# Store-and-forward and pipelining

Subject: Computer Networks

Exam: TEE

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

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

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.

**Definition.** Store-and-forward requires a node to receive the complete forwarding unit before sending it onward; pipelining allows successive units to occupy different links simultaneously.

**Simple meaning.** Pehla packet next link pe jaa raha hai, tab doosra packet first link pe aa sakta hai.

Swipe across the diagram to read its labels ↔

![Packet pipeline: successive packets use different links](https://deep-learning-study-room.vercel.app/cn/fig/pipeline.a023f04605.svg)

Diagram: Packet pipeline: successive packets use different links. Labels: Packet pipeline: successive packets use different links | Packet pipeline: successive packets use different links | Equal serialization intervals; propagation omitted in this sketch. | Link 1 | P1 | P2 | P3 | Link 2 | P1 | P2 | P3 | Link 3 | P1 | P2 | P3 | For h links and k equal packets: h + k − 1 packet times.

Store-and-forward and pipelining

## Understand it step by step

1. With message switching, the whole message is the forwarding unit. On h equal-rate links, a message of M bits needs hM/R seconds of serialization.
2. With k equal packets, the first packet needs h packet-transmission times to cross h links. Each later packet then emerges one packet-transmission time later under ideal equal-rate conditions.
3. Thus k packets of L bits take (h+k−1)L/R plus all link propagation delays. This assumes no losses, no queueing, negligible processing, equal packet sizes and equal rates.
4. Count links, not routers: three intermediate routers between source and destination create four links. Include packet headers in L if the question supplies them.



$$
T_{\mathrm{message}}=hM/R+h\tau
$$





$$
T_{\mathrm{packet}}=(h+k-1)L/R+h\tau
$$



#### Practical example

A three-link path can simultaneously carry packet 1 on the third link, packet 2 on the second, and packet 3 on the first. The frames are not all transmitted on one link at the same instant.

### Practice question. Explain why packet switching can deliver a large message faster than message switching. [3 practice marks]

#### Answer

Packet switching can pipeline smaller units across successive links. After the first packet is forwarded, the next packet can use an earlier link while the first uses a later link. Whole-message store-and-forward must receive the entire message at each node, preventing that packet-level pipeline.

#### Practical example

A three-link path can simultaneously carry packet 1 on the third link, packet 2 on the second, and packet 3 on the first. The frames are not all transmitted on one link at the same instant.

**Yaad rakho:** First packet h times; remaining packets one time each.