--- title: "Constellation diagram significance; E1 vs T1" subject: "Communication Systems" chapter: 5 key: "semester_6/communication_systems:constellation-significance-e1-t1" source: https://www.bctengineer.com/q/semester_6/communication_systems/constellation-significance-e1-t1 site: BCT Engineering --- # Constellation diagram significance; E1 vs T1 Communication Systems · Chapter 5: Multiplexing Techniques - Asked in: 2080 Chaitra, 2+4 marks - Video: [DIGITAL MULTIPLEXING](https://www.youtube.com/watch?v=q3PtzR9kp9U) — Prof. Barapate's Tutorials - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/constellation-significance-e1-t1 - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-5 ## Question What is the significance of constellation diagram? Compare E1 and T1 TDM PCM hierarchy. ## Short answer A constellation diagram plots digital symbols in I–Q space, showing symbol count, minimum distance (noise immunity), power, and impairments. E1 has 32 slots, 30 voice, 2.048 Mbps, A-law; T1 has 24 voice, 1.544 Mbps, µ-law, with robbed-bit signalling. Both have hierarchical multiplexing: E1/2/3/4 and T1/2/3/4. ## Answer #### Significance of constellation diagram A **constellation diagram** plots every possible symbol of a digital modulation scheme as a point in the I–Q (in-phase vs quadrature) plane. The distance of a point from the origin shows the symbol's amplitude, and its angle shows the symbol's phase. ```text Q Q | o | o o---+---o I ----+---- I | o | o BPSK QPSK ``` Why it matters: - It shows the **number of symbols M**, and so the bits per symbol ($\log_2 M$), at a glance. - The **minimum distance between points** sets the noise margin. Points that are farther apart give a lower bit error rate for the same noise. - It shows the **average and peak power** of the scheme: the mean and maximum squared distance from the origin. - On a received-signal display, spread-out clouds show noise. A rotated pattern shows phase error, and a squeezed pattern shows amplitude non-linearity. This makes it a practical test tool. - It lets us compare schemes such as BPSK, QPSK and 16-QAM by bandwidth efficiency and noise immunity. #### Comparison of E1 and T1 hierarchy | Point | T1 (North America, Japan) | E1 (Europe, ITU-T, Nepal) | |---|---|---| | Voice channels | 24 | 30 (+2 overhead slots) | | Time slots per frame | 24 + 1 framing bit | 32 | | Bits per frame | $24 \times 8 + 1 = 193$ | $32 \times 8 = 256$ | | Frame duration | 125 µs | 125 µs | | Bit rate | 1.544 Mb/s | 2.048 Mb/s | | Framing | 1 F-bit per frame, pattern over 12/24 frames | TS0 carries frame alignment word | | Signalling | Robbed-bit (LSB of frames 6 and 12) | Separate slot TS16 (CAS) | | Multiframe | Superframe 12 frames / ESF 24 frames | 16 frames (2 ms) | | Companding law | µ-law, $\mu = 255$ | A-law, $A = 87.6$ | | Line code | AMI / B8ZS | HDB3 | | Higher levels | 6.312, 44.736, 274.176 Mb/s | 8.448, 34.368, 139.264, 564.992 Mb/s | Frame structures: ```text |<--------- one frame = 125 us = 193 bits --------->| +---+--------+--------+--------+-------+--------+ | F | Ch 1 | Ch 2 | Ch 3 | ... | Ch 24 | +---+--------+--------+--------+-------+--------+ 1 bit 8 bits 8 bits 8 bits 8 bits F = framing bit (pattern spread over 12 frames) ``` ```text |<-------- one frame = 125 us = 256 bits -------->| +-----+-----+-----+-----+------+------+-----+------+ | TS0 | TS1 | ... |TS15 | TS16 | TS17 | ... | TS31 | +-----+-----+-----+-----+------+------+-----+------+ sync voice voice signal voice voice each time slot = 8 bits; 32 slots per frame voice channels: TS1-TS15 and TS17-TS31 (30) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What do the distance and angle of a point in a constellation diagram show? A. Distance = phase, angle = amplitude B. Distance from origin = amplitude, angle = phase C. Distance = frequency, angle = bit rate D. Distance = noise, angle = power Answer: B. A point's distance from the origin gives symbol amplitude and its angle gives phase in the I-Q plane. (Key point: Constellation meaning) 2. What does a larger minimum distance between constellation points give? A. Higher bandwidth efficiency B. Lower bit error rate for the same noise C. More bits per symbol D. Lower peak power always Answer: B. Points farther apart need more noise to be confused, so the noise margin is larger and BER lower. (Key point: Minimum distance) 3. On a received-signal display, what does a rotated constellation pattern indicate? A. Noise B. Phase error C. Amplitude non-linearity D. More symbols Answer: B. Spread clouds show noise, rotation shows phase error, and squeezing shows amplitude non-linearity. (Key point: Diagnosing impairments) 4. Which comparison of T1 and E1 is correct? A. T1: 193 bits, robbed-bit signalling; E1: 256 bits, TS16 signalling B. T1: 256 bits; E1: 193 bits C. T1: frame 125 us; E1: frame 250 us D. T1: A-law; E1: mu-law Answer: A. T1 has 193-bit frames with robbed-bit signalling, E1 has 256-bit frames with separate TS16; both frames last 125 us. (Key point: T1 vs E1) ## Related in Communication Systems - [T1 and E1 TDM-PCM hierarchy with frame diagrams](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-e1-frame-diagrams) — Communication Systems, chapter 5 - [Companding; T1 hierarchy of TDM-PCM telephony](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acompanding-t1) — Communication Systems, chapter 5 - [TDM vs FDM for voice; E1 and T1 hierarchies](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Atdm-fdm-voice-e1-t1) — Communication Systems, chapter 5 - [FDMA vs TDMA; T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afdma-tdma-t1-e1) — Communication Systems, chapter 5 - [E1 hierarchy; PCM of 4.2 MHz TV signal with 512 levels](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-tv-4-2mhz-512) — Communication Systems, chapter 4 - [E1 frame, TDM hierarchy and signaling rate](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-frame-hierarchy) — Communication Systems, chapter 5 ## Related in other subjects - [North American T1 vs European E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3At1-e1-hierarchy-compare) — Telecommunications, chapter 3, asked 2× - [TDM and T1/E1 carrier standards](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-t1-e1-carriers) — Telecommunications, chapter 3 - [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 - [Compare star, snowflake, fact constellation](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Astar-snowflake-constellation) — Data Mining, chapter 1