Communication Basics #22 Worked Example: DWDM Library Download

Instructor: Dr. Süleyman Burak ÇELİK

Fourth midterm preparation video in the Communication Basics series. We work through the classic DWDM library download problem step by step in light mode. A digital library contains ten million books. Each book has 100 pages, each page has 100 words, each word has 4 letters, and each letter is encoded with 8 bits. A DWDM system with 100 channels at 10 Gbps each is used to download the data with no control bits or redundancy. We solve five parts: first we count the total bits in the library (3.2 Tbits), then calculate the DWDM download time (3.2 seconds for 10 million books), find how many channels are needed for a 32 millisecond download (10,000 channels), compute how long a 56 Kbps dial-up modem would take (1.812 years), and finally identify which xDSL technology matches a 17.094 hour download (52 Mbps = VDSL). We finish with a speed comparison: DWDM at 3.2 seconds versus VDSL at 17 hours versus dial-up at nearly 2 years, showing why fiber DWDM is indispensable for large-scale data transfer. Topics covered: - total information content calculation (books, pages, words, letters, bits) - DWDM system throughput: 100 channels at 10 Gbps = 1 Tbps - download time derivation from total bits and throughput - inverse problem: finding the number of channels for a target download time - 56 Kbps dial-up modem: seconds to years conversion - xDSL identification from download rate (52 Mbps = VDSL) - technology speed comparison: DWDM vs VDSL vs dial-up Part of the AcEdumy Communication Basics series.