Transcript Communication and Networksx

Communication Basics
Learning Objectives
• What serial and parallel data transmission are.
• What bandwidth, bit rate and baud rate are and the
differences between them.
• How latency affects the actual speed of
transmission.
• What synchronous and asynchronous data
transmission are.
• How devices establish communication through
handshaking.
• Which protocols areused to establish rules and
standards by which transmissions can take place.
Communication Basics
Data Transmission
The best ways that computers can transmit data is to reduce it to its binary
form, although some medium is analogue – whether it is text, images or
video. Moving data from one place to another is called DATA
TRANSMISSION
Transmission Medium
Transmitter
Receiver
Communication Channel
 Tranmission Media
 Guided – cabling, twisted pairs, coaxial, optic
 Unguided – wireless, radio, microwaves
Communication Basics
Data Transmission
Data transmission can happen between communication applications,
networks and on the motherboard itself between the CPU and various other
components and peripheral devices via the buses
Two types of data transmission
1) Serial
2) Parallel
Communication Basics
Serial Communication
Serial transmission sends and received data one bit at a time in
sequence
101001
101111
100001
111001
Speed of transmission will depend on the cabling. Not necessarily slow.
Universal Serial Bus (USB) uses a high speed serial connection – capable of
transmission rates of 1Gbps
Serial data transmission is used for long-distance communication. The main
reason for using serial data transmission is that it needs only one signal
pathway each way. This makes it easier to regenerate the signal. Signal
strength can fall significantly over long distances.
Communication Basics
Parallel Transmission
This uses a number of wires to send a number of bits simultaneously.
The more wires, the more data can be sent.
This method is used inside the computer on the buses. Depending
on the word length a computer can send 8,16,32 or 64 bits of data at
a time.
Used for short distances – for instance buses or printers nearby.
Faster than serial data tranmisssion.
Different properties on each line can lead to skew.
Communication Basics
Parallel Transmission
010101
110101
001111
101001
111101
101010
110101
111001
Communication Basics
Advantages of serial over parallel Transmission
• The significant reduction in soze and complexity of connectors in
serial transmission results in lower costs. Better for smaller
devices as well
• “Crosstalk” can occur over parallel lines leading to corrupted
data – therefore need to be resent.
• Serial links are more reliable over greater distances
• Lack of interference at higher frequencies means frequencies can
be higher
Communication Basics
Bandwidth
• Amount of data that can be transmitted along the medium at any
one time. Measured in Hz but closely associated with BIT RATE
(bps)
 Bit Rate
• The speed at which a particular transmission is taking place.
Measured in (bps) Bits per second. Bandwidth is the range of
frequencies that can be transmitted and bit rate is the number of
bits that can be transmitted per unit of time. Bit Rate is therefore
directly proportional to Bandwidth.
Communication Basics
Baud Rate
•
Another term to describe the speed at which data can be transmitted.
•
Baud rate is the electronic state change per second.
•
Usually equivalent to 1 bit of information but sometimes a electronic state
change can represent a number of bits – so you are effectively sending
symbols rather than bits.
Communication Basics
Latency
•
This is the time delay between the moment the first byte or data packet of
a communication starts and when it is received at its destination.
•
Communication sometimes has to travel via satellites, via cable, through
busses and logic gates.
•
Latencies from using peripheral devices might be unnoticeable but using
the internet or playing online games – distances are greater therefore it
might be more noticeable.
•
Three types
• Propagation Latency – Amount of time it takes for a logic gate within
a circuit to transmit the data.
• Transmission Latency – Amount of time it takes to pass through a
particular communication medium, e.g. cable
• Processing latency – The amount of time it takes data to pass around
a network depending on how many servers or devices it has to pass
through.
Communication Basics
Study tasks
1) Explain why you rarely achieve the connection speed that is
theoretically possible from your internet connection.
2) What are the highest bit rates that are theoretically possible
between two devices using
a) Serial communication
b) Parallel communication
3) Which country has the highest average internet speeds? Where does the
UK come in the world rankings and why?
Communication Basics
Synchronous Data Transmission
• Data is transmitted by having the sending device and receiving
device synchronising their transmission signals.
• Sending device uses the system clock to control the transmission
rate to be in time with the device or computer receiving the
signal.
• If the devices are not synchronised then data can be lost.
• Devices are kept in time permanently.
Sender
Data
Receiver
Communication Basics
Asynchronous Data Transmission
• Does not require the permanent synchronisation of the sender and
receiver’s clocks.
• Synchronises for the duration of the communication only.
• Uses start and stop bits to synchronise.
• May also include a parity bit for validation of data.
Sender
STOP
Data
Start
STOP
Data
Start
Receiver
Communication Basics
Asynchronous Data Transmission
• Start bit causes receiver to synchronise its clock to the same rate
as the sender
• Both devices agree on how many bits will follow, whether a parity
bit is used and how many stop bits
• Stop bits arrive so receiver processor can handle those bits. Can
now also identify when the next start bit will arrive.
• If more data – another start bit will be sent and the process
repeated.
• Sender just keeps sending data and does not have to wait for
synchronisation signal from receiver.
Sender
STOP
Data
Start
STOP
Data
Start
Receiver
Communication Basics
Protocols
1) A set of rules relating to communication between devices.
2) So many different devices involved in communication all around
the world from so many different manufacturers - that ways of
them communicating to each other needs to be standardised.
3) This is why standardised protocols are established.
4) Internet Protocols:
a) TCP/IP (Transmission Control Protocol/Internet
Protocol)Relates to a set of rules that govern the transmission
of data around the internet. Data sent around the internet are
split into packets and this protocol handles the routing and
re-assembly of these data packets.
b) HTTP (Hypertext Transfer Protocol)
Set of rules governing the exchange of the different types fo
files that make up displayable web pages
c) FTP (File Transfer Protocol) – Provides rule for transfer of
files on the internet.
Communication Basics
Study tasks
1) Investigate how the TCP/IP Protocol actually works. What are the
different layers that it uses. Create a visual presentation of your
findings.
Networks
Learning Objectives
• What components are needed to construct a
network, including servers, clients, routers,
switches and network cards.
• How networks can be constructed using different
topologies
• The benefits of using different topologies
• How networks can be client-server or peer to peer
• How wireless networks work
• How data is transmitted in frames and packets
• How data is being transmitted using protocols to
prevent collisions of data and keep data safe.
Networks
LAN
1) A number of computers and peripherals connected over a small
geographical area.
2) Common in businesses, hospitals and educational
establishments.
3) Usually made up of a number of servers and clients.
Components of a LAN – LINK
Networks
Wide Area Networks
• Were invented to solve the problem of connecting a LAN to a distant
workstation or another remote LAN.
• WAN are used to share information over a large geographical area.
Connections used could be :
 The public switched telephone network
 High-speed, high bandwidth dedicated leased lines
 High-speed fibre optic cable
 Microwave transmission links
 Satellite links
 Radio Waves
 The Internet
Carried out through either Circuit switching or packet switching.
Communicating over a WAN –
Connection oriented networks: Circuit Switching:
A circuit is established
between two nodes and
all data follows this path.
Communicating over a WAN –
Connectionless oriented networks: Packet Switching:
The data is split into
a number of smaller
packets that may take
different routes to
reach the destination.
Networks
TOPOLOGIES – STAR Network
Star network topology:
Hub or switch
Networks
TOPOLOGIES – STAR Network
Star network client connection hardware:
Hub
Networks
Star Networks
• All clients are connected to a central server via an individual connection
• Usually this is done through a switch or a hub
• Server is a high specification machine.
• Different servers used – Data, Software, Printer, Web, Email etc.
• In reality the network will not look like a star – but this is a conceptual
interpretation of it.
Networks
TOPOLOGIES – Bus Network
Terminator
‘Workstation’ Backbone
‘Client’
‘Terminal’
‘Node’
Computer
Terminator
Server
(optional)
Networks
TOPOLOGIES – Bus Network
Network interface card (NIC)
Terminator
Workstation Backbone
Client
Terminator
Server
(optional)
Terminator
T connector – plugs into NIC
Networks
TOPOLOGIES – Bus Network
Bus network operation:
•All components are connected via a backbone.
•At each end of the backbone is a terminator which stops signals ‘bouncing
back’ down the network.
•Each network interface card is passive: they all listen to the backbone for
signals addressed for them.
•Signals are sent in both directions of the backbone at the same time.
•Only one computer can transmit successfully at any one time, however this is
not regulated so collisions and data corruption do occur.
•A protocol called CSMA-CD (carrier sense multiple access collision detection)
is used to handle this problem.
Networks
TOPOLOGIES – Bus Network
Networks
CSMA/CD Summary
1 - A collision is detected.
2 - Devices involved in the collision keep transmitting for a short period of time, to
make sure all devices on the network see the collision (also referred to as the
jamming signal)
3 - Each device sees the jamming signal, and invokes the back-off algorithm. Each
device will have a random timer that determines when it can transmit again.
4 - When the back-off timer expires, devices are free to transmit data again.
Devices involved in the collision earlier do not have priority to transmit data.
Networks
Client Server Networks
• In a client server networks, one or more computers known as Clients are
connected to a powerful central computer known as the server.
• How much the client holds with regards to data and software depends on
whether they are fat or thin clients. Fat – most of the software, thin – server
contains all the software.
• Several type of servers might be connected to a network,
File
Web
Mail
Printer
Networks
Client Server Networks - Advantages
• Security is better, since all files are stored in a a central location and
access rights are managed by the server
• Backups are done centrally so there is no need for individual users to
back up their data. If there is a breakdown and some data is lost, recovery
procedures will enable it to be restored.
• Data and other resources can be shared.
Networks
Client Server Networks - Disadvantages
• It is expensive to install and manage
• Professional IT Staff are needed to maintain the servers and run the
network.
Networks
Peer to peer networks
• No central server.
• Individual computers are connected together either locally or over a
WAN.
• Cheap to set up (especially a home one)
• Enables resources to be shared
• Not difficult to maintain.
• Used in video on demand. Hundreds of computers are used to hold parts
of the video to share the load.
Networks
Peer to peer networks - Disadvantages
• Used for piracy for downloading films and other copyrighted material.
Networks
Wireless Networks
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•
•
•
Does not use cables
Data sent using radio waves
Can be used over LANs and WANs (WLANS and WWANs)
Every device attached to a WiFi (WLAN) has a unique MAC Address
(Media Access Control) This is coded onto the network interface card
(NIC) of the device and is used in the addressing of packets and routing
data)
 WiFi
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•
•
•
•
A standard for connecting computers together wirelessly.
IEEE 802.11
Different devices in a WLAN have different ranges – 802.11n and 802.11g
WiFi hotspots are used by ISP providers (e.g. BT)
WWANs make use of mobile phone networks and satellites (3G,4G,5G)
Networks
WiFi
• Each Wireless Network Access Point (WAP) has a Service Set
Identification (SSID) name – which is an informal name for the network.
Networks
 WiFi
• Most networks will be protected by a password (WiFi Protected Access
(WPA)) or WPA2
• WPA2 is a 128-bit key encryption for data transmission.
• Having the password allows you to send data from or to that WAP.
Networks
 WiFi
• To ensure the reliability of WiFi transmission certain protocols are in
effect for instance the CSMA/CA to avoid data collisions.
Start
End
Assemble
a frame
Transmit
Data
Channel
idle?
Yes
Transmit RTS
(Request to
Send)
No
WAIT
No
Yes
CTS
(Clear to
Send)
received?
Communication Basics
Study tasks
1) “No more wires”. How likely is it that wireless networks will
completely replace networks with cables. Build up a case that
this my be true. Include the following in your presentation –
a) The history of the growth of the use of WiFi
b)
c)
d)
e)
WiFi Technologies – new and current,
Advantages and disadvantages of Wireless Networking.
The growing trend of the use of WiFi in society.
The growing trend of the use of WiFi in businesses