bài giảng mạng máy tính căn bản chương 7 các công nghệ ethernet ethernet techologies phan vĩnh thuần

The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100 Mbps Fast Ethernet.. Optical fiber to support Gigabit Ethernet is considere

CHƯƠNG 7

CÁC CÔNG NGHỆ ETHERNET

ETHERNET TECHNOLOGIES

• Ethernet has been the most successful LAN technology largely because of its simplicity of implementation compared to other technologies Ethernet has also been successful because it has been a flexible technology that has evolved

to meet changing needs and media capabilities This module introduces the specifics of the most important varieties of Ethernet The goal is not

to convey all the facts about each type of Ethernet, but rather to develop a sense of what

is common to all forms of Ethernet.

• Changes in Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the early 1980s The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100 Mbps Fast Ethernet In recent years, an even more rapid growth in media speed has moved the transition from Fast Ethernet to Gigabit Ethernet The standards for Gigabit Ethernet emerged in only three years An even faster Ethernet version, 10 Gigabit Ethernet, is now widely available and still faster versions are being

• In these faster versions of Ethernet, MAC addressing, CSMA/CD, and the frame format have not been changed from earlier versions of Ethernet However, other aspects of the MAC sublayer, physical layer, and medium have changed Copper-based network interface card (NICs) capable of 10/100/1000 operation are now common Gigabit switch and router ports are becoming the standard for wiring closets Optical fiber to support Gigabit Ethernet is considered a standard for

• Students completing this module should be able to:

– Describe the differences and similarities among 10BASE5, 10BASE2, and 10BASE-

T Ethernet

– Define Manchester encoding

– List the factors affecting Ethernet timing limits

– List 10BASE-T wiring parameters

– Describe the key characteristics and varieties of 100-Mbps Ethernet

– Describe the evolution of Ethernet

– Explain the MAC methods, frame formats, and transmission process of Gigabit Ethernet

– Describe the uses of specific media and encoding with Gigabit Ethernet

– Identify the pinouts and wiring typical to the various implementations of Gigabit Ethernet

– Describe the similarities and differences between Gigabit and 10 Gigabit Ethernet

– Describe the basic architectural considerations

7.1.10-Mbps and 100-Mbps Ethernet

7.1.1 10Mbps Ethernet

• 10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet The four common features of Legacy Ethernet are timing parameters, frame format, transmission process, and a basic design rule

• 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters, as shown in the figure (1 bit time at 10 Mbps =

100 nsec = 0.1 µsec = 1 ten-millionth of a second.)

• 10BASE5, 10BASE2, and 10BASE-T also have a common frame format.

• The Legacy Ethernet transmission process

is identical until the lower part of the OSI physical layer The Layer 2 frame data is converted from hex to binary As the frame passes from the MAC sublayer to the physical layer, further processes occur prior to the bits being placed from the physical layer onto the medium One important process is the signal quality error (SQE) signal SQE is always used in half-duplex SQE can be used in full-duplex operation but is not required

• SQE is active:

– Within 4 to 8 microseconds following a normal transmission to indicate that the outbound frame was successfully transmitted

– Whenever there is a collision on the medium – Whenever there is an improper signal on the medium Improper signals might include jabber, or reflections that result from a cable short

– Whenever a transmission has been interrupted

• All 10 Mbps forms of Ethernet take octets received from the MAC sublayer and perform a process called line encoding Line encoding describes how the bits are actually signaled on the wire The simplest encodings have undesirable timing and electrical characteristics So line codes have been designed to have desirable transmission properties This form of encoding used in 10 Mbps systems is

• Manchester encoding relies on the direction

of the edge transition in the middle of the timing window to determine the binary value for that bit period The top waveform has a falling edge, so it is interpreted as a binary 0 The second waveform shows a rising edge, which is interpreted as a binary

1 In the third waveform, there is an alternating binary sequence With alternating binary data, there is no need to return to the previous voltage level

• As can be seen from the third and fourth wave forms in the graphic, the binary bit values are indicated by the direction of change during any given bit period The waveform voltage levels at the beginning or end of any bit period are not factors when determining binary values

• Legacy Ethernet has common architectural features Networks usually contain multiple types of media The standard ensures that interoperability is maintained The overall architectural design is of the utmost importance when implementing a mixed-media network It becomes easier to violate maximum delay limits as the network grows

• The timing limits are based on parameters such as:

– Cable length and its propagation delay

– Delay of repeaters

– Delay of transceivers

– Interframe gap shrinkage

– Delays within the station

• 10-Mbps Ethernet operates within the timing limits offered by a series of not more than five segments separated by no more than four repeaters This is known as the 5-4-3 rule No more than four repeaters may

be connected in series between any two distant stations There can also be no more than three populated segments between any two distant stations

7.1.2 10Base5

• The original 1980 Ethernet product 10BASE5 transmitted 10 Mbps over a single thick coaxial cable bus 10BASE5 is important because it was the first medium used for Ethernet 10BASE5 was part of the original 802.3 standard The primary benefit of 10BASE5 was length Today it may be found in legacy installations, but would not be recommended for new installations

• 10BASE5 systems are inexpensive and require no configuration, but basic components like NICs are very difficult to find as well as the fact that it is sensitive to signal reflections on the cable 10BASE5 systems also represent a single point of failure.

• 10BASE5 uses Manchester encoding It has a solid central conductor Each of the maximum five segments of thick coax may

be up to 500 m (1640.4 ft) in length The cable is large, heavy, and difficult to install However, the distance limitations were favorable and this prolonged its use in certain applications

• Because the medium is a single coaxial cable, only one station can transmit at a time or else a collision will occur Therefore, 10BASE5 only runs in half-duplex resulting in a maximum of 10 Mbps

of data transfer

7.1.3 10 BASE2

• 10BASE2 was introduced in 1985 Installation was easier because of its smaller size, lighter weight, and greater flexibility It still exists in legacy networks Like 10BASE5, it is not recommended for installations in networks today It has a low cost and a lack of need for hubs Again, NICs are also difficult to obtain for this

• 10BASE2 also uses Manchester encoding Computers on the LAN were linked together by an unbroken series of coaxial cable lengths These lengths were attached by BNC connectors to a T-shaped connector on the NIC

• 10BASE2 has a stranded central conductor 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

• There may be up to 30 stations on any individual 10BASE2 segment Out of the five consecutive segments in series between any two distant stations, only three may have stations attached

7.1.4 10BASE-T

• 10BASE-T was introduced in 1990 10BASE-T used cheaper and easier to install Category 3 unshielded twisted pair (UTP) copper cable rather than coax cable The cable plugged into a central connection device that contained the shared bus This device was a hub

• It was at the center of a set of cables that radiated out to the PCs like the spokes on a wheel This is referred to as a star topology The distances the cables could extend from the hub and the way in which the UTP was installed increasingly used stars made up of stars, referred to as an extended star topology Originally 10BASE-T was a half-duplex protocol, but full-duplex features were added later The explosion in the popularity of Ethernet in the mid-to-late 1990s was when Ethernet came to dominate LAN technology

• 10BASE-T also uses Manchester encoding A 10BASE-T UTP cable has a solid conductor for each wire in the maximum 90 meter horizontal cable UTP cable uses eight-pin RJ-45 connectors Though Category 3 cable is adequate for use on 10BASE-T networks, it is strongly recommended that any new cable installations be made with Category 5e or better All four pairs of wires should be used either with the T568-A or T568-B cable pinout arrangement With this type of cable installation, supports the use of multiple protocols without rewiring.

• The figure shows the pinout arrangement for a 10BASE-T connection The transmitting pair on the receiving side are connected to the receiving pair on the attached device.

• Half duplex or full duplex is a configuration choice 10BASE-T carries 10 Mbps of traffic in half-duplex mode and 20 Mbps in full-duplex mode

7.1.5 10BASE-T wiring and architecture

• 10BASE-T links generally consist of a connection between the station and a hub

or switch Hubs are multi-port repeaters and count toward the limit on repeaters between distant stations Hubs do not divide network segments into separate collision domains

• Because hubs or repeaters merely extend the length of a network segment within a single collision domain, there is a limit on how many hubs may be used in that segment Bridges and switches divide a segment into separate collision domains, only leaving the media limitations to determine the distance between the switches 10BASE-T limits the distance between switches to 100 m (328 ft).

• Although hubs may be linked, it is best to avoid this arrangement This is to prevent exceeding the limit for maximum delay between distant stations When multiple hubs are required, it is best to arrange them in hierarchical order as to create a tree structure Performance will be improved if fewer repeaters separate stations

• An architectural example is shown in the figure All distances between stations are acceptable However, the total distance from one end of the network to the other, places the architecture at its limit The most important aspect to consider is how

to keep the delay between distant stations

to a minimum, regardless of the architecture and media types involved A shorter maximum delay will provide better

• 10BASE-T links can have unrepeated distances up to 100 m While this may seem like a long distance, it is typically

“used up” when wiring an actual building Hubs can solve the distance issue but will allow collisions to propagate The widespread introduction of switches has made the distance limitation less important

As long as workstations are located within

100 m of a switch, the 100 m distance

7.1.6 100 Mbps Ethernet

• 100-Mbps Ethernet is also known as Fast Ethernet 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

• Three characteristics common to 100BASE-TX and 100BASE-FX are the timing parameters, the frame format, and parts of the transmission process 100BASE-TX and 100-BASE-FX both share timing parameters Note that one bit time in 100-Mbps Ethernet is 10nsec = 01 microseconds = 1 100-millionth of a second.

• The 100-Mbps frame format is the same as the 10-Mbps frame.

• Fast Ethernet represents a 10-fold increase in speed over 10BASE-T Because of the increase in speed, extra care must be taken because the bits being sent are getting shorter in duration and occurring more frequently These higher frequency signals are more susceptible to noise.

7.1.7 100BASE-TX

• In 1995, 100BASE-TX was the standard, using Cat 5 UTP cable, which became commercially successful

• The original coaxial Ethernet used half-duplex transmission so only one device could transmit

at a time However, in 1997, Ethernet was expanded to include a full duplex capability that allowed more than one PC on a network to transmit at the same time Switches increasingly replaced hubs These switches had the capability of full duplex and rapid

• 100BASE-TX uses 4B/5B encoding, which is then scrambled and converted to multi-level transmit-3 levels or MLT-3 In the example, the highlighted window shows four waveform examples The top waveform has no transition in the center of the timing window No transition indicates that a binary 0 is present The second waveform shows

a transition in the center of the timing window A binary 1 is represented by a transition The third waveform shows an alternating binary sequence The absence of binary transition indicates a binary 0, and the presence of a transition indicates a binary 1 Rising or falling edges indicate 1s Very steep signal changes indicate

• The figure shows the pinout for a 100BASE-TX connection Notice that the two separate transmit-receive paths exist This is identical to the 10BASE-T configuration

• 100BASE-TX carries 100 Mbps of traffic in half-duplex mode In full-duplex mode, 100BASE-TX can exchange 200 Mbps of traffic The concept of full duplex will become increasingly important as Ethernet