10Base-T%20Ethernet%20LAN

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10Base-T Ethernet LAN
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LANs

• Local Area Networks
• Limited Geographical Area
• Single office
• Single building
• University campus or industrial park
• Generally, high speeds
• Now, most operate at around 10 Mbps
• 100 Mbps is emerging as the new base speed
• Most Data Traffic is Local

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Standards Setting

• LANs are Subnets (single networks)
• Subnet technology is Dominated by OSI Standards
  (true for LANs)
• IEEE Creates most LAN Standards
• Institute for Electrical and Electronic Engineers
• Submits its standards to ISO and ITU-T for
  ratification
• IEEE 802 Committee
• LAN standards are set by the IEEE 802 Standards
  Committee.
• 802.3 for Ethernet Standards
• 802.5 for Token-Ring LAN Standards
• 802.11 for Radio and Infrared Wireless LANs

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LANs and OSI Architecture

• OSI is a 7-layer architecture
• LAN transmission only uses Layers 1 and 2
• Layer 1 Physical Layer
• Connectors, Media, Electrical signaling
• Layer 2 Data Link Layer
• Packaging data into frames
• Managing transmission over link (error handling,
  etc.)
• Access control when each station may transmit

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OSI Physical and Data Link Layers
Physical Layer (OSI Layer 1) Physical (plugs,
media, etc.) Electrical (voltages, timing, etc.)
Electrical Signal
Station A
Station B
Transmission Medium (telephone wire, etc.)
Connector Plug
Connector Plug
F4-1
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OSI Physical and Data Link Layers
Data Link Layer (OSI Layer 2)
Frame 2
Frame 1
Station A
Station B
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Data Link Layer

• For point-to-point transmission
• A point-to-point connection is a data link
• So is a transmission system shared by multiple
  devices, only one of which can transmit at a time
  because of collisions

Transmission
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Data Link Layer

• First function Packaging of Data (1s and 0s)
• PDU at Data Link Layer is called a frame
• Second Function Access Control
• Only one station can transmit at any time
• If another transmitted, their signals would
  scramble one another
• Must control access to (transmission into) the
  transmission medium

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Data Link Layer
Logical Link Control Layer
OSI Data Link Layer (Layer 2)
Media Access Control (MAC) Layer
OSI Physical Layer (Layer 1)
802.3 10Base-T
802.3 10Base-5
802.3 Other Physical Layer
802.5 Physical Layer 4 Mbps
802.5 Physical Layer 16 Mbps
Other Physical Layer
OSI Data Link Layer is subdivided into two
layers Media access control Logical
link control
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MAC Layer
Media Access Control (MAC) Layer
Media Access Control
802.3 10Base-T
802.3 10Base-5
802.3 Other Physical Layer
802.5 Physical Layer 4 Mbps
802.5 Physical Layer 16 Mbps
Other Physical Layer
OSI Physical (Layer 1)
MAC layer implements media access control
When a station may transmit Controls the
framing of data along the wire
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Logical Link Control Layer
802.2 Logical Link Control Layer
LLC
802.3 Media Access Control (MAC Layer)
802.5 MAC 4 Mbps
802.5 MAC 16 Mbps
Other MAC
802.3 10Base-T
802.3 10Base-5
802.3 Other Physical Layer
802.5 Physical Layer 4 Mbps
802.5 Physical Layer 16 Mbps
Other Physical Layer
OSI Physical (Layer 1)
Provides Control Function Begin/end
connections between stations Error
correction (optional)
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Simple LAN Using Ethernet 10Base-T
RJ-45 jacks
10Base-T Hub (Multiport Repeater)
10Base-T UTP Wiring Bundles 4 Pairs EIA Category
3, 4, or 5
Unshielded Twisted Pair (UTP) Wiring (4-Pair
Bundle)
PC
Network Interface Card
RJ-45 Jack
NIC
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Ethernet 10Base-T (802.3u)

• Physical Layer Standard
• 10 Mbps (10 in 10Base-T)
• Baseband signaling Injects voltage changes
  directly into the wires (Base in 10Base-T)
• Hubs (Multiport Repeaters)
• Connect the stations together

10Base-T Hub
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NICs

• Network Interface Cards
• Implement Physical Layer
• Plug and Electrical Signaling
• Implements the Data Link Layer (data packaging,
  access control, etc.)
• LLC (802.2)
• MAC (802.3 MAC)

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Wiring

• Unshielded Twisted Pair (UTP)
• Twisted several times per foot to reduce
  interference, T in 10Base- and Unshielded
• No protection except for plastic coating
• Distance limitation 100 meters (attenuation,
  distortion, noise and interference, crosstalk) --
  propagation
• Categories of UTP Wiring
• Category 5 The best. Good for 100 Mbps
• Category 3 and 4 lower. May be OK for 100 Mbps
• Wiring Plugs RJ-45 Standard
• Similar to home (RJ-11) jacks, but wider

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Attenuation

• As signal travels, gets weaker
• If too weak, cannot tell 1s and 0s

Distance
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Distortion

• As signal travels, it become distorted
• Changes shape
• Successive bits may merge, making reception
  difficult

Distance
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Noise

• Unwanted energy on line. Always present
• Noise floor is average level
• Noise spikes will cause problems

Signal
Signal Strength
Spike
Noise
Noise Floor
Time
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Signal to Noise Ratio (SNR)

• Ratio of Signal Power to Noise Power
• If SNR is high, few noise errors

100
Error Rate
0
1
SNR
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Interference

• Unwanted signal from outside sources
• Often intermittent, difficult to diagnose

Signal Strength
Signal
Interference
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Physical Layer Transmission in 10Base-T
10Base-T Hub
Step 1 Station A Transmits on Upstream Pair (Wires
12)
Station A
Station B
Station C
F4-4
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Physical Layer Transmission in 10Base-T
10Base-T Hub
Step 2 Hub Repeats (Broadcasts) The Message To
All Stations On Downstream Pairs (Wires 36)
Bus transmission means broadcasting
Station A
Station B
Station C
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Manchester Encoding

• Inefficient
• Transmits 10 million bits per second
• Must change line state 20 million times per
  second
• 20 Mbaud (baud is a change in the line state)
• Technology limits the baud rate
• Only 10 Mbps for 20 Mbaud
• Bit rate is less than the baud rate (opposite in
  modems)

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MAC LayerCSMA/CD Media Access Control

• Controls when stations may transmit
• If two transmit at once, signals will be scrambled

Collision
Collisions will grow rapidly above 30 of line
utilization. Keep traffic moderate, or
throughput will be affected.
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CSMA/CD Media Access Control

• CS Carrier Sense
• Each NIC always listens for traffic on the line
• This lets it recognize messages sent to its
  address
• This also lets it know if the line is free
• CSMA Carrier Sense Media Access
• A station may transmit if it hears no traffic on
  the network

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CSMA/CD Media Access Control in Ethernet
10Base-T Hub
Station A is Transmitting Station B must wait
Must Wait
Station A
Station B
Station C
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CSMA/CD Media Access Control

• CD Collision Detection
• If two stations transmit at once .
• Their signals collide, scrambling one another
• Because each sender listens (senses the carrier),
  both know that there has been a collision
• Both stop and wait a random amount of time.

101010
001110
X
Collision
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802.3 MAC Layer Frame
In asynchronous transmission, each byte is sent
separately, in a 10-bit frame. In 802.3 MAC Layer
frames, transmission is synchronous. Many data
bytes are sent in each frame of variable length.
Frames must be between 64 octets and 1518 octets
long, divided into groups of bits called
fields. Field lengths are measured in octets,
eight bits. Octet is a synonym for byte
Preamble
Start of Frame Delimiter
Destination Address
Source Address
Length
Data
Pad
Frame Check Sequence
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Error Detection

• MAC Layer Process Provides error detection
• Determines that an error exists
• Does not provide error correction
• Merely discards the frame
• No request for retransmission if there is an
  error
• Logical Link Control Layer may do error
  correction
• Receiving LLC process detects discarded frames
• Receiving LLC process asks for retransmissions

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Logical Link Control (LLC) Layer
802.2 can ask for retransmission of lost MAC
frames. Optional.
Next Higher Layer (Usually Internet)
Next Higher Layer (Usually Internet)
LLC Frame
LLC Layer Process
LLC Layer Process
LLC frame placed within MAC frame
MAC Layer Process
MAC Layer Process
MAC Frame
Station A
Station B
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Ethernet 10Base-T Network with Two Hubs
Hub 1
Hub 2
100 m Segment Maximum
Station A Transmits to Hub 1
Hub 2 Broadcasts the Message Out All of Its Ports
100 m Segment Maximum
100 m Segment Maximum
Hub 1 Transmits Out All Ports, Including the Port
to Hub 2
Station C Receives the Message
Station A
Station B
Station C
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Ethernet 10Base-T LAN with Multiple Hubs
Hub 2
Hub 3
Hub 1
UTP Wire
UTP Wire
UTP Wire
Station C
UTP Wire
Hub 4
Daisy chain, no Loops allowed!
UTP Wire
Station A
Station B
Station D
Maximum distance between farthest Stations is 4
Hubs/5 100 meter segments
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Speed and Distance

• Transmission speed worsens problems
• Error rates increase because bit periods are
  smaller and are more likely to be damaged by
  brief noise spikes and interference
• High speeds create high-frequency components in
  the signal that attenuate more rapidly than
  lower-frequency components
• In general, as speed increases, maximum distance
  decreases, although improving technology can
  lessen the decrease

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Dealing with Propagation Effects

• Use High-Quality, High-Cost Media
• Use media designed for long-distance propagation
• Optical fiber, coaxial cable
• Too expensive for runs to many desktops, good for
  hub-to-hub
• Use Inexpensive Media to the Desktop
• Improve the technology (allows 100 Mbps on UTP)
• Accept distance limitations (100 meters for UTP)
• More popular alternative to desktop because of
  low cost