Communication Links: Our First Networked Concept

1
Communication LinksOur First Networked Concept
2
Review

• How do you prevent the distribution of a specific
  file in a file sharing network with the following
  architecture
• Client-server
• Peer-to-peer (P2P) with centralized file list
• Pure P2P
• What happens in a P2P file sharing system when
  those who download dont subsequently make the
  downloaded file available to others?
• If you run a chain of Internet cafes
• What are the advantages of using an ASP for your
  accounting system?
• What are the disadvantages of using an ASP for
  your accounting system?

3
Learning Objectives

• Understand how bits are transmitted
• Understand two key parameters of transmission
• Bitrate
• Propagation delay
• Understand spectrum allocation
• Identify gating factor(s) limiting performance
  and choose appropriate remedies
• Understand protocols for coordination of shared
  media links

4
Communication Links

• Computers work with digital information
• Communication media carry analog signals
• Wired
• Fiber optic
• Coax
• Twisted pair (copper)
• Wireless
• Broadcast via airwaves
• Directional satellite or local transmitter/receive
  r station

5
Digital Communication Over Analog Media?

• In order for computers to communicate with one
  another, we must have a way to convert digital
  information to an analog signal and back again
• The reverse of representing an analog signal,
  such as sound, digitally

6
Representing Sound Digitally

• To represent sound, which is analog, as bits
• Sample at some frequency (x times per second)
• For each sample, n-bits encode signal level
• Recall session 3, slides 32-38 for more detail

Represent/ encode
Interpret/regenerate
Store/manipulate
Bit-string
Bit-string
7
Transmitting Digital Information Over An Analog
Medium

• Represent digital information as analog signals
• Treat the bits as information
• Represent each bit as an analog signal (e.g.,
  sound)

Encode/represent
Decode/ interpret
Transmit
8
Representing 10111001100011100111
11
11
11
10
10
10
10
Time
01
01
00
11
11
11
10
10
10
10
Time
01
01
00
9
Questions
10
Parameters Of Transmission

• Speed of propagation
• Distance
• Bandwidth
• Spectral efficiency

11
Conveyor Belt Analogy
Blocks waiting to be sent
10
11
01
10
11
11
01
10
00
01
00
11
01
10
10
00
Conveyer belt
00
11
01
Blocks that have already been received
12
Key Concepts(With Animations)

• Message latency
• Time to get whole message to receiver
• Equal to transmission time plus propagation delay
• Propagation delay (seconds)
• Time for each bit (e.g., the last) to reach
  destination
• Bit-rate (bits/second)
• Bits accepted per unit of time
• Transmission time (NumBits/Bitrate)
• Time to get all bits accepted
• A function of bit-rate and message size

13
Questions
14
Conveyor Belt Variables
Blocks waiting to be sent
10

• Speed
• Length
• Block size
• Bits per block

11
01
10
11
11
01
10
00
01
00
11
01
10
10
00
Conveyer belt
00
11
01
Blocks that have already been received
15
Determinants of Propagation Delay

• Speed of propagation
• Conveyor analogy speed of conveyor belt
• Music analogy speed of sound through air
• Really limited by medium
• Distance
• Conveyor analogy length of conveyor belt
• Music analogy distance from musician to audience
• Really closer is faster
• Remember were only talking about one
  communication link
• Interconnections (routers/switches) introduce
  extra delay (next weeks topic)

16
Determinants of Bit-Rate

• In popular usage, people treat bandwidth as
  synonymous with bit-rate
• There are actually two factors that determine
  bit-rate
• Bandwidth
• Spectral efficiency

17
Determinants of Bit-Rate

• Bandwidth
• Conveyor analogy size of each block (determines
  how many blocks per inch can be on the belt)
• Music analogy tempo
• Really how often a new signal can be sent (and
  detected at the other end
• Described in terms of the symbol interval, which
  is the time between the start of the signal
  representing one symbol to the start of the
  signal representing the next symbol

18
Bandwidths Analogy
Blocks waiting to be sent
10

• Little blocks high bandwidth

11
01
10
11
11
01
10
00
01
00
11
01
10
10
00
Conveyer belt
00
11
01
Blocks that have already been received
19
Bandwidths Analogy
10
11
01
Blocks waiting to be sent

• Big blocks low bandwidth

01
10
00
01
11
00
Conveyer belt
00
11
01
Blocks that have already been received
20
Determinants of Bit-Rate

• Spectral efficiency
• Conveyor analogy number of bits written on each
  block
• Music analogy number of different notes number
  of volume levels
• Really how many distinguishable signal levels
• Have to know that the difference between two
  signals is intentional, not the product of a
  signal plus random background noise
• Depends on signal-to-noise ratio (s/n) of medium

21
Representing 10111001100011100111
Bandwidth Signal change every interval
Symbol interval
11
11
11
10
10
10
10
Time
01
01
00
11
11
11
10
10
10
10
Spectral Efficiency Having 4 distinguishable
signals allows 2-bit symbols
Time
01
01
00
22
Higher Bandwidth
Bandwidth Signal change every interval
Symbol interval
11
11
11
10
10
10
10
Time
01
01
00
11
11
11
Spectral Efficiency Having 4 distinguishable
signals allows 2 bit symbols
10
10
10
10
Time
01
01
00
23
Questions
24
Digital Versus AnalogDemonstration

• Whisper a bit and pass it down the row
• Each person knows message will be a 1 or a 0
• Whisper a complicated phrase and pass it on
• Key points illustrated
• Less chance of garbling if fewer signals to
  distinguish among (but this is lower spectral
  efficiency)
• Digital regeneration
• If some noise while whispering, but bit
  understood, the noise will not be passed on, just
  the bit
• Contrast with analog amplification, where noise
  gets amplified at each stage (this was a problem
  with early long-distance phone calls now they
  digitize)

25
Summary

• Understand How Bits are Transmitted
• Understand two key parameters of transmission
• Bitrate
• Propagation delay
• Next session spectrum allocation, network
  bottlenecks, sharing communication media

26
Communication Links, Part II
27
Review

• Digital v. analog
• Are wires/air waves digital or analog?
• How do we transmit digital signal over analog
  medium?
• Bit-rate v. propagation
• On a single link, what is the relationship
  between the order in which bits are sent and the
  order in which they are received?
• How do you reduce message latency over a fixed
  distance if speed is given?
• Hint First identify all factors influencing
  latency
• Spectral efficiency
• How many bits can be represented by each signal
  change if there are 4 distinguishable signal
  levels? 8? 256?

28
Learning Objectives

• Understand How Bits are Transmitted
• Understand two key parameters of transmission
• Bitrate
• Propagation delay
• Understand spectrum allocation
• Identify gating factor(s) limiting performance
  and choose appropriate remedies
• Understand protocols for coordination of shared
  media links

29
Sharing Communication Media

• Any medium is capable of sustaining one signal
  with a huge bandwidth
• For example, the air can transmit signals with
  very low frequencies (20Hz is a low audio
  frequency) to very high frequencies (100 GHz is
  the start of visible light, 1025 is around the
  top of the gamma ray frequencies)
• Often more useful to divide the medium into
  several channels with a fraction of the available
  bandwidth
• For example, radio frequencies range from 100KHz
  to 100 GHz FM radio can be broadcast between
  88-108 MHz
• Modulation and spectrum allocation help to do this

30
Modulation

• Analog signal with bandwidth f
• Modulate onto carrier signal of frequency g
• E.g., FM Radio
• Each channel has bandwidth 200 kHz .2MHz
• Station WUOM Carrier signal 91.7 MHz .1
• Many carrier signals can be used simultaneously
• Decoder (e.g., FM tuner) knows
• Where in overall signal to look
• How to demodulate desired signal
• While ignoring everything else
• Many modulation methods exist sender and
  receiver must agree

See also http//www.howstuffworks.com/radio-spectr
um.htm/printable
31
Amplitude Modulation
Amplitude of modulated signal is a function of
the carrier signal amplitude and the signal
amplitude
Carrier signal
Signal
Modulatedsignal
32
Frequency Modulation
Frequency of modulated signal is a function of
the carrier signal frequency and the signal
amplitude
Signal
Modulatedsignal
Carrier signal
33
Spectrum Allocation

• Multiple users of nearby carrier signals appear
  as noise to each other
• More difficult to extract signals
• Broadcast spectrum use is regulated to avoid
  conflicts
• Allocation of frequency bands to particular uses
  and users
• 91.7 allocated to WUOM
• 91.6 not allocated why?

34
(No Transcript)
35
Diagnosing Bottlenecks

• Determining which factor is functioning as a
  bottleneck in a communication link means knowing
  which is causing more of the total message
  latency, bit-rate (transmission time) or
  propagation delay

A real bottleneck limits the flow of liquid out
of a bottle
36
Diagnosing Bottlenecks
1
0
1
Animated comparison
1
Bitrate-limited case
1
0
1
0
0
0
1
0
1
Propagation delay-limited delay
37
Check Your Understanding

• Bitrate or delay limited?
• Sending a few bits to China over a fiber optic
  cable
• Sending a 100MB file across town over a dial-up
  modem

38
Remedying Bottlenecks

• Remedying Delay Problems
• Reduce delay
• Reduce number of data transfers
• Large messages or batch send
• Local processing (mobile code)
• Remedying Low Bitrate Problems
• Increase bitrate
• Reduce size of data transfers
• Small messages or streaming
• Compression
• Remedying Both Problems
• Avoid data transfers caching

39
Small Versus Large Messages

• Delay limited
• Send large files or batches of small files
• Only have to wait for propagation delay once
• Bit-rate limited
• Break message into smaller part, use pieces as
  soon as they arrive
• i.e., streaming
• Only have to wait for a portion of total
  transmission time before using the file
• Compression

40
Discussion Questions

• Is streaming useful for sending a 10-second
  recording to China over a fiber optic cable?
• How much sooner would playback start?
• An hour-long lecture across town on a dial-up
  modem?
• How much sooner would playback start?

41
Remedying Delay Mobile Code
Send code
Execute locally
Communication bottleneck

• Especially useful for remedying delay problems
• Fewer separate transmissions means less overall
  delay
• This is what JavaScript is used for most
• e.g., dynamic hierarchical menus on SI home page

42
Caching
Communication bottleneck
Future references (where possible)
Remote data being accessed
Local cache

• Make a local copy of frequently used data
• E.g., web browsers keep a local cache of visited
  web pages
• The less data you send, the less network
  performance affects you

43
Multiple Access Media
Ethernet
Bus
Switch
Hosts
Tree
Broadcast
44
Protocols for Sharing a Channel

• Random (Ethernet)
• Hosts transmit any time (maybe listen first),
  retry on conflict with random back off
• Bad if congested
• Polling
• Switch asks each host if it has content to
  transmit
• Centralized bad if messages require immediate
  attention
• Token passing
• A token is passed from host to host only host
  with token may transmit
• Bad if bad actors or if there is a long or slow
  loop
• Reservations
• Hosts make advanced reservations with the switch
• Centralized requires synchronized clocks and
  separate channel for scheduling

45
In-Class Demonstration

• Each of you is a host
• Need 5 volunteers to act as hosts with a message
  to transmit
• When can hosts speak under each protocol?

46
Switched Ethernet

• Ethernet protocol, but not really a shared
  channel
• Each device has dedicated wire to a switch
• Switch connects any pair of devices for private
  connection

47
High-Speed Access at Home

• Bandwidth
• Assume that home users download more than they
  upload
• Allocate more capacity to downlink than uplink
• Key difference between DSL and cable is where
  usage is aggregated
• With cable, users on a common network loop, often
  an entire neighborhood, share a single channel to
  the cable office
• With DSL, each user has a private channel to the
  central office (CO)
• DSL users do share a single high-speed connection
  from the CO to the Internet

48
Summary

• Spectrum allocation
• Identify gating factor(s) limiting performance
  and choose appropriate remedies
• Bit-rate or propagation delay
• Reducing number or size of data transfers
• Understand protocols for coordination of shared
  media links
• Random, polling, token passing, reservations