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Communication media
Thanks to A. Tanenbaum
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Fourier Coefficients
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Bandwidth-Limited Signals

• A binary signal and its root-mean-square Fourier
  amplitudes.
• (b) (c) Successive approximations to the
  original signal.

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Bandwidth-Limited Signals (2)

• (d) (e) Successive approximations to the
  original signal.

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Twisted Pair

• (a) Category 3 UTP.
• (b) Category 5 UTP.

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Coaxial Cable

• A coaxial cable.

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Fiber Optics

• (a) Three examples of a light ray from inside a
  silica fiber impinging on the air/silica boundary
  at different angles.
• (b) Light trapped by total internal reflection.

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Fiber Optic Networks

• A fiber optic ring with active repeaters.

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Fiber Optic Networks (2)

• A passive star connection in a fiber optics
  network.

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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Politics of the Electromagnetic Spectrum

• The ISM bands in the United States.

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The Local Loop Modems, ADSL, and Wireless

• The use of both analog and digital transmissions
  for a computer to computer call. Conversion is
  done by the modems and codecs.

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Data transmission

• Receiver needs to know about sender
• duration of each bit
• length of elements in bits
• duration of a frame

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Modems

• (a) A binary signal
• (b) Amplitude modulation

• (c) Frequency modulation
• (d) Phase modulation

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Bit encoding(1)

• Asynchronous data transmission

• Used for
• character oriented devices
• large indeterminate intervals between characters
• receiver resynchronizes with sender on start and
  stop bits
• polarity of stop bit different from polarity of
  start bit

1 0 0 1 0 0 1 0
Start bit
Stop bit(s)
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Bit encoding(2)

• (a) Binary encoding, (b) Manchester encoding,
  (c) Differential Manchester encoding.

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Modems (2)

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

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Frequency Division Multiplexing

• (a) The original bandwidths. (c) The multiplexed
  channel.
• (b) The bandwidths raised in frequency.

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Wavelength Division Multiplexing

• Wavelength division multiplexing.

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Time Division Multiplexing

• The T1 carrier (1.544 Mbps).

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Time Division Multiplexing (2)

• Delta modulation.

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Time Division Multiplexing (3)

• Multiplexing T1 streams into higher carriers.

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Code Division Multiple Access (CDMA)
Every single bit Xor with individual Walsh code 1
bit is extended to 64 or 128 chips Consequence 64
(128) more bits transmitted Walsh codes are
orthogonal So assume two codes A and B then A.B
0
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Code Division Multiple Access (CDMA) (2)
Example, two stations with Walsh codes A and B A
-1 -1 -1 1 1 -1 1 1 B -1 -1 1 -1 1 1 1
-1 A and B both transmit 1, Xor with Walsh
code Addition of signals then gives S -2 -2
0 0 2 0 2 0 S.A (22002020)/8 1
S.B (22002020)/8 1
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Wireless coding
Direct Sequence Spread Spectrum (DSSS) basis for
CDMA Frequency Hopping Spread Spectrum (FHSS)
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Advanced Mobile Phone System

• (a) Frequencies are not reused in adjacent cells.
• (b) To add more users, smaller cells can be used.

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Multiple Access Protocols

1. ALOHA
2. Carrier Sense Multiple Access Protocols
3. Collision-Free Protocols
4. Limited-Contention Protocols
5. Wavelength Division Multiple Access Protocols
6. Wireless LAN Protocols

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Dynamic Channel Allocation

1. Terminals
2. Single Channel
3. Collision
4. Continuous time vs slotted time
5. Carrier sense or not

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Pure ALOHA

• In pure ALOHA, frames are transmitted at
  completely arbitrary times.

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Pure ALOHA (2)

• Vulnerable period for the shaded frame.

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Pure ALOHA (3)

• Throughput versus offered traffic for ALOHA
  systems.

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Persistent and Nonpersistent CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

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Collision detection interval
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CSMA with Collision Detection

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

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Collision-Free Protocols

• The basic bit-map protocol.

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Collision-Free Protocols (2)

• The binary countdown protocol. A dash indicates
  silence.

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Limited-Contention Protocols

• Acquisition probability for a symmetric
  contention channel.

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Adaptive Tree Walk Protocol

• The tree for eight stations.

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Wireless LAN protocols

• (a) The hidden station problem.
• (b) The exposed station problem.

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Wireless LAN Protocols (2)

• The MACA protocol. (a) A sending an RTS to B.
• (b) B responding with a CTS to A.

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Wireless Lan Protocol CSMA/CA

• The use of virtual channel sensing using CSMA/CA.

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Wireless LAN Protocol CSMA/CA (2)

• A fragment burst.

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Ethernet MAC Sublayer Protocol

• Frame formats. (a) DIX Ethernet, (b) IEEE 802.3.

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Ethernet Performance

• Efficiency of Ethernet at 10 Mbps with 512-bit
  slot times.

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IEEE 802.2 Logical Link Control

• (a) Position of LLC. (b) Protocol formats.

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The 802.11 Protocol Stack

• Part of the 802.11 protocol stack.

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The 802.11 MAC Sublayer Protocol (4)

• Interframe spacing in 802.11.

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The 802.11 Frame Structure

• The 802.11 data frame.

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Bluetooth Architecture

• Two piconets can be connected to form a
  scatternet.

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Bridges from 802.x to 802.y (2)

• The IEEE 802 frame formats. The drawing is not
  to scale.