  We examined the sky confusion noise in 40 sky regions by analysing 175
far-infrared (90--200 $\mu$m) maps obtained with ISOPHOT, the photometer
on-board the Infrared Space Observatory. For cirrus fields with $\rm < \rm B >
> 5$ MJysr$^{-1}$ the formula based on IRAS data (Helou & Beichman,
\cite{Helou+Beichman_90}) predicts confusion noise values within a factor of 2
to our measurements. The dependence of the sky confusion noise on the surface
brightness was determined for the wavelength range 90 $\le \lambda \le$ 200
$\mu$m. We verified that the confusion noise scales as N ~ <B>^{1.5},
independent of the wavelength and confirmed N ~ lambda^{2.5}$ for lambda >=
100$um. The scaling of the noise value at different separations between target
and reference positions was investigated for the first time, providing a
practical formula. Since our results confirm the applicability of the Helou &
Beichman (1990) formula, the cirrus confusion noise predictions made for future
space missions with telescopes of a similar size can be trusted. At 90 and
170um a noise term with a Poissonian spatial distribution was detected in the
faintest fields (<B> <= 3-5 MJysr^{-1}), which we interpret as fluctuations in
the Cosmic Far-Infrared Background (CFIRB). Applying ratios of the fluctuation
amplitude to the absolute level of 10% and 7% at 90 and 170um, respectively, as
supported by model calculations, we achieved a new simultaneous determination
of the fluctuation amplitudes and the surface brightness of the CFIRB. The
fluctuation amplitudes are 7(+/-)2 mJy and 15(+/-)4 mJy at 90 and 170um,
respectively. We obtained a CFIRB surface brightness of B(0)=0.8(+/-)0.2
MJysr^{-1} (nuI_nu=14(+/-)3nWm^-2sr^-1) at 170um and an upper limit of 1.1
MJysr^{-1} (nuI_nu=37 nWm^-2sr^-1) at 90um.
