Ethernet

Ethernet

Ethernet is a standard communications protocol

embedded in software and hardware devices,

intended for building a local area network (LAN)

Ethernet is a contention media access method that

allows all hosts on a network to share the same bandwidth of a

link.

Ethernet is popular because it’s readily scalable,

meaning it’s comparatively easy to integrate new

technologies, such as FastEthernet and Gigabit Ethernet, into

an existing network infrastructure.

Ethernet defines the lower two layers of the OSI Reference

Model

The IEEE divides the data link layer into two sub-layers, the

logical link control (LLC) layer and the media access control

(MAC) layer The MAC layer varies for different network types

and is defined by standards IEEE 802.3 through IEEE 802.5

IEEE 802.3 to ISO

IEEE 802.3 to ISO

                              Ethernet's Logical Relationship to the ISO Reference Model

Ethernet Network Topologies and Structures

Ethernet uses a bus or star topology and supports

data transfer rates of 10/100/1000 Mbps

                                                         Ethernet Network Topologies and

Structures

Ethernet networking uses Carrier Sense Multiple Access with

Collision Detection (CSMA/CD), a protocol that helps devices

share the bandwidth evenly without having two devices

transmit at the same time on the network medium.

CSMA/CD

Network access regulated by the CSMA/CD procedure:

Carrier Sense = “listen” to the cable

Multiple Access = all stations can send at the same time

• Collision Detect = collisions must be recognized

MAC Types

There are two Media Access Control(MAC)

protocols defined for Ethernet:

- Half-Duplex

- Full-Duplex

Half-Duplex

Refers to the transmission of data in just one

direction at a time

Half-Duplex Ethernet is the traditional form of

Ethernet that uses the CSMA/CD

Carrier Sense Multiple Access Collision Detect(Half-Duplex)

The network is monitored for presence of a transmitting

station (carrier sense)

Adapter does not transmit if it senses that some other

adapter is transmitting

The transmission is delayed if an active carrier is detected

The station continues to monitor the network until the carrier

stops

If an active carrier is not detected, then the station

immediately begins transmission

While the transmitting station is sending the frame, it

monitors the medium for a collision.

If a collision is detected, the transmitting station stops

sending the frame data

Before attempting a retransmission, adapter waits a

random time, that is, random access

If repeated collisions occur, then transmission

is repeated

But the random delay is increased with each

Attempt

This process repeats until a station transmits a

frame without collision

State Diagram for CSMA/CD

State Diagram for CSMA/CD

Full-Duplex

Based on the IEEE 802.3x standard, “Full- Duplex” MAC

type bypasses the CSMA/CD protocol

Full-duplex mode allows two stations to simultaneously

exchange data over a point to point link

The aggregate throughput of the link is effectively

doubled

A full-Duplex 100 Mb/s station provides 200

Mb/s of bandwidth

Use of Full-duplex Ethernet

Full-duplex Ethernet can be used in three situations:

- With a connection from a switch to a host

- With a connection from a switch to a switch

- With a connection from a host to a host using a

crossover cable

Ethernet at the Physical Layer

Ethernet was first implemented by a group called DIX

(Digital, Intel, and Xerox).

They created and implemented the first Ethernet LAN

specification, which the IEEE used to create the IEEE

802.3 Committee.

This was a 10Mbps network that ran on coax, and then

eventually twisted-pair, and fiber physical media.

Ethernet (IEEE802.3) Frames

Ethernet (IEEE802.3) Frames

Preamble

- A sequences of 64 bits used for synchronization

- Detects the presence of a signal

- Begin reading the signal before the frame data

arrives

Destination & Source MAC Addresses

– The Destination MAC Address field identifies

the station or stations that are to receive the

Frame

– The Source MAC Address identifies the station

that originated the frame

Length

- Specifies the length of packet

MAC Client Data

- Contains the data transferred from the source

station to the destination station or stations

Frame Check Sequence

– Contains a 4-byte cyclical redundancy check

(CRC) value used for error checking

Cyclical Redundancy Check

The CRC method operates on blocks of data called

frames.

Basically, the sender appends a bit sequence to every

frame, called the FCS (frame check sequence).

The resulting frame is exactly divisible by a predetermined

number.

The receiving computer divides the frame by the

predetermined number.

If there is a remainder, the frame is considered corrupted

and a retransmission is requested.

Ether net Standards

IEEE 802.1: Standards related to network management

IEEE 802.2: General standard for the data link layer in

the OSI Reference Model

– The IEEE divides this layer into two sublayers

- Logical link control (LLC) layer

- Media Access Control (MAC) layer

– The MAC layer varies for different network types and

is defined by standards IEEE 802.3 through IEEE 802.5

IEEE 802.3: Defines the MAC layer for bus networks that use

CSMA/CD. This is the basis of the Ethernet

Standard IEEE 802.4: Defines the MAC layer for bus

networks that use a token-passing mechanism

(token bus networks)

IEEE 802.5: Defines the MAC layer for token-ring networks

IEEE 802.6: Standard for Metropolitan Area

Networks (MANs)