From: jcmorris@mwunix.mitre.org (Joe Morris)
Subject: Re: Class A and Class B devices at home
Organization: The MITRE Corporation
Date: Thu, 21 Jan 1993 13:40:16 GMT

jeh@cmkrnl.com writes:

>In article <1993Jan20.112151.1316@yvax.byu.edu>,
> feijai@endor.byu.edu (Sean Luke) writes:
>> The NeXT does not have Class A certification because it is expensive and  
>> difficult to obtain, not because the NeXT isn't a Class A - quality machine.   
>As I understand it, this isn't quite how it works:

>Using a Class B computing device in a residence is in no way illegal -- unless
>it generates unacceptable interference to someone else's radio or tv reception.
> In that case the operator of the Class B computing device can be required by
>the FCC to either cease using the equipment or make adjustments to eliminate
>the interference. 

Um...you've got the definitions of classes A and B reversed.  A class A 
device is intended for business/industrial environments.  Class B
certification permits use in a residential area.  With this correction 
the comment about certification being expensive and difficult is
correct.

However...parts of the FCC regs explicitly prohibit the sale of some products
without class B certification (such as laptops) or in some contexts (such
as a computer being advertised for non-business applications) on the 
assumption that these products are almost certain to be used in a residential
area.

>Using a class A device, otoh, means that even if your equipment causes an
>unacceptable level of interference in a neighbor's reception, it isn't up to
>you to fix it, because the RFI generated by the device has been shown to be at
>an acceptable level for residences.  In that case the owner of the tv or radio
>has to do the fixing (reorient the receiving antenna, get a better set, etc.).

>(This assumes that you are using the equipment in its configuration "as
>approved" -- covers in place, shielded cables, etc.  Class A acceptance is
>*very* iffy for computers; even if you buy a "Class A" computer, connecting,
>say, an external disk drive to it will likely make it lose "Class A" status,
>unless the particular disk drive *and* connecting cable were tested as a system
>with that computer, and the whole assembly passed the "Class A" requirements.)

Again substituting "B" for "A", your protection against a neighbor's 
complaint of interference is good only if your box is working properly
and the certification has not been compromised by the inclusion of
noncompliant devices.  Even if you have nothing but certified class
B equipment it can still break and flood the radio spectrum with noise;
it's your responsibility to keep the RFI within specifications.

I don't know of any case law on the subject, but I expect that if your
equipment appears to be Class B compliant, it is the neighbor's responsibility
to show that the RFI is outside of the legal limits.

Amateur radio operators have had problems with neighbors' complaints for
decades, and in a depressing number of cases the problems have been found
to have absolutely nothing to do with the radio operator's facility.  The
cause is often found to be a woefully out-of-tolerance receiver in the
complaintant's house, or bad AC wiring in the house, or in one case I
recall a decades-old carbon filament light bulb in the basement.

Joe Morris / MITRE