MAINTENANCE OF INTERNATIONAL VIDEOCONFERENCE SYSTEMS
OPERATING AT TRANSMISSION BIT RATES OF 1544 AND 2048 KBIT/S
1. Scope
This Recommendation deals with the maintenance procedures to be applied
to international videoconferencing systems operating at transmission bit rates
of 1544 and 2048 kbit/s.
2. General
2.1 The configuration shown in Figure l/N.zz is a simplified depiction of a
typical connection.
VS = Videoconference studio
IVC = International videoconference centre
FS = Frontier station, e.g. earth station
FIGURE 1/N.zz
The constituent parts of
an international videoconference connection
2.2 Each participating administration shall establish an international
videoconference centre (IVC) with the same general responsibilities and functions as
those set out in Recommendation N.55 for an international television centre (ITC)
in respect of international television connections. However, as a videoconference
connection is a bidirectional transmission path, the choice of control and sub-control IVCs shall always be established by mutual agreement. This choice should
hold for all the international videoconference connections serving two
administrations.
2.3 The maintenance procedures are designed to facilitate restoration of
service when a videoconference connection is broken, or unacceptably degraded, during
a videoconference call. At this time it is essential that close
cooperation is maintained between control and sub-control IVCs in order to
minimize disruption to the call.
2.4 The IVCs will require to be provided with equipment such as codecs,
monitors and cameras so as to be able to make limited functional checks (visual and
audio) for confirmation of call establishment and rapid fault sectionization.
Additionally, testers should be available for 1544 and 2048 kbit/s data
performance tests. The testers should be able to send a structured test signal in
accordance with either the Recommendation G.732 or G.733 structures. The test
pattern should be a pseudo random test signal of sequence length 2^15-1 as
described in CCITT Recommendation O.151.
The tester conforming to Recommendation G.733 should be able to work at
1536 kbit/s information rate, sending the test signal in time slots 1 to 24.
The tester conforming to Recommendation G.732 should be able to work at
1536 or 1984 kbit/s information rates. When switched to 1536 kbit/s it should send
the test signal in time slots 1 to 15 and 17 to 25. When switched to 1984 kbit/s
it should use time slots 1 to 31.
3. Pre-call tests
It is essential that pre-call tests are made to confirm that a scheduled
call will be satisfactory. All equipments and all parts of the international
videoconference connection to be used in a call should be checked beforehand on the
day of the call. For this purpose tests should be made of the complete
transmission path between the appropriate videoconference studio(s) and frontier station(s)
by each administration and between the frontier stations.
These pre-call tests are intended to be simple checks to establish that
the transmission paths are acceptable, for example, that the transmitted and
received rf carrier levels at each earth station are within their prescribed limits.
Additionally, where the international videoconference connection is new
or where previous, similar calls have suffered problems a complete studio-to-
studio functional test should be made. This should take the form of a visual and, if
appropriate, audio subjective assessment of the end-to-end performance. Such
tests should be sufficiently in advance of the start of the call
(15 to 30 minutes) to give some opportunity for clearing any problem which may
be found. As experience and confidence is gained the time required for pre-call
testing should reduce.
4. Fault localization
4.1 A general outline of the fault localization procedure for international
videoconference connections is given below. This approach aims to rapidly localize
a fault to a circuit section.
4.2 Fault reports may be received by either IVC on a connection but shall
only be accepted from points within their own country.
4.3 On receipt of a fault report the IVC shall monitor the connection where
possible to broadly localize the fault unless this is obvious from the fault
report or from other information, e.g. system alarms. The IVC should then immediately
advise the distant IVC of the reported fault and any information which may assist
to determine the course of action to be taken.
4.4 If the source of the problem is not known then both IVCs shall work
together to determine in which circuit section the fault lies e.g. by means of loops.
Preferably this should be in accordance with an agreed planned procedure. If
these procedures do not localize the fault then the IVCs shall agree the further
action to be taken.
4.5 If at any stage the fault is localized then the appropriate clearance
procedures should be put in hand.
4.6 Both IVCs should be in constant telephone contact during these
procedures. The IVCs should report to their respective VSs no longer than
10 minutes after receipt of the fault report advising on localization progress
and estimated time to restore service. A second status report should be given
after a further maximum period of 10 minutes. If, at 20 minutes following the fault
report, the time to restore service has not been determined then the customers
should be so advised and a decision made as to whether or not to abort the call.
4.7 If, during localization, the overall connection is found to be
satisfactory, the problem may be caused by the interworking of the studio equipment (e.g.
codecs). In this case it may prove necessary to monitor and test the connection VS
to VS.
4.8 Minor problems that are reported for correction but which do not make a
call unusable, shall be accepted for clearance and entered in the fault record but
shall not count against the service availability of the connection. Action to
deal with such problems should not interrupt an ongoing call, except at the
direction of the control IVC.
4.9 Where a videoconference call transmission is encrypted, the VSs shall be
required to remove the encryption for fault localization and clearance purposes
when requested by the IVCs.
5. Maintenance parameters
5.1 The maintenance limits for 1544 and 2048 kbit/s transmission paths are
given in Table l/N.zz. Where the national videoconference circuit includes wideband
analogue or higher order digital system sections then the national maintenance
standards should apply to these sections.
5.2 Because of the need to restrict the time taken for dealing with faults
during scheduled videoconference calls, any assessment of the 1544 and 2048 kbit/s
performance that may be required should be based on bit error ratio (BER)
measurements only. The measuring time should be kept to the minimum necessary for the
investigation in hand.
5.3 Where maintenance activities do not risk the establishment or completion
of a scheduled videoconference call then any assessment of the
1544 and 2048 kbit/s performance should be against all the parameters shown in
Table 1/N.zz. Such measurements should be made over a minimum period of 15
minutes.
TABLE 1/N.zz
Maintenance Limits
(These limits are provisional and subject to further study)
