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Ethernet Fundamentals

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Introduction to Ethernet

Ethernet is now the dominant LAN technology in the world.

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• From its beginning in the 1970s, Ethernet hasevolved to meet the increasing demand for highspeed LANs.

•

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,optical fiber, Ethernet adapted to take advantageof the superior bandwidth and low error rate thatfiber offers.

• Now, the same protocol that transported data at3 Mbps in 1973 is carrying data at 10 Gbps.

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• The success of Ethernet is due to thefollowing factors:

Simplicity and ease of maintenance

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Ability to incorporate new technologies

Reliability

Low cost of installation and upgrade

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Ethernet beginning

• The original idea for Ethernet grew out of the problem ofallowing two or more hosts to use the same medium andprevent the signals from interfering with each other.

•

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was studied in the early 1970s at the University of Hawaii.

• A system called Alohanet was developed to allow variousstations on the Hawaiian Islands structured access to theshared radio frequency band in the atmosphere.

• This work later formed the basis for the Ethernet access

method known as CSMA/CD.

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Ethernet beginning

• The first LAN in the world was the original version ofEthernet made at Xerox more than thirty years ago.

• The first Ethernet standard was published in 1980 by aconsortium of Di ital E ui ment Com an , Intel, and

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Xerox (DIX).

• Ethernet 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.

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Ethernet standardization – 802.3

• In 1985, the IEEE standards committee for Local andMetropolitan Networks published standards for LANs.

• The standard for Ethernet is 802.3.

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.

lower portion of Layer 2 of the OSI model.

• As a result, some small modifications to the original Ethernetstandard were made in 802.3.

• The differences between the two standards were so minorthat any Ethernet network interface card (NIC) can transmitand receive both Ethernet and 802.3 frames.

• Essentially, Ethernet and IEEE 802.3 are the same standards.

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New Ethernet technologies

• In 1995, IEEE announced a standard for a 100-Mbps Ethernet.

• 1 Gbps Ethernet was announced by IEEE in 1998 and 1999.• All the standards are essentially compatible with the original

Ethernet standard.

• An Ethernet frame could leave an older coax 10-Mb s NIC in

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a PC, be placed onto a 10-Gbps Ethernet fiber link, and endup at a 100-Mbps NIC.

• As long as the packet stays on Ethernet networks it is notchanged.

• The bandwidth of the network could be increased manytimes without changing the underlying Ethernet technology.

• The original Ethernet standard has been amended a numberof times in order to manage new transmission media andhigher transmission rates

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IEEE Ethernet Naming Rules

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Baseband signaling uses the entire bandwidth of the transmission

medium - the data signal is transmitted directly over the transmissionmedium.

In broadband signaling, not used by Ethernet, the data signal is neverplaced directly on the transmission medium - it is modulated

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Ethernet and the OSI Model

Ethernet operates in two areas of theOSI model, the lower half of the datalink layer, known as the MAC sublayerand the h sical la er.

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IEEE 802.x Standards

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• Ethernet at Layer 1 involves interfacing with media, signals,bit streams that travel on the media, components that putsignals on media, and various topologies.

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• erne ayer per orms a ey ro e n e commun ca onthat takes place between devices.

• The MAC sublayer is concerned with the physicalcomponents that will be used to communicate theinformation.

• The Logical Link Control (LLC) sublayer remains relativelyindependent of the physical equipment that will be used forthe communication process.

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131313

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141414

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MAC Address

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MAC Address

• The NIC uses the MAC address to assess whether the message shouldbe 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

• As data propagates along the network media the NIC in each device onthe network checks to see if the MAC address matches the physical

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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 andpasses the frame up the OSI layers.

• On an Ethernet network, all nodes must examine the MAC header evenif the communicating nodes are side by side.

• All devices that are connected to the Ethernet LAN have MACaddressed interfaces including workstations, printers, routers, andswitches

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Manufacturer Codes

181818

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Multicast MAC Addresses

191919

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Layer 2 Framing

• Framing helps obtain essential information that

could not, otherwise, be obtained with coded bitstreams alone.

• Examples of such information are:

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Which computers are communicating with one another

When communication between individual computersbegins and when it terminates

Provides a method for detection of errors that occurredduring the communication

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

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Frame check sequence

• There are three primary ways to calculate theFrame Check Sequence number:

Cyclic Redundancy Check (CRC) – performs

222222

.

Two-dimensional parity – adds an 8th bit that makes an8 bit sequence have an odd or even number of binary1s.

Internet checksum – adds the values of all of the databits to arrive at a sum.

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Ethernet Frame Structures

• At the data link layer the frame structure isnearly identical for all speeds of Ethernetfrom 10 Mbps to 10,000 Mbps.

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• However, at the physical layer almost allversions of Ethernet are substantiallydifferent from one another with eachspeed having a distinct set of architecturedesign rules.

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Ethernet Frame Structures

252525

The Ethernet II Type field is incorporated into the current 802.3frame definition.The receiving node must determine which higher-layer protocol is

present in an incoming frame by examining the Length/Type field.If the two-octet value is equal to or greater than 0x600(hexadecimal), then the frame is interpreted according to the

Ethernet II type code indicated.

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Ethernet Frame Structures – 802.3

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The field labeled Length/Type was only listed as Length in the early

IEEE versions and only as Type in the DIX version.

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Ethernet Frame Fields

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Preamble & Start Frame Delimiter

• The Preamble is an alternating pattern of ones and zeroesused for timing synchronization in the asynchronous 10Mbps and slower implementations of Ethernet. Faster

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versions of Ethernet are synchronous, and this timinginformation is redundant but retained for compatibility.

• A Start Frame Delimiter consists of a one-octet field thatmarks the end of the timing information, and contains the bit

sequence 10101011.

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Length/Type

• The Length/Type field supports two different uses. If the value isless than 1536 decimal, 0x600 (hexadecimal), then the valueindicates length.

• The length interpretation is used where the LLC Layer providesthe protocol identification.

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• The type value specifies the upper-layer protocol to receive thedata after Ethernet processing is completed.

• The length indicates the number of bytes of data that followsthis field.

• If the value is equal to or greater than 1536 decimal (0600hexadecimal), the value indicates that the type and contents ofthe Data field are decoded per the protocol indicated.

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Data / Pad

• The Data and Pad field may be of any length that does notcause the frame to exceed the maximum frame size.

•

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octets, so the data should not exceed that size.• An unspecified pad is inserted immediately after the user

data when there is not enough user data for the frame tomeet the minimum frame length.

• Ethernet requires that the frame be not less than 64 octets ormore than 1518 octets.

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FCS

• A FCS contains a four byte CRC value that is created by thesending device and is recalculated by the receiving device tocheck for damaged frames.

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beginning of the Destination Address through the end of theFCS field will cause the checksum to be different, the coverageof the FCS includes itself.

• It is not possible to distinguish between corruption of the FCS

itself and corruption of any preceding field used in thecalculation.

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323232

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LLC Header

333333

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DSAP and SSAP

343434

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Values of DSAP and SSAP

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Values of DSAP and SSAP

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SNAP

373737

To accommodate more protocols, IEEE allowed the use of an extraheader, called Subnetwork Access Protocol (SNAP) header. TheDSAP field is AA, which implies that the SNAP header follows the802.2 header and the SNAP includes a 2-byte protocol type field.

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Media Access Control (MAC)

MAC refers to protocols thatdetermine which computer on ashared-medium environment, orcollision domain is allowed to

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transmit the data.

There are two broad categoriesof Media Access Control:- deterministic (taking turns)

- non-deterministic (first come,first served

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MAC Rules and Collision Detection/BackoffThe access method CSMA/CD usedin Ethernet performs threefunctions:

• Transmitting and receiving datapackets•

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checking them for valid addressesbefore passing them to the upperlayers of the OSI model• Detecting errors within datapackets or on the network

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MAC Rules and Collision Detection/Backofflisten-before-transmit mode

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Collision

• Networking devices detect a collision has occurred when the

amplitude of the signal on the networking media increases.• When a collision occurs, each node that is transmitting will

continue to transmit for a short time to ensure that all

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.

• Once all the devices have detected the collision a backoffalgorithm is invoked and transmission is stopped.

• The nodes stop transmitting for a random period of time,

which is different for each device.• When the delay period expires, all devices on the network

can attempt to gain access to the networking media.

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Ethernet Timing

As a rough estimate, 20.3 cm (8 in) per nanosecond is often used forcalculating propagation delay down a UTP cable.

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10 Mbps version of Ethernet is asynchronous.Asynchronous means that each receiving station will use the eight

octets of timing information to synchronize the receive circuit to theincoming data, and then discard it.100 Mbps and higher speed implementations of Ethernet aresynchronous – do not need Preamble and SFD, but use them for

compatibility reasons

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Slot time

• The actual calculated slot time is justlonger than the theoretical amount of timerequired to travel between the furthest

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po n s o e co s on oma n, co e wanother transmission at the last possibleinstant, and then have the collisionfragments return to the sending station

and be detected.

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Slot time

• For all speeds of Ethernet transmission at or below 1000

Mbps, the standard describes how a transmission may be nosmaller than the slot time.

• Slot time for 10 and 100-Mbps Ethernet is 512 bit-times, or 64

454545

.

• Slot time for 1000-Mbps Ethernet is 4096 bit-times, or 512octets.

• Slot time is calculated assuming maximum cable lengths on

the largest legal network architecture.• All hardware propagation delay times are at the legal

maximum and the 32-bit jam signal is used when collisionsare detected.

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Interframe Spacing and Backoff

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The minimum spacing between two non-colliding frames is also calledthe interframe spacing. This is measured from the last bit of the FCS

field of the first frame to the first bit of the preamble of the secondframe.After collision ALL STATIONS wait for Interframe spacing. After thatstations that collided wait for additional backoff time

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Bursting

• At operating speeds above 100 Mb/s, an implementation mayoptionally transmit a series of frames without relinquishing

control of the transmission medium.

• This mode of operation is referred to as burst mode .

• Once a frame has been successfull transmitted, the

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transmitting station can begin transmission of another frame

without contending for the medium

• The transmitting station fills the interframe spacing interval withextension bits, which are readily distinguished from data bits atthe receiving stations, and which maintain the detection ofcarrier in the receiving stations.

• The transmitting station is allowed to initiate frametransmission until a specified limit, referred to as burstLimit, isreached.

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Max 16 attempts!!!

• If the MAC layer is unable to send the frame aftersixteen attempts, it gives up and generates an error

515151

.

• Such an occurrence is fairly rare and would happenonly under extremely heavy network loads, or when aphysical problem exists on the network.

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Collisions

• The considerable majority of collisions occur very early inthe frame, often before the SFD.

• Collisions occurring before the SFD are usually not reported

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to the higher layers, as if the collision did not occur.

• As soon as a collision is detected, the sending stationstransmit a 32-bit “jam” signal that will enforce the collision.

• The most commonly observed data pattern for a jam signal

is simply a repeating one, zero, one, zero pattern, the sameas Preamble.

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Error Handling

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Types of Collisions

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UTP Collisions

• On UTP cable, such as 10BASE-T, 100BASE-TX

and 1000BASE-T, a collision is detected on thelocal segment only when a station detects asignal on the RX pair at the same time it is

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sen ng on e pa r.

• Since the two signals are on different pairs thereis no characteristic change in the signal.

• Collisions are only recognized on UTP when thestation is operating in half duplex.

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Types of Collisions

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Remote collisions

• The characteristics of a remote collision are a frame that isless than the minimum length, has an invalid FCS checksum,but does not exhibit the local collision symptom of over-voltage or simultaneous RX/TX activity.

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• s sor o co s on usua y resu s rom co s ons

occurring on the far side of a repeated connection.• A repeater will not forward an over-voltage state, and cannot

cause a station to have both the TX and RX pairs active atthe same time.

• The station would have to be transmitting to have both pairsactive, and that would constitute a local collision. On UTPnetworks this is the most common sort of collisionobserved.

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Late collisions

• The most significant difference between late collisions andcollisions occurring before the first 64 octets is that theEthernet NIC will retransmit a normally collided frame

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,

that was collided late.• As far as the NIC is concerned everything went out fine, and

the upper layers of the protocol stack must determine thatthe frame was lost.

• Other than retransmission, a station detecting a late collisionhandles it in exactly the same way as a normal collision.

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Eth t A t N ti ti

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Ethernet Auto-Negotiation

• -

606060

every 16 milliseconds, whenever the station was not engagedin transmitting a message. Auto-Negotiation adopted thissignal and renamed it a Normal Link Pulse (NLP).

• When a series of NLPs are sent in a group for the purpose ofAuto-Negotiation, the group is called a Fast Link Pulse (FLP)burst.

• Each FLP burst is sent at the same timing interval as an NLP,and is intended to allow older 10BASE-T devices to operatenormally in the event they should receive an FLP burst.

T i i P i it R k

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Transmission Priority Rank

• 1000BASE-T full duplex• 1000BASE-T half duplex• 100BASE-TX full duplex• 100BASE-TX half duplex

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• 10BASE-T half duplex

A t ti ti

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Auto-negotiation

• After both stations have interpreted what the otherpartner is offering, both switch to the highest

performance common configuration and establisha link at that speed.

•

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is lost, the two link partners first attempt to linkagain at the last negotiated speed.

• If that fails, or if it has been too long since the link

was lost, the Auto-Negotiation process starts over.• The link may be lost due to external influences,

such as a cable fault, or due to one of the partnersissuing a reset

Manual duplex/speed configuration

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Manual duplex/speed configuration

• Link partners are allowed to skip offeringconfigurations of which they are capable.

• This allows the network administrator to forceorts to a selected s eed and du lex settin

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without disabling Auto-Negotiation

• Auto-Negotiation is optional for most Ethernetimplementations.

• Gigabit Ethernet requires its implementation,

though the user may disable it.

Full/Half Duplex

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Full/Half Duplex

• There are two duplex modes, half and full.• For shared media, the half-duplex mode is

mandatory.

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• All coaxial implementations are half duplex innature and cannot operate in full duplex.

• UTP and fiber implementations may be operatedin half duplex.

• 10-Gbps implementations are specified for fullduplex only.

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Ethernet Technologies

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Types of Ethernet

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Types of Ethernet

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Legacy Ethernet

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Legacy Ethernet

• 10BASE5, 10BASE2, and 10BASE-T Ethernet areconsidered Legacy Ethernet.

• The four common features of Legacy Ethernet are

696969

, ,

process, and a basic design rule.• 10BASE5, 10BASE2, and 10BASE-T all share the

same timing parameters, as shown in Figure (1 bittime at 10 Mbps = 100 nsec = 0.1 µsec)

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

Parameters for 10 Mbps Ethernet Operation

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Parameters for 10 Mbps Ethernet Operation

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Manchester code

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Manchester code

• The simplest encodings haveundesirable timing andelectrical characteristics.

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• Line codes have been

designed to have desirabletransmission properties.

• This form of encoding used in

10 Mbps systems is called“Manchester.”

10BASE5 Architecture Example

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5-4-3 ruleMax 5 segments, 4 repeaters, 3 segments with computers

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• 10BASE5 uses Manchester encoding.

• Each of the maximum five segments of thick coax may be upto 500 m (1640.4 ft) in length.

•

737373

, , .

• However, the distance limitations were favorable and thisprolonged its use in certain applications.

• Because the medium is a single coaxial cable, only onestation can transmit at a time or else a collision will occur.

• Therefore, 10BASE5 only runs in half-duplex resulting in amaximum of 10 Mbps of data transfer.

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10BASE2

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• 10BASE2 was introduced in 1985. Installation was easierbecause 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.

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• It has a low cost and a lack of need for hubs. Again, NICs are

also difficult to obtain for this medium.• 10BASE2 also uses Manchester encoding. 10BASE2 has a

stranded central conductor.

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

185 meters long. There may be up to 30 stations on anyindividual 10BASE2 segment.

• Out of the five consecutive segments in series between any twodistant stations, only three may have stations attached.

10BASE-T Modular Jack Pinouts

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767676

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.

Originally 10BASE-T was a half-duplex protocol, but full-duplex features were

added later

10BaseT

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• 10BASE-T also uses Manchester encoding.

• A 10BASE-T UTP cable has a solid conductor for each wirein the maximum 90 meter horizontal cable.

• UTP cable uses ei ht- in RJ-45 connectors.

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• Though Category 3 cable is adequate for use on 10BASE-Tnetworks, it is strongly recommended that any new cableinstallations be made with Category 5e or better.

• 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.

10BASE-T Repeated Network Design Limits

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p g

787878

10BASE-T Repeated Network Design Limits

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• Hubs do not divide network segments into separate collisiondomains

• Because hubs or repeaters merely extend the length of anetwork segment within a single collision domain, there is alimit on how many hubs may be used in that segment.

797979

• ,arrangement.

• This is to prevent exceeding the limit for maximum delaybetween 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. (lower delay – better performance)

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Parameters for 100-Mbps Ethernet Operation

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• Three characteristics common to 100BASE-TX and100BASE-FX are:

the timing parameters,the frame format,

and arts of the transmission rocess.

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• 100BASE-TX and 100-BASE-FX both share timingparameters.

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

• 100 BASE-TX uses a 4B/5B binary encoding rather than a

Manchester encoding to encode the 100 Mbps data streamwith a 125 MHz signal

• 4B/5B is then scrambled and converted to multi-leveltransmit-3 levels or MLT-3 (100BaseTX).

4B/5B

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4B/5B Encoding Table

Data (Hex) (Binary) 4B/5B Code

0 0000 11110

828282

2 0010 10100

... ... ...

D 1101 11011

E 1110 11100

F 1111 11101

MLT-3

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• MLT-3 encodes a bit as presence or lack of transition

• What makes MLT-3 different is that the base waveform is

a 3-state alternating wave.• Rather than alternating between 0 and 1 as in Manchester

encoding and NRZI, MLT-3 alternates from -1 to 0 to +1,-

838383

, , .

MLT-3

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The encoded data provides:

- synchronization,- efficient usage of bandwidth- improved Signal-to-Noise

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a o c arac er s cs.

100BASE-TX Modular Jack Pinout

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858585

100BASE-FX

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• The timing, frame format, andtransmission are all common to

both versions of 100 Mbps FastEthernet. 100BASE-FX also uses4B/5B encoding.

868686

• 100BASE-FX uses NRZI encoding

• The top waveform has notransition, which indicates that abinary 0 is present. In the secondwaveform, a transition is in the

center of the timing window. Abinary 1 is represented by atransition.

100BASE-FX Pinout

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878787

A fiber version was meant be used for backbone applications, connectionsbetween floors and buildings where copper is less desirable, and also inhigh noise environments.

100BASE-FX was introduced to satisfy this desire.

However, 100BASE-FX was never adopted successfully.This was due to the timely introduction of Gigabit Ethernet copper and fiberstandards.

100BaseX repeaters (hubs)

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• A Class I repeater may introduce up to 140 bit-times oflatency. Any repeater that changes between one Ethernetimplementation and another is a Class I repeater.

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-times latency. Because of the reduced latency it is possible

to have two Class II repeaters in series, but only if the cablebetween them is very short.

• Modification of the architecture rules is strongly discouragedfor 100BASE-TX.

• 100BASE-TX cable between Class II repeaters may notexceed 5 meters.

Example of Architecture Configuration andCable Distances

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898989

Gigabit Ethernet

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• The 1000BASE-T standard, IEEE 802.3ab,specifies 1 Gbps full duplex over UTP.

909090

• The 1000BASE-(SL)X standard, IEEE 802.3z,

specifies 1 Gbps full duplex over optical fiber.

• The Gigabit Ethernet frame has the same formatas is used for 10 and 100-Mbps Ethernet.

Gigabit Ethernet

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• Due to the increased speeds of these newer

standards, the shorter duration bit times requirespecial considerations.

• Since the bits are introduced on the medium for

919191

a s orter urat on an more o ten, t m ng s

critical.• This high-speed transmission requires

frequencies closer to copper medium bandwidthlimitations.

• This causes the bits to be more susceptible tonoise on copper media.

Parameters for Gigabit Ethernet Operation

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929292

1000BaseT

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• Because Cat 5e cable can reliably carry up to 125 Mbps of

traffic, getting 1000 Mbps (Gigabit) of bandwidth was adesign challenge.

• The first step to accomplish 1000BASE-T is to use all four

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pa rs o w res nstea o t e tra t ona two pa rs o w resused by 10BASE-T and 100BASE-TX.

• This is done using complex circuitry to allow full duplextransmissions on the same wire pair. This provides 250Mbps per pair.

• Since the information travels simultaneously across the fourpaths, the circuitry has to divide frames at the transmitterand reassemble them at the receiver.

1000BaseT signal transmission

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949494

Outbound (Tx) 1000Base-T Signal

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959595

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1000BaseT

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• In idle periods there are nine voltage levels found on the

cable, and during data transmission periods there are 17voltage levels found on the cable.

• With this lar e number of states and the effects of noise the

979797

signal on the wire looks more analog than digital. Like

analog, the system is more susceptible to noise due to cableand termination problems.

• The data from the sending station is carefully divided intofour parallel streams, encoded, transmitted and detected in

parallel, and then reassembled into one received bit stream.

1000BASE-SX and LX

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989898

The short-wavelength uses an 850 nm laser or LED source in multimode

optical fiber (1000BASE-SX).

The long-wavelength 1310 nm laser source uses either single-mode or

multimode optical fiber (1000BASE-LX).

1000BaseX

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• Fiber-based Gigabit Ethernet (1000BASE-X) uses 8B/10B encoding which is similar

999999

.

• This is followed by the simple Non-Returnto Zero (NRZ) line encoding of light onoptical fiber.

Gigabit Ethernet Media Comparison

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100100100

Gigabit Ethernet

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• The Media Access Control method treatsthe link as point-to-point.

•

101101101

transmitting (Tx) and receiving (Rx) theconnection is inherently full duplex.

• Gigabit Ethernet permits only a singlerepeater between two stations.

Gigabit Ethernet Architecture

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Maximum 1000BASE-SX Cable Distances

102102102

Maximum 1000BASE-LX Cable Distances

Gigabit Ethernet distance

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• The distance limitations of full-duplex links are

only limited by the medium, and not the round-trip delay.

• -

103103103

, ,topologies are all allowed.

• It is recommended that all links between astation and a hub or switch be configured forAuto-Negotiation to permit the highest common

performance.

Parameters for 10-Gbps Ethernet Operation

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104104104

10-Gbps Ethernet (IEEE 802.3ae) was standardized inJune 2002.

10G Ethernet

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• 10GbE physical layer standards allow both an extension in

distance to 40 km over single-mode fiber and compatibility withsynchronous optical network (SONET) and synchronous digitalhierarch (SDH) networks.

105105105

• Operation at 40 km distance makes 10GbE a viable MAN

technology.

• Compatibility with SONET/SDH networks operating up to OC-192 speeds (9.584640 Gbps) make 10GbE a viable WANtechnology.

10G Ethernet

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• Frame format is the same, allowing interoperability between allvarieties of legacy, fast, gigabit, and 10 Gigabit, with noreframing or protocol conversions.

• Bit time is now 0.1 nanoseconds. All other time variables scale

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accordingly.

• Since only full-duplex fiber connections are used, CSMA/CD isnot necessary

• The IEEE 802.3 sublayers within OSI Layers 1 and 2 are mostlypreserved, with a few additions to accommodate 40 km fiber

links and interoperability with SONET/SDH technologies.

10G Ethernet

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• 10GBASE-SR – Intended for short distances over already-installed multimode fiber, supports a range between 26 m to 82

m

• 10GBASE-LX4 – Uses wavelength division multiplexing (WDM),-

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and 10 km over single-mode fiber

• 10GBASE-LR and 10GBASE-ER – Support 10 km and 40 kmover single-mode fiber

• 10GBASE-SW, 10GBASE-LW, and 10GBASE-EW – Known

collectively as 10GBASE-W are intended to work with OC-192synchronous transport module (STM) SONET/SDH WANequipment.

10-Gigabit Ethernet Implementations

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10-Gigabit Ethernet technical issues

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• Because of the short duration of the 10 GbE data bit, it is

often difficult to separate a data bit from noise.• 10 GbE data transmissions rely on exact bit timing to

se arate the data from the effects of noise on the h sical

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layer.

• In response to these issues of synchronization, bandwidth,and Signal-to-Noise Ratio, 10-Gigabit Ethernet uses twoseparate encoding steps.

• By using codes to represent the user data, transmission ismade more efficient.

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10GBASE-LX4 transmit characteristics

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The Future of Ethernet

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• the IEEE and the 10-Gigabit Ethernet Alliance are

working on 40, 100, or even 160 Gbps standards.• Proposals for Ethernet arbitration schemes other

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• The future of networking media is three-fold:Copper (up to 1000 Mbps, perhaps more)

Wireless (approaching 100 Mbps, perhaps more)

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

The Future of Ethernet

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• When Ethernet was slower, half-duplex, subject to collisions

and a “democratic” process for prioritization, was notconsidered to have the Quality of Service (QoS) capabilitiesrequired to handle certain types of traffic.

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• The full-duplex high-speed Ethernet technologies that now

dominate the market are proving to be sufficient atsupporting even QoS-intensive applications.

• Ironically end-to-end QoS capability helped drive a push forATM to the desktop and to the WAN in the mid-1990s, butnow it is Ethernet, not ATM that is approaching this goal.

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