NGUYÊN lý cơ bản của ETHERNET (MẠNG máy TÍNH cơ bản SLIDE)

• The success of Ethernet is due to the following factors: – Simplicity and ease of maintenance – Ability to incorporate new technologies – Reliability – Low cost of installation and

Chương 6

NGUYÊN LÝ CƠ BẢN CỦA ETHERNET (ETHERNET FUNDAMENTALS)

• Ethernet is now the dominant LAN

technology in the world Ethernet is not one technology but a family of LAN technologies and may be best understood by using the OSI reference model

• All LANs must deal with the basic

issue of how individual stations (nodes) are named, and Ethernet is

no exception Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations However, the basic frame format and addressing scheme is the same for all varieties of Ethernet.

• For multiple stations to access

physical media and other networking devices, various media access control strategies have been invented Understanding how network devices gain access to the network media is essential for understanding and troubleshooting the operation of the entire network

• Students completing this module should

– Describe the Ethernet framing process

and frame structure

– Identify the characteristics of CSMA/CD

– Describe the key aspects of Ethernet

timing, interframe spacing and backoff time after a collision

– Define Ethernet errors and collisions – Explain the concept of auto-

negotiation in relation to speed and duplex

6.1 Ethernet Fundamentals

6.1.1 Introduction to Ethernet

originates and ends with Ethernet connections From its beginning in the 1970s, Ethernet has evolved to meet the increasing demand for high speed LANs When a new media was produced, such

as optical fiber, Ethernet adapted to take advantage of the superior bandwidth and low error rate that fiber offers Now, the

• The success of Ethernet is due to the

following factors:

– Simplicity and ease of maintenance

– Ability to incorporate new technologies

– Reliability

– Low cost of installation and upgrade

• With the introduction of Gigabit Ethernet,

what started as a LAN technology now extends out to distances that make Ethernet a metropolitan-area network (MAN) and wide-area network (WAN) standard

• The original idea for Ethernet grew out

of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other This problem of multiple user access to a shared medium was studied in the early 1970s

at the University of Hawaii

• A system called Alohanet was

developed to allow various stations

on the Hawaiian Islands structured access to the shared radio frequency band in the atmosphere.  This work later formed the basis for the Ethernet access method known as CSMA/CD (Carrier Sense Multiple Access/Collision Detection)

• The first LAN in the world was the

original version of Ethernet Robert Metcalfe and his coworkers at Xerox designed it more than thirty years ago The first Ethernet standard was published in 1980 by a consortium of Digital Equipment Company, Intel, and Xerox (DIX) Metcalfe wanted Ethernet

to be a shared standard from which

• The first products developed using the

Ethernet standard were sold during the early 1980s Ethernet transmitted at up to

10 Mbps over thick coaxial cable up to a distance of two kilometers This type of coaxial cable was referred to as thicknet

• In 1985, the Institute of Electrical and

Electronics Engineers (IEEE) standards committee for Local and Metropolitan Networks published standards for LANs.

• These standards start with the number

802 The standard for Ethernet is 802.3 The IEEE wanted to make sure that its standards were compatible with the International Standards Organization (ISO)/OSI model To do this, the IEEE 802.3 standard had to address the needs of Layer 1 and the lower portion

of Layer 2 of the OSI model As a result, some small modifications to the original

• The differences between the two

standards were so minor that any Ethernet network interface card (NIC) can transmit and receive both Ethernet and 802.3 frames Essentially, Ethernet and IEEE 802.3 are the same standards

• The 10-Mbps bandwidth of Ethernet was more than enough for the slow personal computers (PCs) of the 1980s By the early 1990s PCs became much faster, file sizes increased, and data flow bottlenecks were occurring Most were caused by the low availability of bandwidth In 1995, IEEE announced a standard for a 100-Mbps Ethernet This was followed by standards for gigabit

• All the standards are essentially

compatible with the original Ethernet standard An Ethernet frame could leave

an older coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps Ethernet fiber link, and end up at a 100-Mbps NIC As long as the packet stays on Ethernet networks it

is not changed For this reason Ethernet

is considered very scalable The bandwidth of the network could be increased many times without changing the underlying Ethernet technology

• The original Ethernet standard has

been amended a number of times in order to manage new transmission media and higher transmission rates These amendments provide standards for the emerging technologies and maintain compatibility between Ethernet variations

6.1.2 IEEE Ethernet naming rules

• Ethernet is not one networking

technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet Ethernet speeds can be 10, 100, 1000,

or 10,000 Mbps The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet.

• When Ethernet needs to be

expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard The new supplements are given a one or two letter designation such

as 802.3u An abbreviated description (called an identifier) is also assigned to the supplement

• The abbreviated description consists

• Ethernet relies on baseband signaling,

which uses the entire bandwidth of the transmission medium The data signal

is transmitted directly over the transmission medium In broadband signaling, not used by Ethernet, the data signal is never placed directly on the transmission medium An analog signal (carrier signal) is modulated by the data signal and the modulated carrier signal is transmitted Radio

6.1.3 Ethernet and the OSI model

• Ethernet operates in two areas of the

OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer

• To move data between one Ethernet

station and another, the data often passes through a repeater All other stations in the same collision domain see traffic that passes through a repeater A collision domain is then a shared resource Problems originating

in one part of the collision domain will

• A repeater is responsible for

forwarding all traffic to all other ports Traffic received by a repeater is never sent out the originating port Any signal detected by a repeater will be forwarded If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct

• Standards guarantee minimum

bandwidth and operability by specifying the maximum number of stations per segment, maximum segment length, maximum number of repeaters between stations, etc Stations separated by repeaters are within the same collision domain Stations separated by bridges

or routers are in different collision domains

• The figure maps a variety of Ethernet

technologies to the lower half of OSI Layer 2 and all of Layer 1 Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various topologies Ethernet Layer 1 performs a key role in the communication that takes place between devices, but each of its

• Data link sublayers contribute

significantly to technology compatibility and computer communication The MAC sublayer is concerned with the physical components that will be used to communicate the information The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for

• The figure maps a variety of Ethernet

technologies to the lower half of OSI Layer 2 and all of Layer 1 While there are other varieties of Ethernet, the ones shown are the most widely used.

 Interactive Media Activity

6.1.4 Đặt tên (Naming)

• To allow for local delivery of frames on

the Ethernet, there must be an addressing system, a way of uniquely identifying computers and interfaces Ethernet uses MAC addresses that are

48 bits in length and expressed as twelve hexadecimal digits

• The first six hexadecimal digits, which

are administered by the IEEE, identify the manufacturer or vendor This portion of the MAC address is known as the Organizational Unique Identifier (OUI) The remaining six hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer MAC addresses are sometimes referred to as burned-in addresses (BIA) because they are burned into read-only memory (ROM) and are copied into random-access memory (RAM) when the NIC initializes

• At the data link layer MAC headers and trailers

are added to upper layer data The header and trailer contain control information intended for the data link layer in the destination system Data from upper layer entities is encapsulated

in the data link layer header and trailer

• The NIC uses the MAC address to assess

whether the message should be passed onto the upper layers of the OSI model The NIC makes this assessment without using CPU processing time, enabling better communication times on an Ethernet network

• On an Ethernet network, when one device

sends data it can open a communication pathway to the other device by using the destination MAC address The source device attaches a header with the MAC address of the intended destination and sends data onto the network As this data propagates along the network media the NIC in each device on the network checks

to see if the MAC address matches the

• If there is no match, the NIC discards the data frame When the data reaches the destination node, the NIC makes a copy and passes the frame up the OSI layers

On an Ethernet network, all nodes must examine the MAC header even if the communicating nodes are side by side

• All devices that are connected to the Ethernet LAN have MAC addressed interfaces including workstations, printers, routers, and switches

6.1.5 Tạo frame ở lớp 2 (Layer 2 framing)

• Encoded bit streams (data) on physical

media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen Framing helps obtain essential information that could not, otherwise, be obtained with coded

• Examples of such information are:

– Which computers are communicating with

one another

– When communication between individual

computers begins and when it terminates

– Provides a method for detection of errors

that occurred during the communication

– Whose turn it is to "talk" in a computer

"conversation"

• Framing is the Layer 2 encapsulation

process A frame is the Layer 2 protocol data unit

• There are many different types of

frames described by various standards A single generic frame has sections called fields, and each field is composed of bytes The names of the fields are as follows:

– Start frame field

– Address field

– Length / type field

• When computers are connected to a

physical medium, there must be a way they can grab the attention of other computers

to broadcast the message, "Here comes a frame!" Various technologies have different ways of doing this process, but all frames, regardless of technology, have a beginning signaling sequence of bytes

• All frames contain naming information,

such as the name of the source node (MAC

• Most frames have some specialized

fields In some technologies, a length field specifies the exact length of a frame in bytes Some frames have a type field, which specifies the Layer 3 protocol making the sending request

• The reason for sending frames is to

get upper layer data, ultimately the user application data, from the source to the destination The data package has two parts, the user

encapsulated bytes to be sent to the destination computer Padding bytes

• Logical link control (LLC) bytes are

also included with the data field in the IEEE standard frames The LLC sub-layer takes the network protocol data, an IP packet, and adds control information to help deliver that IP packet to the destination node Layer

2 communicates with the upper-level layers through LLC

• All frames and the bits, bytes, and

fields contained within them, are susceptible to errors from a variety

of sources The Frame Check Sequence (FCS) field contains a number that is calculated by the source node based on the data in the frame This FCS is then added

to the end of the frame that is

• When the destination node receives

the frame the FCS number is recalculated and compared with the FCS number included in the frame If the two numbers are different, an error is assumed, the frame is discarded, and the source is asked

to retransmit

• There are three primary ways to

calculate the Frame Check Sequence number:

– Cyclic Redundancy Check (CRC) –

performs calculations on the data

– Two-dimensional parity – adds an 8th

bit that makes an 8 bit sequence have

an odd or even number of binary 1s

• The node that transmits data must get

the attention of other devices, in order to start a frame, and to end the frame The length field implies the end, and the frame is considered ended after the FCS Sometimes there

is a formal byte sequence referred to

as an end-frame delimiter

6.1.6 Cấu trúc frame Ethernet (Ethernet frame structure)

• At the data link layer the frame

structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps However, at the physical layer almost all versions of Ethernet are substantially different from one another