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Ethernet Basics

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The diagram .. was drawn by Dr. Robert M. Metcalfe in 1976 to present Ethernet to the National Computer Conference in June of that year.

Ethernet

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Topics

• History, Standards, Terminology• Transmission media• Topologies• Protocol• Access methods, Collision management

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What is Ethernet ?

Ethernet is a certain type of a local area network (LAN) which was developed in 1972 in the renowned PARC-research facility of Xerox in Palo Alto by Robert Metcalfe. In the meantime the companies Intel, DEC and Xerox have specified a common standard that has been established in the IEEE-standard 802.3.

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History

• 1969 student Robert Metcalfe (founder of 3Com in 1979) develops a Host Interface Controller for DARPA (Defense Advanced Research Projects Agency) in the company DEC.

• 1970 the ALOHA-Net (multiple access protocol) is developed and tested at the university of Hawaii

• 1972 the idea is picked up by the XEROX Palo Alto Research Center (Metcalfe works there by then). The project goal is: experimental Ethernet

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History

• 1976 the results of the project are published. The companies DEC, Intel and Xerox join in the company DIX and complete Ethernet to the market entry stage.

• 1980 Ethernet version 1.0 is passed. • 1981 IEEE starts standardization efforts. The Ethernet

specification is accepted without major modifications.

• 1982 Publication of Ethernet version 2.0 • 1985 worldwide recognition of the Ethernet standard as

ISO/DIS 8802/3

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History

• 1986 Publication of the 10Base2- and 10BroadT standards

• 1987 Standardization of the 10BaseTspezification

• 1991 Publication of the 10BaseF standard• 1994 more than 10.000 suppliers support the

Ethernet globally• 1995 Standardization of the 100 Mbit/s Ethernet• 1997 Standardization efforts for the Gigabit

Ethernet and presentation of first products prior to the completion of the standard

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Ethernet TCP/IP

Ethernet-Header

Ethernet-DATA FCS

IP-Header

IP-DATA

IP-frame

TCP-Header

TCP-DATA

TCP-frame

Ethernet-frame

LAYER 7 Modbus etc.

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Access method: CSMA/CD

Station is ready to send

check“Ether”

Sending of data and checking the “Ether”

Waiting accordingto back-off algorithm

Medium occupied

Discovered collision

mediumavailable

sendjam signal

No collision

New attempt

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Back-Off Algorithm

• If a collision has occurred, the stations try to send again after a certain period of time.

• After the first collision there a two different back-off times available, from which one is chosen at random. Transmission probability is 50%

• After the second consecutive collision there are four different back-off times available, from which one is chosen at random.

• The transmission probability now is 75%

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Truncated Binary Exponential Back-Off-Algorithm

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

...

......

.........

...............

............

...............

...............

...............

...............

...............

...............

...............

Nuber of back-off times to be selected at random

2

4

8

16

32

64

128

256

512

1024

1024

1024

1024

1024

1024

1024

50%

75%

87,5%

93,75%

96,88%

98,44

99,22%

99,61%

99,80%

99,90%

99,90%

99,90%

99,90%

99,90%

99,90%

99,90%

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Which waiting times are used ?

• The 0...1024 fold of the double max. signal travel time between the most remote stations + Offset is used

• With 10 Mbit/s Ethernet that means:

• The waiting time is also called collision window, the offset (9.6µs) is called gap.

• Only after the time of the collision window has passed, you can be certain that there will be no more collision.

Station 1 Station 225.6µs

25.6µs

51.2µs

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Example

• After the first collision the stations willing to send select a random waiting time of either 9.6µs or 9.6µs plus 51.2µs (duration of the collision window). Condition: Only two stations are involved, no new stations enter the scene in the collision management phase.

Waiting time(A) waiting time(B) transmission9.6µs 9.6µs NO9.6µs 9.6µs+51.2µs YES9.6µs+51.2µs 9.6µs YES9.6µs+51.2µs 9.6µs+51.2µs NO

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Delay depending on the network load

delay

20 % 40 %30 %10 % 50 % 60 % 70 % 80 %

Network load

low

high Highthroughput

Beginningproblems

Overload

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Ethernet address

• Also called "MAC address" • Globally unique ID for each device • Burnt into ROM, cannot be modified• Six Bytes in which manufacturer, device model and serial

number are coded • Readable with many auxiliary tools e.g. WINIPCFG

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Ethernet frame

Preamble SFD DA SA LEN FCSPadData

7 1 6 6 2 >=46 4

Preamble DA SA Type FCSPadData

8 6 6 2 >=46 4

Ethernet II DIX Frame:

IEEE 802.3 Frame:

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Ethernet frame

PreambleTrailer consisting of the bit sequence “0101010101...” serving the bit synchronization of the receiver.

SFD (Start Frame Delimiter)Start character consisting of the bit pattern “10101011” showing the recipient that the actual information will follow now.

DA (Destination Address)Evaluated by the recipient‘s address filter; only data frames destined for this recipient will be passed on to the communication software.

SA (Source Address)Sender‘s address

LEN (Length) Indicates the length of the subsequent data field in Bytes according to IEEE 802.3.

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Ethernet frame

Data and PadThe data field may contain 46 to 1500 user data bytes. Are there less than 46 bytes the Ethernet controller independently adds padding bytes, until the total amount (data + pad) is 46. This miminum length is crucial for the CSMA/CD procedure to work faultlessly. The data field can be used at will, it only has to contain complete bytes.

FCS (Frame Check Sequence)A check character. It is obtained by taking the rest of the division operation from the formula representing the wide-spread cyclic- redundancy-check procedure. This formula is applied to the bit sequence including the address field through to the padding field. In case of en error the whole frame is ignored, i.e. not passed on to the application program.

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Ethernet Address

WIN-NT: ipconfig /all

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Naming of the cable types

• Example: 10base5– 10 Transmission rate in Mbytes/s– base Base or Broadband– 5 Segment length in 100 meters

• UTP unshielded twisted pair• STP shielded twisted pair• S/STP screened shielded twisted pair

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Ethernet topologies

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Ethernet Media

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Extension

• The maximum extension depends on the medium and the transmission rate; here some examples:

– 10base5 Segment: 500mTotal: 2500m (with 4

repeaters)

– 100baseTX UTP Hub-Station: 100m

– 100baseFX Hub-Station: 400m25km (with Mono mode fibre)

– 1000baseSX Hub-Station: 550m

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Repeater / Hub

RepeaterHub

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Bridge

Bridge

A

B

C

D

E F

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Switch = Multiport Bridge

Bridge

Switch

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Hub - Switch

Switch

AB

C

DEF

G

H

Hub

A

B

C

DE

F

G

H

Hub

A

B

C

DE

F

G

H

Tim

e

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Characteristics of the switches

• Cut-Trough Switch – noc cheking of the data frames

• Store-and-Forward – checking of the data frames

• Frames with same destination– kept in internal short term memory thus queueing them– discard them or create collision

• Broadcast messages– go to all stations anyway (z.B. ARP) so switches are of

no advantage here– there are specific approaches of different switch

manufacturers to reduce broadcast data traffic

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Typical office wiring

Hub/Switch

Hub/Switch

Pat

ch f

ield

Pat

ch f

ield

Network socket

Patch cable

„normal“ Cat 5 cable

RJ 45

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www

• http://www.gigabit-ethernet.org• http://wwwhost.ots.utexas.edu/ethernet/• http://www.3com.com/technology/tech_net/white_papers/index.html#ethernet

• http://www.iaopennetworking.com/