Day10
LAN
Why?• Allow more than one machine to
share– Resources– Internet connectivity– Information
Good• LANs are abstract from the
hardware– HP printer will work with an IBM PC or
a MAC or Linux– You usually don’t know what type of
machine you are connecting to, it’s irrelevant since you both talk the same protocols.
• More web servers run on Linux (apache) than windows (IIS), but you can access them all from windows.
Shared Bus• Anytime we see a network where
everyone shares a single wire (either physically or logically) – Traffic from any host to any host goes
to all hosts– Everyone is supposed to ignore it
unless it is addressed to them
Computer Computer Computer Computer
Topologies• Tree
– Cable TV– Single wire, bidirectional, can be
broadband– Need Taps (jaws)– One break it all goes down– Shared Bus
Computer
Computer
ComputerComputer
Computer Computer
Star Wired Bus• Works like a bus (single wire)• Looks like a star (each
workstation connects to central point)
• Most networks use this today– Ethernet– Token Ring
• Shared busComputer Computer Computer
Hubs• Workstations connect to hubs
– Dumb repeaters– Take data in one port, broadcast it to
all ports
• Can be connected to other hubs via a crossover cable
• Mostly obsolete
HUB A
Computer 1 Computer 2 Computer 3
HUB B
Computer 4 Computer 5
Media Access Control Protocol• Carrier Sense Multiple Access
(CSMA)• Collision Detection (CD)
– CSMA/CD– E.g. Ethernet
• Listen for Carrier• Transmit and listen for collision• Wait and retransmit
• 100Mbps Ethernet has max speed of 40Mbps because of collisions
CSMA/CA• Collision Avoidance:
– a station that intends to transmit sends a jam signal
– after waiting a sufficient time for all stations to receive the jam signal, the data station transmits a frame
– while transmitting, if the data station detects a jam signal from another station, it stops transmitting for a random time and then tries again.
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Star-Wired Ring• Token ring is the most common
example– Every workstation connects to a Hub
like device (MAU).– MAU creates a circle between
workstations• Empty ports are shorted
– Workstation adds data to ring, and removes it when it gets back.
– Token decides who can talkComputer 1Computer 2
Token Ring 802.5• Round Robin Protocol
– Single token– No collisions– More efficient– More expensive – Slower speeds– Basically dead
Wireless• Access point
– Converts from wireless to wired– Can be more than one used to blanket
a larger area
Computer
IEEE 802.3 (Ethernet) Frame
Ethernet– [speed][base/broad][distance (in 100
meters)• 10Base2
– 10Mbps baseband signal for 200 meters
• 10BaseT– 10Mbps baseband signal for 100 meters
• 100baseT– 100Mbps baseband for 100 meters
• 1000baseT• 1000BaseFx – 1000m, SX/LX – 100m, LX• 10Gbase-fiber/T
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