Download CCNA 1 Module 6 Ethernet Fundamentals

Chapter 3: Layer 2 Ethernet
Objectives:

Ethernet Fundamentals
Ethernet Operations
Carrier Sense Multiple Access/Collision
Detection
Switches and switching methods
Copyright 2003
www.ciscopress.com
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.
Copyright 2003
www.ciscopress.com
Comparing LAN Standards
Copyright 2003
www.ciscopress.com
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
Copyright 2003
–
–
–
–
–
–
Access methods
Access protocols
Access technologies
Media access
LAN protocols
WAN protocols
www.ciscopress.com
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
Copyright 2003
www.ciscopress.com
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
Copyright 2003
www.ciscopress.com
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.
Copyright 2003
www.ciscopress.com
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.
Copyright 2003
www.ciscopress.com
Framing
• Why framing is necessary
• Frame format diagram
• Generic frame format
Copyright 2003
www.ciscopress.com
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.
Copyright 2003
www.ciscopress.com
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
Copyright 2003
www.ciscopress.com
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
Copyright 2003
www.ciscopress.com
Ethernet Operation
Copyright 2003
www.ciscopress.com
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)
Copyright 2003
www.ciscopress.com
Naming (MAC address)
• 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.
Copyright 2003
www.ciscopress.com
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
Copyright 2003
www.ciscopress.com
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)
Copyright 2003
www.ciscopress.com
FCS and Beyond
• A received frame that has a bad frame check
sequence, also referred to as a checksum
Redundancy Check (CRC) error, differs from the
original transmission by at least 1 bit.
Copyright 2003
www.ciscopress.com
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.
Copyright 2003
www.ciscopress.com
CSMA/CD
1
Two nodes transmit
at the same time
2
Nodes detect there
has been a collision
3
4
Nodes transmit a
jamming signal
Nodes wait a random
period before retransmitting
Copyright 2003
CSMA/CD
Ethernet uses carrier sense, multiple access
with collision detection (CSMA/CD).
Nodes monitor the bus (or Ether) to
determine if it is busy. A node wishing to send
data waits for an idle condition then transmits
its message.
Collisions can occur when two nodes
transmit at the same time, thus nodes must
monitor the cable when they transmit.
When a collision occurs, both nodes stop
transmitting frames and transmit a jamming
signal. This informs all nodes on the network
that a collision has occurred.
Each of the nodes involved in the collision
then waits a random period of time before
attempting a re-transmission.
As each node has a random delay time then
there can be a prioritization of the nodes on
the network.
www.ciscopress.com
Ethernet (CSMA/CD)
Carrier sense multiple access with collision detection
Copyright 2003
www.ciscopress.com
•
Switch Memory buffering
•
switch
•
•
•
1111
3333
Abbreviated
MAC
addresses
•
2222
4444
•
Rick Graziani [email protected]
An Ethernet switch may use a buffering
technique to store and forward frames.
Buffering may also be used when the
destination port is busy.
The area of memory where the switch
stores the data is called the memory
buffer.
This memory buffer can use two methods
for forwarding frame:
– port-based memory buffering
– shared memory buffering
In port-based memory buffering frames
are stored in queues that are linked to
specific incoming ports.
Shared memory buffering deposits all
frames into a common memory buffer
which all the ports on the switch share.
•
Two switching methods
• Store-and-forward – The entire frame is received before
any forwarding takes place.
– The destination and source addresses are read and
filters are applied before the frame is forwarded.
– CRC Check done
• Cut-through – The frame is forwarded through the switch
before the entire frame is received.
– This mode decreases the latency of the transmission,
but also reduces error detection.
Rick Graziani [email protected]
•
Cut-through
Cut-through
• Fast-forward – Offers the lowest level of latency.
– Fast-forward switching immediately forwards a packet after reading
the destination address.
– There may be times when packets are relayed with errors.
– Although this occurs infrequently and the destination network
adapter will discard the faulty packet upon receipt.
Rick Graziani [email protected]
•
Cut-through
Cut-through
• Fragment-free – Fragment-free switching filters out collision fragments before
forwarding begins.
– Collision fragments are the majority of packet errors.
– In a properly functioning network, collision fragments must be smaller than
64 bytes.
– Anything greater than 64 bytes is a valid packet and is usually received
without error.
– Fragment-free switching waits until the packet is determined not to be a
collision fragment before forwarding.
Rick Graziani [email protected]
•
Two switching methods
•
Adaptive cut-through
– In this mode, the switch uses cut-through until it detects
a given number of errors.
– Once the error threshold is reached, the switch changes
to store-and-forward mode.
Rick Graziani [email protected]
•
Switches and broadcast domains
These are logical not
physical representations
of what happens to
these frames.
• Switches flood frames that are:
– Unknown unicasts
– Layer 2 broadcasts
– Multicasts (unless running multicast snooping or IGMP)
• Multicast are special layer 2 and layer 3 addresses that are sent
to devices that belong to that “group”.
Rick Graziani [email protected]
•
Symmetric and asymmetric switching
Note: Most switches are now
10/100, which allow you to use
them symmetrically or
asymmetrically.
Rick Graziani [email protected]