Communication Basics #31 Worked Example: E-Carrier Selection — Voice + Video on One Backbone Link

Instructor: Dr. Süleyman Burak ÇELİK

A short, very practical engineering exercise: how do you pick the right transmission carrier for a real backbone link? We are designing a link between two cities. The traffic is a mix — 60 voice telephone channels (point-to-point trunks for the local exchanges) plus one full television channel (a continuous video broadcast feed riding the same link). Three questions: total bandwidth demanded, smallest E-carrier from the European hierarchy that fits, and how much spare capacity is left over. We start by laying the E-carrier ladder on the table — five rungs from E-1 (2.048 Mbps, 30 voice channels) up to E-5 (565 Mbps). Each rung is a multiple of the one below: E-2 = 4×E-1, E-3 = 16×E-1, E-4 = 64×E-1. When you pick a carrier, you don't get to pick a custom number — you climb until your traffic fits. Then we compute. Voice: each call is 64 kbps via the Nyquist + PCM chain (4 kHz spectrum × 2 sampling × 8-bit PCM). 60 channels × 64 kbps = 3.84 Mbps. Video: one analog TV channel uses 6 MHz, so 12 million samples/sec × 8 bits = 96 Mbps per TV channel. Total = 99.84 Mbps. Notice how dominant the video is — over 96% of the link, voice is the rounding noise. Then we climb the ladder. E-1 (2 Mbps) doesn't fit; voice alone almost equals it. E-2 (8 Mbps) doesn't fit; video overflows it. E-3 (34 Mbps) doesn't fit; video alone overflows. **E-4 (139.264 Mbps) finally fits.** Headroom = 139.264 − 99.84 = 39.42 Mbps. Utilization = 99.84 / 139.264 ≈ 72% — exactly the engineering sweet spot (60–80% target). Closing covers three takeaways: (1) capacity planning is ladder climbing, hierarchies like E-carrier and SDH make pricing and interop tractable; (2) 60–80% utilization is healthy, below 60% is wasteful, above 80% queues during peaks; (3) video dominates raw mixes — 1 uncompressed TV ≈ 1,500 voice calls — which is the underlying reason every real video service is compressed (MPEG, H.264, AV1). Topics covered: - E-carrier hierarchy (E-1 through E-5) — rates, voice capacity, multiplication factors - voice rate from Nyquist + PCM = 64 kbps per call - TV channel rate = 96 Mbps from 6 MHz × 12 Msps × 8 bits - ladder-climb selection method - headroom and utilization as engineering constraints - 60–80% sweet spot and why - video vs voice asymmetry (≈1500:1) and the role of compression Sources: ECE 271 HW Q7 + HW Q9 + ExQ9. Part of the AcEdumy Communication Basics series.