CCNA1_M6_Ethernet_Fundamentals (1130698)
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CCNA – Semester1Module 6Ethernet FundamentalsObjectives• Ethernet fundamentals• Ethernet operationEthernet FundamentalsIntroduction to Ethernet• The success of Ethernet is due to thefollowing factors:–––––Simplicity and ease of maintenanceAbility to incorporate new technologiesReliabilityLow cost of installation and upgradeBandwidth can be increased without changingunderlying technologyIEEE 802 Committees802.0 SEC802.1 High Level Interface (HILI)802.2 Logical Link Control (LLC)802.3 CSMA/CD Working Group802.4 Token Bus802.5 Token Ring802.6 Metropolitan Area Network (MAN)802.7 BroadBand Technical Adv.
Group (BBTAG)802.8 Fiber Optics Technical Adv. Group (FOTAG)802.9 Integrated Services LAN (ISLAN)802.10 Standard for Interoperable LAN Security (SILS)801.11 Wireless LAN (WLAN)802.12 Demand Priority802.14 Cable-TV Based Broadband CommunicationNetwork802.15 Wireless Personal Area Network (WPAN)802.16 Broadband Wireless Access (BBWA)RPRSG Resilient Packet Ring Study Group (RPRSG)Ethernet StandardsIEEE 802.3 - 10MbitIEEE 802.3u - 100 MbitIEEE 802.3z - 1000 MbitEthernet SpecificationsDesignationDescription10Base-210 Mbps baseband Ethernet over coaxial cable with a maximum distance of 185 meters.Also referred to as Thin Ethernet or Thinnet or Thinwire.10Base-510 Mbps baseband Ethernet over coaxial cable with a maximum distance of 500 meters.Also referred to as Thick Ethernet or Thicknet or Thickwire.10Base-T10 Mbps baseband Ethernet over twisted pair cables with a maximum length of 100meters.100Base-FX100 Mbps baseband Ethernet over two multimode optical fibers.100Base-T100 Mbps baseband Ethernet over twisted pair cable.100Base-T4100 Mbps baseband Ethernet over four pairs of Category 3 or higher unshieldedtwisted pair cable.100Base-TX100 Mbps baseband Ethernet over two pairs of shielded twisted pair or Category 4twisted pair cable.1000Base-CX1000 Mbps baseband Ethernet over two pairs of 150 shielded twisted pair cable.1000Base-LX1000 Mbps baseband Ethernet over two multimode or single-mode optical fibers usinglongwave laser optics.1000Base-SX1000 Mbps baseband Ethernet over two multimode optical fibers using shortwave laseroptics.1000Base-T1000 Mbps baseband Ethernet over four pairs of Category 5 unshielded twisted paircable.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 layerLLC sublayerPacket••PacketLLC PDUPacketFrameLLC PDU includes:– DSAP: Destination service access point– SSAP: Source service access pointSupports connection control methods specified by upperprotocolsMAC sublayerPacketPacket802.3Packet• Naming.• Framing.• Media access control rules.802.5PacketMAC address• MAC addresses are burned into read-onlymemory (ROM) and are copied intorandom-access memory (RAM) when theNIC initializes.• Presentation formats: 0000.0c12.3456 or00-00-0c-12-34-56.• Broadcast address: FFFF.FFFF.FFFFLayer 2 Framing•Framing helps obtain essentialinformation that could not,otherwise, be obtained withcoded bit streams alone.Examples of such informationare:– Which computers arecommunicating with one another– When communication betweenindividual computers begins andwhen it terminates– Provides a method for detectionof errors that occurred during thecommunication– Whose turn it is to "talk" in acomputer "conversation“Generic Frame Format• Framing is the Layer 2 encapsulation process.
Aframe is the Layer 2 protocol data unit.Ethernet Frame Format••At the data link layer the frame structure is nearly identical for allspeeds of Ethernet from 10 Mbps to 10,000 Mbps.Ethernet requires that the frame be not less than 46 octets or morethan 1518 octets.IEEE 802.3 EthernetEthernet IIEthernet OperationMedia Access Control (MAC)• MAC refers to protocols that determinewhich computer on a shared-mediumenvironment, or collision domain, is allowedto transmit the data.• There are two broad categories of MediaAccess Control, deterministic (taking turns)and non-deterministic (first come, firstserved).Media Access Control (MAC) Protocols•Ethernet (IEEE 802.3)– Logical bus topology– Physical star or extended star– Nondeterministic• First-come, first-served•Token Ring (IEEE 802.5)– Logical ring– Physical star topology– Deterministic• Token controls traffic– Older declining technology•FDDI (IEEE 802.5)– Logical ring topology– Physical dual-ring topology– Deterministic• Token controls traffic– Near-end-of-life technologyCSMA/CD ProcessHost wantsto transmitBroadcastjam signalIs carriersensed?AssembleframeStarttransmittingIsa collisiondetected?attempts =attempts + 1Attempts>too many?Wait fort secondsAlgorithmcalculatebackoffKeeptransmittingIstransmissiondone?Too manycollisions; aborttransmissionTransmissioncompletedEthernet Transmission Mode• Full duplex :– send and receive simultaneously– no collisions occur.• Half duplex:– only send or receive at a specific moment– transmit 64 bits of timing synchronization information that is known asthe preamble to make sure collision do not occur.Ethernet Timing• For CSMA/CD Ethernet to operate, collision must besensed before completing transmission of aminimum-sized frame.• At 100 Mbps the system timing is barely able toaccommodate 100 meter cables.
For this reason halfduplex is not permitted in 10-Gigabit Ethernet.Slot time•Ethernet standardspecifications limit– maximum segment length– maximum number of stationsper segment– maximum number of repeatersbetween segments•Slot time >= round trip delayInterframe Spacing• The minimum time space between two non-collidingframes is also called the interframe spacing.• After a frame has been sent, all stations on a 10-MbpsEthernet are required to wait a minimum of 96 bit-times(9.6 microseconds) before any station may legallytransmit the next frame.Error Handling• Collisions are to resole contention fornetwork access which results in networkbandwidth loss.• When collision occurs, the devices with datato transmit return to a listen-before-transmitmode and no device have priority to transmitdata.Types of Collisions• The results of collisions, collision fragments, arepartial or corrupted frames that are less than 64octets and have an invalid FCS.
Three types ofcollisions are:– Local– Remote– LateLocal Collision• Local collision is detected on the local segmentwhen a station detects a signal on the RX pair atthe same time it is sending on the TX pair.• Local collision is sensed as over-voltagecondition on the local cable area when twosignal from different stations collide.Remote Collison• The characteristics of a remote collision are aframe that is less than the minimum length, hasan invalid FCS checksum.• This sort of collision usually results fromcollisions occurring on the far side of a repeatedconnection.Late Collision• Collisions occurring after the first 64 octets arecalled late collisions.• Difference between late collisions and othercollisions is that the Ethernet NIC will notautomatically retransmit a frame that wascollided late.FCS and Beyond•••A received frame that has a bad frame check sequence, alsoreferred to as a checksum or CRC error, differs from the originaltransmission by at least 1 bit.High numbers of FCS errors from a single station usually indicatesa faulty NIC and/or faulty or corrupted software drivers, or a badcable connecting that station to the network.If FCS errors are associated with many stations, they are generallytraceable to bad cabling, a faulty version of the NIC driver, a faultyhub port, or induced noise in the cable system.Ethernet Autonegotiation• This process defines how two link partners mayautomatically negotiate a configuration offering the bestcommon performance level.• It has the additional advantage of only involving thelowest part of the physical layer.Summary••••••••The basics of Ethernet technologyHow Ethernet and the OSI model interactEthernet frame field names and purposesThe characteristics and function of CSMA/CDEthernet timingThe backoff algorithm and time after a collisionEthernet errors and collisionsAuto-negotiation in relation to speed and duplex.
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