David G. Messerschmitt

1
Chapter 20

• by
• David G. Messerschmitt

2
Outline

• Industry structure
• Communication link characteristics
• Mitigating the impact of a link
• Compression
• Caching
• Mobile code

3
Industry Structure

• At the link layer, bits are bits
• Voice over IP
• IP over voice
• New technologies for access links
• Deregulation
• Turbulent times ahead

4
IP over Voice
Public telephone network
?
Computer w/modem
Computer w/modem
Gateway
?
?
Internet
5
Voice over IP
Public telephone network
?
Plain old telephone
Gateway
IP telephone
?
?
Internet
6
Communication Link

• by
• David G. Messerschmitt

7
Simplest link fiber optics
Speed of light
Absence of pulse 0
Pulse of light 1
8
Key Concepts

• Bitrate
• bits accepted per unit of time
• Transmission time
• Time to get all bits transmitted on link
• Propagation delay
• Time for each bit to reach destination
• Message latency
• transmission time propagation delay

9
Conveyer belt analogy
Bits waiting to be sent
0
1
Conveyer belt
Bits that have already been received
Constant speed
10
Conveyer belt parameters
Conveyer belt
Speed predetermined by Einstein Length
predetermined by distance Size of block Bits
communicated per block
Where we have control
11
Determinants of Propagation Time

• Speed of propagation
• speed of conveyor belt
• limited by medium
• Distance
• length of conveyor belt
• closer is faster
• Remember this is one link
• Routers/switches introduce extra delay

12
Determinants of Bitrate

• Bandwidth
• size of blocks on conveyor belt
• how rapidly the signal changes
• Spectral efficiency
• number of bits written on each block
• how many distinguishable signal levels
• depends on s/n ratio of medium
• Popular usage Bandwidth bitrate

13
Ways to increase bitrate
Make blocks smaller and put them on belt faster
Conveyer belt
Use blocks with more shades of gray (more bits
per block)
Conveyer belt
14
Wavelength division multiplexing (WDM)
Stack blocks with different colors
Conveyer belt
Today in commercial use 40 wavelengths, each at
10 Gbps
15
Numerical parameters
B Bitrate rate bits put on belt
Conveyer belt
? Propagation delay distance/speed
?B Bitrate x delay bits in transit
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The Graphical Version
Transmit time
Message latency
Message in transit
Propagation latency
Distance
Time
17
Message latency
Message latency Time to transmit message
(m/B) Time for last bit to reach destination
(?) These two components are equal when Message
length bitrate x delay (m ?B)
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Bitrate-limited case
Propagation delay-limited delay
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m ltlt ?B
?
Propagation delay-limited
Entire message in transit most of the time
m gtgt?B
Only part of message in transit at any time
Bitrate-limited
Time
Distance
20
Two situations

• Bitrate limited
• Increasing bit rate decreases message latency
• True for long messages
• Propagation delay limited
• Increasing bit rate has little impact on message
  latency
• True to short messages
• Over time, with technology advances, we become
  delay limited!

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B? number of bits in transit
Bitrate (b/s)
28.8 kb/s ltlt1 b ltlt1 b 891 b 4460 b
1.5 Mb/s ltlt1 b ltlt1 b 4780 b 239 kb
622 Mb/s ltlt1 5 19 Mb 96 Mb
Distance
Across chip One meter Across U.S.A. Halfway
around world
Large!
Assumption v c/2 1.5 x 108 meters/sec)
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Physical Media

• Wired
• fiber optics
• coax
• twisted pair (copper)
• Wireless
• Terrestrial radio
• Satellite (long propagation delays)

23
Fundamental limitation

• Over time
• Bitrates increase with electronics and fiber
  optics advances
• Message latencies become delay-limited
• Further technological advances have little impact
  on application performance
• Exception wireless

24
Mitigating communication bottlenecks

• Bitrate-limited regime
• Compression
• Delay-limited regime
• Caching
• Mobile code
• In the future, these techniques will be very
  important

25
Compression

• Lossless
• Typically 2x or 3x
• Lossy (but imperceptible)
• Easily 10x for audio and 100x for video
• Discards perceptually unimportant information
• Important for todays bitrate-limited regime

26
Caching
Communication bottleneck
Future references (where possible)
Remote data being accessed
Local cache
Predictive caching becomes very important in the
delay-limited regime
27
Mobile code
Send code
Communication bottleneck
Execute locally
Mobile code is especially useful to enable faster
interaction in the delay limited regime
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1
0
1
1
Bitrate-limited case
1
0
1
0
0
0
1
0
1
Propagation delay-limited delay
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Supplements

• by
• David G. Messerschmitt

30
Time
Period T
Frequency f 1/T
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Symbol interval
11
11
11
10
10
10
10
Time
01
01
00
11
11
11
10
10
10
10
Time
01
01
00
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Ethernet
Bus
Switch
Hosts
Tree
Broadcast
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Web server
Wireless access protocol
Proxy
Filter
HTML or XML
WML
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Access time
S
2C
C
Hit rate H
1
S 100?C ? H 0.989