Transcript Network Environments - Computing and ICT in a Nutshell

2.9 Network Environments
In this section you must be able to:
• Describe the characteristics and relative
advantages of network and standalone
environments
• Describe the difference between a Local area
and a Wide area network
• Describe the elements of network
environments
Standalone Systems
• Everyone has their own PC – processing and
data are dispersed throughout the organisation
• Lack of centralisation leads to problems:
– No standardisation – e.g. shared templates
– Data redundancy/integrity problems – multiple
copies of data could be changed separately
– Communication between departments is not
possible
What is a network?
A network is two or more computers with a common
electrical connection!
Local Area Network
Examples
A network in one building
or on one site
Company, school or
home networks
Wide Area Network
Examples
A network covering a
larger area than just one
site
ATM networks, lottery
machines, the internet!
Advantages and Disadvantages
• Shared hardware,
software and data –
leading to savings?
• Easier to install
applications, etc.
• Improved
communications
• Centralised
management of
users, backups,
software, etc.
• Dependency - server or
cable failure affects
everyone
• Security issues – can
be difficult to make
secure
• Performance - network
slows down under load
• Difficult to manage –
may require a network
manager
Networking Terminology
• A client-server network is one that has one or
more file servers storing data and applications
and print servers - this is the most common type
for anything but very small networks
• A peer-to-peer server has no server:
– files and peripherals (e.g. printers) are shared
between workstations on the network
– all workstations may need to be switched on
– suitable for small networks due to reduced cost
• A gateway is used to link two different networks
together, e.g. to connect a LAN to the internet
• A bridge links together two similar networks
Client-Server or Peer-to-Peer?
• Choose client-server if:
– you have a large network that can absorb the cost
of the server
– you want centralised management of users and
resources
– users are mobile and/or don’t always have their
PCs turned on
• Choose peer-to-peer if:
– the network is small and you can’t justify the
expense of a dedicated server
– you have a workforce that generally uses the same
PC and works the same hours so that all
workstations are on at the same time
Topologies
The topology is how the computers are connected – there are 3 main ways:
Bus
Star
Ring
In a bus network, a single cable
running the length of the network
connects all of the nodes.
Each node in a star network is
connected to a central computer,
usually the file server.
In a ring network, the nodes are
connected in a circle. In a token ring
network, the information travels around
the network in one direction only. A
“token” travels around the network, like
a train, carrying the information.
Bus Networks
• Any node can try to
send a message at
any time, but the
bus can only carry
one at a time
The
bus is
free
Nothin
g
coming
!
1
3
P
Boom!
2
S
• Before a node sends a message, it must check that the
bus isn’t being used – it senses the data carrier
• If the bus is not being used, the message can be sent –
but two nodes could start at the same time, resulting in
a collision of the two messages – the nodes need to
detect when this happens and re-send the message
• This protocol is called carrier sense multiple access
with collision detection – or CSMA/CD
Token Ring Network
• So-called because a
token passes around the
ring to carry messages
between the nodes
• Nodes can only send
when the token passes
and is empty
S
2
3
1
P
• There will be no collisions, but a node may have to wait
for the empty token to arrive, so may be slower than a
bus network under light loads but will slow down less
when it gets busier
• Any breaks in the cable will affect all users
Star Networks
• All nodes are
connected directly
to the server by a
dedicated server –
this means there are
no collisions
1
P
S
2
3
• This means that if a cable is cut, only one of the nodes is
affected
• Messages might be send by the server polling each
node in turn to see whether they have a message to
send, and if they have, delivering it.
• Logically, this is the same as a token ring!
Cabling/Connection
The type of cable used can also affect the speed at which data
travels around the network. There are three main types of
cabling used:
Twisted pair – this is cable with two wires next to each other, like
bell or speaker wire.
Slower
Co-axial cable – this has one cable in the form of a braid around
the outside of the other, with a (usually plastic) spacer in between,
like television aerial cable. The wire around the outside shields the
inside one from interference, and allows higher transmission rates.
Faster
Fibre-optic – information is sent down a glass fibre in the form of
light. This is obviously faster, as light travels at the speed of light!
Light is also less prone to interference and can travel further
without amplification.
Glossary
File server
The main computer on the network (although big
networks may have several servers) where the
files are stored
Print server
The computer on the network that controls the
printing – it will manage the queue where print jobs
wait before they are printed
Peer-to-peer
A type of network that has no file server – files on
the workstations can be shared with other users of
the network – as long as the computer is on!
Intranet
Provides web-pages on a local area network, e.g.
for company notices. Like a local version of the
world-wide web
Proxy server
A computer that fetches web-pages from the
internet and distributes them to the workstations
on the network.
Security
What security measures do networks use?
• Training and user awareness
• Usernames and passwords
• Firewalls
• Proxy servers
• Auditing / audit logs
• Monitoring performance
• Access rights for users
• Encryption
• Physical methods – lock the server away
Wide Area Networks
• Some large organisations are national or multinational, and data might need to be exchanged
globally
• Large wide area networks (WANs) therefore
exist to make this data transfer:
– possible
– secure
• Examples of WANs include:
–
–
–
–
airline booking systems
credit card payment systems
national lottery machines
Police National Computer
The Internet and the World Wide Web
• The internet and the world-wide web are not the
same! The internet also supports:
–
–
–
–
–
e-mail
newsgroups
IRC chat
FTP - file transfer
telnet - for logging into other computers remotely
• Through a variety of protocols, e.g.:
– TCP/IP - transfer control protocol/internet protocol
– POP3 (post office protocol) and SMTP (simple mail
transfer protocol)
– HTTP - hyper-text transfer protocol
– FTP - file transfer protocol
Internet Infrastructure
• Consumers connect to
Internet Service
Providers (ISPs), e.g.
Freeserve, AOL,
Tiscali
• ISPs connect to
Network Service
Providers (NSPs),
e.g. BT
• The links between the
NSPs is the internet
backbone
Consumers
ISP
NSP
NSP
Internet
backbone
NSP
NSP
ISP
Other ISPs
ISP
Connecting to the Internet
• PSTN (public service telephone network)
– connect using a modem
– allows speeds up to 56kbps
• ISDN (integrated services digital network)
– digital phone line - dialling of number required
– 64kbps per channel - usually 2 channels = 128 kbps.
– Connect using an ISDN or terminal adapter
• ADSL (asymmetric digital subscriber line)
–
–
–
–
connect using a terminal/network adapter
“always on” - no need to dial connection
faster to download than to upload (asymmetric!)
not charged per minute (unlike PSTN and ISDN)
Synchronous and Asynchronous
• With synchronous transmission, the bits are sent
at regular intervals governed by a clock in the
computer.
– This is the method usually employed by mainframe
computers
– The rate of transmission is governed by the speed of
the clock
• Asynchronous transmission is used by PCs
– Messages are preceded by a start message, e.g.
“ready to send”
– Messages have a stop message at the end
– Parity bits or check digits can be added to detect
errors in the communication
Network Protocol
• In general, a protocol is a set of rules governing
communication – this term doesn’t only apply to
computers, but also to diplomatic communication
• If both communicating parties are using the same
protocol, communication is much easier
• An agreed protocol could include:
– The transmission mode – synchronous or
asynchronous
– How characters are represent, e.g. ASCII or EBCDIC
– Any encryption and keys used
– Error detection and correction methods used, e.g.
parity, check digits
Factors Affecting Transmission Rate
Transmission rate will be affected by:
• The hardware used – e.g. how fast your network
card, modem, etc., is
• The type of connection you’re using – e.g. PSTN,
ADSL, wireless
• The type of cable you’re using, e.g. twisted pair,
coaxial or fibre-optic
• The transmission mode – synchronous or
asynchronous
• You can also make your transmission rate appear
to be higher by using compression techniques to
compress the data before you send it