Common Question Answer for Telecom 


1. Why Don’t All Phones Ring at the Same Time?

In a telecommunication network, each device is uniquely identified by a Temporary Mobile Subscriber Identity (TMSI) or International Mobile Subscriber Identity (IMSI), which is assigned during network attachment. When there is an incoming call or notification, the network performs a paging process:

  • Paging Message: The network sends a paging signal to the cells within the user’s last known Tracking Area (TA). This message contains the TMSI or IMSI of the target device.
  • Paging Occasion and Paging Frame: Each phone monitors the Paging Channel (PCH) during specific intervals defined by its Paging Occasion (PO). This scheduling reduces power consumption and ensures only the intended phone responds.
  • Selective Response: The device with a matching TMSI or IMSI replies to the paging request, while others ignore it.

This process ensures only the target phone rings, and others remain unaffected.

2. What Happens if Two Users Talk Simultaneously?

In LTE, simultaneous communication is achieved using Orthogonal Frequency Division Multiple Access (OFDMA) and dynamic resource allocation:

  • OFDMA: The frequency spectrum is divided into orthogonal subcarriers, and each user is assigned a unique set of subcarriers for uplink and downlink transmission. This ensures no overlap between users.
  • Resource Blocks (RBs): The available bandwidth is further divided into resource blocks in both time and frequency domains. The eNodeB dynamically schedules these RBs for each user.
  • Hybrid Automatic Repeat Request (HARQ): If packet collisions or errors occur, HARQ retransmits corrupted packets using error correction mechanisms.

These methods prevent interference, enabling seamless communication for multiple users.

3. Why Don’t I Get the Bill from My Neighbor?

Billing integrity is maintained by associating all usage with the subscriber’s unique identifiers:

  • IMSI and EPS Bearers: Each SIM card has a unique IMSI that is authenticated during network attachment. During data sessions, a dedicated Evolved Packet System (EPS) Bearer is established for each user.
  • Session Tracking: The Serving Gateway (SGW) and Packet Data Network Gateway (PGW) record all usage details, such as data volume, session duration, and destination IP addresses, against the IMSI.
  • Call Detail Records (CDRs): The core network generates CDRs, which are processed by the Charging Gateway Function (CGF). This ensures each subscriber is billed accurately based on their usage.

This system eliminates the possibility of cross-account billing errors.

4. Why Can an Indian Use Her Phone in Paris?

Global roaming is enabled by 3GPP standards, inter-operator agreements, and network protocols:

  • Roaming Agreements: Indian and French operators have a roaming agreement that allows subscribers to access services abroad. The visited network (VPLMN) queries the home network (HPLMN) for authentication and billing coordination.
  • HSS Authentication: The visited network uses the IMSI to authenticate the subscriber by querying the Home Subscriber Server (HSS) in the Indian operator’s core network via the Diameter Protocol.
  • Session Establishment: After authentication, the user’s sessions are managed using the S8 interface, which connects the visited PGW to the home operator’s core network.
  • Universal Standards: LTE operates on global 3GPP standards, enabling device compatibility worldwide.

Roaming ensures seamless communication, even across borders.

5. Why Can’t Two Phones Have the Same Phone Number?

Phone numbers (MSISDNs) are tied to unique subscriber profiles:

  • IMSI-MSISDN Mapping: Each MSISDN is mapped to a unique IMSI in the Home Location Register (HLR) or Home Subscriber Server (HSS).
  • Call Routing: When a call is made, the network identifies the IMSI associated with the MSISDN and routes the call to the correct device via the Serving Gateway (SGW) and eNodeB.
  • Duplicate Conflict Prevention: Having duplicate MSISDNs would cause routing conflicts, which are prevented by enforcing one-to-one mappings in the core network.

This ensures that every MSISDN is linked to a single subscriber and device.

6. Why Can’t I Simply Overhear the Neighbor’s Communication?

LTE ensures communication privacy through advanced security mechanisms:

  • Encryption: User data and signaling messages are encrypted using AES-128 (Advanced Encryption Standard). Encryption keys are dynamically generated during the Authentication and Key Agreement (AKA) process and are never transmitted over the air.
  • Mutual Authentication: Both the device and the network authenticate each other using credentials stored on the SIM card and in the network’s HSS.
  • Dynamic Resource Allocation: Communication channels are dynamically assigned using orthogonal resource blocks, making it impossible to predict or intercept specific transmissions.

These measures ensure that communications are secure and cannot be overheard.

7. Why Doesn’t My Phone Always Connect to the Closest Tower?

The network prioritizes signal quality and resource availability over physical proximity:

  • Signal Quality Metrics: The phone evaluates metrics such as Reference Signal Received Power (RSRP) and Signal-to-Interference-plus-Noise Ratio (SINR) to determine the best connection.
  • Load Balancing: If the closest tower is congested, the network may redirect the device to a less-loaded tower to optimize performance.
  • Handover Optimization: The network may prioritize towers that offer better long-term connection stability, especially in high-mobility scenarios.

This ensures the best possible user experience rather than relying solely on proximity.

8. Why Do Phones Show Different Signal Strengths in the Same Location?

Signal strength variations are influenced by device-specific factors and environmental conditions:

  • Antenna Design: Different phones have different antenna designs and sensitivities, affecting how they receive signals.
  • Frequency Bands: Phones may connect to different LTE frequency bands (e.g., 700 MHz, 1800 MHz), each of which has distinct propagation characteristics. Lower frequencies offer better penetration, while higher frequencies may degrade faster.
  • Multipath Propagation: Signals reflect off surfaces, causing constructive or destructive interference that varies with device position and orientation.

These factors explain why signal strength varies between devices.

9. Why Doesn’t My Old 4G Phone Work on a 5G Network?

5G networks require devices with specific capabilities:

  • New Radio (NR): 5G uses a new air interface with advanced technologies like massive MIMO and beamforming, which are incompatible with 4G-only hardware.
  • Frequency Bands: 5G operates on additional bands, including sub-6 GHz and millimeter wave, which require new radio hardware.
  • Protocol Differences: 5G uses new signaling protocols and architectures, such as Service-Based Architecture (SBA), that are not supported by 4G devices.

This necessitates hardware upgrades for 5G compatibility.

10. Why Does My Phone Use More Battery in Low Signal Areas?

In low signal conditions, the phone expends more energy to maintain connectivity:

  • Increased Transmission Power: The phone increases its uplink transmission power to overcome poor signal quality and maintain a connection with the base station.
  • Frequent Cell Search: The device continuously scans for stronger signals, consuming significant power.
  • RRC State Transitions: Weak signals often cause frequent RRC (Radio Resource Control) state transitions, such as reconnections and re-authentication attempts, further draining the battery.

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