Networks and Communication Lecture 2: Protocol Stacks

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Networks and CommunicationLecture 2 Protocol
Stacks

• Peter Steenkiste
• School of Computer Science
• Carnegie Mellon University
• ECOM, Summer 2000

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Todays Lecture

• Protocol stacks.
• Standards.
• Physical layer.

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Is Networking Hard?
Application
Application
Router Software (many protocols)
Operating System
Operating System
Links
Computer
Network Interface
Router Hardware
Network Interface
Computer
Bridge HW/SW
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Protocols
The Open Systems Interconnection (OSI) Model.
Application
Application
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Presentation
Presentation
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Session
Session
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Transport
Transport
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Network
Network
3
Data link
Data link
2
Physical
Physical
1
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Module Interfaces

• Each layer provides services to the layer above,
  using services provided by the layer below.
• Interoperability two ways
• higher level protocols (e.g. TCP/IP, Appletalk)
  can run on multiple lower layers
• multiple higher level protocols can share a
  single physical network
• Interfaces naturally creates independent modules.
• layers can be implemented and modified in
  isolation
• sometimes not true in practice

Service provided to next layer
Interface to peer on other node
Layer i
Service received from lower layer
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OSI Functions

• (1) Physical transmission of a bit stream.
• (2) Data link flow control, framing, error
  detection.
• (3) Network switching and routing.
• (4) Transport reliable end to end delivery.
• (5) Session managing logical connections.
• (6) Presentation data transformations.
• (7) Application specific uses, e.g. mail, file
  transfer, telnet, network management.

Multiplexing takes place in multiple layers
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Packet Headers

• Each layer encapsulates/encodes/fragments data
  from the layer above.
• During transmit, each layer adds a header with
  information for its peer on the next node.
• Router or switch for core network protocols
• End-host for end-to-end protocols
• Headers are consumed on the receiving node as
  packets travel up the stack.
• Layers use the header information for protocol
  processsing

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A TCP / IP / 802.3 Packet
Ethernet preamble
MAC header
LLC / SNAP header
IP header
TCP header
Data
Ethernet CRC
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Example Sending a Web Page
Http hdr
Web page
. . .
DL header
IP header
TCP header
Application payload
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Limitations of theLayered Model

• Some layers are not always cleanly separated.
• Inter-layer dependencies in implementations for
  performance reasons
• Some dependencies in the standards (header
  checksums)
• Higher layers not always well defined.
• Session, presentation, application layers
• Lower layers have sublayers.
• Sublayers well defined in the standards
• Interfaces are not really standardized.
• It would be hard to mix and match layers from
  independent implementations
• Many cross-layer assumptions, e.g. buffer
  management

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Local Area Network Protocols
IEEE 802 standards refine the OSI data link layer.
Application
Presentation
Upper Layer Protocols
Session
Transport
link service access points
Network
LLC
Data link
MAC
Physical
Physical
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IEEE 802 Networks

• Common addressing at MAC level 48 bit IEEE
  address.
• 802.3 (Ethernet)
• 802.5 (Token ring)
• 802.6 (Distributed queue dual bus)
• 8.02.11 (Wireless)

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The Internet Protocol Suite
Application
Applications
Presentation
Presentation
Session
Session
Transport
Network
Data link
Data Link
Physical
Physical
The Hourglass Model
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Internetworking Options
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4
4
data link
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3
3
3
physical
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2
2
2
2
1
1
1
1
1
1
1
repeater
bridge (e.g. 802 MAC)
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7
7
7
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6
6
6
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5
5
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. . .
network
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2
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1
router
gateway
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Relevant Standardization Bodies

• Trend toward a priori standards.
• a mixed blessing
• ITU-TS (formerly CCITT) - Telecommunications
  Sector of the International Telecommunications
  Union.
• government representatives (PTTs/State
  Department)
• responsible for international recommendations
• T1 - telecom committee reporting to American
  National Standards Institute.
• T1/ANSI formulate US positions
• interpret/adapt ITU standards for US use
• represents US in ISO

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More Bodies

• IEEE - Institute of Electrical and Electronics
  Engineers.
• responsible for many LAN physical layer and
  datalink layer standards
• IETF - Internet Engineering Task Force.
• standards for network layer and higher
• ATM Forum.
• voting membership mostly manufacturers
• comparatively rapid evolution of recommendations
• ISO - International Standards Organization.
• covers a broad area

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From Signals to Packets
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Physical Layer

• Modulation.
• Frequency spectrum and its use.
• Copper.
• Fiber.
• Wireless.

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Modulation

• Sender changes the nature of the signal in a way
  that the receiver can recognize.
• Similar to radio AM or FM
• Amplitude modulation change the strength of the
  signal, typically between on and off.
• Sender and receiver agree on a rate
• On means 1, Off means 0
• Similar frequency of phase modulation.

0 0 1 1 0 0 1 1 0 0 0 1 1 1 0 0 0 1 1 0 0 0 1 1
1 0
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The Frequency Domain

• A (periodic) signal can be viewed as a sum of
  sine waves of different strengths.
• See figure 2.1, page 80, in the textbook
• Every signal has an equivalent representation in
  the frequency domain.
• What frequencies are present and what is their
  strength
• Similar to radio and TV signals

Amplitude
Time
Frequency
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Baseband versus Carrier Modulation

• Baseband modulation send the bare signal.
• Carrier modulation use the signal to modulate a
  higher frequency signal (carrier).
• Can be viewed as the product of the two signals
• Corresponds to a shift in the frequency domain
• Some idea applies to frequency and phase
  modulation.
• E.g. change frequency of the carrier instead of
  its amplitude

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Amplitude Carrier Modulation
Amplitude
Amplitude
Signal
Carrier Frequency
Modulated Carrier
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Why Do We Care?

• How much bandwidth can I get out of a specific
  wire (transmission medium)?
• What limits the physical size of the network that
  I can
• How can I have multiple hosts communicate at the
  same time?
• How can I manage bandwidth on a particular
  transmission medium?
• How do the properties of copper, fiber, and
  wireless compare?

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The Nyquist Limit

• A noiseless channel of width H can at most
  transmit a binary signal at a rate 2 x H.
• E.g. a 3000 Hz channel can transmit data at a
  rate of at most 6000 bits/second
• Shannon extended this result by accounting for
  the effects of noise.
• Every transmission medium corresponds to a
  channel of a certain width.
• The width of the channel is determined both by
  the transmission medium itself and the
  characteristics of the transmitter and the
  receivers

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Limits to Speed and Distance

• Noise random energy is added to the signal.
• Attenuation some of the energy in the signal
  leaks away.
• Dispersion attenuation and propagation speed are
  frequency dependent.
• Changes the shape of the signal
• Effects limit the speed and distance of
  transmission.
• Affects different technologies in different ways.

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Supporting Multiple Channels

• Multiple channels can coexist if they transmit at
  a different frequency, or at a different time, or
  in a different part of the space.
• Compare with planes height, (horizontal) space,
  time
• Space can be limited using wires or using
  transmit power of wireless transmitters.
• Frequency is controlled by standards or law.
• See figure 2.11, page 95
• Controlling time is a datalink protocol issue.
• Media Access Control (MAC) who gets to send when?

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Copper Wire

• Unshielded twisted pair
• Two copper wires twisted - avoid antenna effect
• Grouped into cables multiple pairs with common
  sheath
• Category 3 versus category 5
• 100 Mbps up to 100 m, 1 Mbps up to a few km
• Coax cables.
• One connector is placed inside the other
  connector
• Holds the signal in place and keeps out noise
• Gigabit up to a km
• Signaling processing research pushes the
  capabilities of a specific technology.
• E.g. modems, use of cat 5

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Light Transmission in Silica Fiber
1.0
tens of THz
loss (dB/km)
0.5
1.3?
1.55?
0.0
1000
1500
wavelength (nm)
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Ray Propagation
cladding
core
lower index of refraction
(note minimum bend radius of a few cm)
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Fiber Types

• Multimode fiber.
• 62.5 or 50 micron core carries multiple modes
• used at 1.3 microns, usually LED source
• subject to mode dispersion different propagation
  modes travel at different speeds
• typical limit 1 Gbps at 100m
• Single mode
• 8 micron core carries a single mode
• used at 1.3 or 1.55 microns, usually laser diode
  source
• typical limit 1 Gbps at 10 km or more
• still subject to chromatic dispersion

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Example Interfaces for a Gigabit Ethernet Switch
GBIC Wavelength Fiber Type Core Size
Modal BW Cable Distance
(nm) (um)
(MHz.km) SX 850
MMF1 62.5
160 722 ft
(220 m)
62.5
20 902 ft
(275 m)
50.0
400 1640 ft
(500 m)
50.0
500 1804 ft
(550 m) LX/LH 1300 MMF2
62.5
500 804 ft (550 m)

62.5
500 1804 ft (550 m)


50.0 400
1804 ft (550 m)

50.0 500
1804 ft (550 m)
SMF3 9/10
-
6.2 miles (10 km) ZX 1550
SMF 9/10
- 43.5
miles (70 km)
SMF4 8
- 62.1
miles (100 km)
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Regeneration and Amplification

• At end of span, either regenerate electronically
  or amplify.
• Electronic repeaters are potentially slow, but
  can eliminate noise.
• Amplification over long distances made practical
  by erbium doped fiber amplifiers offering up to
  40 dB gain, linear response over a broad
  spectrum. Ex 10 Gbps at 500 km.

pump laser
source
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Wavelength Division Multiplexing

• Send multiple wavelengths through the same fiber.
• Multiplex and demultiplex the optical signal on
  the fiber
• Each wavelength represents an optical carrier
  that can carry a separate signal.
• E.g., 16 colors of 2.4 Gbit/second
• Like radio, but optical and much faster

Optical Splitter
Frequency
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Wireless Good News Bad News

• Great technology no wires to install, convenient
  mobility, ..
• High attenuation limits distances.
• Wave propagates out as a sphere
• Signal strength reduces quickly (1/distance)2
• High noise due to interference from other
  transmitters.
• Use MAC and other rules to limit interference
• Aggressive encoding techniques to make signal
  less sensitive to noise
• Other effects multipath fading, security, ..
• Ether has limited bandwidth.
• Try to maximize its use

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Frequency Spectrum

• Use of the frequency spectrum is regulated by the
  Federal Communications Commission.
• ITU world-wide
• Lots of different user communities.
• Emergency response, FAA, military, commercial
  radio and television, unlicensed bands, ..
• Each band typically has a range of (technical)
  constraints.
• Transmit power limits range frequency reuse
• Width of the band limit interference
• Unlicensed bands around 900 MHz, 2.4 GHz, 5.8
  GHz.
• Large numbers of types of users

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Example Technologies

• Microwave transmission.
• Can cover long distances and can be directed
• Communication often through a basestation
• More bandwidth available than with radio
  transmission
• Buildings, cause attenuation
• Cordless, analog and digital cellular phones.
• Personal communication systems.
• Communication satellites.
• Lots of bandwidth but high delay
• Inherently broadcast medium but can focus beams
  on specific areas
• Infrared and millimeter waves.
• Appropriate for short range communication