Download Tutorial - University of Hawaii

Multimedia on the Internet
Edo Biagioni
University of Hawaii at Mānoa
Information and Computer Sciences
Overview of the Tutorial
Multimedia on the Internet
• Encoding Principles
• Storage and Transmission
• Media Characteristics
How the Internet works
• Protocols, including TCP/IP
• How to build an Internet
Part 1: Multimedia on the Internet
Types of Multimedia
• Voice and – generally – sound
• Still Images
• Video
Use for Internet
• Live communication
• Storage, Retrieval, Backup etc
Multimedia -- Outline
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Compression
Quality of Service
Real-Time and non-Real-Time
Relevant Internet Characteristics
Specific Formats: GIF, MPEG
Voice Quality and Compression
• Telephone-Quality uncompressed voice:
64Kb/s -- 8KByte every second: 8000
samples/second, 1 byte/sample
• high-quality sound: 40,000
samples/second, 2 bytes/sample, 80,000
bytes/second, 160,000 Kb/s
• Compression can reduce this data rate
considerably, e.g. MP-3 (MPEG version 3)
Compression
• Lossless compression: eliminate
redundancies in the data stream,
reconstruct the original data exactly
• Lossy compression: eliminate
“unimportant” data in the data stream,
reconstruct an approximation of the
original data
• Lossy compression gives better
compression, lossless gives better signal
Lossless Compression
• Sound at one millisecond usually
resembles (same frequency and volume)
the sound in the next few milliseconds
• So, only send the difference, using fewer
bits
• If all the preceding data has been received
correctly, can reconstruct the sound
• Note: more vulnerable to data losses
Lossy Compression
• Humans “hear” more information in the
lower frequencies (down to about 20Hz)
• Use more bits to encode information about
lower frequencies, fewer bits for
information about higher frequencies
• Humans think reconstructed signal sounds
the same as (or similar to) the original
• High quality requires high data rate
Quality of Service -- Scenario
• Voice over IP sends real-time voice data
over the Internet (or an Intranet)
• If the network is congested, it will start
discarding packets
• Discarding VoIP packets can be very
disruptive
• We need a way to tell the network what
kind of service we need
Quality of Service – Traffic Types
• Data traffic is best effort: if packets are dropped,
we will recover, not much disruption
• Uncompressed voice (video) is constant bit rate
(CBR): each second produces n bits
• Compressed voice (video) is variable bit rate
(VBR): the number of bits produced each
second varies with the data being sent
Quality of Service – Other Factors
• Bandwidth: higher for video, not exactly
predictable for VBR traffic
• Burstiness: what is the maximum shortterm bandwidth?
• Delay: can the network guarantee delivery
in 100ms or less?
• Error rate: can the network guarantee no
packet loss? At most 1% packet loss?
Real-Time Multimedia
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Interactive Communication
Voice
Voice and Video
“Shared Workspaces”: drawings,
documents, bank accounts, etc.
• Near-real time: Real-time source, but a
delay is acceptable (e.g. pay-per-view)
Non-Real-Time Multimedia
• Image databases
• Napster and friends
• Distance Learning
Multimedia over the Internet:
A fundamental Choice
• Data for the internet will be broken into
small (a few K Byte) packets before
transmission
• Some of these packets may be lost
If we retransmit, we lose (real-) time
If we don’t retransmit, we lose data
Adapting to Packet Loss
• A real-time stream can adapt to lost (or
delayed) packets by reproducing the most
recent signal (e.g. tone, screen), giving
graceful quality degradation
• A non-real-time stream can buffer n
seconds of data while retransmission
occurs
No strategy works if many packets are lost
Other Internet Characteristics
Pros
• Nearly Universal Connectivity
• Well-defined, open standards
Cons
• Congestion is essentially unpredictable
• QoS is (essentially) nonexistent
• Large variations in available bandwidth
Internet of the near future
Multicast
• Efficient single sender, many receivers
• Adaptive to congestion, to variable QoS
IPv6
• More addresses, networks
• Autoconfiguration
• Encryption and Authentication
Specific Formats
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Graphic Interchange Format (GIF)
Joint Picture Experts Group (JPEG)
Moving Picture Experts Group (MPEG)
All three are lossy compression of images
(MPEG is for motion pictures)
GIF
• Most (small) images have a small number
of distinct colors
• Pick 256 of the most common colors, and
use them to encode each pixel in one byte
-- approximate where necessary
• Use a lossless compression (LZ) over the
resulting pixel values
JPEG
• Divide image into 8x8 blocks
• Do a spatial-frequency analysis on each
block (Discrete Fourier Transform, DFT)
• Use more bits for the lower spatial
frequencies (is the picture light or dark?)
than for the higher spatial frequencies
(where is that edge?)
• Run-length encoding efficiently codes
“zero” frequencies (no energy)
JPEG – What does it all Mean?
• Many 8x8 blocks
have very simple
structure (in the
frequency domain)
• Others have only a
few relevant features,
captured in few nonzero frequency
components
→Good compression
MPEG
• Encode each frame in a manner similar to
JPEG: this is an I (Intrapicture) frame
• Successive frames tend to resemble each
other, so only encode the differences from
the I frame: this is a P (Predicted) frame
• Predict in both directions: a B
(Bidirectional) frame
MPEG Characteristics
• Excellent compression!
• You need the preceding I frame to
reconstruct P or B frames
• You need the following P or I frame to
reconstruct a B frame
• I frames tend to be the largest
• Example: one I frame , 3 B frames, 1 P
frame, 3 B frames, 1 P frame, 3 B, 1 I
MPEG Characteristics (continued)
• B frames cannot be used for real-time
(forward prediction is difficult)
• Good encodings take time (or hardware)
• Loss of an I frame means loss of all the P
frames that follow, and the B frames
immediately before and after
• If we have priorities or QoS, we can mark I
frames as “more important” so the network
will only drop them as a last resort
Multimedia Summary
• Many different types of traffic
• Lossy compression makes bandwidth
demands tolerable, introduces issues
• Real-time is not well-suited to congested
Internet, works well on uncongested
networks
• QoS describes both type of traffic and type
of service
Part II: The Internet
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Basic Internet Connectivity
End-to-end data transfers
What is the Internet? Internet standards
What can (or can’t) the Internet run on?
Other technologies: Ethernet, ATM,
Frame Relay, Modems, Cable, Wireless,
Cellular
Basic Internet Connectivity
We are on the Internet if:
• We can send/receive Internet Protocol
packets (IP packets)
• to another machine which is connected to
the Internet: a router
• Recursive, decentralized definition
• Any machine that forwards packets among
two networks is a router
Internet Example
Net 1
Host 1
Net 3
Router
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Host 2
Router 1
Net 2
Net 5
Net 4
Router
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Net 6
Routing
• The IP address has a network part and a
host part
• A router must know where to forward
packets destined for other networks
• To do so, it runs a routing protocol: RIP,
OSPF, BGP, etc
• Manual configuration is also possible
• Most hosts have a default route(r)
Internet Protocol
• Each packet carries source and
destination address
• Large packets (up to 65KB) can be
fragmented
• TimeToLive kills packets after n seconds
• Protocol field identifies next higher level
• There are no sequence numbers, no
retransmissions, no error detection
Common IP errors
Packets may be:
• Dropped
• Duplicated
• Delivered out of order
• Delivered with bit errors or missing bytes
• Arbitrarily delayed
• Misdelivered
Some of these are infrequent, but observed
Types of Internet Addresses
• IP address: 1.2.3.4 is a dotted decimal
notation, where each decimal (0..255)
represents one byte of a 4-byte address
• DNS name: www.biagioni.org
• “hardware” addresses such as
00:FF:33:53:78:21 (Ethernet)
• IPv6 addresses: 128-bit address, hex
1234:5678:9ABC:DEF0::33:5AB8
More on Internet Addresses
The above addresses are globally unique
An address can be permanent or temporary
Network Address Translation (NAT): a
single machine forwards packets for other
machines that don’t have an IP address,
pretending the packets are from itself
Firewalling is (usually) NAT with additional
checking
Domain Name System
• Independent of IP addresses
• Hierarchical system
• Each DNS name can be mapped to one IP
address
• Authoritative DNS servers maintain the
mapping for each organization
• Servers will query each other, and clients
will query servers, to find the mapping
End-to-end Data Transfers
Transmission Control Protocol:
• Reliable (always delivers if possible)
• Stream-Oriented (re-packetizes)
• Congestion control lessens congestion
User Datagram Protocol
• Very simple and efficient
• Unreliable, packet oriented
User Datagram Protocol
• Good for Real-Time
• Same classes of errors as for IP, except
can protect against data corruption and
mis-delivery
• Users of UDP must be prepared to deal
with packet loss
• Users of UDP should be prepared to slow
down if congestion is present
Transmission Control Protocol
• Used by most services, such as WWW,
email, telnet/ssh
• Usually, sub-second response
• Used for reliable transfers, e.g. files
• Adapts to congestion by slowing down
• Connection-oriented: connection
established before any data is sent
TCP and UDP Ports
• Ports are used to identify the intended
servers (also for UDP) – some ports are
“well known” to correspond to specific
services
• HTTP (www) is usually on port 80, but
• http://host.com:6431/file identifies a server
on port 6431
Internet Standards
• IP, TCP, DNS are all open standards
• Defined by Internet Engineering Task
Force (IETF), mostly via email longdistance communication
• “Rough consensus and running code”
• Originally “Request For Comments”, RFC
• Available online at www.ietf.org
• Delegate power to other organizations: IP,
DNS addresses
The Internet connects other
Networks
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IP doesn’t say how we reach our router
Carrier pigeon? (RFC 1149)
Serial Line (SLIP, PPP)
Broadcast Medium (Aloha, Ethernet,
Cable, Wireless, Cellular)
• Telephone Lines (Modems)
• Virtual Connections (ATM, Frame Relay)
• Satellite point-to-point
Ethernet
• Can broadcast or send to a specific
address
• How do I find the address of my router?
• I broadcast a request containing my
router’s IP address: Address Resolution
Protocol, ARP
• Speeds: 10Mb/s, 100Mb/s, 1Gb/s, 10Gb/s
• Hubs connect computers to each other to
form a Local Area Network: LAN
Aloha and Satellites
• When two Ethernet NICs (network
interface cards) send at the same time,
they can detect the collision and retransmit
• When sending to a satellite, senders
cannot detect a collision
• Satellite broadcasts to everyone, so a
ground station can retransmit if it doesn’t
see its message in the broadcast
ATM: Asynchronous Transfer Mode
• Much better support than IP for real-time,
multicast, and QoS
• Point-to-Point, Point-to-Multipoint, and
Multipoint-to-Multipoint virtual channels
allow reservation of QoS for a specific
stream of traffic
• No support for broadcast, so hard to ARP
• Virtual Private Networks, VPNs
Frame Relay
• ATM has fixed-size cells (48-byte
payloads)
• Frame relay has frames large enough to
carry an average-sized packet
• Like ATM, FR uses virtual circuits,
requiring a connection setup step before
transmission can occur
Modems and Cable Modems
• Carrying data over a medium (a wire) with
given frequency characteristics
• Encode the bits in such a way that they
are received at the opposite side
• Framing: where is the start of a packet?
• Byte framing: where is the start of a byte?
Wireless Internet
• Connect a modem to a radio to give a
wireless connection
• Collisions are worse than for Ethernet but
better than for Aloha, since detection is
faster
• 802.11 protocol (up to 11Mb/s, 100+m)
• In the future, maybe Bluetooth (10m)
Cellular Internet
• Connect a modem to a cell phone to give
cellular internet access
• Cellular protocols know how to avoid
collisions
• Many cellular protocols are connectionoriented, reserving the bandwidth even
when it is not needed
Summary
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Many choices for encoding multimedia
Can choose reliability or speed, not both
Multicast, IPv6 may help
Understanding the Internet may help
understand multimedia performance
• These slides are at
http://www.ics.hawaii.edu/~esb/talk/2001mmin.ppt or
http://www.ics.hawaii.edu/~esb/talk/2001mmin.html