Multimedia on the internet
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Multimedia on the Internet. Edo Biagioni University of Hawaii at M ānoa 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

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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

  • 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 short-term 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

  • 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

  • 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 non-zero 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

  • 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

Router

2

Net 3

Net 5

Net 1

Host 1

Host 2

Router 1

Router

3

Net 6

Net 4

Net 2


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 long-distance 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

  • 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 connection-oriented, reserving the bandwidth even when it is not needed


Summary

  • 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 athttp://www.ics.hawaii.edu/~esb/talk/2001mmin.ppt or http://www.ics.hawaii.edu/~esb/talk/2001mmin.html


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