someone shouting by using the exact opposing wavelengths... in theory.
About stacking jammers: its kinda like when person J (jammer) is shouting LALALA! thereby making mr Grenade miss the command from person G (grenade launching psycko) to detonate himself. It might be possible for mr grenade to hear that command, if it came on the L in LALALA! as the L isnt as loud as the A. Having 2 persons shout at the same time would diminish L detonations as the other person would be yelling A at that time, but the 2 persons combined wont be shouting twice as high.
I believe that in SR the jammers are good enough to yell Ls as loudly as As, and thereby not stacking with each other. (and also for balance as you stated )

On another note: As it is possible for a wireless enabled jammer to ignore ones own equipment, wouldnt it be possible for an opponent then to hack those "blind spots" and use them himself?

[Michael Chandra]

Yeah, that'd work just fine and make you a fool. Better to get wireless datajacks and eat the Noise yourself as well.

As for stacking jammers: A GM could go with a DB-scale, so +1 = x10. In other words, you'd need 10 Rating X Jammers to create a rating X+1 effect. This means 100 Rating 6's for a Rating 8 effect. Might be better off just getting wifi-inhibiting, like used in SRM 5A-01.