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Ethernet and 802.3 Networking Technology:
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Currently Ethernet is the most popular Networking technology
Cheapest to implement
Was developed by XEROX in 1972
Modified and re-developed by DIGITAL, INTEL and XEROX in 1980 and
renamed DIX ETHERNET Ver1
IEEE developed the Standards for Ethernet in 1984 called 802.3 and uses
BUS Topology
DIX revised Version 1 to confirm with IEEE and named it VERSION 2.
Fig shown earlier illustrates how Ethernet fits into OSI model. The Data link
Layer is divided into two sub layers called the LLC and MAC as described
earlier.
The function of the LLC is to establish a logical connection between the
Source and destination
The function of the MAC sub layer is to access the Network. Ethernet uses
CSMA/CD for bus access as explained earlier
Ethernet Operation:
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Every Station has a Network Card which connects to the Cable as shown
below
Station
NIC
BUS
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Each Network Card also has a physical address called MAC Address
When a Station transmits a frame, all stations on the Network copy the
frame and look at the destination address. If it matches its own physical
address, it accepts the frame, other wise discards it.
Ethernet uses CSMA/CD Protocol to gain control of the bus, which works as
follows:
(a)
(b)
(c)
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When a station wants to transmit, it listens on the channel (senses the
carier). If it figures that no body is transmitting, it begins to transmit.
If two or more Stations decide to transmit at the same time thinking
that no body is transmitting, there will be collision.
The Station that detects the collision first sends a Jamming signal on
the bus (making Noise that thee is a collision, there by informing every
body that there is a collision). This Jamming signal is 32 bits of all 1’s.
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(d)
(e)
The Stations involved in the collision then back –off and wait according
to a back –off algorithm, then restart transmission.
System works well for 99% of the time.
ETHERNET 802.3 FRAME FORMAT:
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A BLOCK OF DATA TRANSMITTED ON THE NETWORK IS CALLED
A FRAME AS SHOWN BELOW:
7 BYTES
PREAMBLE
1 BYTE
2 OR 6
2OR 6
SFD
DA
SA
0 – 1500 BYTES
2
LENGTH
INFORMATION
1
DSAP
1
1–2
0 – 46
4
PAD
FCS
42 - 1497
SSAP CONTROL
DATA
Preamble: Seven bytes of alternating 1’s and 0’s for synchronization
SFD ( Start Frame Delimiter): Indicates the end of frame synchronizing pattern
(10101011)
Destination Address ( DA): Physical address or Hardware Address or the MAC
Address of the Destination Card (NIC).. fig below shows the format of the MAC
Address.
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Six bytes for the unique address of the individual card
22 bits indicate manufacturer’s ID
The first 2 bits have special meaning
CARD ADDRESS
DA47
DA46
DA45 – DA 24
DA23-----------------DA0
MANUFACTURER’S ID
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DA47 and DA 46 have Special meaning.
If DA47 = 0, means the individual address also called unicast address
DA47 = 1, means Multicast Address. A multicast address is assigned to a
group of Stations by the Network operating system. It is given by IEEE to
Manufacturers e.g multicast address for Cabletron NIC’s is 010010ffff20
Hex.
DA46 = 0 means the address is locally administered. Used only for closed
networks
DA46 = 1means the address is Universally administered-meaning globally
unique IEEE assigned address
DA45 to DA24 represents the Manufacturer’s ID
DA45 to DA0 are burned into the ROM chip of the NIC and indicate the
physical address of the card.
SA (SOURCE ADDRESS): Physical address of the source machine.
LENGTH: Indicates the number of bytes in the Data field.
DATA: Actual Data – min 46 bytes and Max 1500 bytes. Thus Ethernet allows an
MTU (Maximum Transfer Unit ) of 1546 bytes including overhead.
PAD: If data field is less than 46 bytes, dummy data is filled as padding.
FCS(Frame Check Sequence):CRC code for Error detection. Ethernet uses CRC-32
DSAP: ( Destination Service Access point): Address of the application such as IP or
Netware or Decnet
SSAP ( Source Service Access point) : Address of the originating application
protocol.
Control Field: Indicates the type of information in the Data field.
ETHERNET CHARACTERISTICS:
Data Rate
Encoding
Slot Time or Prop. Delay
Inter frame gap
Back-off limit
Jam Size
Maximum Size
Min Frame size
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10 Mbps
Manchester
512 bit time
9.6 
10 successive collisions
32 bits of 1’s
1526 bytes
64 bytes
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ETHERNET CABLING AND COMPONENTS:
Ethernet uses 3 different cabling methods:
(a)
(b)
(c)
(d)
10BT: 10 Mbps, Baseband Signal, Twisted pair, uses Twisted pair UTP or
STP wiring (also called CAT 4or CAT 5) in Star topology withRJ-45 or RJ
-11 connector
10B2: 10Mbps, Baseband Signal , 200 meter (185 meter to be exact). This
is also called ThinNet, uses Bus topology and BNC connector
10B5 : 10Mbps, Baseband Signal, 500 meter. This is also called ThickNet.
Uses coax cable with D Connector or Nconnector
Fig below shows an NIC which comes with 3 types of connectors.
ThinNet:Go to Web site www.wown.com/ i_ helmig/thin.htm
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Fig below shows a 10B2 system
Uses thin coax with BNC –T connector
Maximum length of one segment is 185 meters
The maximum length of the total network is 185  5 = 925 meters with 4
Repeaters
The transceiver is built into the NIC
The maximum distance between the T- connectors is 0.5 meter
Maximum 30 connectors per segment
The first and last station on each segment must be terminated with a 50 Ohm
resistor called BNC Terminator.
The function of the terminator is to prevent signal reflection on the cable
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Thick Net (10B5):
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10B5 is occasionally used for Network backbone
The transceiver is a separate device attached to RG-8 or RG –11 thick Coax
Cable as shown in the fig
The transceiver performs following functions
Transmit, Receive, Collision detect, enforce back – off
The maximum length of one segment is 500 m
Stations are attached to the backbone via transceiver
The maximum length of AUI (Attached Unit Interface Cable) is 50 m
The minimum distance between transceiver = 2.5 m
Max 100 Transceivers per Segment
Both ends of the segment must be terminated with a 50 ohm resistor
The AUI cable uses DB-15 Connector
Fig shows 10b5 Transceiver with Signal Quality Error (SQE)
The function of the SQE is to inform the station that the Collision detection
Mechanism is working properly
The function of the Jabber control is to disconnect the transceiver from the
cable in case of a short circuit in the transceiver.
10BT:
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10BT uses UTP or STP cable for transmission and all stations are connected
to a HUB or a SWITCH as shown in the fig below
S
HUB or SWITCH
WS
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WS
WS
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PIN
1
2
3
4
5
6
7
8
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A HUB repeats signals from one station to all other stations on the Network,
whereas a SWITCH Reads into the frame and transfers signals only to the
destination station
A 10BT hub or a switch uses RJ-45 connectors. Fig below shows PIN
connections of RJ-45 connector
SIGNAL
RD +
RD TD +
NC
NC
TD NC
NC
Fig below shows Twisted pair wiring.
1.
2.
3.
4.
5.
6.
7.
8.
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W/O
O/W
W/G
B/W
W/B
G/W
W/BROWN
BROWN/W
The mean length of a segment is 100 m
The transceiver is built into NIC
The cable is #22 to #26 AWG UTP CAT 4 or CAT 5 or CAT 5e
Stations are connected to a 10BT HUB or a SWITCH in a STAR topology
10BT topology allows a maximum of 4 repeaters, with a maximum diameter
of 500 m
To connect HUB to HUB See fig below
Cross Cable if no UP-LINK port
HUB #1
HUB # 2
Regular cable with an UP=LINK port
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Table below shows a comparison of 10BT, 10B2 and 10B5 configurations.
10B5
10B2
10BT
Medium
RG-8 or RG – 11
Thick Coax Cable
50 OHM 10 mm diameter
UTP or
STP
CAT 3, 4,
5 or 5e
Signal
10 MBPS
Baseband Signal with
MANCHESTER encoding
500 m
RG 58 or 59
Thin Coax
Cable
50 Ohm 5 mm
diameter
Same
185 m
100m
2.5KM
0.925KM
500m
Nodes/Segment
100
30
N/A
Topology
BUS
BUS
STAR
Maximum Segment
Length
Maximum distance
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