Computer Networks · TEE · practice

Packet pipeline with header overhead

Switching techniques · optional practice

Optional extension · no matching calculation in the two supplied papers

Optional worked extension. This calculation format was not found in the supplied December 2025 final or February 2026 re-exam. The numbers and practice marks are authored. Study the core concept and actual-paper questions first; this is extra explanation, not a TEE prediction.

Practice question. A 12,000-bit message is divided into 1,000-bit payloads. Each packet adds a 100-bit header. Packets cross four store-and-forward links of 1 Mbps, with 1 ms propagation per link. Find packet count, transmitted bits per packet, total bits sent on one link and final completion time. Ignore queueing and processing. [3 practice marks]

Answer

Given

M=12,000 bits; payload=1,000 bits; header=100 bits; h=4; R=1,000,000 bits/s; τ=0.001 s.

To find

k, L, total serialized bits on one link and completion time.

Formula and why

k=M/P,\quad L=P+H

T=(h+k-1)L/R+h\tau

The payload determines packet count; payload plus header determines packet transmission time.

Step-by-step solution

Step 1. Count packets

k=12,000/1,000=12.

Step 2. Add the header to each packet

L=1,000+100=1,100 bits.

Step 3. Count all bits sent on one link

12×1,100=13,200 bits, of which 1,200 are headers.

Step 4. Find one packet time

1,100/1,000,000=0.0011 s.

Step 5. Apply the pipeline

(4+12−1)×0.0011+4×0.001=0.0205 s=20.5 ms.

Final answer: 12 packets; 1,100 bits each; 13,200 bits on one link; completion=20.5 ms.