3.3 - Network Characteristics

Exam Board:

OCR

Specification:

2016 - Unit 1 

Network Topologies

Network topology refers to the arrangement of computer systems on a network.

 

Devices in a network topology diagram are often called 'nodes'.

Client-Server Network

Clients make requests to a server, the server manages that request and responds. For example, if the user (client) makes a request to access www.bbc.co.uk to a web server.

Large services like Amazon and Google will need very powerful servers to handle millions of requests a second.  

The client is completely dependent on the server to provide and manage the information. The server controls network security, backups and can be upgraded to manage higher demand.

Advantages:

  • Centralisation of control for ease of back up and updates.

  • Shared resources e.g. printer / files / software.

  • Scalability allows more clients to be added to the network

Disadvantages:

  • Traffic congestion will cause the network to slow down.

  • Server fault will result in the whole network going down.

  • Technical staff are required to manage the network.

  • Viruses can spread quickly around the network. 

Peer-to-Peer Network

For peer to peer networks, no single system is solely responsible for being the server. Instead, each computer acts as a server and is equally responsible for providing data.

Peer to peer is best suited for sharing files.

 

Without a dedicated server, however, there is no central device to manage security or backups. Users are not dependent on the server, but performance will decrease when more devices are connected, causing the network to run at a slower speed.

Bus Topology

The nodes are connected to a bus (a central cable which transfers all data on the network).

 

How it works: 

  • The bus transfers data packets along the cable.

  • As the data packets arrive at each computer system, the computer checks the destination address contained in the packet to see if it matches its own address.

  • If the address does not match, the computer system passes the data packet to the next system.

  • If the address of the computer system matches the destination address in the data packet, it is accepted and processed.

  • At both ends of the cable are terminators to mark the end of the bus.

Advantages: 

  • Because of the simple layout, it is easy to attach another system to the main cable without disrupting the whole network.
     

  • A bus topology is quick to set up once the main cable has been established making it optimal for temporary networks.
     

  • A bus topology is cost-effective because it usually contains less cabling than other topologies and requires no additional hardware (like a hub or switch).

Disadvantages:

  • Poor security as data packets are passed on to each system on the network.
     

  • Data collisions are likely - this is when two systems attempt to transfer data on the same line at the exact same time. Resending the data wastes time and slows down the network.
     

  • The main cable will only have a limited length which can become crowded and slows network speed as more systems are attached. The main cable must also be terminated properly.

Token Ring Topology

In a token ring network, computer systems are connected in a ring or a loop.

 

How it works:

A token (small data packet) is sent around the ring in one direction, being passed from one computer system to the next. A computer seizes the token and includes its own data when it transfers data. As the token arrives at each computer system, the system checks the destination address contained in the packet to see if it matches its own. If the addresses match, the computer processes the data otherwise it ignores it.

Advantages:

  • Data collisions are avoided as data packets are transmitted in one direction around the ring.
     

  • Attaching more systems to a ring topology won't affect the transfer speed as much as other layouts like a bus topology because the data is transferred at a consistent speed.

Disadvantages:

  • If any system on the network fails then the whole network fails as the loop is broken and data can't be transferred to all systems.
     

  • To add a new system to a ring topology the network must be temporarily shut down.

Star Topology

In a star network, each computer system is connected to a central node: a hub or switch.

 

How it works:

Each node is connected to the central node (usually a hub or switch) and transfers its data packets here. The hub/switch looks at the destination address and transfers the packets to the intended computer only.

Advantages:

  • A star topology has improved security because data packets are sent directly to and from the hub / switch in the centre and not necessarily all devices like in a bus or ring topology.
     

  • New systems can be attached directly to the central system so the network doesn't need to be shut down. System failures of attached computers won't usually cause complete network failure.
     

  • Transfer speeds are generally fast in a star topology as there are minimal network collisions.

Disadvantages:

  • Extra hardware (the hub or switch) is required to be purchased, installed and maintained.
     

  • If the central system (the hub or switch) fails then the whole network will be unusable until the error is fixed.

Mesh Topology

In a mesh network, each computer system is connected to every other computer system.

 

How it works:

Data packets are transferred to the destination address along the quickest path, travelling from node to node. If a pathway is broken, there are many alternative paths that the packets can take.

Advantages:

  • If one cable or system fails then data packets can take an alternative route and still reach the destination address.
     

  • Because of the large possible number of systems and connections, a mesh topology can usually withstand large amounts of data traffic.
     

  • New systems can be added to the network without disrupting the entire topology.

Disadvantages:

  • Because of the possibly large amount of cables required (especially in a complete mesh topology) this network layout can be expensive to install and maintain.
     

  • Redundant cabling should be avoided - this is when cables are connected between systems that won't ever need to communicate.

Configuration

Before a computer system can use a network, three pieces of information must be configured (set up) correctly.

IP Address

An IP address is used to uniquely identify computer systems on a network, allowing communication between them.

​An example of an IP address is 195.10.213.120.

Default Gateway

When data is to be sent from one network to another, it must be sent through a default gateway.

 

This default gateway is usually a router that connects the local network to another network.

Network managers can use automatic configuration which is quicker and easier to set up.

A new device can connect to and use a network automatically, such as free WiFi in an airport.

Network managers can also set manual configuration which improves security as new devices can’t be used until the addresses have been configured by a technician. This stops unauthorised devices from connecting to the network.

Subnet Mask

Subnetting is the act of dividing a physical network into smaller 'sub' networks (known as subnets). This helps to reduce traffic and means that users can externally access parts of a network (e.g. emails from home) without having to open the entire network.

A subnet mask is used to define these subnets. The mask is used to determine the start and end address of each IP address in a subnet.

A common subnet mask is 255.255.255.0 as making the first 3 sections full restricts the fourth section to 256 unique values. For example 113.12.14.230 and 113.12.14.157 are in the same subnet but 114.12.14.127 wouldn't be.

Questo's Questions

3.3 - Network Characteristics:

1a. Describe how peer-to-peer networks and client-server networks function.

1b. Give one use for both types of network.

2a. Draw and label a diagram for all 6 network topologies.

2b. Describe 2 advantages and 2 disadvantages of each network topology.

3. What is an IP address? Why is it necessary for networks?

4. Describe what is meant by a default gateway.

5a. What is subnetting?

5b. What is the purpose of a subnet mask?

5c. State a common subnet mask. How many unique devices can be used on a network with this subnet mask?

 

6. Describe 1 reason why a network manager may use automatic configuration and 1 reason why they may use manual configuration.

© CSNewbs 2020