Transcript Document 7281631

Lecture 7
Communications Basics (Part I)
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
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
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
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
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 crosscoupling of electrical signals between adjacent wires in the
same cable. This is known as crosstalk
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)
Transmission Media – Unshielded
twisted pair
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
Transmission Media – Unshielded and
Shielded twisted pair
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
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
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
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
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
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
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
Transmission Media – Optical fibre
transmission media
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
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 VSAt’s 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
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
Transmission Media
Transmission Media – Satellite System:
data communications
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
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
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
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
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
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
Delay distortion - ISI
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
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
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
Asynchronous Transmission – principle
of operation
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)
Frame synchronization
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
Frame synchronization
Smart Home Technologies
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
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
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.
Internet Refrigerator?
• South Korea’s 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.
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
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
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
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
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
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.
• 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.
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
• 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 party’s 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
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
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
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