# Packet pipeline with header overhead

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/practice/switching/headers

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/practice/switching--headers/

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.

#### What this result means

Packet headers consume link capacity. Ignoring them gives an unrealistically small completion time for this question.