Telecommunication Fundamentals

(a) GSM

GSM (Global System for Mobile Communications) is the world's first cellular system to specify digital modulation and network-level architecture. AMPS is analog, while IS-54 and CDMA came later.

(C) cell size

In GSM, macrocells, microcells and picocells classify cells by their coverage area (macro = large, up to tens of km; micro = hundreds of metres; pico = indoor, tens of metres).

(b) Message delay is low

In CDMA every user transmits continuously over the whole band using a distinct code, so a message need not wait for an assigned slot. In TDMA a user can transmit only in its own time slot, which adds waiting delay. Hence low message delay is the advantage of CDMA over TDMA.

(d) Voice, video and message

IP (Internet Protocol) is a packet-based network-layer protocol that carries any type of digitised traffic — voice (VoIP), video (streaming, video calls) and messages/data — as IP packets. Hence it is not limited to voice or data alone.

(b) High speed data transmission over long distance

SDH is a synchronous multiplexing standard designed for the backbone transport network — it carries high-speed data and voice traffic over long distances (typically over optical fibre) with high reliability and built-in protection switching. LAN management, telephone switching and short-range radio are handled by other technologies.

(a) refractive index profile that is uniform within the core

In a step index fiber, the core has a single uniform refractive index throughout, and the cladding has a lower constant refractive index. The refractive index changes abruptly (in a step) at the core–cladding boundary.

(c) Splice closure

A splice closure is installed in a manhole to house and protect the spliced joints of multiple cables, keeping the fibres organized and shielded from moisture, dust and mechanical damage.

(d) To connect telephone lines to network equipment

A distribution frame (main distribution frame, MDF) is the termination point where incoming outside-plant telephone lines are connected and cross-connected to the exchange's switching equipment. It allows any subscriber line to be jumpered to any switch port, making line testing, rearrangement and fault isolation easy.

(a) a telephone line

A modem (modulator–demodulator) converts digital data from a computer into signals that can travel over the telephone network, so it is connected to a telephone line (the wall jack) to send and receive data.

ADSL Tester — 2.5 marks

ADSL (Asymmetric Digital Subscriber Line) Tester is a handheld instrument used by a telecom technician to test and verify an ADSL (broadband) line at the subscriber end or at the exchange/DP side.

Main functions:

1. Line qualification — measures loop resistance, insulation resistance, capacitance and line voltage to check whether the copper pair is healthy enough to carry ADSL service.
2. Synchronisation (sync) test — checks whether the modem/line can train (achieve sync) with the DSLAM and displays the achieved upstream and downstream sync rates.
3. Noise and attenuation measurement — shows line attenuation (dB) and noise margin / SNR (dB). A low noise margin (below about 6 dB) or high attenuation indicates a noisy or long loop.
4. Tone/bit loading display — shows the DMT sub-carrier (tone) graph, so the technician can see which frequency bands are affected by interference such as AM radio or crosstalk.
5. Fault localisation — by comparing results at the exchange, DP and subscriber premises, it helps decide whether the fault lies in the cable pair, the drop wire, the splitter, the modem or the ISP side.
6. Modem/PPP test — some models also emulate a modem to verify authentication (PPPoE login) and internet connectivity.

Use: It is connected in place of the subscriber modem at the telephone socket (through the splitter) and readings are taken. It saves time by confirming whether a broadband complaint is a line fault or a modem/config problem before replacing equipment.

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VSWR Meter — 2.5 marks

VSWR (Voltage Standing Wave Ratio) meter is an instrument used to measure the impedance matching between a transmitter/receiver and its antenna or transmission line (feeder cable).

Principle: When the load (antenna) impedance is not equal to the characteristic impedance of the feeder, part of the forward wave is reflected back. The forward and reflected waves combine to form a standing wave. The ratio of maximum to minimum voltage of this standing wave is the VSWR:

$\text{VSWR} = \frac{V_{\max}}{V_{\min}} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$

where $\Gamma$ is the reflection coefficient. For a perfect match, VSWR = 1 (ideal); in practice 1 to 1.5 is acceptable. A high VSWR means poor matching, loss of transmitted power and possible damage to the transmitter.

Construction: It consists of a slotted coaxial line or a directional coupler section with a diode detector probe, a calibrated sensitive meter (µA), and controls for sensitivity and range selection. Modern sets are often combined "power–VSWR" meters.

Working/use:

1. The meter is inserted in the feeder line between the transmitter and antenna.
2. Forward power is noted by adjusting the probe/coupler in the forward direction and setting the meter to full scale (set FSD).
3. The probe is then switched to the reflected direction and the reflected reading is noted.
4. VSWR is read directly from the calibrated scale, or calculated as $\text{VSWR} = \frac{1 + \sqrt{P_r/P_f}}{1 - \sqrt{P_r/P_f}}$ where $P_f$ = forward power and $P_r$ = reflected power.

Applications: tuning and testing antennas, checking feeder cables and connectors in microwave, VHF/UHF and mobile base station (BTS) installations, and detecting cable damage, water ingress or loose connectors.

Fiber-to-the-Home (FTTH) is a broadband network architecture in which an optical fiber is laid all the way from the telecom operator's central office (OLT) directly up to the subscriber's premises, replacing the traditional copper pair. It uses light signals through glass fiber, so it can deliver very high bandwidth (typically 100 Mbps to 1 Gbps and above), supports triple-play services (voice, internet, IPTV) over a single fiber, and is immune to electromagnetic interference. It is the passive optical network (PON) technology most widely deployed by Nepal Telecom.

Main components of FTTH:

1. OLT (Optical Line Terminal): Installed at the central office/exchange. It is the main active equipment that converts electrical signals from the core network into optical signals, controls the PON, and manages all ONUs on the network.

2. ODF (Optical Distribution Frame): A rack at the exchange where incoming and outgoing fibers are terminated, spliced and patched for easy management and testing.

3. Feeder Cable: The main optical cable that runs from the OLT/ODF to the FDC (Feeder Distribution Cabinet) in the field. It is usually a high-count cable.

4. FDC / FDT (Fiber Distribution Cabinet/Terminal): A cabinet in the field where the feeder cable is split and distributed to different areas through distribution cables.

5. Distribution Cable: Optical cable running from the FDC to the FAT (Fiber Access Terminal) near the subscriber's locality.

6. Splitter (Optical Power Splitter): A passive device (1:2, 1:4, 1:8, 1:16, 1:32) that divides one optical signal among many subscribers without using power. It is the key element that makes PON economical.

7. FAT (Fiber Access Terminal) / Drop Box: A small terminal near the customer's building where the distribution cable is terminated and the drop cable is connected.

8. Drop Cable: A thin, flexible optical cable that connects the FAT to the subscriber's ONT/ONU, often entering through a wall or duct.

9. ONT/ONU (Optical Network Terminal/Unit): Installed at the customer's premises. It converts the optical signal back into electrical signals for telephone, internet (Ethernet/Wi-Fi) and TV services.

10. Accessories: Splice closures, connectors (SC/APC), pigtails, patch cords, and OTDR/power meter for testing and maintenance.

Operation of VSAT (2 marks)

VSAT (Very Small Aperture Terminal) is a two-way satellite earth station with a small dish antenna (typically 0.75–2.4 m diameter) used to transmit and receive data, voice and video via satellite.

Operation:

1. Uplink: The VSAT earth station (indoor unit + outdoor unit) sends data to the satellite in the Ku-band (12–14 GHz) or C-band (4–6 GHz) using a modulated carrier.
2. Transponder: The satellite's transponder receives the weak uplink signal, shifts its frequency (e.g. 14 GHz → 11 GHz), amplifies it and retransmits it back to earth as a downlink beam.
3. Downlink: The receiving VSAT (or hub) captures the downlink signal with its LNB, down-converts it to L-band (950–1450 MHz) and demodulates it to recover the original data.
4. Topology: In a star topology, all remote VSATs communicate through a central hub (double hop). In a mesh topology, VSATs communicate directly with each other through the satellite (single hop).
5. Access: Multiple users share the transponder using FDMA, TDMA or DAMA (Demand Assigned Multiple Access), so bandwidth is allotted only when needed.

Thus VSAT provides instant, remote-area communication independent of terrestrial links.

Difference between PDM and SDM (3 marks)

Key point: PDM separates signals by polarization on one carrier, while SDM separates them by physical space/path, allowing many independent channels to run in parallel.

NGN (Next Generation Network)

NGN (Next Generation Network) is a packet-based, multi-service telecommunication network that uses a single unified network to carry voice, data, and video (triple-play) services. It is defined by ITU-T as a network that provides telecommunication services using multiple broadband, QoS-enabled transport technologies, in which service-related functions are independent of the underlying transport technologies. In NGN, the traditional circuit-switched PSTN is replaced by packet switching (IP/MPLS), and the intelligence (call control, service logic) is separated from the transport (switching and transmission) layer.

Benefits of implementing NGN in telecommunication

1. Single converged network (cost saving): Voice, data, and video run over one IP-based network, so separate networks for telephony and data are no longer needed. This reduces capital expenditure (CAPEX) and operation and maintenance (OPEX) costs.

2. Efficient bandwidth utilization: Packet switching (statistical multiplexing) uses bandwidth only when data is actually sent, unlike circuit switching which reserves a fixed channel for the whole call. This gives much higher bandwidth efficiency.

3. Easy and fast service creation: Because service logic is separated from transport (open, layered architecture), new services such as VoIP, IPTV, video-on-demand, VPN, and multimedia messaging can be added quickly by software upgrade without changing the underlying network.

4. Convergence of services (triple/quadruple play): A single connection can deliver voice, internet, and IPTV to the subscriber, increasing average revenue per user (ARPU) and customer satisfaction.

5. Scalability and flexibility: NGN is modular and standards-based (softswitch, media gateway, SIP), so capacity can be expanded easily as subscriber numbers grow, and different access technologies (PSTN, mobile, broadband, Wi-Fi) can be integrated.

6. Interoperability and open standards: NGN supports interworking with existing PSTN/PLMN networks through media gateways, allowing smooth migration from legacy networks without losing existing subscribers.

7. Better QoS and network management: Centralized, software-based control enables efficient traffic management, quality of service control, and remote monitoring/fault management.

8. Support for mobility and future technologies: NGN provides a platform for fixed–mobile convergence (FMC) and smooth evolution towards all-IP (IMS/4G/5G) networks.

In short, NGN gives Nepal Telecom a single, scalable, IP-based platform that lowers cost, uses bandwidth efficiently, and enables rapid introduction of modern multimedia services.

WiMAX Communication System

WiMAX (Worldwide Interoperability for Microwave Access) is a wireless broadband technology based on the IEEE 802.16 standard. It provides high-speed internet access over a wide area using microwave radio signals, as an alternative to cable and DSL.

Working principle:

• A WiMAX tower (Base Station) is connected to the core network (internet/ISP backbone) through a high-capacity wired link.
• Subscriber terminals (CPE — Customer Premises Equipment) or WiMAX-enabled devices communicate with the base station by microwave radio using OFDM (Orthogonal Frequency Division Multiplexing) modulation.
• The base station allocates bandwidth to subscribers on demand (using OFDMA in mobile WiMAX), so many users share the same channel efficiently.
• NLOS (Non-Line-of-Sight) operation is possible in mobile WiMAX, while fixed WiMAX (802.16d) generally needs LOS.

Types:

Features: High data rate (up to ~70 Mbps per base station), long coverage range, supports both fixed and mobile users, QoS (Quality of Service) support, NLOS operation, and easy deployment where cable laying is difficult.

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Features of GSM System

GSM (Global System for Mobile Communication) is a 2G digital cellular standard.

1. Digital cellular technology — voice is digitized and transmitted, giving better quality and security than analog.
2. Frequency bands — 900 MHz and 1800 MHz (also 850/1900 MHz in some regions).
3. Multiple access — uses TDMA/FDMA; each carrier (200 kHz wide) is divided into 8 time slots.
4. SIM (Subscriber Identity Module) — subscriber identity is stored in a removable SIM card, so the number can be moved to another handset.
5. Roaming — automatic international and national roaming between networks.
6. Security — authentication (A3), encryption of air interface (A5) and signalling (A8).
7. Data capability — circuit-switched data up to 9.6 kbps; extended by GPRS (2.5G) and EDGE.
8. Network architecture — BTS, BSC, MSC, HLR, VLR, AUC, EIR.
9. Supplementary services — call waiting, call forwarding, call barring, conference call, CLIP/CLIR.
10. SMS — Short Message Service up to 160 characters.

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Services of GSM System

1. Bearer (Tele) Services — basic data transport:

• Voice telephony (full rate / half rate / enhanced full rate)
• Circuit-switched data (up to 9.6 kbps)
• Short Message Service (SMS)
• Fax (group 3)

2. Supplementary Services:

• Call forwarding (CFU, CFB, CFNR)
• Call waiting and call hold
• Call barring (outgoing/incoming)
• Multi-party conference call
• Calling Line Identification Presentation/Restriction (CLIP/CLIR)
• Closed User Group (CUG)
• Advice of Charge (AoC)

3. Value Added / Other Services:

• GPRS and EDGE packet data (internet, MMS, WAP)
• Voice mail / voice mail box
• Prepaid and postpaid billing services
• Emergency call (112/100)
• Roaming service
• Cell broadcast (information services)