Multimedia Communication Systems

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Multimedia Communication Systems

• Part 2

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The Network

• Internet / Broadband Internet
• 56Kbps for normal dial-up connection and up to
  2Mbps for broadband Internet
• Asynchronous transmission bursty packet
  transmission
• Low available bandwidth
• Internet can subject to many delays

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The Network

• Local Area Network (LAN)
• Asynchronous packet based system
• Provides multipoint desktop conferencing within
  organisation cheaper than ISDN
• Similar to Internet/broadband Internet (using
  Internet Protocol, IP)
• Networks can subject to many delays

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The Network

• Integrated Services Digital Network (ISDN)
• Higher bandwidth and quality than Public Service
  Telephone Network (PSTN)
• Allow multiple digital channels to be operated
  simultaneously
• One important feature of ISDN is its Guaranteed
  Quality of Service (GQoS) - for example a
  384kbit/s link is dialled then unless there is a
  fault condition the full 384kbit/s will be
  provided for the duration of the conference.

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The Network

• High Speed Networks
• Examples
• Asynchronous Transfer Mode (ATM) Network
• JANET Network
• SuperJANET Network
• Offers speed up to 2.4Gbps
• Offers Guaranteed Quality of Service (GQoS) and
  continuous data transfer rate

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Video Transmission (example)

• True colour image 24bits per pixel
• Full screen 640x480 required 7 million bits
  (about 7M bit)
• Smooth video clip at least 25 frames/second (25
  images transmit in a sec)
• So, 7 million bits x25 175 million bits/second
  required (175 Mbps) !!!

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The Compression Standards

• Video conference link requires
• Equipments
• An intervening network to carry the signal
• ITU-T has produced several umbrella
  videoconferencing standards, collectively known
  as the H.3xx videoconferencing standards.

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The Compression Standards
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IP based video conferencing standards
H.261 Video CODECDefines the way in which the
picture information is compressed and coded to
enable transmission over low bandwidth networks.
It is the baseline coding which is mandatory for
most videoconferencing systems to ensure
interoperability at a basic level. G.711To
ensure interoperability between systems G.711 is
the baseline audio coding algorithm. It is
mandatory in most videoconferencing systems. This
coding produces an upper frequency limit of
3.4kHz/s (i.e. telephone quality) and occupies up
to 64kbit/s of data.
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ATM based Video Conferencing standards
More standards (more features are included )
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CODEC

• Coder Decoders (CODECs) used to perform
  digitisation as well as compression and
  decompression
• Available in the forms of hardware and software
• Hardware no burden on CPU
• Software relies on CPU easy to install and
  cheaper

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Block Diagram of an H.323 CODEC