--- title: "CCITT Q.40 transmission plan guidelines" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:ccitt-q40-transmission-plan" source: https://www.bctengineer.com/q/semester_8/telecommunications/ccitt-q40-transmission-plan site: BCT Engineering --- # CCITT Q.40 transmission plan guidelines Telecommunications · Chapter 2: Transmission Media - Asked in: 2069 Chaitra, 4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/ccitt-q40-transmission-plan - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question Write the guidelines for transmission plan defined by CCITT in its recommendation Q.40. ## 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. ## Answer 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): 1. **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. 2. **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). 3. **Four-wire working:** international circuits and international switching centres are four-wire; the 2W/4W conversion is done inside the national system. 4. **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. 5. **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. 6. **Stability:** each circuit must have a margin against singing; loss at 2W/4W points is controlled. 7. **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. 8. **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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is an international connection structured under the Q.40 guidelines? A. A national system at each end plus an international chain between the international exchanges B. Only one international chain with no national parts C. Two-wire throughout D. Several national chains and no international exchange Answer: A. The connection consists of national systems at both ends and an international chain between CT exchanges. (Key point: Connection structure) 2. What limit is placed on circuits in tandem? A. At most 4 in the whole connection B. At most 12 in the whole connection, normally not more than 4 in each national extension and 4 in the international chain C. Unlimited if loss is low D. At most 12 in each national extension Answer: B. Few circuits keep delay, loss and noise acceptable: 12 total, normally 4+4+4. (Key point: Number of circuits) 3. Where is the 2W/4W conversion done in the plan? A. Inside the international chain B. At every international switching centre C. Inside the national system, since international circuits are four-wire D. Nowhere Answer: C. International circuits and centres are four-wire; conversion is in the national systems. (Key point: Four-wire working) 4. When are echo suppressors or cancellers required? A. When one-way delay is below 5 ms B. Only on 2-wire circuits C. When loss exceeds 0.5 dB D. When one-way propagation time exceeds about 25 ms Answer: D. Beyond about 25 ms one-way delay echo becomes annoying; mean delay should preferably stay below 150 ms and never exceed 400 ms. 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