Transcript CCNA 1 Module 6 Ethernet Fundamentals

CCNA 1 Chapter 5
Ethernet Fundamentals
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Objectives
• Ethernet fundamentals
• Ethernet operation
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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.
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Comparing LAN Standards
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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:
Ethernet, Fast Ethernet, Gigabit
Ethernet, Token Ring, FDDI, Frame
Relay, ATM, PPP, HDLC, and so on
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Access methods
Access protocols
Access technologies
Media access
LAN protocols
WAN protocols
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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
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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
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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 connectionoriented upper-layer protocols.
• Allows multiple higher-layer protocols to share a
single physical data link.
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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.
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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.
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Framing
• Why framing is necessary
• Frame format diagram
• Generic frame format
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Why Framing Is Necessary
• Binary data is a stream of 1s and 0s.
• Framing breaks the stream into decipherable
groupings:
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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.
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Generic Frame Format
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Start Frame field
Address fields (source and destination MAC)
Type/Length field
Data field
FCS (Frame Check Sequence) field
Frame Stop field
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Ethernet Frame Fields
• Some of the fields permitted or required in an
802.3 Ethernet frame are as follows:
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Preamble
Start Frame Delimiter
Destination Address
Source Address
Length/Type
Data and Pad
Frame Check Sequence (FCS)
Extension
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Ethernet Operation
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Media Access Control (MAC)
• Provides MAC addressing (naming)
• Depending on access technology (Ethernet,
Token Ring, FDDI), provides the following:
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Data transmission control
Collision resolution (retransmission)
Layer 2 frame preparation (data framing)
Frame check sequence (FCS)
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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
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Ethernet (CSMA/CD)
Carrier sense multiple access with collision detection
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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.
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Interframe Spacing and Backoff
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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.
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Types of Collisions
• Three types of collisions are:
– Local
– Remote
– Late
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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)
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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.
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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.
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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
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