Telecommunications Techniques Corporation interceptor 147, Reference Manual, Page: 260 / 280

Understanding G.821 Performance Analysis
D-2
D
INTERCEPTOR 147 2 Mbit/s and Data Analyzer
D.3 DETERMINING AVAILABLE AND
UNAVAILABLE SECONDS
CCITT Recommendation G.821 defines available and
unavailable time as follows.
“A period of unavailable time begins when the bit error
rate (BER) in each second is worse than 10
-3
for a period
of 10 consecutive seconds. These 10 seconds are considered
to be unavailable time. The period of unavailable time
terminates when the BER in each second is better than 10
-
3
for a period of 10 consecutive seconds. These 10 seconds
are considered to be available time.”
Available and unavailable time are measured in seconds
— available seconds (Avl Sec) and unavailable seconds
(Unavl Sec), respectively. All seconds after initial pattern
synchronization must fall into one of the two categories
(total Avl Sec + total Unavl Sec = total seconds after initial
pattern synchronization).
After initial pattern synchronization, seconds are
considered to be available time; the Avl Secs begin counting
(seconds that occur before initial pattern synchronization
are not included in performance analysis). These seconds
continue to be counted until 10 consecutive seconds, each
with a BER greater than 10
-3
, occur. A sliding window, 10
seconds in length, is used to detect this transition from
available time to unavailable time and vice versa.
As an example, assume a test begins and continues to run
for 25 seconds and each of those 25 seconds has a BER
greater than or equal to 10
-3
. After the initial pattern
synchronization, the seconds are considered to be available
time, so the Avl Sec count at this point is 25 as shown in
Figure D-2. In the 26th second, the BER becomes greater
than 10
-3
. The same for the 27th and 28th seconds. In the
29th second, the BER improves to less than or equal to 10
-
3
. All 29 seconds are a part of available time and are,
therefore, counted as Avl Sec.