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Document Title: Testing n x 64 Kb/s Services (AN 1211-2)
Part Number: 5091-2069E
Revision Date: August 1991
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Testing n x 64 kb/s services
Application Note 1211-2
1
n x 64 kb/s traffic
Wideband services such as
videoconference, LAN
interconnection, and high-speed
computer links usually require a
bandwidth greater than 64 kb/s, but
maybe less than the full 2 Mb/s, for
example, 384 kb/s. These wideband The most critical factor when using For example using a test set like the
signals can be sent in a 30-channel 2 n x 64 kb/s aggregate channels is to Hewlett-Packard 37722A digital
Mb/s frame by "sharing" the signal ensure all equipment is configured telecom analyzer or the HP 37732A
between several "aggregated" for the same sequence of timeslots. telecom/datacom analyzer, a
64 kb/s channels. According to Furthermore, the integrity of this multiplexer can be stimulated with
CCITT recommendation G.704, the n sequence must be maintained, a 64 kb/s signal and a check made
x 64 kb/s signal is accom-modated particularly if the signal passes in the appropriate timeslot of the
in "n" contiguous time-slots, through switching equipment. outgoing 2 Mb/s stream. Similarly,
omitting TS16, and with each you can send a PRBS test signal in
timeslot taking consecutive octets of You need to know that each one or more
the traffic signal. If the remaining timeslot is delayed by the same 64 kb/s timeslots, and check for
timeslots are unused for traffic, they amount, and that timeslots have not timeslot integrity at the output of a
should be filled with all 1s. Of been interchanged by a switch. crossconnect switch.
course, more than one n x 64 kb/s When problems occur, you need to
signal may be carried in the 2 Mb/s identify the position of a received Tests on n x 64 kb/s
frame, depending on the bandwidth. timeslot for a known transmit circuits
timeslot.
In practice, it is not necessary When testing n x 64 kb/s circuits,
to use contiguous timeslots provided Conventional PRBS tests at leased the foremost requirement is to
they are filled in an agreed sequence line rates of 64 kb/s, 704 kb/s and 2 check the integrity of a wideband
and demultiplexed sequentially at Mb/s are well understood. signal spread across several
the far end. An example of a However, when framing at 2 Mb/s timeslots that make up an aggregate
non-contiguous plan is the or 704 kb/s is added, the value of channel.
recommendation for five 384 kb/s the test can be enhanced.
channels (six timeslots each) given
in CCITT G.735:
1-2-3 + 17-18-19, 4-5-6 + 20-21-22 and
so on.
2
Two ways to test n x 64 kb/s
When doing a "loop-back" test at n
x 64 kb/s, it is possible that the
return path may use a different
timeslot allocation. In this case, the
Some network equipment may treat just as the live signal would test set would need to have
the individual 64 kb/s channels as (method 2 above). In this approach, independent settings of transmitter
independent entities, switching the first octet of the PRBS would go and receiver timeslots. The
them to different timeslot positions to the first timeslot, the second HP 37722A and 37732A have this
or even rerouting them through octet to the second timeslot in the facility. The above shows the setup
different paths. If this happens, the plan, and so on. The unused for transmission of a 384 kb/s
multiple timeslots of the timeslots are usually filled with all channel in timeslots
n x 64 kb/s signal will not arrive at 1s, though a PRBS such as 26-1 can 1, 2, 3 and 17, 18, 19, and reception
the destination in the right also be used. in timeslots 4, 5, 6 and 20, 21, 22, as
sequence, and some could be in CCITT recommendation G.735.
missing altogether. It will be For the wideband signal to be
difficult or impossible to received without error at the far Drop and insert
reconstruct the wideband signal. end, not only would the timeslot
allocations have to be maintained, One final application of a framed 2
At present there is little or no but the timeslots would also need Mb/s test set capable of "through-
switching in digital leased lines, but to arrive in the right sequence. The data" mode is to drop and insert
this might increase in the future integrity of the n x 64 kb/s would single or multiple 64 kb/s test
with greater use of virtual private thus be proved. The second method channels while the remaining
networks and ISDN. is the one used in the HP 37722A channels carry revenue-earning
and 37732A. traffic. This requires the test set to
To test an n x 64 kb/s channel, you be placed in-circuit with the 2 Mb/s
can inject a separate PRBS pattern If a timeslot has been misplaced, line.
in each allocated timeslot, and then you would need to send an A much more detailed analysis is
check for continuity and error-free identifiable pattern (an 8-bit word, then possible on one or more 64 kb/
reception at the far-end on a for example) in each timeslot and s channels while still providing a
channel-by-channel basis (method search for it at the receive end. partial service. Again, the HP
1 above). However, it is more 37722A and 37732A have this
realistic to "spread" a single PRBS capability.
across the sequence of timeslots
allocated to the wideband channel
3
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