Cisco – Chapter 7 – Layer 2
Technologies
IEEE LAN Standards
LAN Technologies
• Token Ring 802.5
– IBM 1970s
– Large installed base but losing favor
• FDDI
– Popular as campus backbone
– 4 specifications
• Ethernet 802.3
– Many flavors
Token Ring
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Ring Logical topology
Star Physical topology
Access though MAC
Manchester Encoding
– Combines bit and timing
• 0 = high to low transition
• 1 = low to high transition
IEEE 802.5 and Token Ring
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4MB to 16 MB
260 stations
Star Topology
Twisted Pair
Baseband
Token Passing
Differential
Manchester
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Same
250 stations
Not specified
Not specified
Baseband
Token Passing
Differential
Mancheseter
Frame Fields
• Token is a flag bit
– Start delimiter, access control, end delimiter
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Frame control
Source address
Destination address
FCS
Data
Token Process
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Host seizes the token
Alters 1 bit which becomes start of frame
Appends data
Sends token to next station
Token continues until destination is reached
No other hosts can transmit data
Early release can release token when data
transmission is complete
Priority Systems - Token
• Priority field
– Only stations with priority value or higher can
seize the token
– Once token is seized, only stations with higher
priority can reserve token
– Next token has higher priority value
– Previous value reinstated when transmission is
complete
MSAU
• All active workstations can see
communications
• Stations can be selectively removed when
there is a transmission problem
• Beaconing detects and attempts to repair
network faults
• A beacon frame can define a failure domain
Signal Encoding
• Combining bits and timing into one stream
of signals
• Manchester differential encoding used by
token ring
FDDI
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Common in backbone installations
Connects high speed computers in LAN
Continues to grow as price decreased
Four Specifications
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MAC frames, token, addressing, FCS algorithm
Physical Layer Protocol – clocking & framing
Physical media – fiber, power, error rates
Station Management – configuration issues
FDDI Frame Format
• Token
– Preamble, start delimiter, frame control, end
delimiter
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Destination address
Source Address
Data
FCS
FDDI MAC
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Token grants permission to transmit data
Maximum period of time token can be held
Token becomes start of frame sequence
No collisions
Token circles ring until destination is
reached
• Deterministic and reliable
4B/5B Encoding
• Algorithm to prevent long durations of high
or low signals
– Danger of losing clocking mechanism
• 4 bits encoded as 5
– You don’t want to know the details
• Only important because it may be a test
question
Fiber Details
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100 Megabits per second
Ring logical and ring physical topologies
Token is MAC mechanism
Medium is fiber
– Laser for single mode and LED for multimode
• Advantages
– Speed, Security, Distance, Reliability
Ethernet – IEEE 802.3
• Most common LAN technology
• Well suited to applications where a local
communication medium must carry
sporadic, occasionally heavy traffic at high
peak data rates
• CSMA/CD
• Broadcast
– All stations can see all communications
Differences 802.3 & IEEE
• Ethernet provides services corresponding to
layers 1 and 2 of OSI model
• IEEE does not specify a LLC protocol
• Both are implemented through hardware
Ethernet Family Tree
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Refer to 7.3.2
10 Base 5
Coaxial
Cat 5 UTP
Twisted
10 Base FL Fiber
100 Base TX Twisted
100 Base FX Fiber
1000 Base TX Twisted
500 m
100 m
2000 m
100 m
2000 m
100 m
10mbs
10mbs
10 mbs
100mb
100mb
1000
Ethernet and 802.3 Frame Format
• Essentially the same
• Includes:
– Preamble, Start, Source and Destination Add
– Type, Data, and FCS
• Type specifies upper layer protocol
• Main difference is minimum number of data
bits; maximum is the same
CSMA/CD
• Performs three functions
– transmitting and receiving data packets
– decoding data packets and checking them for valid
addresses before passing them to the upper layers of
the OSI model
– detecting errors within data packets or on the network
• Connectionless – best delivery
• No notification
Ethernet Binary Encoding
• Manchester
– results in 0 being encoded as a high-to-low
transition and 1 being encoded as a low-to-high
– transition.
– Because both 0's and 1's result in a transition to
the signal, the clock can be effectively
recovered at the receiver
Star Topology
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Easy to design and install
Easy to maintain
Layout is easy to modify and troubleshoot
Reliable
Easy to add hosts
Requires more media
Hub is single point of failure
TIA/EIA 568 Standards
• Requires horizontal cabling in star topology
• All cabling must be terminated at patch
panel
• Repeaters can be used to extend length of
cable run
• 100 meters total length (horizontal length
modified by need to run cable down walls
and around periphery of room)
NIC Selection Criteria
• LAN technology used
– Ethernet, Token Ring, or FDDI
• Type of media used
– UTP, Fiber, Coaxial
• Type of bus
– PCI or ISA
NIC Layer 2 Functions
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Logical Link Control
Unique identifier – MAC address
Framing
MAC – access to shared medium
Signaling – creates signal to be placed on
medium
Bridge
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Minimizes collisions
Reduces traffic
Filters based on MAC address
Concerned with frames – not packets
Data Link Layer
• Controls data flow, handles transmission
errors, provides physical addressing, and
manages access to the physical medium
• Bridges use various link layer protocols that
dictate specific flow control, error handling,
addressing, and media access algorithms
• Data link layer protocols include Ethernet,
Token Ring, and FDDI.
Bridge Functions
• Reduce large collision domains
• Work best when traffic between segments is
low
• Always spread a broadcast packet
• Too many broadcasts result in broadcast
storm
– Can cause time outs, low performance, traffic
slowdowns
Switches
• Can replace hubs without disrupting the
network or requiring new installation
• Build and maintain switching tables
• Switch data frames
• Connect network segments, use table of
MAC addresses, and reduce traffic
• Faster than bridges
• Can support virtual LANs
LAN Switches
• Are considered multiport bridge with no collision
domain
– Because virtual LANs can be created
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Low latency levels
High speed for packet forwarding
Creates virtual circuits or PP connections
Microsegmentation provides full bandwidth to each host
• All hosts are still part of same broadcast domain
Segment LANs To:
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Isolate traffic between segments
Achieve more bandwidth per host
Create smaller collision domains
Can act as firewall for potentially dangerous
network errors
• Can allow more hosts to use network
Switches vs Bridges
• Switches are faster
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Operate in hardware rather than software
Can connect traffic with different bandwidths
Can support higher port densities
Can support cut-through switching
• Reduces latency
– Reduce collisions and increase bandwidth
• Because they can create virtual LANs
Bridges, Routers, & Switches
• Bridges and switches are layer 2
• Routers are layer 3
• Bridges pass data frames regardless of
protocol (operate at layer 2)
• Bridges increase latency – store & forward
– Thus cause propagation delay
Ethernet Performance
• Perform best when kept under 30-40% of
full capacity
• Bandwidth usage that exceeds the
recommended limitation results in increased
collisions
Router Segmentation
• Layer 3 – makes decisions based on IP address
• Make exact determinations of where to send data
packet
• Use routing table to make forwarding decisions
• Protocols that require acknowledgement have 3040% throughput
• Minimal acknowledgments (sliding windows)
have 20-30% loss in throughput
Troubleshooting
• Start at layer 1 – move up layer by layer to
layer 7