1. OCR Computing for GCSE © Hodder Education 2011
Chapter 6: Networks
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LAN
A LAN is a Local Area Network.
It is a connected set of computers and other devices.
Each device is called a node.
A LAN is installed on one site.
It does not cover a wide geographical area.
All the infrastructure is owned by the organisation or individual who owns the LAN.
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Advantages of a LAN
It allows communications between workers.
It allows data to be shared.
It allows peripherals to be shared.
It allows computers to be upgraded more easily.
It allows distributed processing.
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LAN hardware
NIC – network interface card
Each node needs a NIC to connect to the LAN.
Produces the electrical signals that carry data.
Hub
A device that connects nodes together.
Makes them act as a single segment.
Not smart – all data packets are transmitted.
Cheap devices – USB hubs useful in home computer installations.
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LAN hardware
Wireless access point
Wireless technology has many advantages over wired networks.
No building work needed
Quick to add new workstations
Flexible deployment of nodes
Visitors can access network / internet
But there are drawbacks
Slower performance
Security issues
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6. Peer-to-peer networks
All computers have the same status.
May be very slow because of
data collisions
shared processor power
Only suitable for small low traffic networks.
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7. Client–server networks
One or more computer is designated as a server.
A high specification machine.
It offers services to client machines, such as centrally held software, data, file storage and peripheral access.
Servers control network traffic.
Log usage of resources.
Provide security.
Clients have to log in – this identifies users and assigns rights.
Big networks may have hundreds of servers.
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Network topologies
The topology is the physical layout of the network.
It affects
cost
performance
ease of installation.
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Star topology
Shared link to server(s)
Most common
Few data collisions
Fast
Can set up independent segments
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Ring topology
Ring topology
One common linking cable
Passes all nodes
One direction traffic
Fast performance
But any problems and the whole network goes down
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Bus topology
Bus topology
One common linking cable
Nodes connected off the cable
Cheap
But
Slows down when heavy traffic
Lots of data collisions
Breakage to cable affect whole network
Limited distance covered
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LANs and WANs
LAN
Local Area Network
Located on one site
Owner of network owns the infrastructure
WAN
Wide Area Network
Covers a large geographical area – may be worldwide
Infrastructure may be provided by telecoms companies
VPN
Virtual Private Network
Uses internet for infrastructure
Need for enhanced security
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Protocols
Protocol
Meaning
Application
DNS
Domain Name System
Translates domain names such as ocr.org.uk into IP addresses.
TLS/SSL
Transport Layer Security / Secure Sockets Layer
Cryptographic protocols designed for secure communications.
FTP
File Transfer Protocol
For copying files from one host to another.
Gopher
An early means of searching for files on the Internet.
HTTP
Hypertext Transfer Protocol
For distributing hypermedia files – essentially web pages.
IMAP
Internet Message Access Protocol
One method for accessing s.
POP3
Post Office Protocol (version 3)
Another method for accessing s, used by most webmail services.
Telnet
Allows bidirectional text communications on a network.
Protocols are the rules for communication between devices.
Standard protocols encourage network development.
Network protocols are constructed in layers.
This allows independent development of small parts of a protocol.
Internet Protocol is a group of protocols that underlies the internet.
TCP/IP now the most important set of protocols.
Includes many special protocols.
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IP addressing
Each node on a network running IP has an IP address.
It uniquely identifies the node.
Consists of four octets (8-bit bytes).
Written as four numbers separated by dots, e.g
DNS servers convert domain names such as OCR.org.uk into IP addresses.
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MAC addressing
MAC – Media Access Control
This is a unique number on network interfaces.
Used to communicate with physical layer of network.
Can use it to restrict access to a network.
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Packets
Data is split into packets on an IP network.
Packets get sent by different routes according to availability.
Packets reassembled at receiving end.
Typical packet structure
source address
destination address
Packet sequence number
data
checksum
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Security
Transmitted data is vulnerable.
Users on a network do not need to see all the data.
Access levels
Users granted privileges.
Associated with login names.
Passwords
Must be changed regularly.
Passwords should be strong.
Encryption
Especially important on wireless networks.
WEP or WPA should be used.
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Policies
Network owners should have policies to protect their assets.
Users have responsibilities.
Acceptable use policies are common.
Policy ensures that users understand what is expected of them.
Sanctions used if users misuse resources.
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Disasters
Disasters do happen.
Need to plan for worst case scenario.
Example strategies:
mirrors of systems
use of disk protection technology such as RAID
surge protectors — to minimise the effect of power surges on delicate electronic equipment
uninterruptible power supply (UPS)
back-up generator in case of a power failure
fire preventions — alarms, fire extinguishers
anti-virus software and other security measures
failover systems
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