6 Ethernet Basics

8/12/2019 6 Ethernet Basics

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Objectives

Ethernet fundamentals

Ethernet operation


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Introduction to Ethernet

The success of Ethernet is due to its simplicity

and ease of maintenance, as well as its ability to

incorporate new technologies, reliability, and low

cost of installation and upgrade.


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Comparing LAN Standards


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OSI Layer 1 and 2 Together Are the

Access Protocols

These are the deliverysystem protocols.

Independent of:

Network OS

Upper-level protocols TCP/IP, IPX/SPX

Sometimes called:

Access methods

Access protocols

Access technologies

Media access

LAN protocols

WAN protocols

Ethernet, Fast Ethernet, GigabitEthernet, Token Ring, FDDI, FrameRelay, ATM, PPP, HDLC, and so on


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IEEE Standard

Divided OSI Layer 2 into two sublayers Media Access Control (MAC)Traditional L2 features Transitions down to media

Logical link control (LLC)New L2 features Transitions up to the network layer


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Logical Link Control (LLC)

Allows part of the data link layer to function

independent of LAN access technologies

(protocols / methods)

Provides services to network layer protocols, whilecommunicating with access technologies below it

LAN access technologies: Ethernet

Token Ring FDDI


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Logical Link Control (LLC)

Participates in the data encapsulation process LLC PDU between Layer 3 and MAC sublayer.

Adds control information to the network layer data to

help deliver the packet. It adds two fields:

Destination Service Access Point (DSAP)

Source Service Access Point (SSAP)

Supports both connectionless and connection-

oriented upper-layer protocols.

Allows multiple higher-layer protocols to share a

single physical data link.


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Naming

Ethernet uses MAC addresses that are 48 bits inlength and expressed as 12 hexadecimal digits.

The first 6 hexadecimal digits, which are

administered by the IEEE, identify themanufacturer or vendor and thus comprise theorganizational unique identifier (OUI).

The remaining 6 hexadecimal digits represent

the interface serial number, or another valueadministered by the specific equipmentmanufacturer.


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Layer 2 Framing

Framing is the Layer

2 encapsulation

process; a frame is

the Layer 2 protocoldata unit.

A single generic

frame has sections

called fields, andeach field is

composed of bytes.


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Framing

Why framing is necessary

Frame format diagram

Generic frame format


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Ethernet Frame Fields

Some of the fields permitted or required in an

802.3 Ethernet frame are as follows:

Preamble

Start Frame Delimiter Destination Address

Source Address

Length/Type

Data and Pad

Frame Check Sequence (FCS)

Extension


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Ethernet Operation


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Media Access Control (MAC)

Provides MAC addressing (naming)

Depending on access technology (Ethernet,Token Ring, FDDI), provides the following: Data transmission control

Collision resolution (retransmission)

Layer 2 frame preparation (data framing)

Frame check sequence (FCS)


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Media Access Control (MAC)

Protocols Ethernet (IEEE 802.3)

Logical bus topology Physical star or extended star Nondeterministic

First-come, first-served

Token Ring (IEEE 802.5) Logical ring Physical star topology Deterministic

Token controls traffic Older declining technology

FDDI (IEEE 802.5)

Logical ring topology Physical dual-ring topology Deterministic

Token controls traffic Near-end-of-life technology


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Ethernet (CSMA/CD)

Carrier sense multiple access with collision detection


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Ethernet Timing

Any station on an Ethernet network wanting to

transmit a message first listens to ensure that

no other station is currently transmitting.

If the cable is quiet, the station beginstransmitting immediately.


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Interframe Spacing and Backoff


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Error Handling

Collisions are the mechanism for resolving

contention for network access.

Collisions result in network bandwidth loss that

is equal to the initial transmission and thecollision jam signal. This affects all network

nodes, possibly causing significant reduction in

network throughput.


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Types of Collisions

Three types of collisions are:

Local

Remote

Late


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Ethernet Errors

The following are the sources of Ethernet error: Simultaneous transmission occurring before slot time haselapsed (collision or runt)

Simultaneous transmission occurring after slot time haselapsed (late collision)

Excessively or illegally long transmission (jabber, longframe and range errors)

Illegally short transmission (short frame, collision fragmentor runt)

Corrupted transmission (FCS error)

Insufficient or excessive number of bits transmitted(alignment error)

Actual and reported number of octets in frame don't match(range error)

Unusually long preamble or jam event (ghostor jabber)


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Ethernet Auto negotiation

A process called autonegotiation (of speeds at

half or full duplex) was developed.

This process defines how two link partners may

automatically negotiate a configuration offeringthe best common performance level.

It has the additional advantage of only involving

the lowest part of the physical layer.


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Link Establishment

There are only two ways to achieve a full-duplex

link:

Through a completed cycle of autonegotiation

Or, by administratively forcing both link partners to fullduplex

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6 Ethernet Basics