6 Ethernet Basics
Transcript of 6 Ethernet Basics
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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 deliverysystem 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, GigabitEthernet, Token Ring, FDDI, FrameRelay, ATM, PPP, HDLC, and so on
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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, whilecommunicating 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 connection-
oriented 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 inlength and expressed as 12 hexadecimal digits.
The first 6 hexadecimal digits, which are
administered by the IEEE, identify themanufacturer or vendor and thus comprise theorganizational unique identifier (OUI).
The remaining 6 hexadecimal digits represent
the interface serial number, or another valueadministered by the specific equipmentmanufacturer.
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Layer 2 Framing
Framing is the Layer
2 encapsulation
process; a frame is
the Layer 2 protocoldata unit.
A single generic
frame has sections
called fields, andeach field is
composed of bytes.
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Framing
Why framing is necessary
Frame format diagram
Generic frame format
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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
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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: 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 beginstransmitting 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 thecollision 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 haselapsed (collision or runt)
Simultaneous transmission occurring after slot time haselapsed (late collision)
Excessively or illegally long transmission (jabber, longframe and range errors)
Illegally short transmission (short frame, collision fragmentor 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 (ghostor jabber)
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Ethernet Auto negotiation
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 offeringthe 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 fullduplex