Transcript Document 7399931

Ethernet Technology
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Electromagnetic Spectrum
Guided Transmission Media
– Twisted Pair
– Coaxial cable
– Optical fiber
Unguided Transmission Media
– Terrestrial Microwave
– Satellite
– Radio
– IR
– Cellular Telephony
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Electromagnetic Spectrum
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Guided Transmission Media
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Twisted Pair
Coaxial cable
Optical fiber
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Twisted Pair
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Most common medium
Telephone network
– Between house and local exchange (subscriber loop)
Within buildings
– To private branch exchange (PBX)
For local area networks (LAN)
– 10Mbps or 100Mbps
Pros and Cons:
– Cheap
– Easy to work with
– Low data rate
– Short range, about 100 meters.
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Unshielded and Shielded TP
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Unshielded Twisted Pair (UTP)
– Ordinary telephone wire
– Cheapest
– Easiest to install
– Suffers from external EM interference
Shielded Twisted Pair (STP)
– Metal braid or sheathing that reduces
interference
– More expensive
– Harder to handle (thick, heavy)
– Capacity 10-155 Mbps
UTP Categories
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Cat 3
– up to 16MHz
– Voice grade found in most
offices
Cat 4
– up to 20 MHz
Cat 5
– up to 100MHz
– Commonly pre-installed in
new office buildings
RJ-11 vs. RJ-45
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RJ-11 is a typical UTP phone
connector. Has 2 pairs.
RJ-45 is a UTP connector.
Has 4 pairs.
Coaxial Cable
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Most versatile medium
Television distribution
– Ariel to TV
– Cable TV
Long distance telephone
transmission
– Can carry 10,000
voice calls
simultaneously
– Being replaced by
fiber optic
Transmission Characteristics
 Analog
 Amplifiers every few km
 Closer if higher frequency
 Up to 500MHz
 Digital
 Repeater every 1km
 Closer for higher data rates
Short distance computer
systems links
Local area networks
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Optical Fiber
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Greater capacity
– Data rates of
hundreds of Gbps
Smaller size & weight
Lower attenuation
Electromagnetic isolation
Greater repeater spacing
– 10s of km at least
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Transmission Characteristics
 Act as wave guide for
1014 to 1015 Hz
 Portions of
infrared and
visible spectrum
 Light Emitting Diode
(LED) used in MMF
 Cheaper
 Wider operating
temp range
 Last longer
 Injection Laser Diode
(ILD) used in SMF
 More efficient
 Greater data rate
 Wavelength Division
Multiplexing (WDM)
using light prisms.
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MMF vs. SMF
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Wavelength Division Multiplexing
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Used for fiber optics
Multiplexing and demultiplexing involve light signals
Combining and splitting of light sources are done by prisms.
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Comparison
Cable Type
Cost
1 = lowest
5 = highest
Typical
Capacity
Installation
1 = easy
5 = hard
Max Cable
Length
EMI
Resistance
1 = low
5 = high
CoaxialThinnet
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10 Mbps
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185 meters
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CoaxialThicknet
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10 Mbps
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500 meters
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UTP
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10-100 Mbps
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100 meters
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STP
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16-155 Mbps
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100 meters
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Fiber-optic
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100+ Mbps
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Multiple
kilometers
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Data Link Layer
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The Logical Link Control (LLC) sublayer
– Framing
– Flow Control
– Error Control
The Media Access Control (MAC) sublayer
– Random Access (CSMA)
– Token Passing
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LLC Framing
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Framing is partitioning a bit stream into discrete units or blocks of
data.
To distinguish between same bit pattern of real user data and the
start-stop, bit stuffing is employed.
– E.g., stuff a bit after every 5th consecutive ones.
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Ethernet/802.3 Frame
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Preamble is for synchronization
Source and destination are 48 bits each and have the MAC
address, e.g, 08:00:20:01:D6:2A. The left 3 bytes are vendor
specific, and the others are serial number of device assigned by
Sun. 00:00:0C is CISCO.
This MAC address is different than the IP address !!!
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CSMA/CD
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Step 1. “Listen before talking”.
Step 2. If channel is quiet for a certain time, called
interframe gap (IFG), then transmit. “Talk if quiet”.
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Step 3.
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Step 4.
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Step 5.
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Step 6.
If channel is busy, monitor the channel
until it is quiet for IFG period before transmitting.
“Wait for quiet before talking.”
Monitor the channel continuously during
transmission to detect collisions. “Listen while
talking”.
If collision occurs, the first node
recognizes it will send a jam signal to ensure that all
other stations detect the collision. Transmitters
should stop immediately, receivers should reject
data, and others wishing to transmit should
recalculate their backoff period. “A buzzer sounds off
indicating we have more than one talker at a time.”
All nodes wish to transmit must now wait
a random period called “backoff” and attempt again
to spread out collisions. “Backoff” is based on BEB
algorithm (Binary Exponential Backoff). After 16
collisions, drop the frame to be transmitted.
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Notes
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On baseband bus, collision produces
much higher signal voltage than signal
Collision detected if cable signal greater
than single station signal
Signal attenuated over distance
Limit distance to 500m (10Base5) or
200m (10Base2)
For twisted pair (star-topology) activity on
more than one port is collision
Why largest network diameter for
Ethernet LAN 10Base5 is 2500 meters?
– 5-4-3 Repeater Placement Rule.
 5 segments
 4 repeaters or hubs
 and only 3 populated
segments
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Repeaters, Bridges, & Switches
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Repeater
Hub
NIC
Bridges
Switches
VLANs
GbE
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Hardware Components
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Repeater
– Layer 1 device that provides physical and electrical
connections.
– It receives signals from one cable segment, regenerates,
retimes, and amplifies them, and then transmits these
“revitalized” signals to another cable segment.
– Transmits in both directions
– Joins two segments of cable
– No buffering
– No logical isolation of segments
Hub
– Used to describe a repeater
– Can be “repeater hub”, “switching hub”, bridging hub”.
NIC
– Network Interface Card
– Performs layer-2 functions: framing, error detection, and
flow control.
– Performs layer-1 functions by converting the bits into
electrical signals using appropriate coding scheme.
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Bridges
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Layer 2 devices
Interconnects two or more individual LANs or LAN segments.
Desirable for separating traffic among segments. A segment is part of a LAN in which
traffic is common to all nodes, i.e. it is a single continuous conductor, though it may
include repeaters.
Split the segment with bridges/switches, if link utilization is more than 30%.
Store-and-forward devices. They capture the entire frame before deciding whether to
filter or forward it. Frames with bad CRC are not forwarded.
Minimal buffering to meet peak demand
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Bridges Standards
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Transparent Bridges
– Operate in promiscuous mode.
– Bridging is transparent to stations, as if they are on one single LAN.
– “plug and play” unit, learns addresses connecting to ports by examining source and
destination addresses.
– examines the destination address to forward or filter frames.
– All broadcast and multicast frames are forwarded.
Source Routing Bridges
– Sender provides routing information for frames.
– Routing information includes local or remote destination address, and list of
intermediate route designators.
– A route designator contain 12-bit LAN number and 4-bit bridge number.
– Every station has a map of the network (different routes to get to different
destinations).
– Routes can be configured manually or by performing route discovery.
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Bridges Standards (cont.)
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Network loops can happen. Can cause
broadcast storms that can bring the LAN down.
Spanning Tree algorithm resolves network
loops.
Algorithm is based on graph theory.
– Any connected graph, consisting of nodes
and edges connecting pairs of nodes,
there is a spanning tree of edges that
maintains the connectivity of the graph but
contains no closed loops.
– Each LAN is a node and each bridge is an
edge.
– Specified in IEEE 802.1. It involves a brief
exchange of messages among all bridges
to discover the minimum-cost spanning
tree. Whenever there is a change in
topology, the bridges automatically
recalculate the spanning tree.
Disabling B3-LAN4 port will result in a spanning
tree. If B4 fails, the algorithm should enable this
port again.
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Spanning Tree
Switches
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Switches can operate at different layers: layer 2, 3, 4, and 7..
Basically a switch is hardware based, not software based.
Three types of layer 2 switches:
– Store-and-Forward Switch
 Similar to store-and-forward bridge. Store entire frame, check for errors,
and then switch to the other ports, based on the destination MAC
address.
– Cut-Through Switch
 The transmission of frame begins as soon as it reads the destination
MAC address. Two switch fabric/matrix designs:
 Crossbar
 Backplane with bus speed > aggregate port speeds
– Hybrid Switch
 Reliability: store-and-forward. Turn ON when errors are high.
 Low latency: cut-through. Turn ON when errors are low.
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VLANs
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VLAN is a logical grouping of nodes
using Ethernet switches. Nodes don’t
need to be connected physically to the
same switch. A broadcast frame will be
heard by all nodes within VLAN.
Benefits:
– Isolates broadcasts
– Frees up network from physical
locations
– Easily shares resources. A server
can be part of multiple VLANs.
– Performance. Easily can be
enhanced by creating new VLANs.
– Security. By containing who can
listen to broadcast.
VLAN Membership (implicit tagging)
– Port-based
– MAC-based
– Layer 3/IP
– Combination of the above
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Gigabit Ethernet
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With GbE CSMA/CD
– Network diameter shrinks
to 25 meter. This is not a
good option.
– Therefore, minimum frame
time was increased to 512
bytes. This gives network
diameter of 200 meter, but
waste in bandwidth
especially for small size
data.
Most common use is point-topoint fame switching.
– No CSMA/CD
We have now 10GbE that can
go more than 50 km over SMF.
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