ESE250: Digital Audio Basics

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ESE250Digital Audio Basics

• Week 12 December 1, 2009
• Networking

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Networked Systems

• Today
• We expect our computers to be networked
• Google, wikipedia, Email, IM,
• Can work stand alone
• Airplane mode?
• But, are crippled when not connected
• Phone isnt a phone unless its networked

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Course Map
Numbers correspond to course weeks
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Outline

• Communicating between machines
• Technology and Costs
• Layering
• Transport
• Network
• Routing and what can go wrong and what to do
  about it
• Physical
• physical layer independence
• What can go wrong and what to do about it
• Running simulation will punctuate points
• (no interlude)

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Communicating between Machines
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Minimal Setup

• Have two computers
• think raw processors for the moment
• Want them to communicate
• Send an mp3 file

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Physical Connection

• Place an IO peripheral in each computer
• String wire between computers
• E.g. one wire from A?B, another B?A

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Signaling

• Communicate with Voltage pulses
• A pulls line low
• B senses low line
• Encode data as series of pulses or voltages on
  the line

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Communication

• Start program on B to receive data (file)
• Start program on A to send data (file)
• B waits for valid characters
• A sends data
• B receives
• A sends out-of-band signal to indicate end of data

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Multiple Tasks

• Could have a separate interface/wire for each
  application
• Process allocates hardware when needs to
  communicate
• Holds exclusive access during transfer

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Connect to Multiple Machines

• Add interface/wire for every machine want to talk
  to
• Talk to machine through its dedicated wire

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Scalability

• Do we like where this is going?
• How many machines are there?
• July 2008 570M
• http//www.ciolek.com/GLOBAL/2000ten.html

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How many connections?

• What machines do you connect to regularly?
• E-mail server
• Eniac
• Web servers?
• Yahoo, cnn, google, YouTube, facebook
• iTunes
• How many friends do you IM?

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Technology and Costs
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Wires

• How fast can I send data over a wire?
• Consider a Category-5 ethernet cable
• Bandwidth (bits/s)
• 100Mbits/s 100Base-T (100Mb ethernet)
• 1Gbit/s 1000Base-T (Gigabit ethernet)
• Latency/transit time (distance/time)
• 0.64 c cspeed of light 3108 m/s
• 0.192 m/ns or roughly 5ns/m

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Comparison computer parts

• Transit time to back of room?
• 5m ? 25ns
• 3GHz processor ? 75 machine instructions
• Hard Solid-State Disk Bandwidth
• 1.52 Gbits/s

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Comparison Audio

• Real-Time, single-channel Audio
• 44KHz _at_ 16b 504Kbits/s
• How many can share 1Gbit/s link?
• 1Gbits/s / 0.5Mbist/s 2000
• Real-Time single-channel MP3
• 64Kbist/s
• How many can share 1Gbit/s link?
• 1Gbits/s / 64Kbits/s 16,000

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Comparison Video

• HDTV (uncompressed)
• 1920x1080 2M pixel 316b/pixel
• 100Mb/frame
• 30 frames/second
• 3Gbits/second
• HDTV compressed
• Around 36Mbits/s
• 1Gbits/s / 36Mbits/s 30

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Costs

• Cat 5e per foot 0.20/foot
• Say 0.6/m
• Raw wire
• Ignoring handling to run
• Ignoring rent/lease/buy land to run
• Philly ? San Francisco 4,000km
• 2.4M for wire run

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Implications?

• Todays wire bandwidth exceeds the throughput
  needs of any real-time single-stream data
• Can afford to share the wire
• Wires not cheap
• Cannot afford not to share the wire

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Sharing (Virtualizing) Connections
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Sharing Link

• Idea Tag data with target
• this is for process 34
• this is for process 45
• Have transport layer deal with
• Mixing data from separate streams
• Separating data out into individual streams
• Delivering to individual processes

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Packet

• Begin to form a packet
• Header says where to go
• Data payload the data to send
• Header
• Added, consumed by network handling in routing
• Data payload
• Only thing seen by the application processes

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Packets
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Transport Layer

• Call this the Transport Layer
• responsible for delivering data to the individual
  application process on the computer

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Possible Role Assignment
SA1
SA2
SA3
SA4
T1
T3
N2
N1
R1
W1/R2
R3
T2
N2
N4
T4
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Simulation 1
SA1
SA2

• Send 4 verses or digits from each
• from song-server-app, p-server-app
• to song-listener-app, p-consumer

T1
W1
T2
CA1
CA2
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Virtualize Physical Wires
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Start Simple

• Add more computers to same pair of wires
• Like the memory bus
• How do I know who the message is for?

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Extended Packet

• Extend our packet header
• Destination computer
• Process on destination computer
• Sending computer
• Process on sending computer

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Network Layer

• responsible for end-to-end (source to
  destination) packet delivery

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Virtualization Effect

• Each pair of processes on different computers
• Has the view of a point-to-point connection

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(No Transcript)
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Simulation 2

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients

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Simulation 2
N1
N3
W1
N2
N4
T4
CA3
CA4
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Simulation 2
N1
N3
W1
N2
N4
T4
CA3
CA4
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Extending the Virtual Link
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Indirect Connections

• A and B are connected
• B and C are connected
• How get message from A to C?

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Indirect Connection

• Run process on B to forward messages from A to C

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Routing

• B runs a general program
• If packet destined for B, takes it
• Otherwise, sends on to (toward) destination
• Extension of the network handling process that is
  sorting data to processes

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Routing
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Reachability

• If everyone plays along
• Can communicate with any computer reachable
  transitively from my computer
• Dont need direct connections

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Routing ? Route Tables

• To make efficient
• Each computer should route close to destination
• and not route in circles
• E.g. compute all-pairs shortest paths
• Store result, each machines knows where to send
  packet next
• How much storage?
• Cleverness to compress/summarize
• What happens when links break, new computers
  added?
• Cleverness to incrementally recompute

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Network Layer

• responsible for end-to-end (source to
  destination) packet delivery including routing
  through intermediate hosts

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Simulation 3

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients

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Sim 3
R1
R2
N2
N4
T4
CA3
CA4
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Simulation 4

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients
• Roles
• 4 server apps
• Network Interface for each of 2 servers
• 3 routers ???
• Network Interface for each of 2 clients
• 4 client apps

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Sim 4
R1
R2
R3
N2
N4
T4
CA3
CA4
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Where are we now?

• Can communicate
• From one process on a computer
• To any other process on any other computer
• If the two are transitively connected
• By a set of participating computers which route
  data
• Abstraction
• Processes just see streams of data between the
  endpoints

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Layers
(abstraction)
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Protocols

• Mostly sketched key buildings blocks and
  functionality of IP
• IP Internet Protocol
• Delivery to process (rather than hosts) UDP
• UDP Unreliable Datagram Protocol

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Simulation 5

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients

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Sim 5
R1
R2
R3
R3 delayed
N2
N4
T4
CA3
CA4
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What can go wrong?

• Packets arrive out of order
• Solution?
• Add a sequence number

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Simulation 6

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients

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Sim 6
R1
R2
R3
R3 delayed
N2
N4
T4
CA3
CA4
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Abstracting Physical Layer

• Application, transport, network
• Dont really care how the bits are moved from
  machine-to-machine
• What are other ways we send bits?
• Optically
• RF/wireless
• Pneumatic tubes, passing paper notes, SMS Text
  messages

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Simulation 7

• Send 4 verses or digits from each
• from song-server serving 2 songs
• And digit-server serving 2 fundamental constants
• To two clients
• Roles
• 4 server apps
• Network Interface for each of 2 servers
• 3 routers, connect to both servers and endpoints
• One link is via text messaging
• Network Interface for each of 2 clients
• 4 client apps

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Sim 7
R1
wireless
R2
R3
N2
N4
T4
CA3
CA4
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What else can go wrong?

• Bits get corrupted
• Intermediate machines holding messages can crash
• Messages can get misrouted

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(No Transcript)
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Data Corruption

• How do we deal with data corruption?
• Use redundancy
• Two strategies
• Use enough redundancy to correct
• Use just enough redundancy to detect it
• Have the sender resend

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Data Corruption

• Relatively uncommon
• Most packets are fine
• We have efficient (low overhead) ways to detect
• Compute a hash of the message data
• Highly unlikely one (few) message bit errors will
  result in same hash
• ? checksum

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Revised Packet

• Header
• Data payload
• Checksum

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Lost Packet

• How can we deal with lost packets?

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Lost Packet Strategy

• Sender sends packet
• But keeps a copy
• Receiver gets packet
• Checks checksum
• OK, uses packet and sends ACK
• got your last packet in tact
• Not ok, discard packet
• Sender
• Receives ACK, can discard packet and send next
• No ACK (after timeout), resend packet

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Retransmission Discipline

• Dont depend on receiver to request
  retransmission
• Why?
• Header may be corrupted
• Not deliver to receiver
• Only know receiver got it when it says it got it

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Corrupted ACK

• What if the ack is lost?
• Sender resends
• Receiver receives a second copy
• Oops, dont want that to be interpreted as new
  data
• i.e. send rm cd ..\n
• Receive rm cd ..\n rm cd ..\n

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Avoid Duplication

• How can we avoid duplication?

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Accommodating Duplication

• Use packet sequence number
• Keep track of last packet received
• If receive packet again,
• Discard the packet

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Sequence Numbers
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TCP

• TCP Transmission Control Protocol
• Provides Reliable delivery
• Deals with
• Retransmission
• Duplication
• Out of sequence / resequence / reconstruction

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Transport Layer

• Call this the Transport Layer
• responsible for reliably delivering data to the
  individual application process on the computer

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Layers
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Learn More

• Online reading/pointers
• Courses
• ESE404 Intro Networks and Protocols
• ESE575 Intro to Wireless Systems

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Big Ideas

• Sharing Network interface, wires
• Virtualization datastream abstracts physical
  point-to-point link
• Layering
• Divide-and-conquer functionality
• Implementation hiding/technology independence
• Reliable communication link from unreliable
  elements