Communications Basics Part I

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• Lecture 7
• Communications Basics (Part I)

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Introduction

• When transmitting any type of electrical signal
  over a transmission line, the signal is
  attenuated (decreased in amplitude) and distorted
  by the transmission medium
• Present with all types of transmission medium is
  an electrical signal known as noise
• The amplitude of the noise signal varies randomly
  with time and adds itself to the electrical
  signal being transmitted over the line
• Due to this noise effect at some stage the
  receiver is unable to determine from the
  attenuated received signal whether the
  transmitted signal was a binary 1 or 0

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Example Effect of attenuation, distortion, and
noise on transmitted signal

• If the amplitude of the received signal falls
  below the noise signal level, then the received
  signal may be incorrectly interpreted and a
  transmission/bit error will result

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Introduction

• The level of signal impairment is determined by
• - the type of transmission medium
• - the length of the transmission medium
• - the bandwidth of the medium
• - the bit rate of the data being transmitted

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Transmission media

• The type of transmission medium is important,
  since various types of media have different
  bandwidth associated with them
• The associated bandwidth will determine the
  maximum bit rate that can be used
• The common types of transmission media are
• - two-wire open lines
• - twisted-pair lines
• - Coaxial cable
• - Optical Fibre
• - Satellites
• - Terrestrial microwave
• - Radio

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Two-wire open lines

• In two-wire open lines each wire is insulated
  from the other and both are open to free space
• This type of line is adequate for connecting
  equipment that is up to 50m apart using moderate
  bit rates (less than, say, 19.2kbps)
• Two-wire open lines are used normally to connect
  a DTE to local data circuit-terminating equipment
  (DCE) for example modem
• With this type of line, care must be taken to
  avoid cross-coupling of electrical signals
  between adjacent wires in the same cable. This is
  known as crosstalk

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Transmission Media Copper Wire

• Is the simplest transmission medium each wire
  is insulated from the other and both are open to
  free space. This type of line is adequate for
  connecting equipment that is up to 50 m apart
  using moderate bit rates (less than 19.2kbps)

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Transmission Media Unshielded twisted pair
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Twisted-pair lines

• Much better immunity to spurious noise signals
  can be achieved by using the twisted-pair line
• In this a pair of wires are twisted together
• The proximity of the signal and ground reference
  wires means that any interference signal is
  picked up by both the signal and reference wires
• If multiple twisted pairs are enclosed within
  the same cable, the twisting of each pair within
  the cable reduces crosstalk
• Twisted pairs are suitable for bit rates in the
  order of 1Mbps over short distances (less than
  100m) and lower bit rates over long distances

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Transmission Media Unshielded and Shielded
twisted pair
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Twisted-pair lines

• The limiting factor of a twisted-pair line are
  its capacity and a phenomenon known as the skin
  effect
• As the bit rate of the transmitted signal
  increases, the current flowing in the wires tends
  to flow only on the outer surface of the wire,
  thus using less of the available cross section
• This increases the electrical resistance of the
  wires for higher frequency signals, leading to
  higher attenuation
• In addition at higher frequencies more signal
  power is lost as a result of radiation effects
• Hence for applications that demand a high bit
  rate over long distances, coaxial cable is often
  used as the transmission medium

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Coaxial cable

• Coaxial cable minimizes both these effects
• In this type of cable the signal and ground
  reference wires run concentrically (coaxially)
  inside a solid ( or braided) outer circular
  conductor
• Ideally the space between the two conductors
  should be filled with air, but in practice it is
  filed with a dielectric insulating material with
  a solid or honeycomb structure
• The centre conductor is effectively shielded
  from external interference signals by the outer
  conductor
• Only minimal losses occur as a result of the
  electromagnetic radiation and the skin effect
  because of the presence of the outer conductor
• These cables can be uses with either baseband or
  modulated transmission, but typically 10Mbps over
  several hundred meters

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Optical Fibre

• Although coaxial cable significantly reduces the
  various limiting effects, the maximum frequency,
  and hence the bit rate that can be transmitted
  using a solid conductor, although very high, is
  limited
• The optical fibre cable differs from both these
  transmission media in that it carries the
  transmitted bit stream in the form of a
  fluctuating beam of light in a glass fibre
• Light waves have a much wider bandwidth than
  electrical waves, enabling optical fibre cable to
  achieve transmission rates of hundreds of Mbps
• It is used in the core transmission network of
  PSTNs and LANs and also CATV networks

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Optical Fibre

• Light waves are also immune to electromagnetic
  interference and crosstalk
• Hence optical fibre cable is extremely useful
  for the transmission of lower bit rate signals in
  electrically noisy environments, e.g steel
  plants
• It is also being used increasingly where
  security is important, since it is difficult
  physically to tap
• The light signal is generated by the transmitter
  which uses a light-emitting diode (LED) or laser
  diode (LD)
• The receiver uses a light-sensitive photodiode

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Optical fibre

• The fibre consists of two parts an optical core
  and an optical cladding with a lower refractive
  index
• Light propagates along the optical fibre core in
  one of three ways depending on the type and width
  of the core material used
• In a multimode stepped index fibre the cladding
  and core material each has a different but
  uniform refractive index
• All the light emitted by the diode at an angle
  less than the critical angle is reflected at the
  cladding interface and propagates along the core
  by means of multiple reflections
• Depending on the angle at which it is emitted by
  the diode, the light will take a variable amount
  of time to propagate along the cable

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Optical fibre

• Therefore the received signal has a wider pulse
  width than the input signal with the decrease in
  the maximum permissible bit rate
• This effect is known as dispersion
• Dispersion can be reduced by using a core
  material that has a variable refractive index
• This type of fibre is known as the multimode
  graded index fibre
• In this fibre the light is refracted by an
  increasing amount as it moves away from the core
• This has the effect of narrowing the pulse width
  of the received signal compared to the stepped
  index fibre

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Optical fibre

• Further improvement can be obtained by reducing
  the core diameter to that of a single wavelength
  (3-10 ?m) so that all the emitted light
  propagates along a single path
• Hence the received signal will be of comparable
  width to the input signal and is called the
  monomode fibre
• Alternatively multiple high bit rate
  transmission channels can be derived from the
  same fibre by using different proportions of the
  optical bandwidth for each channel
• This mode of operation is known as
  wavelength-division multiplexing (WDM)
• Using this bit rates in excess of tens of Gbps
  can be achieved

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Transmission Media Optical fibre transmission
media
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Satellites

• In satellite systems the data is transmitted
  using electromagnetic (radio) waves through free
  space
• A collimated microwave beam, onto which the data
  is modulated, is transmitted to the satellite
  from the ground
• This beam is received and retransmitted to the
  predetermined destinations using an on board
  circuit known as a transponder
• A single satellite has many transponders, each
  covering a particular band of frequencies
• A typical satellite has a bandwidth of 500MHz
  and can provide many high bit rate data links
  using TDM

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Satellites

• Satellites are widely used for data transmission
  applications ranging from interconnecting
  different national computer communication
  networks to providing high bit rate paths to link
  communication networks in different parts of the
  same country
• In data communication applications, a more
  common configuration involving a central hub
  ground station that communicates with a number of
  ground stations distributed around the country is
  used
• Each ground station has a small antenna
  associated with it typically 1 metre in
  diameter which receives and transmits signals
• Typically the central site broadcasts to all
  VSAts at a bit rate of 0.5 2 Mbps while in the
  reverse direction each VSAT transmits as a lower
  bit rate of up to 64 kbps

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Satellites

• To communicate with a particular VSAT (very
  small aperture terminal), the central site
  broadcasts the message with the identity of the
  intended VSAT at the head of the message
• In the next generation of satellites direct
  VSAT-to-VSAT communication is possible

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Transmission Media
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Transmission Media Satellite System data
communications
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Radio

• Radio transmission using lower-frequency radio
  waves is also used for the digital information
  transmission in place of the fixed-wire links
• Example applications include mobile telephony
  and more general mobile data applications
• A radio transmitter (base station (BS)) is
  located at a fixed-wire termination point
• This provides a cordless link to the fixed-wire
  termination point for any handset/terminal that
  is within the (radio) field of coverage of the
  base station
• Multiple base stations must be used for wider
  mobile coverage
• The coverage area of the BS is restricted hence
  wider coverage is achieved by arranging multiple
  base stations in a cell structure

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Transmission Media

• Radio transmission using a lower-frequency radio
  waves is also used for the transmission of
  digital information in place of fixed-wire links
  over distances up to several kilometers

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Transmission Media

• Wider coverage is achieved by arranging multiple
  base stations in a cell structure the size of
  each cell varies and is determined by such
  factors as the handset/terminal density and local
  terrain

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Radio

• Each base station operate using a different band
  of frequencies from its neighbours
• Since the field of coverage is limited it is
  possible to use its frequency band in other parts
  of the network
• All base station within a region are connected
  by fixed-wire lines using the mobile switching
  centres (MSCs)
• This is connected to other MSCs in other regions
  and to the fixed telephone network

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Attenuation

• As a signal propagates along a transmission
  medium its amplitude decreases known as signal
  attenuation
• Normally a limit is set on the length of the
  cable that can be used to ensure that the
  receiver circuitry can reliably detect and
  interpret the received attenuated signal
• If the cable is longer then one or more
  amplifiers (repeaters) are inserted at intervals
  along the cable to restore the received signal to
  its original level
• Signal attenuation increases as a function of
  frequency
• To overcome this effect the amplifiers are
  designed to amplify different frequency signals
  by varying amounts
• Devices such as equalizers are used to equalize
  the attenuation across a defined band of
  frequencies

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Delay distortion

• The rate of propagation of the sinusoidal signal
  along a transmission line varies with the
  frequency of the signal
• Consequently, when we transmit a digital signal
  the various frequency components making up the
  signal arrive at the receiver with varying delays
  resulting in delay distortion of the received
  signal
• As the bit rate increases, some of the frequency
  components associated with each bit transition
  are delayed and start to interfere with the
  frequency components associated with a later bit
• Delay distortion is also known as intersymbol
  interference (ISI)
• The level of intersymbol interference associated
  with a transmission channel can be observed by
  means of an eye diagram

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Delay distortion - ISI
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Delay distortion

• The eye diagram is obtained by displaying the
  signal received from the channel on an
  oscilloscope which is triggered by the
  transitions in the signal
• Assuming the received signal contains random
  binary 1 and 0 signal transitions, the
  oscilloscope will display all the possible
  signals superimposed on one another
• Higher the level of interference, the smaller
  the central section known as the eye

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Asynchronous transmission

• With asynchronous transmission each character or
  byte that makes up a block/message is treated
  independently for transmission
• This can be used for transfer of simple
  characters entered at a keyboard, or for the
  transfer of blocks of characters/ bytes across a
  low bit rate transmission line/channel
• Since all transfers that are external to the
  system are carried out bit-serially, the
  transmission control circuit on the network
  interface card (NIC) must do the following
• - parallel-to-serial conversion of each
  character or byte in preparation for its
  transmission on the line
• - serial-to parallel conversion of each
  received character or byte in preparation for its
  storage and processing in the received end system

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Asynchronous transmission

• - a means of the receiver to achieve bit,
  character, and frame synchronization
• - the generation of suitable error check
  digits for error detection and the detection of
  such errors at the receiver should they occur
• Using the PISO shift register a full character
  can be loaded in parallel and shifted out
  bit-serially
• Serial to parallel conversion is carried out by
  the SIPO shift register

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Asynchronous Transmission principle of operation
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Frame synchronization

• When messages comprising blocks of characters or
  bytes normally referred to as information
  frames - are being transmitted, in addition to
  bit and character synchronization, the receiver
  must be able to determine the start and end of
  each frame
• This is known as frame synchronization
• The simplest method of transmitting blocks of
  printable characters is to encapsulate the
  complete block between two special transmission
  control characters STX (start-of-text) which
  indicates the start of a new frame after an idle
  period and ETX (end-of-text)

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Frame synchronization
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Frame synchronization

• Although the above scheme is satisfactory for the
  transmission of block of characters, when
  transmitting blocks of strings of bytes, the use
  of ETX to indicate the end is not sufficient
• In the case of string bytes one of the string
  bytes may be same as the ETX character, which
  would cause the receiver to terminate the
  reception process abnormally
• To overcome this problem, when transmitting this
  type of data the two transmission control
  characters STX and ETX are each preceded by a
  third transmission control character known as
  data link escape (DLE)
• After transmitting the start-of-frame sequence
  (DLE-STX) the transmitter inspects each byte in
  the frame prior to transmission to determine if
  it is the same as the DLE character. If it is,
  irrespective of the next byte, a second DLE
  character is transmitted before the next byte
• This process is known as character or byte
  stuffing

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Frame synchronization
39

• Smart Home Technologies

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Smart Homes

• Imagine a completely networked home in which
  every appliance has its own Internet address and
  can be remotely managed from anywhere on the
  Internet with a simple Web browser
• The general goal of the smart-home movement is
  to use networking technology to integrate the
  devices, appliances and services found in homes
  so that the entire domestic living space can be
  controlled centrally or remotely

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What are smart homes?

• A house with intelligent devices that can obey
  the orders of humans and serves as a reliable
  monitor of home activities
• Smart home networks will bring the integration of
  sophisticated alarm systems, network of sensors
  and small microelectronic devices able to start
  home electrical devices remotely
• The recent emergence in communication
  technologies, especially wireless and fixed small
  range networks, enabled extended use of smart
  homes and their remote control

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What are smart homes?

• In the current smart homes, a large number of
  sensors observe the environment and regulate
  certain parameters of the environment, such as
  temperature
• The next generation of smart homes are emerging
  as a result of sophisticated wireless networking
  technologies that have become reality in recent
  years
• However, the important issue for this smart home
  network of next generation is that it has to
  reach the same level of reliability and user
  availability as previous generations
• User interface is one of the key issues for a
  successful design of a smart home.

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Internet Refrigerator?

• South Koreas LG Electronics made Internet ready
  refrigerator designed to give users the ability
  to surf the Web from the kitchen
• It had two prominent 15-inch LCD displays on the
  front panels, one on each of the two main doors
• Through these panels you can access real-time
  grocery prices, health and nutrition tips,
  cooking information also if the users first tell
  the refrigerator what goods are being stored
  inside, alerts about expiration dates
• Based on the ingredients inside it can also
  provide few recipes for you!! Currently the cost
  is 5000.

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Smart home Technologies and issues

• Smart home systems must run sophisticated
  algorithms which have to be trained to adapt to
  the users
• Most important issues
• - User interface design
• - communication technology
• - location identification
• - automatic decision making
• - On-demand action

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Smart home Technologies and issues

• Tracking and location systems are one of the
  most important services in smart home networking.
• These systems allow users to feel more
  independent and secure since they can be informed
  by the network about possible obstacles and about
  their precise location
• Key issues in the development of the tracking
  system and more generally of the whole smart
  networking systems include
• - Cost
• - Accuracy of the location algorithm
• - size and weight of the devices and power
  consumption

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Technologies for smart homes

• UPnP (Universal Plug and Play)
• UPP is a standard that uses Internet and Web
  protocols to enable devices such as PCs,
  peripherals, intelligent appliances, and wireless
  devices to be plugged into a network and
  automatically know about each other
• With UPnP, when a user plugs a device into the
  network, the device will configure itself,
  acquire a TCP/IP address, and use a discovery
  protocol based on HTTP to announce its presence
  on the network to other devices
• e.g a camera connected to the network will look
  for a printer to print a colour photo

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Technologies for smart homes

• UPnP (Universal Plug and Play)
• The camera and printer will use XML to
  establish a common language, or protocol
  negotiation, to talk to each other and determine
  capabilities
• Microsoft one of the 29 companies sponsoring
  UPnP hopes that UPnP will make it as easy to plug
  a device or appliance into a home or small
  business data network as it is to plug a lamp
  into an electrical outlet

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Bluetooth

• Bluetooth is a telecom standard that describes
  how mobile phones, computers, and PDAs can be
  easily interconnected using a short-range
  wireless connection
• Bluetooth requires that a low-cost transceiver
  chip be included in each device
• Each device has a 48-bit address from the IEEE
  802 standard
• Maximum range is 10 meters
• A frequency hop scheme allows devices to
  communicate even in areas with a great deal of
  electromagnetic interference
• Built-in encryption and verification provided

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Jini

• Jini is a new idea that Sun Microsystems calls
  spontaneous networking.
• Using Jini architecture, users will be able to
  plug printers, storage devices, speakers, and any
  kind of device directly on to the network.
• And the user on the network will know that a new
  device has been added
• Each pluggable device will define itself
  immediately to a network device registry
• When someone wants to use or access the resource,
  their computer will be able to download the
  necessary programming from it to communicate with
  it.

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• No longer will the special device support
  software known as the device driver need to be
  present in an operating system
• The operating system will know about all
  accessible devices through the network registry
• Jini can be viewed as the next step after the
  Java programming language to make the network
  look like one large computer
• A printer could be added to the network with a
  microchip-embedded operating system and that can
  be shared by users of a mix of computers
  Windows, Macintosh, UNIX etc.

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• SIP (Session Initiation Protocol)
• Sip is an IETF standard protocol for initiating
  an interactive user session that involves
  multimedia elements such as video, voice, data,
  chat, gaming and virtual reality
• Like HTTP SIP works in the Application layer of
  the open systems interconnection communication
  model
• SIP can establish multimedia sessions or Internet
  telephony calls, and modify, or terminate them
• The protocol can also invite participants to
  unicast or multicast session that do not
  necessarily involve the initiator

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• SIP is a request response protocol, dealing with
  requests from clients and responses from servers
• Requests can be sent through any transport
  protocol, such as UDP, TCP.
• SIP determines the end system to be used for the
  session. The communication media, media
  parameters, and the called partys desire to
  engage in the communication
• Once these are assured, SIP establishes call
  parameters at either end of the communication,
  and handles call transfer and termination

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Current Smart Home Technologies

• A bus connects classical smart home functions
  (control, command and supervision)
  Telecommunication run separately
• Sensors and other components are connected via
  the bus, which can be implemented over twisted
  pair or power lines, or communication through
  wireless links

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Future Smart Home Technologies

• A new generation control systems/procedures and
  protocols
• Improvements based on wireless networks
• Convergence of communication and control
  functions
• Better user adaptation for example with the aid
  of speech technology and user modelling
  techniques
• Services such as monitoring and localization
• Automatic emergency call systems probably based
  on the analysis of human bio signals
• The introduction of intelligent systems like
  smart clothes

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Wearable computers and smart clothes

• Very small computers which can be embedded in
  clothing or carried in some other unobtrusive way
  are essential development of a flexible smart
  home infrastructure