Data Packets & Packet Switching

Syllabus & Goals 3 min

Cambridge 2.1 · Types and methods of data transmission Paper 1 · Computer Systems

By the end of this lesson you can:

1. Describe the structure of a data packet — header, payload and trailer — and what each part holds.
2. Explain how packet switching sends data, including the role of the router.
3. State the benefits and drawbacks of packet switching and use a trailer error check.

Textbook: Chapter 2, §2.1.1 (pp. 45–49) — data packets and packet switching.

Recap / Warm-Up 5 min

Unit 1 was about how data is represented in binary. Unit 2 is about how that data is moved between devices — quickly, correctly and safely.

Quick starter

You must post a 500-page book to a friend. Would you send one huge parcel, or several small ones? Give one reason.

Key Concept 14 min

1 · Why data is split into packets

Data sent over long distances is broken up into small data packets (sometimes called datagrams). A typical packet carries about 64 KiB of data. Small packets are much easier to control than one long, continuous stream.

Each packet can travel by a different route. That helps when a route is broken or very busy. The one obvious cost is that the receiver must reassemble the packets at the end.

2 · The structure of a packet

Every packet has three parts: a header, the payload and a trailer.

3 · The trailer's CRC check

At IGCSE, the cyclic redundancy check is described like this:

1. The sender adds up all the 1-bits in the payload.
2. It stores that total as a hex value in the trailer, then sends the packet.
3. The receiver counts the 1-bits in the payload again.
4. If its count matches the trailer value, there was no error. If not, the packet is re-sent.

4 · Packet switching

Packet switching breaks a message into packets. Each packet is then sent independently from the start point to the end point. At every stage there is a node containing a router.

• A router receives a packet and reads its header.
• It uses the destination IP address to decide where to send the packet next.
• The choice depends on how many packets are waiting at each node. The shortest available path is picked.
• So each packet may follow its own route and arrive in a different order.
• The receiver uses the sequence numbers to reassemble the original data.

5 · Benefits and drawbacks

Worked Example 12 min

(a) Send a photo by packet switching

Question: Sam sends Alex a 300 KiB photo. Each payload holds 64 KiB. Describe what happens, step by step.

1. Work out the number of packets: 300 ÷ 64 = 4.6875, so 5 packets — four full ones and one of 300 − 4 × 64 = 44 KiB.you cannot send part of a packet, so always round up.
2. Each packet gets a header: Sam's IP, Alex's IP, a sequence number 1…5, and its size. The trailer gets an end marker and a CRC value.the header makes each packet self-addressed, so it can travel alone.
3. At every node, a router reads the destination IP and forwards the packet on the best free route.routes change from moment to moment, so packets split up.
4. The packets arrive in the order 3, 1, 5, 2, 4. Alex's computer checks each CRC and asks for any damaged packet again.a quick route for one packet may be jammed for the next.
5. The computer sorts the packets by sequence number back into 1, 2, 3, 4, 5 and rebuilds the photo.the header's "5 packets in total" tells it nothing is missing.

How the marks are earned: data split into packets (1) · each packet sent independently (1) · routers use the destination IP in the header to choose the route (1) · packets may arrive out of order (1) · reassembled using sequence numbers (1).

(b) Use the trailer's CRC to check a payload

Question: a packet's payload holds these six bytes. Show how the receiver checks it arrived error-free.

1. Sender counts the 1-bits in each byte: 4 + 3 + 7 + 2 + 5 + 3 = 24.the CRC value is built from the whole payload.
2. Convert to hex: 24 = 1 × 16 + 8, so 18 (hex). This goes in the trailer.the textbook method stores the total as a hex value.
3. Receiver recounts the 1-bits in the payload it received. It gets 24 = 18 hex. It matches the trailer, so the packet is accepted.
4. Suppose byte 3 arrived as 11101011 (one bit flipped). The recount is 23 = 17 hex, which does not match 18, so an error is detected and the packet is re-sent.any single changed bit changes the count by exactly 1.

Try It Yourself 12 min

📝 Exam Practice 10 min

Recap & Key Terms 3 min

Data is split into packets: header (IPs, sequence number, size), payload (≈ 64 KiB) and trailer (end marker, CRC). In packet switching, routers read the destination IP and send each packet its own way. The receiver reorders the packets by sequence number.

Data packet

A small part of a message / data that is sent over a network; the packets are reassembled after transmission.

Packet header

The part of a packet holding the sender's and receiver's IP addresses, the sequence number and the packet size.

Payload

The actual data being carried in a data packet.

Packet trailer

The part of a packet that marks its end and holds the CRC error check.

Cyclic redundancy check (CRC)

An error check where the 1-bits in the payload are added and stored in the trailer; the receiver repeats the calculation.

Packet switching

Transmission where a message is broken into packets that are sent independently along different routes.

Router

A device at a network node that reads a packet's header and decides where to send it next.

Homework 1 min

Task (≤ 15 min): give two benefits and two drawbacks of packet switching. Then explain why a live sports stream suffers more than a file download. [6]