Network topologies and hardware
Wiring it together
- A network's topology is how its nodes and links are arranged.
- A handful of hardware boxes — NICs, switches, routers — make it work.
- The arrangement decides cost, speed and how well it survives a failure.
Topologies
- Bus — all devices on one shared cable. Cheap, but the whole LAN fails if the bus fails, and it slows as more devices share the bandwidth.
- Star — every device connects to a central switch. One device failing doesn't affect others, but the switch failing brings everything down. Most common today.
- Mesh — every device links directly to others, with many paths. Very fault-tolerant (traffic reroutes) but needs lots of cabling.
- Hybrid — a mix (a star per office, mesh between offices).
In a star topology, what happens if one device's cable fails?
Each device has its own link to the central switch, so one cable failing affects only that device. (If the switch fails, all go down.)
Why is a mesh topology very fault-tolerant?
Every device links to others directly, giving multiple paths — if one link fails, traffic takes another. The cost is lots of cabling.
LAN hardware
- NIC (network interface card) — lets a device join the network; has a unique MAC address (a 48-bit hardware address).
- Switch — forwards each Ethernet frame only to the port for the destination MAC. (A hub, now obsolete, copied traffic to all ports.)
- Wireless access point (WAP) — lets wireless clients join a wired LAN.
- Cabling — twisted-pair for short runs, fibre-optic for longer/faster runs.
How does a switch differ from a hub?
A switch sends each frame only out the destination's port (efficient, more secure). A hub copied to all ports. Routing by IP is a router's job.
A MAC address is:
The MAC address is a fixed 48-bit hardware identifier on the network interface card. (DHCP hands out IP addresses, which can change.)
Routers
- A router connects different networks and forwards data between them — usually at the LAN–internet boundary. It does:
- Forwarding — reads each packet's destination IP address and sends it the right way, using a routing table.
- NAT — lets many private LAN addresses share one public IP.
- DHCP — hands out private IP addresses to LAN devices; and a firewall blocks unwanted incoming traffic.
The main job of a router is to:
A router joins networks (e.g. a LAN to the internet) and forwards packets using their destination IP and a routing table.
Ethernet and CSMA/CD
- On a shared medium two devices can transmit at once — a collision. Ethernet handled this with CSMA/CD:
- Carrier sense — listen first; wait if the cable is busy.
- Collision detection — keep listening while sending; on a clash, both stop, send a brief jam signal, then wait a random backoff before retrying.
- Modern switched, full-duplex Ethernet uses point-to-point links, so collisions no longer happen.
In CSMA/CD, when a collision is detected the devices:
On a collision both senders stop, send a brief jam signal, and wait a random backoff before trying again — the random wait avoids an immediate repeat clash.
You've got it
- bus (shared cable), star (central switch — most common), mesh (many paths, fault-tolerant)
- NIC has a 48-bit MAC address; a switch forwards a frame only to the destination port
- a router connects networks and forwards by IP address (+ NAT, DHCP, firewall)
- CSMA/CD: listen, detect collisions, jam + random backoff (switched full-duplex avoids them)