nho cac bac dich giup em bai nay sang tieng viet

Typical LAN Features for OSI Layer 1The physical layer defines the standards used to send andreceive bits across a physical network.. Typical LAN Features for OSI Layer 2The data link la

Data Link Layer Fundamentals

Cisco Icons and Symbols

Different Layer 2 features

Ethernet and the LAN standards today.

Operation of hubs and switchs.

Ethernet family and its Future

Typical LAN Features for OSI Layer 1

The physical layer defines the standards used to send andreceive bits across a physical network

The physical layer defines the details of cabling:

The maximum length allowed for each type of cable

The number of wires inside the cable

The shape of the connector on the end of the cable…

Most cable include several conductors (wires) inside thecable

The endpoint of these wires are called pins, which endinside the connector

The physical layer also must define the purpose of eachpin

CAT5 Cable with RJ-45 Connector

RJ-45 is a typical connector used with Ethernet cabling today.

One pair of wires is used for transmitting data, using pins 1 and 2.

One pair of wires is used for receiving data, using pins 3 and pins 6.

Typical LAN Features for OSI Layer 2

The data link layer defines the standards and protocols used to control the transmission of data across a physical network.

Each data link protocol “controls” a particular type of physical layer network, the details of how a data link protocol functions must include some consideration of the physical network.

Most of data link protocols perform the following functions:

Arbitration Addressing Error detection Identification of the encapsulated data

Data Link Function 1: Arbitration

With some type of physical networks, data frames cancollide if devices can send any time they want

When frames collide in a LAN, the data in each frame iscorrupted and the LAN is unusable for a brief moment

The specifications for these data-link protocols define how

to arbitrate the use of the physical medium to avoidcollisions, ar at least to recover from the colisions whenthey occur

Ethernet uses the carrier sense multiple access with collision detection (CSMA/CD) algorithm for arbitration

Data Link Function 2: Addressing

Many physical networks allow more than two devicesattached to the same physical network

Data-link protocols define addresses to make sure that thecorrect device listens and receives the data that is sent

By putting the correct address in the data-link header, thesender of the frame can be relatively sure that the correctreceiver will get the data

Each data-link protocol defines its own unique addressingstructure

Ethernet uses Media Access Control (MAC)

Frame Relay uses data-link connection identifier (DLCI)

Data Link Function 3: Error Detection

Error detection discovers whether bit errors occurredduring the transmission of the frame

Most data-link protocols include a frame check sequence (FCS) or cyclical redundancy check (CRC) field in the data-link trailer

This field contains a value that is the result of amathematical formula applied to the data in the frame

If the FCS sent by the sender matches the FCS calculated

by the receiver, the frame did not have any errors

Error detection does not imply recovery

The FCS allows the receiving device to notice that errorsaccurred and then discard the data frame

Data Link Function 4: Identifying the Encapsulated Data

This function identifies the contents of the Data field in

the frame

Each data-link header has a Type field to identify the type

of protocol that sits inside the frame’s data field

IEEE Ethernet 802.2 Logical Link Control (LLC) uses a field

in its header to identify the type of data in the Data field

Coming up … Ethernet Standard

Early Ethernet Standards

In this section, you learn about the three earliest types of Ethernet networks.

The term Ethernet refers to a family of protocols and standards that together define the physical and data link layers of LAN.

There are many variations of Ethernet

More popular: 10BASET, Fast Ethernet, GiganetOlder types: 10BASE2, 10BASE5

Ethernet Standards Overview

Xerox needed an effective way to allow a new invention, called the personal computer, to be connected in its offices.

Xerox teamed with Intel and Digital Equipment Corp (DEC) to further develop Ethernet.

The original Ethernet became known as DIX Ethernet , meaning DEC, Intel, and Xerox.

The IEEE created a standardized version of Ethernet base on the work performed by DEC, Intel, and Xerox.

Low cost of installation and upgradeBandwidth can be increased without changing underlyingtechnology

Ethernet and the OSI model

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

Original Ethernet Standard: 10BASE5

The original 1980 Ethernet product 10BASE5 transmitted 10 Mbps over a single thick coaxial cable bus.

10BASE5 was the first medium used for Ethernet.

The primary benefit of 10BASE5 was length.

10BASE5 systems also represent a single point of failure.

Each of the maximum five segments of thick coax may be up to

Original Ethernet Standard: 10BASE2

10BASE2 was introduced in 1985.

Installation was easier because of its smaller size, lighter weight, and greater flexibility.

Computers on the LAN were linked together by an unbroken series of coaxial cable lengths.

Each of the maximum five segments of thin coax may be up to

185 meters long and each station is connected directly to the BNC “T” connector on the coax.

Only one station can transmit at a time or else a collision will occur.

10BASE2 also uses half-duplex The maximum transmission rate of 10BASE2 is 10 Mbps

How CSMA/CD works?

After the jamming is complete, each sender randomizes a timer and waits that long ( backoff algorithm).

When each timer expires, the process stars over with step 1.

10BASE-T networks make use of devices called hubs.

The hubs are multiport repeaters

The hub simply regenerates the electrical signal that come inone port and sends the same signal out every other port

By doing so, 10BASE-T creates an electrical bus, just like 10BASE5 and 10BASE2.

Collisions can still occur, so CSMA/CD access rulescontinue to be used

With 10BASE-T, a cable is run from each device to a hub, so

a single cable problem affects only one device

All devices are sharing a single bus, so we call this sharedEthernet

10BASE-T Hub operation

1 NIC sends a frame.

2 NIC loops the frame back.

3 Hub receives the frame.

4 Hub sends it to all ports,

except the received port.

5 Hub sends the frame out.

If two PCs sent a frame

at the same time, a

collision would occur.

Collisions and Duplex Settings

10BASE2, 10BASE5, and 10BASE-T Ethernet would not work without CSMA/CD.

With CSMA/CD algorithm, Ethernet becomes more inefficient under higher loads.

In the next section, you will read about two things that have improved network performance, both relating to the reduction or even elimination of collisions:

LAN SwitchingFull-Duplex Ethernet

Reducing Collisions - LAN Switching

LAN switches overcome the

problems created by collisions and

the CSMA/CD algorithm by removing

the possibility of a collision.

Switches do not create a single

shared bus, they treat each

individual port as a separate bus.

Switches use memory buffers to hold

incoming frame.

Switch performs OSI Layer 2

functions by interpreting Ethernet

header information to make

forwarding decisions.

Full-Duplex Ethernet

The original Ethernet specifications used a shared bus, over which a single device could not be sending a frame and receiving a frame at the same time because it would mean that

a collision was occurring.

That logic is called half-duplex logic.

Ethernet switches allow multiple frames to be sent over different ports at the same time.

If only one device is connected to a switch port, there is never

a possibility that a collision could occur.

LAN switches with only one device cabled to each port of the switch can send and receive concurrently.

This is full-duplex operation.

Ethernet Addressing

Ethernet LAN addressing identifies either individualdevices or groups of devices on a LAN

Unicast Ethernet addresses identify a single LAN card

Each address is 6 bytes long, is usually written inhexadecimal, with periods separating each set of four hexdigits

For example, 0000.0C12.3456 is a valid Ethernet address

Computers use these addresses to identify the sender andreceiver of an Ethernet frame

IEEE calls them MAC addresses

Unicast Ethernet Address

The IEEE requires globally unique unicast MAC addresses

on all LAN interface cards

To ensure a unique MAC address, the Ethernet cardmanufacturers encode the MAC address onto the card,usually in a ROM chip

Is is also called burned-in address (BIA).The first half of the address identifies the manufacturer ofthe card, is called the organizationally unique identifier (OUI)

The second half of the address being assigned a numberthat this manufacturer has never used on another card

Broadcast addresses – the broadcast address, has a

value of FFFF.FFFF.FFFF (hexadecimal notation) Thebroadcast address implies that all devices on the LANshould process the frame

Multicast addresses – Multicast addresses are used to

allow a subset of devices on a LAN to communicate

Recent Ethernet Standards

Fast Ethernet Gigabit Ethernet 10-Gigabit Ethernet

Fast Ethernet

Fast Ethernet, as defined in IEEE 802.3u , is also known as Mbps Ethernet.

100-The two technologies that have become important are

100BASE-TX , which is a copper UTP medium and 100BASE-FX , which is a multimode optical fiber medium.

The CSMA/CD logic can be disabled for full-duplex point topologies.

point-to-The 802.3u specification calls for the use of the same old IEEE 802.3 MAC and 802.2 LLC framing for the LAN headers and trailers.

Both Fast Ethernet shared hubs and switches can be deployed.

Fast Ethernet (cont)

Fast Ethernet’s autonegotiation function allows anEthernet card or switch to negotiate dynamically todiscover whether it should use either 10 or 100 Mbps

Many Ethernet cards and switch ports are called 10/100cards or ports today because they can autonegotiate thespeed

The endpoints autonegotiate whether to use half duplex

or full duplex as well

If autonegotiation fails, it settles for half-duplex operation

at 10 Mbps

Gigabit Ethernet

The IEEE defines Gigabit Ethernet in standards 802.3z for

optical cabling and 802.3ab for electrical cabling

CSMA/CD still is used and can be disabled for full-duplex

support

The 802.3z and 802.3ab standards call for the use of the same

old IEEE 802.3 MAC and 802.2 LLC framing for the LAN headers

and trailers

The most likely place to use Gigabit is between switches,

between switches and a router, and between a switch and a

server

The same Ethernet headers and trailers are used, regardless of

whether it’s 10 Mbps, 100 Mbps, or 1000 Mbps

It have some differences with the other types:

It allows only a point-to-point topology because it isintended for connectivity between switching devices

It allows only full-duplex communication

It specifies only optical fiber - no copper cabling (Supportfor copper wiring might be added later.)

10 Gig Ethernet using SM fiber for up to 40km

Future of Ethernet

Ethernet has gone through an evolution from Legacy → Fast →

Gigabit → MultiGigabit technologies.

The future of networking media is threefold:

Copper (up to 1000 Mbps, perhaps more)

Wireless (approaching 100 Mbps, perhaps more)

Optical fiber (currently at 10,000 Mbps and soon to be more)

Some discussions between IEEE members have begun that suggest the

The basics of Ethernet technology

How Ethernet and the OSI model interact

Ethernet frame field names and purposes

10Base2, 10Base5, 10BaseT

The characteristics and function of CSMA/CD

The backoff algorithm and time after a collision

Ethernet errors and collisions

Auto-negotiation in relation to speed and duplex

Characteristics and varieties of 100-Mbps and 1000-Mbps Ethernet

Stop here!

It’s time for questioning.