CCITT Error Performance Objectives for International ISDN Connections
a Degraded minutes
Fewer than 10% of 1-min intervals to have a bit error ratio worse than
b Severely errored seconds
Fewer than 0.2% of 1-s intervals to have a bit error ratio worse than
c Errored seconds
Fewer than 8% of 1-s intervals to have any errors (equivalent to 92%
The terms degraded minutes, severely errored seconds, and errored seconds are used as a convenient and concise
performance objective ``identifier.'' Their usage is not intended to imply the acceptability, or otherwise, of this level of
The 1-min intervals mentioned in the table and in the notes are derived by removing unavailable time and severely
errored seconds from the total time and then consecutively grouping the remaining seconds into blocks of 60. The basic
1-sec intervals are derived from a fixed time pattern.
The time interval T
(L), over which the percentages are to be assessed, has not been specified since the period may
depend on the application. A period of the order of any one month is suggested as a reference.
For practical reasons, at 64 kbps, a minute containing four errors (equivalent to an error ratio of 1
.04 × 10
) is not
considered degraded. However, this does not imply relaxation of the error ratio objective of 1
Source: CCITT Rec. G.821 (Ref. 12).
nominal clock slip rate is 0. If there is trouble with the primary reference, a maximum of
one slip on any trunk will result from a switched reference or any other rearrangement.
If there is a loss of all references, the maximum slip rate is 255 slips the first day for any
trunk. This occurs when the stratum-3 clocks drift a maximum of 0.37 parts per million
from their reference frequency (Ref. 13).
From a CCITT Perspective
. With plesiochronous operation, the number of
slips on international links will be governed by the sizes of buffer stores and the accuracies
and stabilities of the interconnecting national clocks.
The end-to-end slip performance
should satisfy the service requirements for telephone and nontelephone services on a
64-kbps digital connection in an ISDN.
The slip rate objectives for an international end-to-end connection are specified with
reference to the standard hypothetical reference connection (HRX), which is 27,500 km
The theoretical slip rate is one slip in 70 days per plesiochronous interexchange
link assuming clocks with specified accuracies (see Section 126.96.36.199) and provided that
the performance of the transmission and switching requirements remain within their
In the case where the international connection includes all of the 13 nodes identified
in the HRX and those nodes are all operating together in a plesiochronous mode, the
nominal slip performance of a connection could be 1 in 70/12 days (12 links in tandem)
or 1 in 5.8 days. In practice, however, some nodes in such a connection would be part
of the same synchronized network. Therefore, a better nominal slip performance can be
expected (e.g., where the national networks at each end are synchronized). The nominal
slip performance of the connection would be 1 in 70/4 or 1 in 17.5 days. Note that these
calculations assume a maximum of four international links.
The performance objectives for the rate of octet slips on an international connection
of 27,500 km in length of a corresponding bearer channel are given in Table 6.7. CCITT
(Ref. 14) adds that further study is required to confirm that these values are compatible
with other objectives such as error performance given in Section 188.8.131.52.
CCITT is looking at the problem from an international switching center gateway. It will connect via digital
trunks to many national networks, each with their own primary reference source (PRS).