Download Topic 6: Using Networks - St Andrew`s High School

Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Networking Hardware
Connecting two or more computers together
makes a computer network. Connecting two
computer networks together creates an
internetwork. Connecting millions of
computers and computer networks together
forms a huge internetwork or what we now
refer to as the Internet.
Generally some interconnecting device is
required to take the data from one network,
change it to suit the second network, and then
pass it over to the second network. This
interconnecting device must be capable of
converting the data into a format suitable for
either network, as data is transmitted in both
directions.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Networking Hardware
Stations on networks are recognised by
four digits (octets), so typically a
workstation on a network can be
identified by the address 136.67.64.10.
In the diagram you can visualise a station
with such an address as being node 10 on
network 136.67.64:
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Networking Hardware
Workstations are single-user computers
that are connected to a network.
However, the workstations have an extra
component when compared to standalone computers – a network interface
card (NIC).
The NIC receives data from the network and translates it into a form the
workstation can understand. It also takes workstation data and translates it
into network format ready for transmission.
A number identifies each NIC called a MAC Address. This number uniquely
identifies the computer. These network card numbers are used to identify the
destination for the data. When talking to another computer, the data you send
to that computer is prefixed with the number of the card you are sending the
data to.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hub
A Hub is a simple device that
interconnect groups of users.
A hub in an Ethernet network is a multi-port repeater (boosts a signal).
Hubs are normally used on networks using UTP cabling, limit is 100m
with a speed of 1 Mbps/10Mbps/100Mbps for Cat 2, 3 and 5 cables.
Hubs can also be used to divide a network into Network Segments.
Thus a cable fault will not bring an entire network down and will only
affect the one segment.
Hubs operate at the Physical Layer of the OSI networking model.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hubs – An Example
To understand how a hub serves your business
network, imagine a hotel with just one phone
line available to all guests.
Let’s say one guest wants to call another.
She picks up her phone and the phone rings in
all rooms.
All the other guests have to answer the phone
and determine whether or not the call is
intended for them.
Then, as long as the conversation lasts, no one
else can use the line.
With a few guests, this system is marginally
acceptable.
However, at peak times of the day—say, when
everyone returns to his or her rooms at 6 p.m.—
it becomes difficult to communicate. The phone
line is always busy.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hubs
Hubs broadcast frames
received on one port to all
other
ports.
This
is
illustrated here, where a
small network of six
stations (A through F) is
connected in different ways.
Above, station A transmits a frame with a destination address of Station F.
This does not matter to the four-port hubs, which simply broadcast copies
of the frame from station A to the other five stations (B to F). This amounts
to a good deal of wasted bandwidth. Furthermore, all six stations operate in
the same collision domain, making them compete for bandwidth.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Switches
Switches are smarter than hubs. A switch can receive a data packet and
instead of sending it to every station on the network, forwards data packets
only to the appropriate port for the intended recipient, based on the
destination information in each packet’s header. The switch establishes a
temporary connection between the source and destination, and then
terminates the connection once the conversation is complete..
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
4-port switch
Switches
1
A
2
B
3
4
4-port switch
C
Collision
domains
1
2
D
3
E
4
F
This diagram shows the same network with the four-port hubs replaced by
four-port switches. A frame transmitted from station A with a destination
of station F is forwarded between the switches and sent directly to station
F on port 4. Stations B, C, D, and E do not receive copies of the frame, as
they do with the hubs. Thus, network traffic has been reduced.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
4-port switch
Switches
1
A
2
B
3
4
4-port switch
C
Collision
domains
1
2
D
3
E
4
F
An Example:
A switch would be like a phone system with private lines in place of the
hub’s “party line.” Jane Lipton at the Berkley Hotel calls Bill Johnson in
another room, and the operator or phone switch connects the two of them
on a dedicated line. This allows more conversations at any one time so
more guests can communicate.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
The switches also partition the network into six separate collision
domains. Each station now has unrestricted access to its own dedicated
bandwidth (the speed of the switch port). The switch is capable of
specific forwarding because it learns what MAC addresses are associated
with each port. Recall that every Ethernet frame contains a source MAC
address and a destination MAC address. When a frame is received by a
port on a switch, the switch will save a copy of the source MAC address
and its associated port number in a special internal look-up table.
Although we are storing the source MAC address, it is also a destination
MAC address to every other station in the network.
When a frame requires forwarding, the switch examines the destination
MAC address stored in the frame and looks for it in the look-up table. If
the destination MAC address is found in the table, the frame is forwarded
to the associated port. If the destination MAC address is not found, the
frame is broadcast to all ports. Eventually, the destination station will
most likely respond with its own frame, and its port will be identified.
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Further broadcasts for that station will not be required.
Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
This diagram shows
the results obtained
when a hub and
switch are used
together. Stations A,
B, and C are in one
collision
domain
(competing
for
bandwidth).
Stations D, E, and F are in their own collision domains. Each has full
access to the available bandwidth. Station A transmits a frame destined for
Station F. The frame is broadcast by the hub and forwarded by the switch.
Stations B and C must contend with the broadcast frame, waiting their turn
for access. Neither station B or C is allowed to transmit while the hub is
broadcasting or a collision will result. Even though station A is sending a
frame to station F, stations B and C are affected
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Here station A is sending a frame to station C. The hub still
broadcasts the frame, which affects station B, but not stations D,
E, or F.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Here station F sends a frame to station A. The frame is forwarded by the
switch and broadcast by the hub. Stations B and C are affected by the
frame of station F; however, stations D and E are not affected.
If station F was sending a frame to station D. Station E is unaffected and
may transmit a frame to stations A, B, or C without affecting the F-to-D
transmission.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Routers are smarter than
switches. Routers are
used between networks
ie, in inter-networks
using different protocols
(rules).
Routers provide links between networks that use different protocols, or
ways of communicating. Routers use the packet address to determine
which router or workstation should receive each packet next. Based on a
network route map called a ‘routing table’, routers can help ensure that
packets are travelling the most efficient path to their destinations. If a link
between two routers fails, the sending router can determine an alternative
route to keep traffic moving.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Routers: In Operation – Network has 3 segments (1 with 3 networks)
5 networks are connected to a router. The router has three segments,
segments 1, 2 and 3. Segment 3 comprises of 3 networks. The networks are
also numbered as the diagram, as are the computers on each network. Station
11 on network 136.67.64 wants to send to station 254 on 205.68.150. The
packet is received by the router, which looks at the packet header to see
where it’s going, and then looks up its routing table. Instead of flooding the
packet through its two other segments it can see that the destination station is
accessed via segment 2. It then floods only that network with the packet.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Routers have two major responsibilities
1. Optimising the routing paths. A router uses a routing algorithm to
determine the optimal path to the destination.
2. Switching - transport of packets over networks.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
The diagram above shows a network consisting of 3 segments. Each
segment has its own identifying address, every device in a segment has the
same 3 octets address with the 4th octet identifying the actual device. Eg:136.67.64 could have device 136.67.64.10 or 136.67.64.11
205.68.150 could have device 205.68.150.254 or 205.68.150.22
158.43.23 could have device 158.43.23.21 or 158.43.23.6 (not shown)
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hardware and Software Factors
Coupled with the falling costs more of us are increasingly taking full
advantage of networks (and particularly the Internet).
Processors - Networks are demanding equipment with very high
throughput and need the flexibility to support these applications.
Memory
Backing Storage - Network storage is simply about storing data so that
network clients can access it. The simplest form of storage on a network
is the hard drive of the network server and is very common on small
networks.
Browsers - A browser is the application program that enables a computer
user, to look at, and interact with the World Wide Web.
Network Operating Systems - Network operating systems will usually
run on the server or servers installed in a LAN.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Network Trends
As the economic demand for networking technology has grown, the
trend has been for equipment prices to fall and performance to increase.
Although still in its relative infancy, the development of wireless
networking is likely to follow the same pattern.
Bandwidth
The most obvious technical advance has been the increased bandwidth
available to users of networks. The term bandwidth describes the rate at
which data can be transmitted over a segment of a network. Advances in
cabling technology for local area networks has resulted in the equipment
becoming easier to install, cheaper to purchase and has provided everincreasing bandwidth.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Network Trends
Bandwidth
The most obvious technical advance has been the increased bandwidth
available to users of networks. The term bandwidth describes the rate at
which data can be transmitted over a segment of a network. Advances in
cabling technology for local area networks has resulted in the equipment
becoming easier to install, cheaper to purchase and has provided everincreasing bandwidth.
Users now demand the same services
of a WAN that they get on a LAN.
This has implications for bandwidth.
The demand for bandwidth has always
outstripped the ability of networks to
provide it.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Network Trends - Wireless
Wireless networking in LANs is still slower than conventional cabling,
but the advantages of being able to move around a building and still be
connected to the network often makes wireless networking worthwhile.
Wireless networking can be advantageous for organisations who need to
create a network quickly within a building without suffering the
disruption of installing cabling, or who wish their users to be able to
move around while using laptop computers. A wireless network requires
each computer to be fitted with a wireless network card. The available
bandwidth is often substantially less than is available using a cabled
network, but is still adequate for many applications.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Wireless
Much slower than cabling (less bandwidth).
Advantage is portability.
Easier and faster to set up than cabling.
Computers need wireless network card and
need to be in the range of a wireless hub
(wireless access point).
Bluetooth – allows printers, mobile phones,
and laptop computers to form ad-hoc
networks when they come close enough to
each other.
www.nokia.com/bluetooth/
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Wireless
WAP – Wireless Application Protocol allows mobile
phones to access the internet. Still very slow, but will
improve as bandwidth improves.
http://www.nokia.com/wap/
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Bandwidth
Some service providers charge by the
amount of data a user downloads
rather than the length of time they
use it.
Bandwidth available to users on a
network depends on:
The number of users sharing that
network segment:
The rate at which data can be
transmitted.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Bandwidth
From an economic viewpoint there are 2 possibilities:
Reduce demand:
1 data compression before transmission. Normally removes
redundant data by detecting repeating data, transmit the pattern once
then the no of times it is repeated.
2 Caching – caching a web page is saving it to hard disk.
Subsequent requests for the page are reloaded from local hard disk
rather than going off to the internet (Web caching on our Web Server)
3 Broadcasting – live video/audio data takes up less bandwidth as
users much view immediately and don’t all want to download there
own copy of the data.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Bandwidth
From an economic viewpoint there are 2 possibilities:
Increase Bandwidth:
Use of satellite links and fibre optic cable has made the internet a much
faster service to use. Network OSs with built in networking software make
it much easier to connect to the internet.
Faster modems, and services like ISDN/ADSL have made data
transmission much faster, able to receive multimedia services like
graphics/sound/video over an internet link.
Find out about ADSL from http://www.adsl.com/adsl_tutorial.html
And ISDN from http://www.ralphb.net/ISDN/index.html
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Misuse of Networks
Software can be distributed cheaply and easily over the internet, but so can
illegal copies of software. Software piracy is a major problem in many
parts of the world.
In addition to conventional software licensing, there are a number of
alternative software distribution models in existence, including Shareware
and even Freeware. The Internet has made many of these distribution
models viable because it provides a large enough market and removes the
cost of distribution from the developer.
There is also concern over copyright concerning images and other data
available on the Internet. The fact that this data is on a web page and
anyone can access it does not make it freely available for anyone to take for
him or herself and publish it under their own name.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hacking
The Network OS is responsible for security on the network, eg when
logging on with a User ID and Password. The OS compares the data
entered with the password data in its database. If they don’t match log-in
fails and so access to network resources is denied.
If there is a match then access to network resources then depends upon the
level of access the user has been given by the network manager. This is
determined by the level of permissions in the group the user is assigned to.
Groups can be assigned different permissions.
Eg pupils can read files in the School folder but cannot write to these
folders whereas staff can write to the folder for their department as well as
read any folder inside the School folder. Groups can be restricted to the
printers they can use and hard discs they can access.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Hacking
With increased use of the internet many people are concerned with credit
card fraud when credit card details are being transmitted over the internet
while using e-commerce systems.
A secure web browsing protocol named Secure Hyper Text Transmission
Protocol (SHTTP) has been developed for e-commerce transactions.
SHTTP ensures that data transmitted using this protocol encrypts the data
before it is transmitted using SSL (Secure Socket Layer).
Digital Certificates can also be used which guarantee you are who you say
you are. (See www.verisign.com/)
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Encryption
Coding data to make it difficult/impossible to read.
Encryption is classified according to the number of bits in the ‘key’ used to
encode the data. More bits in the key = longer it takes to crack the code.
As computers become more powerful the codes can be cracked quicker, so
encryption keys will keep increasing in size.
At moment most email is not encrypted, it is plain ASCII text!
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Viruses
A virus is a piece of programming code that causes some unexpected and
usually undesirable event in a computer system.
They are often designed so that they automatically spread to other
computer users on a network.
Viruses can be transmitted as attachments to an e-mail, as a download, or
be present on a disk being used for something else.
Some viruses take effect as soon as their code takes residence in a system
whilst others lie dormant until something triggers their code to be executed
by the computer.
Viruses can be extremely harmful and may erase data or require the
reformatting of a hard disk once they have been removed.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Legislation
Computer Misuse Act
In the United Kingdom, the Computer Misuse Act (1990) covers using
computers to damage or steal data.
The Computer Misuse Act covers crimes such as breaking into computer
systems or networks to destroy or steal data (hacking) and propagating
viruses (spreading) which destroy or damage information or computer
systems.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Legislation
Data Protection Act
In the UK, the DPA (1998) describes the
duties and responsibilities of those
holding data on individuals. It also
describes the right of these individuals.
In general, it is the duty of those holding data on individuals to register
with the Data Protection Registrar, to keep the information secure, make
sure it is accurate, and to divulge it only to those persons who are
authorised to view it. It is the right of an individual who has data stored
concerning them to view that information and to have it changed if it is
inaccurate.
The Police, Customs, National Security and Health
Authorities are exempt from this Act.
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Computing Higher - Unit 1 - Computer Systems: Topic 6: Using Networks
Legislation
Copyright, Designs and Patents Act
Copyright is the ownership of intellectual property
outlined by a particular nation's or international law.
In the UK, the Copyright, Designs and Patents Act
of 1988, and legislation in other countries who
signed the Berne Convention, provide protection.
Intellectual property could be music, films, books
etc. So it is illegal to copy music/MP3/DVDs/CDs
etc!!
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