During the summer of 2025, I spent two months as an engineering intern at Bharat Sanchar Nigam Limited (BSNL). Working at a local telecom exchange gave me a direct look at how a large public sector communication network operates, closing the gap between textbook telecom theory and physical hardware in the field. The exchange runs a mix of legacy copper systems and modern fiber-optic routing equipment. My time was divided between understanding the central office infrastructure and watching field engineers troubleshoot real-world network problems.

Central Office Infrastructure: OLT and ONT

A key focus of my internship was Fiber to the Home (FTTH) technology, which has largely replaced traditional copper-based landlines. At the center of this sits the Optical Line Terminal (OLT). The OLT is the brain of the local optical network. It takes incoming data streams from BSNL's core network and routes them to individual consumer connections. At the customer end, an Optical Network Terminal (ONT) decodes the light signals back into ethernet and Wi-Fi. I learned how passive optical splitters divide a single physical fiber path from the exchange to serve up to 64 customers. This reduces infrastructure costs while maintaining high-speed delivery across the network.

Transitioning to FTTH and Fiber Optics

Moving from copper cables to fiber is a massive upgrade in bandwidth, but it introduces different maintenance challenges. Fiber optic cables carry light pulses through glass cores, making them immune to electrical interference or water ingress, which frequently ruined old copper joints during monsoons. Fiber is fragile, though. A sharp bend causes light leakage, degrading the signal. I watched field teams use an OTDR (Optical Time Domain Reflectometer) to find cable breaks. The OTDR sends a light pulse down the fiber and measures the reflections. By calculating the return time, the device pinpoints the exact distance to a cut or bad splice, down to the meter. I also tried fiber splicing using a fusion splicer, which aligns the microscopic glass cores of two fibers and welds them together with an electric arc. It requires clean preparation and a steady hand.

Mobile Networks: GSM and Base Stations

The training also covered mobile communication. I visited a local BSNL tower site to see the mobile network architecture in action:

• Base Transceiver Station (BTS): The physical tower hardware that communicates directly with mobile handsets over radio frequencies.
• Base Station Controller (BSC): The unit that manages multiple towers, handling frequency allocations and handovers when a user moves between cells.
• Mobile Switching Center (MSC): The central hub that routes voice calls and data packets between different mobile networks and landlines.

Watching these components coordinate in real time to prevent dropped calls as users move between cell zones was a practical lesson in network routing that no textbook fully captures.

Lessons from the Field

What made this internship valuable was seeing how engineers work under pressure. When an excavator cuts a main fiber trunk line, the local exchange loses connectivity. Engineers must find the break, run a patch, and splice dozens of fiber cores in the field, often in bad weather and under time pressure. This experience at BSNL grounded my academic studies. Designing a protocol on paper is only half the problem. The real test of an engineering system is how easily it can be deployed, monitored, and repaired in the field.