Describe networks and how they communicate (part 2: the suffering continues)
This part includes the introduction. Read that before continuing.
LANs and WANs
- LAN – local area network. Covers a small geographical area (typically in a single site) and the hardware is owned by the organisation using it. Usually connected using UTP (unshielded twisted pair) cables, fibre optic cables or wirelessly with Wi-Fi.
- WAN – wide area network. Covers a large geographical area. Multiple LANs connected together form a WAN. Infrastructure forming a WAN is owned and leased from large telecommunication companies. Usually connected with telephone lines, fibre optic cables or wirelessly with satellite links.
Network topologies
Describes the arrangement of computers/devices that make up a network.
Bus topology
- All nodes are connected to a single backbone cable.
- Each end is connected to a terminator which 'terminates' the signal and stops it from bouncing back.
- Each node is passive (i.e. each node simply listens and receives or sends data. An active network would use electrically powered equipment to route the signal from one place to another).
- Data is sent in one direction at a time.
- Only one computer can transmit data at a given time.
- CSMA/CD (carrier sense multiple access with collision detection), handles data collision, thus allowing nodes to transmit on the same cable. This will be talked about later.
| Advantages | Disadvantages |
|---|---|
| Cheap and easy to set up as only one big backbone cable is needed. | The backbone is a single point of failure (i.e. if it breaks, the entire network goes down). |
| Data can only be sent in one direction at a time. | |
| Additional devices slow down the network greatly. |
CSMA/CD
Recommended videoMeans carrier sense multiple access with collision detection.
Process:
- Check if wire is idle. If so, send the data.
- If a collision occurs (i.e. when 2 computers send data at the same time), transmission stops and a jamming signal is sent to stop further transmissions.
- The 2 computers that caused the collision wait a random amount of time before resending the data, and then transmission resumes as normal.
CSMA/CD isn’t particularly relevant anymore. It is only relevant on half-duplex networks, where data can only be sent in one direction at a time.
CSMA/CA will be talked about later. This is not the same as that.
Star topology
Computers are connected to a central node, such as a hub or switch.
- Hub – broadcasts a message to every computer on a LAN.
- Switch – sends each message to the specific recipient it was intended for.
| Advantages | Disadvantages |
|---|---|
| Easy to scale up (only one cable per node). | The central node is a single point of failure. |
| One node failing doesn’t destroy the network. |
Physical and logical topologies
- Physical topology – how devices are physically connected.
- Logical topology – how the devices communicate across the physical topology.
A network with a physical star topology can behave logically as a bus network by using a bus protocol and appropriate physical switching.
MAC addressing
Every networking device contains an NIC (network interface card).
Each NIC has a hardcoded MAC (media access control) address when it is manufactured.
A switch uses MAC addresses of devices connected to the network to direct data packets to the correct devices.
MAC addresses are a unique 12-digit hexadecimal code, each byte being separated with a dash.
Example: 00:00:5e:00:53:af
Network architecture
Things to know: client-server, peer-to-peer.
Client-server
- Contains a central server and clients which request resources and services from the server.
- Servers may become clients themselves if they send requests (e.g. for a web page) to another server.
- Central server manages security.
- Files may be held on the central server.
- Some processing tasks may be performed by the server.
- Can require IT specialists to set up, adding to cost.
A large client-server network can contain many servers, each performing a specialised task. Examples are web servers, print servers, mail servers, file servers and database servers.
Peer-to-peer
- No central server.
- Cheap and easy to set up and maintain, and is therefore suitable for small companies with few computers.
- All computers can easily see files on all other computers.
- All computers communicate directly to each other, without going through a server.
- Data cannot be accessed from a machine that is turned off.
Peer-to-peer networking can also be used on file-sharing websites such as BBC iPlayer, since it would alleviate a lot of stress from BBC’s servers. They are widely used on piracy sites because peer-to-peer networks are extremely difficult to shut down completely.