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

Net 1

Net 4 Net 6

Net 5Net 3

Net 2

Router 1

Router2

Router3

Host 1 Host 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• MisdeliveredSome 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 at http://www.ics.hawaii.edu/~esb/talk/2001mmin.ppt or http://www.ics.hawaii.edu/~esb/talk/2001mmin.html