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