Download Packet mode

Terms used with Multimedia
Media Types
• The information associated with the different
applications can be either continuous or blockmode
• Continuous: The information is played out
directly as it is received continuously (called
streaming or real-time media) (E.g Audio and
video)
•Block-mode: The source information is created
in a time-independent way and is often stored at
the source in, say, a file
• When requested it will be transferred across the
network and displayed at a time specified by the
requesting application (called downloading)
(e.g. email consisting of a block of text)
Communication Modes
• Simplex: The information associated with the
application flows in one direction only.
• Half-Duplex: Information flows in both directions but
alternatively (two-way alternative).
• Duplex: Information flows in both directions
simultaneously (Two-way simultaneous).
Communication Modes
• Broadcast: The information output by a single node is
received by all the other nodes connected to the same
network
• Multicast: The information output by the source is
received by only a specific subset of the nodes (Latter
form known as multicast group)
Communication Modes
Communication mode Examples
• In half-duplex and duplex communications,
the bit rate associated with the flow of
information in each direction can be equal
(symmetric) or different (asymmetric).
•Video Telephony – Symmetric duplex
communication
• Web browsing – Asymmetric half-duplex
mode (as different bit rates for downloading
and uploading)
Network Types (Circuit-mode)
• This operates in a time-dependent manner
and comprises an interconnected set of
switching offices/exchanges to which the
subscriber terminals/computers are connected
Circuit-mode- Operational Principle
Step1: The source must set up the connection first
through the network
Step2: Each subscriber terminal has a unique network
wide address and to make a call the source first enters
this number of the intended communication partner
Step3: The local switching office uses this number to
set up a connection. Depending on the availability of
the destination the connection will be estabilished
Step4: Finally at the end of information exchange the
call will be terminated by the source or the destination
Circuit-mode- Terminology
• Signalling messages – The messages associated
with the setting up and clearing of a connection
• Call/Connection setup delay – The delay
associated with the connection procedures
•Examples of Circuit-mode operation – PSTN and
ISDN
•PSTN – setup delay varies from fraction of a
second to few seconds for international connections
• ISDN – setup delay ranges from tens of
milliseconds through to several hundred milliseconds
Packet mode
• There are two types of packet-mode network
- Connection Oriented (CO)
PSE: Packet Switching
Exchanges
• As the name implies a connection is established prior to
information interchange
• The connection utilizes only a variable portion of the
bandwidth of each link and known as virtual circuit (VC)
Packet mode – Operational Principle
• To set up a VC the source terminal sends a call request
control packet to the local PSE which in addition to the
source and destination addresses holds a short identifier
known as virtual circuit identifier (VCI)
•Each PSE maintains a table that specifies the outgoing
link to use to reach the network address
•On receipt of the call request the PSE uses the destination
address within the packet to determine the outgoing link
• The next free identifier (VCI) for this link is selected and
two entries are made in the routing table
Packet mode – Connectionless
• In connectionless network, the establishment of a
connection is not required and they can exchange
information as and when they arrive
•Each packet must carry the full source and
destination address in its header in order for each
PSE to route the packet onto the appropriate
outgoing link (router term used rather than PSE)
Packet mode – Summary
• In both types each packet is stored in a memory
buffer and a check is performed to determine if any
transmission errors are present in the received
message. (i.e 0 instead of a 1 or vice versa)
• If an error is detected then the packet is discarded
known as best-effort service.
• All packets are transmitted at the maximum link bit
rate
• As packets may need to use the same link to
transfer information an operation known as storeand-forward is used.
Packet mode – Summary
• The sum of the store and forward delays in each
PSE/router contributes to the overall transfer delay
of the packets and the mean of this delay is known
as the mean packet transfer delay.
• The variation about the mean are known as the
delay variation or jitter
• Example of connectionless mode – Internet
• Examples of connection oriented network – X.25
(text) and ATM (multimedia)
Multipoint Conferencing
• Multipoint conferencing is implemented in one of
two ways
- Centralized mode
- Decentralized mode
Centralized mode
• This mode is used with circuit switched networks
such as PSTN and ISDN
Multipoint Conferencing – Centralized
mode
• With this mode a central server is used
• Prior to sending any information each terminal
needs to set up a connection to the server
• The terminal then sends the information to the
server.
• The server then distributes this information to all
the other terminals connected in the conference
Multipoint Conferencing – Decentralized
mode
• The decentralized mode is used with packet-switched
networks that support multicast communications
• E.g – LAN, Intranet, Internet
Decentralized mode Operation
• The output of each terminal is received by all the other
members of the conference/multicast group
• Hence a conference server is not required and it is the
responsibility of each terminal to manage the information
streams that they receive from the other members
Hybrid Mode
• This type of mode is used when the terminals
are connected to different network types
• In this mode the server determines the output
stream to be sent to each terminal
Network Qos
• Network Quality of Service parameters: Operational
parameters associated with a communication channel
through a network that determine the suitability of the
channel in relation to its use for a particular application
• Circuit-switched network: Bit Error Rate (BER) is the
probability of a bit being corrupted during its
transmission in a defined time interval. The transmission
delay is determined by the bit rate used plus the codes
(network interfaces) and propagation delay of the digital
signal
•Packet-switched network: Mean packet transfer rate is a
measure of the average number of packets transferred per
second. Mean Packet Error Rate (PER) is the
probability of a received packet containing one or more
Network Qos
• Most networks (circuit and packet switched) provide an
unreliable service which is also known as a best-try or besteffort service
• If the application accepts only error free blocks then it is
necessary for the sending terminal to divide the source
information into blocks of a defined maximum size and the
destination to detect any missing blocks
•When a block is missing then the destination must request for
a copy of the block from the source. The service is then called
a reliable service
•
Application Qos
• Transmission of
a constant bit rate
stream over a
packet switched
network
• The startup delay defines the amount of time
that elapses between an application making a
request to start a session and the confirmation
being received at the destination
Application Qos
• To transfer a large file from the server to your
home computer using the packet switched (PW) and
circuit switched (CS) networks
- PSTN (28.8kbps) and ISDN (64/128kbps) operate
in CS mode and provide constant bit rate channel
- Cable modem operate in PS mode and the bit rate
of the shared channel is 27Mbps
Application Qos
• Assuming the file size is 100Mbits, the
minimum time to transmit the file using the
different Internet access modes is:
- PSTN and 28.8 kbps modem: 57.8 minutes
- ISDN at 64 kbps:
26 minutes
- ISDN at 128 kbps:
13 minutes
- cable modem at 27 Mbps:
3.7 seconds
Application Qos
• The application quality of service is different
from the network QoS
• For example in an application involving
images the parameters may include a minimum
image resolution and size while a video may
include the digitization format and the refresh
rate
Application QoS - Parameters
• The required bit rate or mean packet transfer
rate
•The maximum startup delay
•The maximum end-to-end delay
•The maximum delay variation/jitter
• The maximum round-trip delay
Application Qos
• To overcome the effect of jitter a technique
known as buffering is used
•The effect of jitter is overcome by retaining a
defined number of packets in a memory buffer
at the destination before playout of the
information bit stream is started
Application QoS - Summary
• In order to determine whether a particular
network can meet the QoS requirements of an
Application a number of standard application
service classes have been defined
• Each service class has an associated set of
QoS parameters defined
• For networks that support different service
classes ( i.e internet), the packets relating to
each class are assigned a different priority
• Real time streams have higher priority than
packets relating to email
Summary
Summary
Summary – Multimedia Communication
Network and Services
Multimedia Information Representation
• Multimedia Information is stored and processed
within a computer in a digital form
• Codeword: Combination of a fixed number of bits that
represents each character, in the case of textual
information
• analogue signal: Signal whose amplitude (magnitude
of the sound/image intensity) varies continuously with
time
• Signal encoder: Electrical circuit used for the
conversion of an analogue signal into a digital form
• Signal decoder: Electrical circuit that converts stored
digitized samples into time-varying analogue form
analogue Signals
• As mentioned earlier the amplitude of the signal
varies continuously with time
• The Fourier analysis can be used to show that any
time varying signal is made up of infinite number of
single-frequency sinusoidal components
• The range of frequencies of the sinusoidal
components that make up the signal is called the
signal bandwidth
• Speech bandwidth: 50Hz – 10kHz
• Music Bandwidth: 15Hz – 20kHz
analogue Signals –Signal Properties
analogue Signals –Signal Properties
• To transmit an analogue signal through a network
the bandwidth of the transmission channel should be
equal to or greater than the signal bandwidth
• If the bandwidth of the channel is less than the
signal bandwidth than channel is called the
bandlimiting channel
Encoder Design
• The Encoder consists of bandlimiting filter
and an analogue-to-digital converter (ADC) (
comprising sample and hold + quantizer)
Encoder Design
• Bandlimiting filter: Removes the selected higher
frequency components from the source signal
• Sample and hold Circuit: Samples amplitude of the
filtered signal at regular intervals and holds the
sampled amplitudes between samples
•Quantizer: Converts the samples into their
corresponding binary form
Encoder Design – Data representation
• The most significant bit of the codeword
represents the sign of the sample
• A binary 0 indicates a positive value and a
binary 1 indicates a negative value
• The signal must be sampled at a much higher
rate than the maximum rate of change of the
signal amplitude
• The number of quantization levels should be as
large as possible to represent the signal accurately
Sampling Rate
• Nyquist sampling theorem: To obtain an accurate
representation of a time-varying analogue signal, its
amplitude must be sampled at a minimum that is
equal to or greater than twice the highest sinusoidal
frequency component that is present in the signal
• Nyquist rate is represented either in Hz or more
correctly in samples per seconds (sps)
• Antialiasing filter: Another name for bandlimiting
filter. Since it passes frequencies that are within the
Nyquist rate
Alias signal generation due to undersampling
• In reality the transmission channel used often has a
lower bandwidth
• To avoid distortion the source signal is first passed
through the BLF which is designed to pass only the
frequency components that are within the channel
bandwidth
• This avoids alias signals caused by undersampling
Quantization Intervals
• Representation of the analogue samples require an
infinite number of digits
Quantization Intervals
• Three bits are used to represent each sample ( 1 bit for the
sign and two bits to represent the magnitude)
• If Vmax is the maximum positive and negative signal
amplitude and n is the number of binary bits used then the
quantization interval, q, is defined as
q = 2Vmax/ 2n
• A signal anywhere within the quantization interval will be
represented by the same binary codeword
• Each cordword is at the centre of the corresponding
quantization interval
• Therefore a difference of q/2 from the actual signal level
is present. This difference is known as the quantization error
Quantization noise polarity
• Quantization error is the difference between the
actual signal amplitude and the corresponding
nominal amplitude (also known as quantization
noise since values vary randomly)
Dynamic Range
• With high-fidelity music it is important to be able
to hear very quiet passages without any distortion
created by quantization noise
• Dynamic range is defined as the ratio of the
maximum signal amplitude to the minimum.
D = 20 log10 (Vmax/Vmin) dB
Decoder Design
Encoder+decode= Codec
• A signal decoder is an electronic circuit that
performs the conversion prior to their output back
again into their analogue form through a digitalto-analogue converter and a low pass filter
• Low-pass filter: Only passes those frequency
components that were filtered through the
bandlimiting filter in the encoder
Text
• Unformatted text: Known as plain text; enables pages
to be created which comprise strings of fixed-sized
characters from a limited character set
• Formatted Text: Known as richtext; enables pages to
be created which comprise of strings of characters of
different styles, sizes and shape with tables, graphics,
and images inserted at appropriate points
• Hypertext: Enables an integrated set of documents
(Each comprising formatted text) to be created which
have defined linkages between them
Unformatted Text – The basic ASCII character set
• Control characters
(Back space, escape,
delete, form feed etc)
• Printable characters
(alphabetic, numeric,
and punctuation)
• The American Standard Code for Information
Interchange is one of the most widely used character sets
and the table includes the binary codewords used to
represent each character (7 bit binary code)
Unformatted Text – Supplementary set of Mosaic
characters
• The characters in columns 010/011 and 110/111 are
replaced with the set of mosaic characters; and then
used, together with the various uppercase characters
illustrated, to create relatively simple graphical images
Unformatted Text – Examples of
Videotext/Teletext
• Although in practice the total page is made up of a
matrix of symbols and characters which all have the
same size, some simple graphical symbols and text of
larger sizes can be constructed by the use of groups of
the basic symbols
Formatted Text
• It is produced by most word processing packages and
used extensively in the publishing sector for the
preparation of papers, books, magazines, journals and so
on..
• Documents of mixed type (characters, different styles,
fonts, shape etc) possible.
•Format control characters are used
Hypertext – Electronic Document in hypertext
• Hypertext can be used to create an electronic version of
documents with the index, descriptions of departments,
courses on offer, library, and other facilities all written in
hypertext as pages with various defined hyperlinks
Hypertext – Electronic Document in hypertext
• An example of a hypertext language is HTML used to
describe how the contents of a document are presented
on a printer or a display; other mark-up languages are:
Postscript, SGML (Standard Generalized Mark-up
language, Tex, Latex
Images
• Images include computer-generated images (referred
to as computer graphics or simply graphics) and
digitized images of both documents and pictures
• All types of images are displayed in the form of a twodimensional matrix of individual picture elements
(pixels or pels), but represented differently within the
computer memory (file)
• Each type of these images is created differently
Graphics
• VGA is a common type of display that consists
of a matrix of 640 horizontal pixels by 480
vertical pixels with for example, 8 bits per pixel
which allows each pixel to have one of 256
different colours
Graphics
• Colouring a solid block
with the same colour is
known as rendering
• All objects are made up of a series of lines that are
connected to each other and, what appear as a curved
line, in practice is a series of short lines each made up
of a string of pixels
• Each object has a number of attributes associated
with it. These include its shape, size in terms of pixel
position, colour of the border etc..
Graphics - Conclusions
• There are two forms of representation
- high-level representation (similar to a source
code of a program) – requires less memory to store the
image and less bandwidth for transmission
- actual picture image of the graphic ( similar to
the low-level machine code and generally known as
bit-map format) – e.g. GIF (graphical interchange
format), TIFF ( tagged image format)
• A graphic can be transferred over the network in
either form
• A software called SRGP (simple raster graphics
package) - used to convert high-level form into a
pixel-image form
Digitized Documents- Fax Principles
• The scanner associated with fax machines operates
by scanning each complete page from left to right to
produce a sequence of scan lines that start at the top of
the page and end at the bottom
• Vertical resolution is either 3.85 (100 lines) or 7.7
mm (200 lines)
Digitized Documents- Digitization format
• Fax machines uses a single binary digit to represent
each pel, a 0 for a white pel and a 1 for a black pel.
Hence the digital representation of a scanned page
produces a stream about 2 million bits
• Single binary digit per pel means fax machines are
best suited for bitonal images
Colour Derivative Principles – additive colour
mixing ( R + G + B)
•Black is produced when all three primary colours
(R,G,B) are zero.
• Useful for producing a colour image on a black
surface as is the case in display applications
Digitised Pictures- Subtractive colour mixing
• White is produced when the three chosen
primary colours cyan,magenta and yellow are all
zero
• Useful for producing a colour image on a white
surface as is the case in printing applications
Digitized Pictures- Television/computer monitor
principles
• The picture tubes used in most television sets
operate using what is known as a raster-scan;
this involves a finely-focussed electron beam
being scanned over the complete screen
Digitized Pictures- Raster Scan
• Progressive scanning is performed by repeating
the scanning operation that starts at the top left
corner of the screen and ends at the bottom right
corner follows by the beam being deflected back
again to the top left corner
Digitized Pictures – Raster scan display architecture
Digitized Pictures-Pixel format on each scan
• The set of three related colour-sensitive
phospors associated with each pixel is called a
phospor triad and the typical arrangement of the
triads on each scan line is shown
Digitized Pictures – Concepts
• Frame: Each complete set of horizontal scan lines
(either 525 for North & South America and most of
Asia, or 625 for Europe and other countries)
•Flicker: Caused by the previous image fading from the
eye retina before the following image is displayed, after
a low refresh rate ( to avoid this a refresh rate of 50
times per second is required)
• Pixel depth: Number of bits per pixel that determines
the range of different colours that can be produced
• Colour Look-up Table (CLUT): Table that stores the
selected colours in the subsets as an address to a location
reducing the amount of memory required to store an
image
Digitized Pictures
• Aspect Ratio: This is the ratio of the screen
width to the screen height ( television tubes and
PC monitors have an aspect ratio of 4/3 and wide
screen television is 16/9)
Digitized Pictures – Screen Resolutions
• NTSC = 525 lines per frame (480 Visible)
• PAL,CCIR,SECAM=625 lines ( 576 visible)
•Example display resolutions: VGA
(640x480x8), XGA (1024x768x8) and SVGA
(1024x768x24)
Digitized Pictures – Colour Image Capture:
Schematic
• Typical arrangement that is used to capture
and store a digital image produced by a scanner
or a digital camera (either a still camera or a
video camera)
Digitized Pictures – Colour Image Capture:
Schematic
• Photosites: Silicon chip which consists of a two
dimensional grid of light-sensitive cells, which stores
the level of intensity of the light that falls on it
• Charge-coupled devices (CCD): Image sensor that
converts the level of light intensity on each photosites
into an equivalent electrical charge