Ethernet

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Ethernet
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Ethernet and IEEE 802.3

• Developed during the mid-1970? at Xerox Palo
  Alto Research
• Center (Bob Metcalfe)
• Later refinement by DEC, Intel and Xerox (DIX
  standard)
• Became IEEE 802.3 in 1985
• Different frame format between Ethernet and
  IEEE 802.3

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Ethenet architecture
Data link layer Physical layer
station interface
data encapsulation
NIC
link management
encoding and decoding
AUI cable
transceiver
transmission and receipt
tap
BNC connector
0.5 ?Coax
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Multi segments network
10Base2 - Thin Ethernet
hub
bridge
10Base5 - Thick Ethernet
hub
router
server
10BaseT-Twisted pair
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IEEE 802.3 specification

• Various standard defined for IEEE802.3 with a
  concise notation
• 10Base5 -- thickwire coaxial
• 10Base2 -- thinwire coaxial or cheapernet
• 10BaseT -- twisted pair
• 10BaseF -- fiber optics
• 10Broad36 -- broadband
• Fast Ethernet
• 100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN

10 Base 5
maximum segment length in hundreds of meters
data rate in Mbps
signaling baseband or broadband
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10Base5

• tap cable does not to be cut
• transceiver send/receive, collison
• detection, electronics isolation
• AUI Attachment Unit Interface
• Use for backbone networks

maximum segment length500m maximum number of
stations per segment100
minimum distance between two stations 2.5 m
maximum network distance between two stations
2.8km
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10Base2
0.25 ?Coax

• BNC connector
• No drop cable
• use for office LAN

BNC T-connector
NIC
maximum segment length185m maximum number of
stations per segment30
minimum distance between two stations 0.5 m
maximum network distance between two stations
925 m
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10BaseT

• A hub functions as a repeater
• use for office LAN

hub
NIC
maximum segment length 100m
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10BaseF

• 10BaseF specification enable long distance
  connections with
• the use of optical fiber. Three standards are
• 10BaseFP - A passive-star topology, up to 1 km
  link
• 10BaseFL - An asynchronous point-to-point link
• (a new FOIRL) , up to 2 km link
• 10BaseFB - A synchonous point-to-point link , up
  to 2 km link
• with 15 cascade repeaters

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Carrier Sense Multiple Access

• Station listens to the medium before
  transmitting (listen before talking LBT)
• Non persistent CSMA
• if medium is idle, transmit
• if medium busy, waits a random periods an then
  resense medium
• 1-persistent CSMA
• if medium is idle, transmit
• if medium busy, continue to listen until the
  channel is sensed idle then transmit immediately

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Media Access Control technique

• Carrier Sense Multiple Access with Collision
  Detection (CSMA/CD)
• listen before talking
• Station senses to the medium before transmitting
  CS
• Topology supports multiple access (listenin)
  MA
• Manchester encoding ensures a transition every
  bit
• 0 high-to-low
• 1 low-to-high

0 1 0 0 1 1
0 0 0 1 1
0 mA
0 V (high)
-90 mA
-2.2 V (low)
100 ns
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Collision

• More than two station send frame at the same
  time
• Station transmits its data while checking the
  cahannel for collision
• If average DC voltage level exeeds the CD
  threshold, collision is detected

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Collision detection

• How to detect
• Station send frame and sense the medium
• Collision detected if
• station sense the exeeded signal strength (coax)
• there is signal on more than one inut (UTP)
• How to do
• Transmitting nodes send a jamming signal after a
  collision is detected.

jam
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Ethernet frame format
7 1 6 6
2 46-1500
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IEEE 802.3
calculation of the FCS
64-1518 bytes
8 6 6
2 46-1500 4
Ethernet
PA Preamble - 10101010s for
synchronization SFD Start of Frame delimiter --
10101011 to start frame DA Destination Address
-- MAC address SA Source Address -- MAC
address LEN Length -- Number of data bytes Type
identify the higher -level protocol LLC PDUpad
-- minimum 46 bytes, maximum 1500 FCS Frame
Check Sequence -- CRC-32
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Ethernet MAC address
1 1 46
I/G U/L 46-bit address
Two addressformat 48 bits 16 bits
1 15
I/G 15-bit address
I/G 0 Individual address I/G 1 Group
address U/L0 Global administered address U/L1
Local administered address Unicast define a
single destination Broadcast FFFFFFFF each
station on the network receive and accept frames
multicast a group address defines multiple
recepient
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Interframe gap
PA
Interframe gap gt 9.6 ms

• Obligatory 9.6ms interval between the emitted
  frame and the new one
• To enable other stations wishing to transmit to
  take over at this time

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Frame transmission
Assemble frame
carrier sense signal ON ?
Y
N
Wait interframe gap time Start trnasmission
Y
send jam sequence Increment attempt.
collision detected?
N
N
Y
transmission done?
Compute backoff and wait backoff time
attempt limited?
Y
N
tranmit OK
Discard frame
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Frame reception
start receiving
done receiving?
N
Y
N
Matched DA
Y
N
Discard frame
FCS and frame size OK?
Y
Pass frame to next layer
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Binary Backoff
delay 2
send frame
N
transmission done?
collision detected?
N
Y
Y
random wait between 0 and delay-1
N
tranmit OK
gt16 attempts
1st wait 0 or 1 slot time 2nd wait 0,1,2 or 3
slot time 3rd wait 0,1,2,..7 slot time kth
wait 0.. 2k slot time
Y
Discard frame
double delay

• after 10th collisions the interval is frozen
  at 1023 slots
• after 16th collisions , frame is discarded and
  report failure
• back to the upper layer

limited delaylt1024
max delay is limited at 102351.2 ms52.4 ms
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Binary Backoff
max_delay is limited at 102451.2 ms52.4 ms
max_delay 1 slot time
Y
limited max_delaylt1024
collision detected?
double max_delay
random wait 0 or max_delay
N
N
transmission done?
gt16 attempts
Y
N
Y
tranmit OK
Discard frame
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Minimum frame size
A and B locates at the far end of the cable
B
A
B
(2) packet almost at B at t-d
A
(1) packet starts at time 0
(3) B send packet collision occurs at t
A
B
B
(4) jam signal gets back to A at 2t
A

• A frame must take more than 2t to send to
  prevent the situation that the sender incorrect
• conclude that the frame was successfully sent.
• This slot times is 51.2 ms corresponds to 512
  bit 64 bytes
• The minimum frame length is 64 bytes (excluding
  preamble)
• This answers why data field must have 46 bytes
  minimum

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Late Collision

• Late collisions are collisions which occur after
  the first 64 bytes have been transmitted on to
  the network
• Primary causes excessive cable lengths and
  repeaters

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Ethernet Connectors
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Do you know?

• A propagation speed 0.77c on 10Base5 and 0.59c
  on twisted pair