Summary
Highlights
The video introduces the relationship between throughput, bandwidth, and data rate, using an 802.11a OFDM-based system as a practical example. It outlines the system components including OFDM with a cyclic prefix, forward error correction, and higher layers like MAC, ARQ, IP, and TCP, which handle re-transmissions and contention.
Using specific values for an 802.11a system, the video explains how the symbol rate is calculated. For a 20 MHz bandwidth channel with 64 sub-channels (48 for data), 64QAM modulation (6 bits per symbol), and a 3/4 rate forward error correction, the data rate is calculated to be 54 megabits per second. This is the advertised data rate for such systems.
The video highlights that the calculated 54 Mbps data rate does not account for all overheads. All layers of the protocol stack add headers to the data. Therefore, the actual throughput is reduced by the ratio of data bits to the total bits (data + headers). This is the first factor in calculating the true throughput.
Another factor reducing throughput is MAC layer contention. When multiple users try to send packets, collisions can occur, leading to packets needing to be resent. The MAC layer detects these collisions by checking headers, and an unsuccessful check results in a retransmission request, reducing effective throughput.
Even if there's no MAC collision, the ARQ layer can request retransmissions due to errors within the packet caused by channel conditions (e.g., being far from the access point or noise). The forward error correction might not be able to overcome all errors, requiring the entire packet to be resent.
Finally, the TCP layer, higher up the protocol stack, can also request retransmissions if the assembled frames do not match the expected application data. The video concludes by emphasizing that throughput is the data rate multiplied by factors accounting for headers, MAC contention, ARQ retransmissions, and TCP retransmissions. This means the actual throughput is significantly lower than the advertised data rate, often dropping from 54 Mbps to 20 Mbps or less.