--- title: "WDM and T carrier system" subject: "Telecommunications" chapter: 3 key: "semester_8/telecommunications:wdm-t-carrier-t1" source: https://www.bctengineer.com/q/semester_8/telecommunications/wdm-t-carrier-t1 site: BCT Engineering --- # WDM and T carrier system Telecommunications · Chapter 3: Signal Multiplexing - Asked in: 2079 Bhadra, 3+5 marks - Video: [Unit 5 T1 Carrier system | A PCM-TDM System | T1 Digital system](https://www.youtube.com/watch?v=SY8ng74yEj4) — Techno Tutorials ( e-Learning) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/wdm-t-carrier-t1 - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-3 ## Question What is wavelength division multiplexing? Describe T carrier system showing the frame structure of T1 level and different multiplexing levels with data rates. ## Short answer WDM multiplexes optical signals on one fibre using different wavelengths, FDM in optical domain. T1 frame has 24 channels x 8 bits plus 1 framing bit = 193 bits at 8000 frames/s, giving 1.544 Mbps. Hierarchy: T1 1.544, T2 6.312, T3 44.736, T4 274.176 Mbps. ## Answer #### Wavelength Division Multiplexing (WDM) **WDM** is the multiplexing of several optical signals on one optical fibre by giving each a different wavelength (colour) of light. It is FDM in the optical domain. ```text l1 -->|\ /|--> l1 l2 -->| MUX ==== fibre ==== DEMUX|--> l2 l3 -->|/ (l1+l2+...+lN) \|--> l3 lN --> --> lN l = wavelength (lambda) ``` - Each channel has its own laser at wavelength $\lambda_1, \lambda_2, \ldots, \lambda_N$ (typically in the 1550 nm band). - A passive multiplexer (prism, diffraction grating or thin-film filter) combines them; at the far end a demultiplexer separates them. Erbium-doped fibre amplifiers boost all wavelengths together. - **CWDM** uses wide spacing (20 nm, up to 18 channels); **DWDM** uses close spacing (0.8 nm/100 GHz or less, 40–160 channels), giving terabits per second on one fibre. #### T carrier system The T carrier system is the North American digital TDM hierarchy (AT&T). The basic unit is a 64 kbps PCM voice channel (DS0): 8000 samples/s × 8 bits. **T1 frame structure:** 24 channels × 8 bits + 1 framing bit = 193 bits per 125 µs frame. ```text 1 bit 8 bits 8 bits 8 bits +-----+-------+-------+-----+--------+ | F | Ch1 | Ch2 | ... | Ch24 | +-----+-------+-------+-----+--------+ <--------- 193 bits in 125 us --------> F = framing bit ``` $$ \begin{aligned} \text{Bits per frame} &= 24 \times 8 + 1 = 193\ \text{bits} \\ \text{Frame rate} &= 8000\ \text{frames/s}\ (T = 125\ \mu\text{s}) \\ R_{T1} &= 193 \times 8000 = 1.544\ \text{Mbps} \end{aligned} $$ - Framing bits from 12 frames form a superframe pattern; signalling is carried by robbing the LSB of each channel in frames 6 and 12. **Multiplexing levels:** ```text 24 x DS0 --> [MUX] --> T1 1.544 Mbps (24 ch) 4 x T1 --> [M12] --> T2 6.312 Mbps (96 ch) 7 x T2 --> [M23] --> T3 44.736 Mbps (672 ch) 6 x T3 --> [M34] --> T4 274.176 Mbps (4032 ch) ``` | Level | Made from | Voice channels | Bit rate | Overhead | |---|---|---|---|---| | DS0 | 1 voice channel | 1 | 64 kbps | – | | DS1 (T1) | 24 DS0 | 24 | 1.544 Mbps | 8 kbps (framing) | | DS2 (T2) | 4 DS1 | 96 | 6.312 Mbps | 136 kbps | | DS3 (T3) | 7 DS2 | 672 | 44.736 Mbps | 552 kbps | | DS4 (T4) | 6 DS3 | 4032 | 274.176 Mbps | 5.76 Mbps | $$ \begin{aligned} T2 &: 4 \times 1.544 = 6.176;\ \ 6.312 - 6.176 = 0.136\ \text{Mbps overhead} \\ T3 &: 7 \times 6.312 = 44.184;\ \ 44.736 - 44.184 = 0.552\ \text{Mbps overhead} \\ T4 &: 6 \times 44.736 = 268.416;\ \ 274.176 - 268.416 = 5.76\ \text{Mbps overhead} \end{aligned} $$ The overhead at each higher level carries framing, pulse-stuffing (justification) control and alarm bits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do CWDM and DWDM differ? A. CWDM has close spacing (0.8 nm), DWDM wide spacing B. CWDM uses wide spacing (about 20 nm, up to 18 channels); DWDM uses close spacing (0.8 nm or less, 40-160 channels) C. They only differ in fibre colour D. DWDM carries fewer channels Answer: B. Close spacing in DWDM packs many more wavelengths for terabit capacity. (Key point: CWDM vs DWDM) 2. What is the T1 line rate and why? A. 1.536 Mbps since 24 x 64 kbps B. 2.048 Mbps from 32 slots C. 1.544 Mbps since 193 bits are sent 8000 times per second D. 6.312 Mbps Answer: C. (24 x 8 + 1) x 8000 = 1.544 Mbps; the 8 kbps extra is framing. (Key point: T1 rate) 3. Which multiplexing sequence is correct for the T carrier? A. 24 DS0 to T1, 4 T1 to T2, 7 T2 to T3, 6 T3 to T4 B. 24 DS0 to T1, 7 T1 to T2, 4 T2 to T3, 6 T3 to T4 C. 30 DS0 to T1, 4 T1 to T2, 4 T2 to T3 D. 4 DS0 to T1, 4 T1 to T2, 7 T2 to T3 Answer: A. T2 = 4 x T1 (6.312 Mbps), T3 = 7 x T2 (44.736 Mbps), T4 = 6 x T3 (274.176 Mbps). (Key point: T-carrier hierarchy) 4. How is T3 overhead found and what is it? A. 44.736 - 7 x 1.544 = 33.9 Mbps B. 44.736 - 4 x 6.312 = 19.5 Mbps C. 44.736 - 7 x 6.312 = 0.552 Mbps, used for framing, stuffing control and alarms D. It has no overhead Answer: C. 7 x 6.312 = 44.184 Mbps, so the extra 0.552 Mbps carries framing, justification control and alarms. (Key point: Overhead at higher levels) ## Close variants set in other exams - [FDM hierarchy and T carrier system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Afdm-hierarchy-t-carrier) — Telecommunications, chapter 3, asked 2× ## Related in Telecommunications - [T1 and E1 frame structures and bit rates](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3At1-e1-frame-structures) — Telecommunications, chapter 3 - [TDM principle and T1 carrier](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-principle-t1-carrier) — Telecommunications, chapter 3 - [Digital carrier system and multiplexing techniques](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adigital-carrier-multiplexing) — Telecommunications, chapter 3 - [TDM and T1/E1 carrier standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-t1-e1-carriers) — Telecommunications, chapter 3 - [Need for multiplexing and WDM](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Amultiplexing-need-wdm) — Telecommunications, chapter 3 - [TDM frame, time slot and 24-channel system](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-frame-24-channel) — Telecommunications, chapter 3 ## Related in other subjects - [T1 digital carrier system; adaptive delta modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-hierarchy-adm) — Communication Systems, chapter 5 - [Time division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Atdm-explain) — Data Communication, chapter 6, asked 3× - [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3× - [Short note: Frequency division multiplexing](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-fdm) — Communication Systems, chapter 5, asked 2×