How is an international connection structured under the Q.40 guidelines?

A national system at each end plus an international chain between the international exchanges

Only one international chain with no national parts

Several national chains and no international exchange

What limit is placed on circuits in tandem?

At most 12 in the whole connection, normally not more than 4 in each national extension and 4 in the international chain

At most 4 in the whole connection

At most 12 in each national extension

Where is the 2W/4W conversion done in the plan?

Inside the national system, since international circuits are four-wire

At every international switching centre

When are echo suppressors or cancellers required?

When one-way propagation time exceeds about 25 ms

When one-way delay is below 5 ms

2069 Chaitra · 4 marks

CCITT Q.40 transmission plan guidelines

Short answer

Q.40 fixes a connection as two national systems plus an international chain, with at most 12 tandem circuits, normally four per national extension and four in the international chain. International circuits are four-wire with fixed nominal loss; loudness, singing margin, echo (over 25 ms needs cancellers), delay and noise limits are all specified.

Write the guidelines for transmission plan defined by CCITT in its recommendation Q.40.

A transmission plan fixes how losses, levels, delays and the number of circuits are shared among the links of a connection so that any two subscribers, national or international, get acceptable speech quality. CCITT Recommendation Q.40 ("The transmission plan", in the Q-series together with G.101) gives these guidelines for international connections (the common textbook version is given below):

Structure of a connection: an international connection is made of a national system at each end plus an international chain between the two international (CT) exchanges.
Number of circuits: the international chain should have as few circuits as possible; at most 12 circuits in tandem in a whole connection, normally not more than 4 in each national extension and 4 in the international chain (6 only in exceptional cases).
Four-wire working: international circuits and international switching centres are four-wire; the 2W/4W conversion is done inside the national system.
Nominal losses: each international circuit has a fixed nominal transmission loss (about 0.5 dB between virtual switching points, at the relative level −3.5 dBr), so the chain loss does not depend on the route.
Loudness (reference equivalent) limits: the national sending and receiving systems up to the international centre must not exceed set limits (classically sending RE about 21 dB and receiving RE about 12 dB), keeping the overall connection loudness within an acceptable figure.
Stability: each circuit must have a margin against singing; loss at 2W/4W points is controlled.
Echo and delay: echo suppressors (now cancellers) are required when the one-way propagation time exceeds about 25 ms; mean one-way delay should preferably be below 150 ms and never above 400 ms.
Noise and levels: circuit noise and transmitted power levels at the reference points are limited, so many circuits in tandem still give adequate signal-to-noise ratio.

Nations then build their own national transmission plans to fit within these limits.

✓ Answer checked by Claude Opus 5.5 (high effort)

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