BSNL SET Telecom Notes
Free chapter for students preparing for BSNL SET.
Multiplexing is the technique of combining multiple independent signals or data streams onto a single shared communication link simultaneously. It is fundamental to telecom — without it, every phone call would need its own dedicated wire or radio channel between the two endpoints.
Side-by-side comparison of FDM (frequency slots), TDM (time slots), and WDM (wavelength channels) multiplexing.
Exam point: Multiplexing is the reason a single telephone exchange port or fiber backbone can serve thousands of simultaneous calls.
FDM divides the available frequency spectrum into separate non-overlapping frequency bands (channels). Each user is permanently assigned one band and can transmit continuously within it.
FDM spectrum view: each user gets a dedicated frequency slot. Guard bands between slots prevent adjacent-channel interference.
Key facts:
Guard bands are mandatory in FDM. Without them, the edges of adjacent channels would overlap — this is called adjacent channel interference (ACI).
Exam point: FDM is an analog technique. Guard bands are needed to prevent interference between channels. Spectrum is wasted when users are idle.
TDM divides the transmission channel by time. All users share the full bandwidth but take turns — each user is allocated a fixed repeating time slot. One complete cycle of slots across all users is called a frame.
TDM timeline: users are interleaved in fixed time slots within each repeating frame. Every user gets one slot per frame.
Key facts:
The E1 line (the standard in India/Europe) is a classic TDM system:
| E1 Property | Value |
|---|---|
| Total bit rate | 2.048 Mbps |
| Number of time slots | 32 per frame |
| Voice channels (traffic) | 30 |
| Signaling channel | Slot 16 |
| Framing/sync channel | Slot 0 |
| Bit rate per slot | 64 kbps |
| Frame duration | 125 µs (8,000 frames/sec) |
Exam point: E1 = 2.048 Mbps = 32 time slots × 64 kbps. It carries 30 voice + 1 signaling + 1 framing channel. This is a very commonly tested fact.
| Type | Slots assignment | Efficiency | Example |
|---|---|---|---|
| Synchronous (fixed) TDM | Fixed; each user always has a slot | Can waste capacity when idle | E1, SONET |
| Asynchronous (statistical) TDM | Dynamic; only active users get slots | Efficient; no wasted slots | Packet networks |
In Statistical Multiplexing, time slots are not pre-assigned. Slots are dynamically allocated only to users who have data to send. If a user is silent, no slot is wasted — another user gets it.
Only active users consume link capacity. Idle users release their slot instantly — so many more users can share the link.
Key facts:
Exam point: Statistical multiplexing is the basis of the Internet. It trades predictable latency (fixed TDM) for higher efficiency. Jitter is an inherent side-effect.
WDM is essentially FDM applied to optical fiber. Different data streams are transmitted on the same fiber using different wavelengths (colors) of light. A wavelength in WDM is often called a lambda (λ) or channel.
WDM: multiple wavelengths enter a multiplexer, travel down one fiber, and are separated by a demultiplexer at the far end.
Key facts:
| Variant | Wavelength spacing | Channels | Use |
|---|---|---|---|
| CWDM (Coarse WDM) | Wide (~20 nm) | 8–18 | Metro networks, shorter distances |
| DWDM (Dense WDM) | Very narrow (~0.8 nm) | 40–160+ | Long-haul backbone, BSNL national network |
Exam point: DWDM is used in long-haul backbone networks (including BSNL's national backbone). It can carry 40–160+ channels on a single fiber, each at 10–100 Gbps. DWDM dramatically reduces the cost per bit of long-distance transmission.
Total capacity of a DWDM system: If a fiber carries 80 DWDM channels each at 100 Gbps → total capacity = 80 × 100 = 8 Tbps on one fiber.
CDM allows multiple users to transmit simultaneously on the same frequency band at the same time. Users are separated by unique mathematical codes (called spreading codes or chip codes). Each receiver uses the same code as the intended transmitter to extract the correct signal.
Key facts:
Exam point: CDMA separates users by code, not frequency or time. 3G mobile networks (WCDMA) use CDMA. The near-far problem is a key challenge in CDMA systems.
OFDMA (Orthogonal Frequency Division Multiple Access) is the multiple-access version of OFDM. It divides the available bandwidth into many narrow subcarriers, and different users are assigned different subsets of subcarriers dynamically.
Key difference from FDM:
| System | Multiple Access | What it divides |
|---|---|---|
| 1G (analog) | FDMA | Frequency |
| 2G (GSM) | TDMA + FDMA | Time + Frequency |
| 3G (WCDMA) | CDMA (WCDMA) | Code |
| 4G (LTE) downlink | OFDMA | Subcarriers |
| 4G (LTE) uplink | SC-FDMA | Subcarriers (single-carrier) |
| 5G NR | OFDMA | Subcarriers (flexible numerology) |
Exam point: The progression from FDMA → TDMA → CDMA → OFDMA maps directly to 1G → 2G → 3G → 4G. This table is high-yield for MCQs.
| Feature | FDM | TDM | WDM | CDM | Statistical MUX |
|---|---|---|---|---|---|
| Resource divided | Frequency | Time | Wavelength (light) | Code | Time (dynamic) |
| Signal type | Analog | Digital | Optical | Digital | Digital |
| Guard bands needed? | Yes | No | No (wavelength spacing) | No | No |
| Idle capacity wasted? | Yes | Yes (fixed TDM) | Yes | No | No |
| Delay type | Constant | Constant | Constant | Variable (power dep.) | Variable (jitter) |
| Example use | FM radio, 1G | E1, SONET, GSM | Fiber backbone | 3G WCDMA | Internet/IP |
| Fact | Value |
|---|---|
| E1 bit rate | 2.048 Mbps |
| E1 time slots | 32 (30 voice + 1 signaling + 1 framing) |
| E1 bit rate per slot | 64 kbps |
| E1 frame rate | 8,000 frames/second |
| T1 bit rate (North America) | 1.544 Mbps (24 channels × 64 kbps) |
| STM-1 (SDH) | 155.52 Mbps (E1 × 63) |
| DWDM typical channels | 40–160 per fiber |
| DWDM channel spacing (ITU-T) | 100 GHz or 50 GHz |
| Topic | Remember this |
|---|---|
| FDM resource | Frequency band |
| TDM resource | Time slot |
| WDM resource | Wavelength (lambda) |
| CDM resource | Spreading code |
| FDM guard bands | Needed — prevent adjacent channel interference |
| FDM signal type | Analog |
| TDM signal type | Digital |
| E1 total bit rate | 2.048 Mbps |
| E1 voice channels | 30 (out of 32 slots) |
| WDM medium | Optical fiber only |
| DWDM use | Long-haul backbone, national networks |
| Statistical MUX drawback | Variable delay / jitter |
| CDMA separates users by | Unique spreading codes |
| LTE downlink multiple access | OFDMA |
| LTE uplink multiple access | SC-FDMA |
| Near-far problem | CDMA challenge — strong nearby signal drowns weak distant one |
| 1G multiple access | FDMA |
| 2G (GSM) multiple access | TDMA + FDMA |
| 3G multiple access | CDMA (WCDMA) |
| 4G multiple access | OFDMA |
