bài giảng mạng máy tính căn bản chương 6 nguyên lý cơ bản của etherne

Phan Vĩnh Thuần Computer Network 1Chương 6 NGUYÊN LÝ CƠ BẢN CỦA ETHERNET ETHERNET FUNDAMENTALS... Phan Vĩnh Thuần Computer Network 5• Students completing this module should be able to:

Phan Vĩnh Thuần Computer Network 1

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

Phan Vĩnh Thuần Computer Network 3

• 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

Phan Vĩnh Thuần Computer Network 5

• Students completing this module should be able to:

– Describe the basics of Ethernet technology – Explain naming rules of Ethernet technology

– Define how Ethernet and the OSI model interact

– Describe the Ethernet framing process and frame structure

– List Ethernet frame field names and purposes

– 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

Phan Vĩnh Thuần Computer Network 7

6.1 Ethernet Fundamentals

6.1.1 Introduction to Ethernet

• Most of the traffic on the Internet 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 same protocol that transported data at 3 Mbps in 1973 is carrying data at 10 Gbps

• 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

Phan Vĩnh Thuần Computer Network 9

• 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/

Access/Collision Detection)

Phan Vĩnh Thuần Computer Network 11

• 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 everyone could benefit, so it was released as an open standard

• 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

Phan Vĩnh Thuần Computer Network 13

• 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 Ethernet standard were made in 802.3

• 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

Phan Vĩnh Thuần Computer Network 15

• 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 per second (Gbps, 1 billion bits per second) Ethernet in 1998 and 1999

• 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

Phan Vĩnh Thuần Computer Network 17

• 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

Phan Vĩnh Thuần Computer Network 19

• 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 of:

–A number indicating the number of Mbps transmitted

–The word base, indicating that baseband signaling is used

–One or more letters of the alphabet indicating the type of medium used (F= fiber optical cable, T = copper unshielded twisted pair)

Phan Vĩnh Thuần Computer Network 21

• 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 broadcasts and cable TV use broadband signaling.

Phan Vĩnh Thuần Computer Network 23

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

Phan Vĩnh Thuần Computer Network 25

• 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 usually impact the entire collision domain

Phan Vĩnh Thuần Computer Network 27

• 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 and regenerate the signal

• 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

Phan Vĩnh Thuần Computer Network 29

• 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 functions has limitations Layer 2 addresses these limitations

Phan Vĩnh Thuần Computer Network 31

• 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 communication process

Phan Vĩnh Thuần Computer Network 33

• 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

Phan Vĩnh Thuần Computer Network 35

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

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

Phan Vĩnh Thuần Computer Network 37

• 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

Phan Vĩnh Thuần Computer Network 39

• 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 physical destination address carried by the data frame

• 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

Phan Vĩnh Thuần Computer Network 41

6.1.5 Tạo frame ở lớp 2 (Layer 2 framing)

• Encoded bit streams (data) on physical

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 bit streams alone

• 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

Phan Vĩnh Thuần Computer Network 43

• 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

Phan Vĩnh Thuần Computer Network 45

• 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 address) and the name of the destination node (MAC address)

• 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

Phan Vĩnh Thuần Computer Network 47

• 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 application data and the encapsulated bytes to be sent to the destination computer Padding bytes may

be added so frames have a minimum length for timing purposes