Day 8

1
Day 8

• Multiplexing

2
More than 1 signal per cable

• Typically a single cable can carry a single
  connection
• Not good if you want a cable to be able to send
  lots of separate
• Telephone calls
• TV channels
• Data connections

3
Multiple ways to slice things

• By Frequency
• Frequency Division Multiplexing
• By time
• Synchronous Time Division Multiplexing
• Statistical Time Division Multiplexing
• Other special application
• Wavelength Division Multiplexing
• Discrete Multitone
• Code Division Multiplexing

4
Frequency Division Multiplexing

• Examples
• FM Radio
• TV/Cable TV
• D-AMPS (old cellular)
• 802.11b/g (3 channels)

5
Frequency Division Multiplexing

6
Guard Bands

• Used to prevent cross talk between the
  channels.
• Empty frequencies between channels.

7
Synchronous time division

• Each station gets a chance to talk.
• If a station has anything to say, their data is
  inserted into their block.
• If nothing to say, the block is left empty
• Everyone gets equal time.

8
Synchronous Time Division Multiplexing

9
T1/DS1 1.544Mb/s

• The T1 is a high speed data line designed to
  interconnect phone switches.
• Also available to end users to provide high speed
  data or lots of phone lines
• It uses synchronous time division multiplexing
• 24 Channels
• Each frame gets 1 byte from each of 24 devices
• 8000 frames per second are transmitted
• 56k/channel for voice, 64k for data.
• Extra control bit is used giving 7 bits which is
  all that is necessary for voice.
• 400/month install

10
ISDN

• 2 B channels (64k)
• 1 D channel (16k)
• Signal
• Control bits
• 8 bits from B1
• Control bits
• D bit
• Control bits
• 8 bits from B2

11
SONET

• Synchronous Optical Network
• Single clock
• Atomic clock
• Synchronous Transport Signals
• OC 1 51.84Mbps
• 8000 frames/second 6480 bits/frame
• OC 3 3OC1 155.52Mbps
• 40k/month
• OC48 2.5Gbps
• 80k/month
• OC192 192 OC1 10Gbps
• 1/4million per month
• 1, 3, 9, 12, 18, 24, 36, 48, 96, 192
• Gige 15k/month

12
Statistical Time Division

• Not everyone has something to say at all times.
• Why not give the traffic to those who do
• If so, we need to add addresses so we know who
  said what.
• Multiplexer must be smart

13
Size of frames

• Either they must be set in stone
• Or you must transmit the size so the other side
  can tell where one frame ends and the next starts

14
Wavelength Division Multiplexing

• Using laser color to differentiate streams
• You can now multiplex multiple OC192s together
  on a single cable.
• Has been used to create up to 1.6Tbps over a
  single fiber pair

15
Discrete Multitone Multiplexing

• Examples
• DSL
• Digital FM Radio
• 802.11a/g
• A specific example of frequency division
  multiplexing
• Data in a particular channel is transmitted on
  multiple different frequencies at once.
• Frequencies are chosen to avoid interference with
  other data

16
Code Division Multiplexing

• Each user has a code
• A 10111001
• B 01101110
• C 11001101
• If you want to send a 1 you send your code, if
  you want to send a 0 you send the inverse of your
  code (flip all 0s and 1s)
• Receiver gets the sum of all this, adds each code
  (1 1 or 0 -1).
• 8 means they sent a 1, -8 means they sent 0