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Ethernet & IEEE 802.3

Cisco Section 7.3

Stephanie HutterOctober 2000

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Ethernet

• Commonly used to refer to all carrier sense multiple access collision detection (CSMA/CD) LANS that generally conform to Ethernet specifications, including IEEE 802.3

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History of Ethernet

• 1960’s– CSMA/CD developed at the

University of Hawaii

• 1970’s– First experimental Ethernet

system at Xerox PARC

• 1980’s– IEEE 802.3 released– Digital Equipment, Intel, and

Xerox jointly develop & release Ethernet Version 2.0

• DIX standard• Substantially compatible with IEEE

802.3

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CSMA/CD MAC

• Stations can access the network at any time

• Stations listen to the network before transmitting

• If no signal on the wire, the station sends

• If two stations send at the same time, a collision occurs– Both transmissions are

damaged– Both stations must retransmit– Backoff algorithms determine

when each station can retransmit

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Broadcast Networks

• All stations see all transmissions

• Each station must examine each frame to determine if they are the destination– If yes, the frame is passed

to upper layers for processing

• Ethernet & IEEE 802.3 are both broadcast networks

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Differences between Ethernet and IEEE 802.3

• Ethernet provides services to all of OSI Layer 1 & Layer 2

• IEEE 802.3 specifies the physical layer and the MAC sublayer of the data link layer.– Does not define a Logical Link

Control sublayer protocol

• Both are implemented through hardware– Typically an interface card or

circuitry on a primary circuit board

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Ethernet Family Tree

• “Ethernet” is a vague term• Includes 18 varieties

– Already specified– In the works

• Legacy networks– 10Base2– 10Base5 (Coax)

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Ethernet Family Tree

• Current installed base– 10Base-T

• 10 Mbps Cat 5 UTP

– 100Base-TX • Fast Ethernet

– 100Base-FX • 100 Mbps fiber

• Future Growth– 1000Base-T

• Gigabit over UTP

– 1000Base-SX • Gigabit over fiber• Short wavelength laser source

– 1000Base-LX• Gigabit over fiber• Long wavelength laser source

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Ethernet Frame Format

• Preamble– Alternating patterns of 1s and 0s– Tells receiving stations whether

frame is Ethernet or IEEE 802.3• Ethernet frame contains an

additional byte– Equivalent to the Start of Frame

(SOF) field specified in 802.3

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Ethernet Frame Format

• Start of Frame– SOF– 802.3 delimiter byte ends

with 2 consecutive 1 bits• Synchronize the frame-

reception

– Explicitly specified in Ethernet

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Ethernet Frame Format

• Destination and source addresses– 1st 3 bytes are vendor-specific

• Specified by IEE

– Last 3 bytes are specified by vendor

• Ethernet or IEEE

– Source address is always unicast– Destination can be unicast,

multicast, or broadcast

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Ethernet Frame Format

• Type– Ethernet only– Specifies the upper-layer

protocol to receive the data

• Length– IEEE 802.3 only– Number of data bytes in

the frame

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Ethernet Frame Format

• Data—Ethernet– At least 46 bytes of data– Padding bytes inserted as

needed

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Ethernet Frame Format

• Data—IEEE 802.3– Upper-layer protocol

destination is defined within the data portion of the frame

– At least 64 bytes– Padding bytes inserted as

needed

• Frame Check Sequence– 4 byte CRC value

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Ethernet MAC

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Ethernet MAC

• Connectionless network architecture– Source device is not

notified of successful delivery of data packet

– “Best-effort” delivery system

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Signaling

• Most common varieties use Manchester encoding

• Newer, faster varieties use more complex encoding schemes

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Half Duplex Ethernet Design

• 10Base-T Transceivers– Built on to NIC– Use 4 wires– 1 pair transmits– 1 pair receives

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10Base-T Media & Topologies

• Why 10Base-T?– Commonly installed– Basic configuration– Not always applicable

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Star Topology

• Media runs from a central hub out to each device

• Central point of control• All communication is via

the hub

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Hubs

• Receive data on one port, broadcast it to all other ports

• Active or passive– Active

• Connection and regeneration• Extends networking

distances• Sometimes called

concentrators

– Passive• Connection only

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Advantages of star topology

• Easy to design and install• Ease of maintenance

– Only area of concentration is at the hub

– Layout is easy to modify and troubleshoot

• Easy to add workstations• Limited network effect if

a device or cable goes down– Increased reliability

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Disadvantages of star topology

• Lots of media needed– Increases setup costs

• Hub represents single point of failure

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Horizontal Cabling

• TIA/EIA 568-A– Horizontal cabling must be

star topology• Mechanical termination for

each jack is located at the patch panel in the wiring closet

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Horizontal cabling

• UTP– Horizontal cable

• 90m maximum

– Patch cables• 3m maximum

– Horizontal cross-connect• (Wiring closet)• 6m maximum

– Total• 100m (approx)

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Network Area

• Cable length limitations limit the size of a star topology network

• Sneakernet outside the 200m2

area

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Extending the network

• Repeaters regenerate and retime signals

• Attenuation is “repaired”• Cable length specification

“restarts”