Communications Systems The topics within this unit are:

1
Communications SystemsThe topics within this
unit are

• Characteristics of communication systems.
• Examples of communication systems.
• Transmitting and receiving in communication
  systems.
• Computer Networks

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Characteristics of Communication Systems

• must be a Sender and Receiver
• A protocol is a set of rules which governs the
  transfer of data between computers. Protocols
  allow communication between computers and
  networks.
• Handshaking is used to establish which protocols
  to use. Handshaking controls the flow of data
  between computers
• protocols will determine the speed of
  transmission, error checking method, size of
  bytes, and whether synchronous or asynchronous
• Examples of protocols are token ring, CSMA/CD,
  X.25, TCP/IP
• Carrier Sense Multiple Access/Collision Detection
  (CSMA/CD)

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5 Basic Components

• Every communication system has 5 basic
  requirements
• Data Source (where the data originates)
• Transmitter (device used to transmit data)
• Transmission Medium (cables or non cable)
• Receiver (device used to receive data)
• Destination (where the data will be placed)

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5 Basic Components
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Transmission Media Speed

• BandwidthThe amount of data which can be
  transmitted on a medium over a fixed amount of
  time (second). It is measured on Bits per Second
  or Baud
• Bits per Second (bps) A measure of transmission
  speed. The number of bits (0 0r 1) which can be
  transmitted in a second
• Baud Rate Is a measure of how fast a change of
  state occurs (i.e. a change from 0 to 1)

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Network devices

• To complete the communication process
• Repeaters amplify the signal
• Routers to route the signal
• Switches intelligent analysis
• Bridges connect two networks together
• Cables
• Gateways gateway to the network

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TRANSMISSION MEDIA

• GUIDED TRANSMISSION (physical transmission with
  wires
• Wires such as Fibre optic cables, the Twisted
  Pair Cables, Coaxial Cables
• Unguided Transmission (Wireless transmission)
• Unguided because signals go to all directions eg
  infrared, satellite, radio, microwaves etc

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Computer Networks

• LAN LOCAL AREA NETWORK
• WAN WIDE AREA NETWORKS
• PAN PERSONAL AREA NETWORKS
• MAN METROPOLITAN AREA NETWORKS
• SAN STORAGE AREA NETWORKS

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TOPOLOGIES

• Physical and logical lay out of computers and
  other devices on the network
• Physical the lay out of nodes (computers) and
  other devices like printers and cables on the
  network

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TOPOLOGIES

• Logical the way information flows between nodes
  and devices
• Examples of topologies included star, ring, bus,
  mesh (full partial mesh), tree and hybrid

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BUS TOPOLOGIES

• All the devices/nodes are connected sequentially
  to the same backbone or transmission line. This
  is a simple, low-cost topology, but its single
  point of failure presents a risk. Easy to install
  and configure, Inexpensive, Easily extended

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Star Topology

• All the nodes in the network are connected to a
  central device like a hub or switch via cables.
  Failure of individual nodes or cables does not
  necessarily create downtime in the network but
  the failure of a central device can.
• This topology is the most preferred and popular
  model, Easy to connect new nodes or devices,
  Centralized management. It helps in monitoring
  the network.

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RING TOPOLOGY

• All network devices are connected sequentially to
  a backbone as in bus topology except that the
  backbone ends at the starting node, forming a
  ring.
• Ring topology shares many of bus topology's
  disadvantages so its use is limited to networks
  that demand high throughput.

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The endless topology
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ADVANTAGES DISADVANTAGES OF THE RING TOPOLOGY

• ADVANTAGES of Ring Topology
• when the load on the network increases, its
  performance is better than that of Bus topology.
• There is no need for network server to control
  the connectivity between workstations.
• Additional components do not affect the
  performance of network.
• Each computer has equal access to resource.
• Token ring technology reduces the need of server
  or central hub to manage the workstations.

• DISADVANTAGES of Ring Topology
• If one node is disrupted then the whole network
  goes down.
• Only one machine can transmit on the network at a
  time.
• The failure of one machine will cause the entire
  network to fail.

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MESH TOPOLOGY

• The topology of a network whose components are
  all connected directly to every other component.
• For sending messages, check the cable connected
  into two devices. A message is send directly from
  sender to receiver because each one has
  individual and separate connection.

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MESH TOPOLOGY

• ADVANTAGES OF MESH TOPOLOGY
• Eliminates traffic problems in links sharing.
• If one link becomes unusable, it does not
  incapacitate the entire system. Thus, act as
  robust. It has privacy and security.
• Point-to-point link make fault identification and
  fault isolation easy.
• DISADVANTAGES OF MESH TOPOLOGY
• Difficult to install and maintain.
• Expensive.

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HYBRID TOPOLOGY

• A combination of two or more different topologies
  makes for a hybrid topology. When different
  topologies are connected to one another, they do
  not display characteristics of any one specific
  topology

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HYBRID TOPOLOGY

• ADVANTAGES OF HYBRID TOPOLOGY
• Any topology can be combined with another without
  making any changes to existing topology.  The
  speed of the topology is compatible because it
  combines the strengths of each of the topologies,
  eliminating weaknesses.  It is also more
  efficient.
• DISADVANTAGES OF HYBRID TOPOLOGY
• Installation and configuration of topology is
  difficult.  Because there are different
  topologies that need to connect.  At the same
  time, make sure none of them fail, which makes
  installation and configuration very difficult.

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NETWORK MODELS

• The term computer network model defines the
  category in which a computer network can be
  grouped into based on the architecture they
  employ
• Each architecture is more suitable for certain
  types of businesse.
• Networks are divided into two broad categories
• Client Server Model and Peer-to-Peer Model

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Client server model

• A network model that offers centralised access to
  services and devices.
• One computer plays the role of a server. It is
  the most common type of network architecture
  today that provides centralized data storage,
  security, manning of applications and network
  administration. Most servers operating system are
  Novel Netware, Windows NT, Apple talk, Linux,
  Banyan VINES etc...

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Client server model DEDICATED SERVERS

• Large networks can have specialised servers
  (dedicated servers) servers to do a specific
  task on the network such as
• Print server manage printers print jobs
• File Servers store and manage files
• Mail servers move and store mails over networks
• Database servers Provide database services to
  other network users

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DEDICATED SERVERS CONTINUED

• Proxy servers allow individuals surfing the net
  to do so anonymously
• Web servers computers that deliver web pages
  facilitated by HTTP protocol
• Application servers..

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ADVANTAGES OF CLIENT SERVER MODEL

• Security - All major server based systems
  provides sophisticated security.
• Administration - Servers are centralized making
  them easier to manage.
• Stability - Server based systems are designed to
  support a wide range of organization sizes.
  Additional servers are added to increase
  capacity.

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ADVANTAGES OF CLIENT SERVER MODEL

• Client server networks offer centralized backup
  where data can be stored in one server.
• Flexibility - New technology can be easily
  integrated into the system.
• Accessibility - Server can be accessed remotely
  and across multiple platforms.

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• DISADVANTAGES OF THE CLIENT SERVER
• MODEL
• --------------------------------------------------
  ------------------------------
• Cost - More expensive in terms of hardware and
  network operating system.
• Complexity - Experienced system administrators
  are required to manage the systems.
• Dependence - When server goes down, operations
  will cease across the network.

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PEER-TO-PEER NETWORKS

• In a peer-to-peer network, there are no dedicated
  servers, and there is no hierarchy among the
  computers.
• All the computers are equal and therefore are
  known as peers.
• Each computer functions as both a client and a
  server, and there is no administrator responsible
  for the entire network.

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ADVANTAGES OF PEER-TO-PEER NETWORKS

• This model is not quite secure and is suited for
  a small computer networks (with 10 computers or
  less)
• They are easy to configure
• Computers communicate easily.
• They dont require additional server hardware or
  software
• Users can manage their own resources.
• They dont require a network administrator
• They reduce total cost of network setup.

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DISADVANTAGES OF PEER TO PEER NETWORKS

• They provide a limited number of connections for
  shared resources.
• Computers with shared resources suffer from
  sluggish performance.
• They dont allow for central management
• Users are responsible for managing resources.
  These resources include data in shared
  directories, printers, fax cards, and so on.
• They offer very poor security.

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Peer to peer Vs Server Client
PEER TO PEER CLIENT-SERVER
Easy to set up Difficult to set up
Less expensive to Install More expensive to install
Can be implemented on a wide range of OS Clients can support a wide range of OS but the server runs on OS that supports networking
Very low levels of security supported High levels of security supported due to server
Ideal for networking with less than 10 computers No limit on the number of computers supported by the network
Does not require a server Requires a server
Demands a moderate level of skill to mange network Demands that the network adminstrator has a high level of IT skills
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Advantages of networked resources

• Increased storage capacity
• File sharing
• Resource sharing
• Costs shared resources means less costs
• Flexibility
• Centralised administration

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Disadvantages of networked resources

• Security concerns
• Virus and Malware
• Needs efficient Handlers
• Lack of independence

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Some Network Administration Tasks

• Adding/removing users
• Assigning users to printers
• Giving users file access rights
• Installation of software and sharing with users
• Client installation and protocol assignment
• Logon and logoff procedures
• Network based applications

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DATA PACKETS

• Transmissions are broken up into smaller units or
  data transmissions called packets

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Packets and OSI

• After the file is divided into packets extra
  information is required to make sure it all goes
  back together correctly. The OSI model helps to
  look after this.
• The OSI model also provides much more information
  which is included with each package.

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OSI 7 Layer Model

• OSI Open System Interconnection
• OSI is not a protocol but a list of protocols
  divided between 7 layers with each layer having a
  different set of functions.
• Each packet is layered/packaged with protocols
  from each of the layers as it is processed.
• The process of layering the protocols around each
  package is called ENCAPSULATION. The final
  encapsulated data packet is called a FRAME.

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Open Systems Interconnection (OSI)

• OSI Reference model
• Layer 7 application
• Layer 6 presentation
• Layer 5 session
• Layer 4 transport
• Layer 3 network
• Layer 2 data link
• Layer 1 physical

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Examples of protocols

• Layer 7 application
• Layer 6 presentation
• Layer 5 session
• Layer 4 transport
• Layer 3 network
• Layer 2 data link
• Layer 1 physical

• E-mail, Web browser, Directory
• POP, SMTP, FTP, HTTP, DNS
• Sockets
• TCP
• IP
• PPP, Ethernet, Token ring
• 100baseT

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Examples of Communication Systems

• - E-mail
• - Voice Mail - Fax
• - Smart Phone - Instant Messaging
• - Telecommuting - Video-confrencing
• - Groupware - Telephony
• - E-Commerce - The Internet
• Bulletin board system - The Web
• - Global positioning system

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Serial Transmission

• Data is transmitted, on a single channel, one bit
  at a time one after another
• - Much faster than parallel because of way bits
  processed (e.g. USB and SATA drives)

Receiver received
Sender transmitted
1
0
1
0
0
1
1
0
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Parallel Transmission

• each bit has its own piece of wire along which
  it travels
• - often used to send data to a printer

1
0
0
Receiver received
1
Sender transmitted
1
0
0
1
All bits are sent simultaneously
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Why Not use Parallel Instead of serial?

• Due to inconsistencies on channels data arrives
  at different times
• Because of the way it is transmitted packet
  switching cannot be used a mode of data
  transmission in which a message is broken into a
  number of parts that are sent independently, over
  whatever route is optimum for each packet, and
  reassembled at the destination.
• The above two points makes parallel slower than
  serial and requires higher bandwidth.
• Parallel transmissions are rarely used anymore

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Transmission Direction
SIMPLEX TRANSMISSION used when data is
transmitted in one direction Mode of flow of
signals is UNIDIRECTIONAL
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Half Duplex Transmission

• half duplex Both directions but only one
  direction at a time

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Full Duplex Transmission

• full duplex send and receive both directions at
  once or simultaneously
• Eg telephone

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Error Detection and Correction

• Remember we said that nothing is perfect on
  planet earth.
• Communication chanels are not perfect, as a
  result, transmitted signal may get to the
  recipient unreadable or damaged
• Error detection is the detection of errors caused
  by noise and other impairements during the signal
  transmission from the transmitter to the receiver

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Error Detection and Correction

• Error correction refers to the detection and
  reconstruction of the original error free data

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Causes of errors on the transmission channel

• Attenuation
• Attenuation refers to the reduction in the
  strength of the signal most commonly as a result
  of transmitting analog or digital signals over
  long distances
• The further the signal travels away from the
  transmitter, the weaker it gets!!!

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Delay Distortion

• When a digital signal with varying frequency
  components is transmitted, it arrives at the
  receiver with varying delays
• (delay distortion)

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Noise

• Noise is any unwanted energy (signals) on the
  communication channel. Noise can originate from
  natural or electrical or technological activity
  around the communication channel e.g. nearby
  cables, radio frequencies, earth quakes,
  lightening
• Types of noise include white noise (which
  affects transmission frequencies equally) and
  narrowband noise (that affects only a small range
  of frequencies)

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Error Detection and Correction

• Parity bit check (redundancy)
• Check sum
• data transmitted in blocks, each block
  added to give a total checksum
• Cycle redundancy check

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REDUNDANCY

• A method used to detect and correct errors on the
  network
• Extra bits of data (redundant bits) are sent with
  the actual data to be sent

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• The redundant bits are added by the sender and
  removed by the receiver its called redundancy
  because the extra bits are redundant to the data
• They are discarded as soon as the accuracy of the
  data is determined.

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REDUNDANCY
sender

10100001
1011101
Data and Redundancy
Communication channel
Receiver

Reject Data
No
Yes
10100001
1011101
Ok?
Data and Redundancy
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Redundancy process

• Data packets are generated and passed through a
  device that adds an appropriate redundancy check,
  and its passed onto the receiver
• Receiver puts it into the checking function and
  if it passes the set criteria, is accepted or
  rejected accordingly

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Types of redundancy checks

• Parity check
• The parity is a bit added to every data unit
  (usually 7 or 8 bits) that are transmitted.
• Parity bits are set so that all bytes have either
  an even or odd number of bits hence ODD or EVEN
  PARITY
• The total number of ones is either odd or even

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Parity check
Sender node

Calculate Parity bit
1011101
Communication channel
Receiver node

Reject Data
No
Even?
Yes
Bits
Count bits
Drop Parity bit and accept data
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Example even / odd parity

• Suppose you want to send the word WORLD
• WORLD in ASCII five character code would be
• 1110111 1101111 1110010 1101100 1100100
• What is even add a 0
• What is odd add a 1 ( to make it even)
• The actual bits sent would be
• 11101110 11011110 1110010 1101100
  11001001

PARITY BIT
PARITY BIT
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TWO DIMENSION PARITY

• A Block of bits is divided into rows and a
  redundant row of bits is added to the whole block
• 1100111 1011101 0111001 0101001 (ORIGINALA
  DATA SET)

ROW PARITIES
1
1
0
0
1
1
1
1
1
0
1
1
1
1
0
1
0
1
1
1
0
1
0
0
0
1
0
1
0
1
0
1
0
1
0
1
0
0
1
1
COLUMN PARITIES
01010101
01010011
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CHECKSUM

• Checksums are used to ensure the integrity of
  data portions.
• The unit of data to be sent is divided into k
  sections of n-bits.
• All sectors are added using 1s complement to get
  the sum. The sum is complemented and becomes the
  checksum. The checksum is sent with the data
• As earlier, the unit is divided into k sections
  with n-bits. All sections are added using 1s
  complement to get the sum. The sum is
  complemented. If the result is zero, the data is
  accepted else its rejected.

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CHECKSUM example1

• Suppose the following block of 16 bits is to be
  sent using checksum of 8 bits (10101001
  00111001)
• 10101001
• 00111001

Numbers added using ones complement
11100010
The Checksum is complemented
00011101
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CHECKSUM example1

• 10101001
• 00111001
• 00011101
• After complementation we get all 0s, therefore
  the data is accepted
• 00000000

11111111
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CYCLIC REDUNDANCY CHECK (CRC)

• It is based on Binary division
• A series of redundant bits called CRC or CRC
  reminder is appended to the end of the data
  predetermined binary number.
• At its destination, the incoming data unit is
  divided by the same number. If at this stage
  there is no reminder, it means that the data unit
  is assumed to be intact and therefore accepted.
• A reminder indicates that the data unit has been
  damaged in transit and must be rejected.