Basics of wired data communication
Data is made of bits: 1 and 0. On a copper cable, a bit is a high or low voltage. In an optical fibre, a bit is light on or off. A cable connects sender and receiver; a protocol is the set of rules both follow so they understand each other.
Common cables: twisted pair (Ethernet, cheap, up to 100 m), coaxial (TV and old networks) and optical fibre (very fast, long distance). Speed is told in bits per second (bps, Mbps, Gbps). The data is cut into packets. Each packet has a header (sender, receiver address) and a payload (the data).
Terminal equipment technology
Terminal equipment is what the user touches at the end of a line: telephone, fax, computer, modem, router at home, and the ONU box for fibre. The place where the carrier network meets your equipment is the demarcation point (the wall socket or box). The terminal must follow technical standards so it does not harm the network.
A modem changes digital bits into a signal the line can carry, and back again. A network card (NIC) lets a computer use a cable; it has a unique MAC address.
Network technology
A LAN is a network in one building. A switch joins computers in a LAN and sends each packet only to the port of the right computer (it learns MAC addresses). A router joins different networks and chooses the path using IP addresses. A WAN joins faraway LANs.
Topologies: star (all to one switch, the most common), bus and ring. If one cable of a star breaks, only one computer is cut off.
Information security technology
Security protects three things: secrecy (only the right person reads), integrity (data is not changed) and availability (service works when needed).
- Firewall: a gate that allows or blocks packets by rules.
- Encryption: scrambles data with a key so thieves cannot read it.
- Passwords and two-step login, updates and backups.
- Anti-virus and safe habits: do not click unknown links.
Connection work techniques
Connection work means laying, joining and testing cables. Steps for an Ethernet cable: cut the cable square, strip the outer jacket, untwist the pairs only a little, arrange the eight wires in the standard order (T568B: orange-white, orange, green-white, blue, blue-white, green, brown-white, brown), push into the RJ45 plug, crimp, and test with a cable tester.
Safety and quality: keep bends gentle, keep cables away from power lines, label both ends, never exceed 100 m for twisted pair, and test every joint. For fibre, ends must be very clean and are joined by fusion or connectors.
Try it
In the 3D, send a packet to A, then C, and say which cable lit the path. Then send a bad packet and see where it stops. At home or school, look at the back of a router: count the ports and find the cable that goes to the wall socket.
Key formulas and definitions
- Transfer time = file size (bits) / speed (bits per second)
- 1 byte = 8 bits
- Packet = header (addresses) + payload (data)
- Twisted-pair length limit: 100 m
Worked examples
1. A 20 MB file is sent over a 100 Mbps link. How long does it take (ignore overhead)?
20 MB = 160 Mb. Time = 160 / 100 = 1.6 s.
2. A computer in a lab wants to talk to another computer in the same lab. Which device sends the packet to the right port: switch or router?
The switch. It reads the address and uses the right port.
3. A file of 1500 bytes is cut into packets of 500 bytes payload. How many packets?
1500 / 500 = 3 packets.
4. Name two jobs of a firewall.
It blocks packets that break the rules (such as unsafe websites) and allows packets that follow the rules.
5. Why must the wire order match at both ends of an Ethernet cable?
Each pin carries a fixed job (send or receive). If the order differs, the signal reaches the wrong pin and communication fails.
Common mistakes
- Mixing bits and bytes: 1 byte = 8 bits, and speeds are in bits.
- Thinking a switch and a router are the same. A switch works inside a network; a router joins networks.
- Untwisting too much of the cable pair near the plug, which adds noise.
- Believing a firewall removes all danger. Passwords, updates and careful habits are also needed.