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Ethernet Frame
Preamble
Destination
Address
Source
Address
Length/
Type
LLC/
Data
Frame
Check
Sequence
Ethernet Addresses
48 bit
 normally shown in HEX
 e.g.: 02-60-8C-44-59-E0
 first 24 bits are obtained from IEEE by hardware
manufacturers
 one bit is used to distinguish unicast from
multicast addresses
 one bit is used to distinguish locally assigned
addresses(rare) from globally assigned
addresses (typical)

Transmission Types
Unicast - intended for one specific station
 Broadcast - intended for all stations
 Multicast - intended for a group of stations

CSMA/CD
Carrier Sense, Multiple Access with
Collision Detection
 Carrier sense - don't transmit if there is
already a signal
 Collision detection - if a collision is
detected jam, wait, and retransmit

No Carrier
Transmission
Transmission
End of Transmission
Transmission
Second Transmission
Collision
Collision detected
Wait
Ethernet HUB
(repeater)
•
•
•
•
•
•
•
Layer 1 device
Repeat signal on all ports
Enforce collision on all segments (jam)
Restore amplitude of the signal
Retime the signal
Single speed
May mix media
Repeater Hub
Repeater Hub
one station transmits
Repeater Hub
signal repeated on all other ports
Repeater Hub
signal repeated on all other ports
Repeater Hub
end of transmission
Repeater Hub
Matrix Module
Collision Domain
• Devices connected by hubs
• Only one device can transmit at a time
• Collisions detected by all devices in the domain
One Collision Domain
Hub
Hub
Hub
Broadcast Domain
•
•
•
•
Devices connected by switches
Can contain multiple collision domains
One transmission in each collision domain
Collisions do not propagate between collision
domains
• Broadcast frames do propagate to all collision
domains
• Can be full duplex if only two devices in collision
domain and both devices are full duplex cabable
Three Collision Domains
One Broadcast Domain
Switch
Hub
Hub
H – Ethernet Hub
S – Ethernet Switch
R – IP Router
R
W – Workstation
_____ Ethernet Segment
S
S
H
S
H
H
H
W
W
W
W
W
W
W
W
W
H – Ethernet Hub
S – Ethernet Switch
R – IP Router
R
W – Workstation
_____ Ethernet Segment
S
S
H
S
H
H
H
W
W
W
W
W
W
W
W
W
H – Ethernet Hub
S – Ethernet Switch
R – IP Router
R
W – Workstation
_____ Ethernet Segment
S
S
H
S
H
H
H
W
W
W
W
W
W
W
W
W
H – Ethernet Hub
S – Ethernet Switch
R – IP Router
R
W – Workstation
_____ Ethernet Segment
S
S
H
S
H
H
H
W
W
W
W
W
W
W
W
W
Ethernet switch
•
•
•
•
•
Function (old name) “bridge”
Forwards or Filters unicasts
Floods (forwards to all ports) all broadcasts
Learns address locations
Can be full duplex if only two devices in collision
domain and both devices are full duplex cabable
• Can mix speeds and media
switch
1
DA
SA
Hub
Hub
Hub
E1
3
2
E2
E3
E4
Action
ADDRESS
FF
E2
E2
E3
E3
E2
FF
E5
E5
E6
E6
E5
E4
E1
E5
PORT
E6
Switch Light Videos
• Switch Lights(3.5 MB)
• Switch Lights – more(5.9 MB)
Cut-Through Mode and
Store and Forward Mode
• Cut-through mode
– Switching mode in which switch reads a frame’s header and
decides where to forward the data before it receives the entire
packet
– Can detect runts, or packet fragments
• Store and forward mode
– Switching mode in which switch reads the entire data frame
into its memory and checks it for accuracy before transmitting
it
Using Switches to Create VLANs
• Virtual local area
networks (VLANs)
– Means by which a
switch can logically
group a number of
ports into a broadcast
domain
• Broadcast domain
– Combination of ports
that make up a Layer
2 segment and must
be connected to a
Layer 3 device
FIGURE 6-24 Simple VLAN design
Trunk Aggregation
Resilient Links
Higher-Layer Switches
• Switch capable of interpreting Layer 3 is called a
Layer 3 switch
• Switch capable of interpreting Layer 4 is called a
Layer 4 switch
• Higher-layer switches may also be called routing
switches or application switches
Routers
• Multiport device
• Can connect dissimilar LANs and WANs running
at different transmission speeds and using a
variety of protocols
Router Features and Functions
• Filter out broadcast transmission to alleviate network
•
•
•
•
•
congestion
Prevent certain types of traffic from getting to a network
Support simultaneous local and remote activity
Provide high network fault tolerance through redundant
components
Monitor network traffic and report statistics to a MIB
Diagnose internal or other connectivity problems and trigger
alarms
Routers
FIGURE 6-26 Placement of routers on a LAN
Routing Protocols
• Means by which routers communicate with each
other about network status
– Convergence time
• The time it takes for a router to recognize a best path in the
event of a change or outage
– Bandwidth overhead
• Burden placed on an underlying network to support the
routing protocol
Routing Protocols
• RIP (Routing Information Protocol) for IP and
IPX
• OSPF (Open Shortest Path First) for IP
– Best path refers to the most efficient route from one
node on a network to another
• EIGRP (Enhanced Interior Gateway Routing
Protocol) for IP, IPX, and AppleTalk
• BGP (Border Gateway Protocol) for IP
Gateways
• Combination of networking hardware and software that
connects two dissimilar kinds of networks
–
–
–
–
E-mail gateway
IBM host gateway
Internet gateway
LAN gateway