Terms used with Multimedia

Media Types The information associated with the different applications can be either continuous or block-mode 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

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 store-and-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 bit errors

Network Qos Most networks (circuit and packet switched) provide an unreliable service which is also known as a best-try or best-effort 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

In reality the transmission channel used often has a lower bandwidth 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 digital-to-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 two-dimensional 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