Broadband Network Fundamentals and Architecture
Broadband Network Fundamentals and Architecture
This domain tests how an HFC broadband network is organized and how RF and data signals move from the headend to the subscriber. Expect questions on headend/hub/node roles, DOCSIS transport, coaxial and fiber segments, amplifiers, taps, splitters, and capacity concepts such as service groups, node splits, and fiber deep.
Core rules and concepts
- The headend or hub houses the CMTS/core, EQAM, combiners, and optical transmitters; fiber carries signals to an optical node, and coax carries them from the node through amplifiers, taps, and the drop to the premises.
- Downstream and upstream share the same plant but use separate spectra; DOCSIS data rides on SC-QAM and OFDM/OFDMA channels, while video and other services may share the downstream.
- Passives split RF power and add loss; amplifiers add gain but also noise and distortion. Design balances levels, tilt, MER, and BER across the cascade.
- Capacity is shared within a service group. Node splits and fiber-deep designs shrink service groups and increase headroom; R-PHY/remote PHY moves physical-layer functions closer to the edge.
Worked example
A node serves 400 homes. At peak, 25% are active and each averages 3 Mbps upstream. Demand = 400 × 0.25 × 3 = 300 Mbps. If return capacity is 300 Mbps, headroom is zero. Split the node in half: 200 × 0.25 × 3 = 150 Mbps, so headroom becomes 150 Mbps, or 50%. That is why node splits relieve upstream congestion.
Traps that cost marks
Confusing hub and headend; assuming fiber reaches every home; treating shared capacity as dedicated; forgetting that a long amplifier cascade adds noise and distortion; ignoring that return-path capacity is also shared.