---
title: "Digital hierarchy of T-carriers"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:t-carriers-hierarchy"
source: https://www.bctengineer.com/q/semester_5/data_communication/t-carriers-hierarchy
site: BCT Engineering
---

# Digital hierarchy of T-carriers

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2070 Chaitra, 2 marks
- Video: [Digital Transmission Hierarchy | T1, T2, T3 & T4 Multiplexing | Data Rate, Chennels, & Applications](https://www.youtube.com/watch?v=huH4sxrkuP8) — Engineering Funda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/t-carriers-hierarchy
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the digital hierarchy of T-carriers.

## Short answer

T-carriers are the North American digital TDM hierarchy. A voice DS‑0 channel is 64 kbps. They are multiplexed in stages: T1 (1.544 Mbps, 24 channels), T2 (6.312 Mbps, 96), T3 (44.736 Mbps, 672), T4 (274.176 Mbps, 4032). T1 frame rate: (24×8+1)×8000 = 1.544 Mbps with one framing bit.

## Answer

T-carriers are the North American digital TDM hierarchy. A voice channel is sampled 8000 times/s with 8 bits, giving a **DS0 = 64 kbps** channel, and these are time-multiplexed in stages:

| Line | Digital signal | Channels | Rate |
|---|---|---|---|
| T1 | DS-1 | 24 | 1.544 Mbps |
| T2 | DS-2 | 96 (4 × T1) | 6.312 Mbps |
| T3 | DS-3 | 672 (7 × T2) | 44.736 Mbps |
| T4 | DS-4 | 4032 (6 × T3) | 274.176 Mbps |

T1 rate = (24 × 8 + 1) × 8000 = 1.544 Mbps, where 1 bit per frame is for framing. Higher levels exceed a plain multiple because of extra synchronisation bits (bit stuffing).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a DS0 channel?
   A. 64 kbps: 8000 samples/s x 8 bits
   B. 1.544 Mbps
   C. 56 kbps
   D. 128 kbps
   Answer: A. DS0 = 8000 x 8 = 64 kbps. (Key point: DS0)

2. How is the T1 rate obtained?
   A. 24 x 8 x 8000
   B. (24 x 8 + 1) x 8000 = 1.544 Mbps
   C. 24 x 64 x 193
   D. 32 x 8 x 8000
   Answer: B. 193-bit frames (24x8 plus 1 framing bit) x 8000 = 1.544 Mbps. (Key point: T1 rate)

3. How many T1 channels form T2, and how many T2 form T3?
   A. T2 = 7 x T1; T3 = 4 x T2
   B. T2 = 6 x T1; T3 = 7 x T2
   C. T2 = 4 x T1; T3 = 7 x T2
   D. T2 = 4 x T1; T3 = 6 x T2
   Answer: C. T2 = 4 x T1 = 96 channels; T3 = 7 x T2 = 672 channels. (Key point: T2 and T3)

## Related in Data Communication

- [Short note: Analog vs digital mux hierarchy](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aanalog-vs-digital-mux-hierarchy) — Data Communication, chapter 6

## Related in other subjects

- [E1 digital hierarchy; SQNR in uniformly quantized PCM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-hierarchy-sqnr-uniform-pcm) — Communication Systems, chapter 5, asked 2×
- [Short note: E1 digital hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-e1-hierarchy) — Communication Systems, chapter 5
- [Differences between T1 and E1 hierarchy](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3At1-vs-e1) — Communication Systems, chapter 5
- [E1 hierarchy; PCM SQNR at 32 kbps](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ae1-pcm-4khz-1-1v-32kbps) — Communication Systems, chapter 5