CCNA 1 Chapter 5 Ethernet Fundamentals

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CCNA 1 Chapter 5

Ethernet Fundamentals

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Objectives

• Ethernet fundamentals

• Ethernet operation


Introduction to Ethernet

• The success of Ethernet is due to its simplicity and ease of maintenance, as well as its ability to incorporate new technologies, reliability, and low cost of installation and upgrade.


Comparing LAN Standards


OSI Layer 1 and 2 Together Are the Access Protocols

• These are the delivery system protocols.

• Independent of:– Network OS– Upper-level protocols

• TCP/IP, IPX/SPX

• Sometimes called:– Access methods– Access protocols– Access technologies– Media access– LAN protocols– WAN protocols

Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, Frame Relay, ATM, PPP, HDLC, and so on


IEEE Standard• 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 layer


Logical Link Control (LLC)

• Allows part of the data link layer to function independent of LAN access technologies (protocols / methods)– Provides services to network layer protocols, while

communicating with access technologies below it • LAN access technologies:

– Ethernet– Token Ring– FDDI


Logical Link Control (LLC)

• Participates in the data encapsulation process– LLC PDU between Layer 3 and MAC sublayer.– Adds control information to the network layer data to

help deliver the packet. It adds two fields:• Destination Service Access Point (DSAP)• Source Service Access Point (SSAP)

• Supports both connectionless and connection-oriented upper-layer protocols.

• Allows multiple higher-layer protocols to share a single physical data link.


Naming

• Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits.

• The first 6 hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor and thus comprise the organizational unique identifier (OUI).

• The remaining 6 hexadecimal digits represent the interface serial number, or another value administered by the specific equipment manufacturer.


Layer 2 Framing

• Framing is the Layer 2 encapsulation process; a frame is the Layer 2 protocol data unit.

• A single generic frame has sections called fields, and each field is composed of bytes.


Framing

• Why framing is necessary• Frame format diagram• Generic frame format


Why Framing Is Necessary

• Binary data is a stream of 1s and 0s.• Framing breaks the stream into decipherable

groupings:– Start and stop indicator fields– Naming or addressing fields– Data fields– Quality-control fields

• Framing is the Layer 2 encapsulation process.• A frame is the Layer 2 protocol data unit.


Generic Frame Format• Start Frame field• Address fields (source and destination MAC)• Type/Length field• Data field• FCS (Frame Check Sequence) field• Frame Stop field


Ethernet Frame Fields

• Some of the fields permitted or required in an 802.3 Ethernet frame are as follows: – Preamble – Start Frame Delimiter – Destination Address – Source Address – Length/Type – Data and Pad – Frame Check Sequence (FCS) – Extension


Ethernet Operation


Media Access Control (MAC)

• Provides MAC addressing (naming)• Depending on access technology (Ethernet,

Token Ring, FDDI), provides the following:– Data transmission control– Collision resolution (retransmission)– Layer 2 frame preparation (data framing)– Frame check sequence (FCS)


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 technology


Ethernet (CSMA/CD)

Carrier sense multiple access with collision detection


Ethernet Timing

• Any station on an Ethernet network wanting to transmit a message first “listens” to ensure that no other station is currently transmitting.

• If the cable is quiet, the station begins transmitting immediately.


Interframe Spacing and Backoff


Error Handling

• Collisions are the mechanism for resolving contention for network access.

• Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This affects all network nodes, possibly causing significant reduction in network throughput.


Types of Collisions

• Three types of collisions are:– Local– Remote– Late


Ethernet Errors

• The following are the sources of Ethernet error: – Simultaneous transmission occurring before slot time has

elapsed (collision or runt)– Simultaneous transmission occurring after slot time has

elapsed (late collision)– Excessively or illegally long transmission (jabber, long

frame and range errors)– Illegally short transmission (short frame, collision fragment

or runt)– Corrupted transmission (FCS error)– Insufficient or excessive number of bits transmitted

(alignment error)– Actual and reported number of octets in frame don't match

(range error)– Unusually long preamble or jam event (ghost or jabber)


FCS and Beyond

• A received frame that has a bad frame check sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least 1 bit.


Ethernet Autonegotiation

• A process called autonegotiation (of speeds at half or full duplex) was developed.

• This process defines how two link partners may automatically negotiate a configuration offering the best common performance level.

• It has the additional advantage of only involving the lowest part of the physical layer.


Link Establishment

• There are only two ways to achieve a full-duplex link:– Through a completed cycle of autonegotiation – Or, by administratively forcing both link partners to full

duplex

CCNA 1 Chapter 5 Ethernet Fundamentals

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