ccnet lec 09 ethernet 3

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Lecture

09Data Communication & Networks

IEEE 802.3 (Ethernet)

Muhammad Yousaf



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LAN Standards:

IEEE has defined many standards for LANsInstitute of Electrical & Electronics Engineers

In the beginning of 1980s IEEE initiated the project802

Aim of this project was to develop standards foremerging LAN technologies

So that the equipment from different vendors can

work together



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LAN Standards: cont…

Following are some of many IEEE standardsIEEE 802.3 (Ethernet)

IEEE 802.4 (Token Bus)

IEEE 802.5 (Token Ring)

IEEE 802.11 (Wireless LAN)

IEEE 802.15 (Bluetooth)

IEEE 802.16 (WiMAX)

Most popular of these is 802.3

Base of 802.3 is CSMA/CD

Carrier Sense Multiple Access with Collision Detection



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Scope of LAN Technologies:

IETF = Internet Engineering Task Force IEEE = Institute of Electrical & Electronic Engineers

Physical

Data Link

IP

TCP UDP

Application

IEEE

IETF



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Bus/Star Configurations:



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Ethernet Transmission:



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Ethernet Card:

RJ=Registered Jack

UTP=Unshielded Twisted Pair

LED=Light Emitting Diode



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Media Access Control (MAC) Address:

Many levels of addressing are used in computer networksDomain Names (highest level of addressing)

IP addresses (for routing)

MAC addresses (lowest level of addressing)

MAC address is the physical Address of Network InterfaceCard (NIC)

It is 48-bit address, written in hexadecimal notation

First 24-bits are organizational identifier

Next 24-bits are network interface identifier



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MAC Addresses: cont…

C:\yousaf>ipconfig /all

Ethernet adapter Local Area Connection:

Media State . . . . . . . : Media disconnected

Description . . . . . . . .: Realtek RTL8139/810x Family Fast Ethernet NIC

Physical Address. . . .: 00-16-D3-03-0A-D4

Ethernet adapter Wireless Network Connection:

Media State . . . . . . . : Media disconnected

Description . . . . . . . .: Intel(R) PRO/Wireless 2200BG Network Connection

Physical Address. . . .: 00-16-6F-52-F3-8B



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802.3 Frame:

Header Trailer

SFD=Start Frame Delimiter MAC=Media Access Control




802.3 Frame Fields:

Preamble + SFD:Used for synchronization

Start of Frame Delimiter

Destination & Source Addresses:

MAC addresses of receiver & sender

MAC = Media Access Control

Length:

Length of Frame in bytes

Type:

Type of next protocol header; e.g. ARP, IP, etc

Frame Check Sequence: (FCS)

CRC Value; used for error detection




Media Access Control:

Two major types of protocols that control rights totransmit over the physical medium

Primary / Secondary Protocols:Primary station controls the access on media

Contention-Free protocolsMedia Access is simple and controlled

Peer-to-Peer Protocols:All stations have equal rights to send data

For transmission, node doesn’t need to take permissionfrom any other node

If more than one nodes try to send data simultaneously,then there will be collision




CSMA/CD:

CSMA/CD is a peer-to-peer protocolAll nodes share a common transmission medium

Only one station can transmit at a time

But which station?

This is controlled by the CSMA/CD protocol

Multiple Access means this protocol is for multipoint links(shared links)

Carrier Sense means that nodes will sense the carrier(medium) before transmission

Collision Detection




CSMA/CD: (look before you speak)

If some node is transmitting, it means that mediumis busy

Else medium is idle

Whenever a node needs to transmit, it senses the

medium before starting to transmit

If medium is idle then start transmitting

If medium is busy then wait

In this way we can avoid almost all collisions

Can collision still occur???




CSMA/CD Transmissions:




CSMA/CD: cont…

Yeah! Collision can still occur in followingconditions:

More than one nodes try to transmit at the same time

Time difference in transmitting from two nodes is less

than the propagation delay between themSo, we need a collision detection mechanism

Nodes sense the medium while transmission

In case of collision, voltage level on the medium willbe much greater than the valid voltage level

In this way, collision can easily be detected




CSMA/CD: cont…

When a node detects the collision, it immediatelystops transmitting data & sends a jamming signal

Jamming signal is a short message, declaring emergencyon medium so that all nodes be aware of collision

Note: this collision happened due to the fact thatmore than one nodes were trying to transmitsimultaneously

Now competing nodes will again try to contend

We need a contention resolution mechanism

The most popular contention resolution algorithm isBinary Exponential Backoff (BEB)




Transmission pattern:




Binary Exponential Backoff:

Base of BEB is the contention window

Initially contending nodes randomly choose from 2-contention slots

Nodes wait for the time equal to the period of contentionslots

After this random time slot nodes again come and sense themedium and if found idle then they are allowed to transmit

If only one node picks the shortest slot then contention isresolved

Otherwise at every collision, contention window size isdoubled to 4, 8, 16, …

But there is maximum window limit e.g. 32 or 128-slots




IP

TCP UDP

Application

Putting 802.3 in protocol stack:

Physical

Data Link

IEEE

IETF

Signal Tx / Rx

Data Encoding

Media Control

Data Encapsulation / Decapsulation




Putting 802.3 in protocol stack: cont…

Data Encapsulation/ Decapsulation:Appends MAC header and trailer

Also appends the synchronization bits

Media Control:

Buffers the upper layer dataMakes decision according to CSMA/CD & BEB

Data Encoding:Conversion of data bits into voltage levels

Ethernet uses Manchester Encoding schemeSignal Transmission/Reception:

Transceivers actually send/receive the signals

Can also sense the medium




Evolution of Ethernet:

Ethernet:10Mbps

Pioneering work done by Dr. Robert Metcalf in 1972

In 1979 Xerox, DEC & Intel formed a consortium for development ofEthernet

In 1983 IEEE standardized it

Implemented in Bus / star topology

Fast Ethernet:100Mbps

Approved by IEEE in 1995

Star topologyGigabit Ethernet:

1Gbps to 10Gbps

Standardized by IEEE in 1998

Star topology




Typical Configuration of Ethernet:




Questions ???

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