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The Workings
of the Internet
CECS 5030
with Cathie Norris,
Jennifer Smolka & Gerald Knezek
Overview
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Layered Organization
Topologies
Network Transports
Access Methods
Routing
ISO/OSI Model
 Developed by International Organization
for Standardization in 1974
 Consists of seven layers
 Each with unique function
 Each hands off functions to adjacent layer
 Modules (layers) may be replaced with
another of equal functionality (Xerox vs.
Novell, for example)
OSI Model Layers
OSI Layer
Function Provided
Application
Network applications such as file
transfer and terminal emulation
Presentation
Data formatting and encryption
Session
Transport
Network
Establishment and maintenance
of sessions
Provision for end-to-end reliable
and unreliable delivery
Delivery of packets of information,
which includes routing
Data Link
Transfer of units of information,
framing, and error checking
Physical
Transmission of binary signal
Network Topologies
 Architectural “drawings” that show the
overall physical configuration for a given
communications system
 Determine access methods and rules used
to design and implement a communication
system
 Represent the drawing of your network
cable plant
 Three main types: star, ring, and bus
Network Topologies
 Linear Bus - Ethernet/IEEE 802.3
10Base2 and 10Base5
 Star Wired Bus - Ethernet/IEEE 802.3i
10BaseT
 Star Wired Ring - Token Ring/IEEE
802.5
 Dual Counter Rotating Ring FDDI/ANSI X3T9.5
 Wireless - Product Specific
Star Topology
 First used with the telephone switches
 Centralized hub with all stations connected
 No single point of failure effects the whole
network, except the hub
 Oldest and most popular topology
 Better network management
Node
Node
Central Hub
Node
Node
Ring Topology
All stations (repeaters) are enclosed in a
loop
Each receives the signal and repeats it on
the other side to its “downstream” neighbor
Data is transmitted in one direction only
Single point of failure when one station
quits repeating
Management processes invoked that
dynamically remove a station allowing the
ring to return to an operational state
Ring Topology
Node
Transmitter
Node
Data Direction
Receiver
Node
Repeater
Node
Bus Topology
Also known as linear bus
Uses a single length of cable with all
stations attached to it
The network is terminated at its endpoints
(not a closed loop)
A break on the single cable will bring
down all attachments on the network
The bus topology is most commonly used
for Ethernet networks
Bus Topology
Node
Node
Node
Star-Wired Bus Topology
Each node is attached to
Node
Node
hub
When one node fails, it
Node
Node
doesn’t affect the other
nodes
Node
Node
The hub is a single point
of failure for all nodes
Concentrator
Hub failure causes all
Hub
nodes to lose connectivity
Physical Media
 Physical media provide the connections
between network devices that make
networking possible
 There are four main types of physical
media in widespread use today:
 Coaxial Cable
 Twisted Pair
 Fiber Optic Cable
 Wireless Media
Thick Coaxial Cable
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Used in the first Ethernet networks
Type RG-11 / 10Base5
Usually orange/black
Thickness of a small garden hose
Very expensive and heavy cable
Two strands along the axis
Conductor down the center
Insulator surrounds conductor
Shielded mesh serves as outside
Thin Coaxial Cable
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Alternative to Thick Ethernet Cable
Type RG-58 / 10Base2 / “Cheapnet”
Usually black
Thickness of a pencil
More flexible than thick Ethernet
Reduced the cost of the cabling
Flexible
Twisted Pair Cable
 Phone Systems
 Twisted Pair Cable consists of two copper
wires, usually twisted around each other
to cancel out any noise in the circuit
 Two main type of Twisted Pair Cabling
 Shielded Twisted Pair (STP)
 Unshielded Twisted Pair (UTP)
Shielded Twisted Pair (STP)
 Shielded twisted pair is the original
media used for token ring networks
 STP can be used for high-speed
networks, such as FDDI or ATM, where
shielding is important
Unshielded Twisted Pair
(UTP)
 Most commonly used twisted pair cable
 Uses common telephone wire
 UTP was standardized by the IEEE 802.3
committee in October of 1990
 UTP for LANs is now classified as:
 Category 3 - used for LANs up to 10 Mbps
 Category 4 - used for LANs up to 16 Mbps
 Category 5 - used for LANs up to 100 Mbps
Fiber Optic Cable
 Uses light signals transmitted over a
very thin filament, usually made of glass
 Advantage over other types of media
 security against eavesdropping
 immunity to interference
 maximum length of a single
segment
 Most expensive of all media
Wireless Media
 Connect your computer to your cell
phone?
 Problems with stability of connection
 Have wireless for a long time
 Commercial Satellite
 Geostationary Orbit
 Microwave Wavelength
 Expensive
Wireless Media
 A number of wireless media are used in
internetworking, e.g.:
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Microwave
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Commercial Radio wave
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Infrared signaling (Palm Synching)
Concentrators/Hubs
 Hubs allow multiple users to be connected
to a single network as a shared device
 The more users on a hub the slower the
response time
Network Transports
 Ethernet / Fast Ethernet / IEEE 802.3
 Token Ring / IEEE 802.5
 FDDI / FDDI/ANSI X3T9.5
 Wireless/IEEE 802.11
Ethernet Cable Names
Unshielded
Twisted
Name
Thick Coaxial Thin Coaxial
Pair
Fiber
Wire Type
RG-8
RG-58
22 - 26 AWG 62.5/125 micron
IEEE Name
10BASE5
10BASE2
10BASET
10BASEF
Standard Number IEEE 802.3
IEEE 802.3a IEEE 802.3i
NA
Other Names
Thick net
Thin net
UTP
How Ethernet Works
 Sent the message and listens for a
response
 An access method based on the Carrier
Sense Multiple Access with Collision
Detection (CSMA/CD) algorithm
 Cooperative effort between Digital, Intel,
and Xerox produced Ethernet version 1.0
in 1980
How Ethernet Works
 Ethernet was adopted with modifications
by the standards committees IEEE 802.3
and ANSI 8802/3
 Most widely used network system today
Normal Ethernet Operation
B
C
Address mismatch
packet discarded
Address mismatch
packet discarded
Send data
to node D
Transmitted packet seen
A
by all stations on the LAN
(broadcast medium)
Data
Address match
packet processed
D
Final Ethernet Issues
 Ethernet is an access method that
strictly adheres to the CSMA/CD
algorithm
 Ethernet is a multiprotocol solution
 Ethernet is usually hardware (firmware),
not software
How Token Ring Works
 Token Ring controls which PC can send
messages by passing a token from
station to station around the ring
 When a PC wants to transmit it will
replace the token with a “frame”
(message)
 The frame is passed from PC to PC until
it reaches its destination
How Token Ring Works
 The destination PC makes a copy of the
“frame” (message) and marks the frame
to indicate that it got the message
 The frame circulates around the
network until it gets back to the sender
 The sender, seeing that the message
has been received, replaces it with a
new token
Wide Area Network (WAN)
Topologies
Dedicated Circuits
 56Kb
 T-1
 DS-3
Frame-Relay
 56Kb to T-1 speeds
Integrated Services Digital Network (ISDN)
Inter-networking
 Networks have their restrictions
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Thick coaxial cable maximum length is
500 meters
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LANs are broadcast-oriented
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Proper network design is impossible
using repeaters
Inter-networking
 Properly extending the LAN requires
special devices known as bridges and
routers
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A LAN that uses bridges is called an
extended LAN
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A LAN that uses routers is called an
internet or inter-network
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A gateway between dissimilar networks
Inter-networking
 Bridges and routers are data-forwarding
devices that forward packets to one or
more LANs
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They allow for more efficient
networks to be designed
Inter-networking Categories
Application
Presentation
Gateways
Session
Transport
Network
Routers
Data Link
Bridges
Physical
Repeaters
Repeaters
 Extend the network by interconnecting
multiple segments
 Have transformed into wiring
concentrators (hubs)
 Low cost
 Can be used to interconnect different
wiring types but not different access
methods
 e.g. Coax to twisted pair
Bridge Designs
 Cascaded
 Locates on bridge next to another in a
pillar fashion
 Backbone
 For networks with many LANs
 Backbone cable is run vertically in
building’s riser
 LAN “ribs” run on each floor
 Star
 Used in wide area networks or remote
bridged networks
Cascaded
Terminal Server
Workstation
Terminal
Host
Cable segment 1
Cable segment 2
Cable segment 3
File Server
Backbone
Workstation
Floor 20
Host
Fiber
backbone
Workstation
Terminal
Floor 1
Star
California
Serial line
Serial line
Texas
Virginia
Serial line
North Carolina
Introduction to Routers
 Routers are data forwarding devices but operate
differently than a bridge
 Routers separate networks into regions.
 Each region is assigned a unique network number
 These network numbers are unique for each network
they are assigned to
 Packet forwarding is based on these network Ids
 Routers route packets based on a protocol as well as
a network ID
 Most routers today are multiprotocol in that one box
can forward different protocol packets
 Routers, like bridges, can be used locally or remotely
Routing
 Most network protocols were designed with network-layer
routing
 Routers base forwarding decisions on an embedded
network number in the network layer header of the packet
 Network numbers can be thought of as area codes in the
phone system
 Must use the area code to call different areas
 Any number of end stations may be assigned to one
network number
 Most routers do not keep track of individual end
stations’ addresses
 Network numbers group network stations into one or more
network numbers
 Taken as a whole, routers combine networks and form
internets
Routers - Operation
Node D
Router sends
packet directly
to the end station
MAC address
for the router
Destination
network
number
is different
Network 2
C
Router Z
Find router B
and give packet
to the router
Node A
Node P
Destination network address is local
transmit packet directly to the end station
Network 1
Routing Diagram
G
Network 4
Router X
H
F
E
Network 3
Router Y
D
Network 2
C
MAC Addresses
Router Z
B
A
Network 1
Multiprotocol Routers
 LANs currently operate with many different types of
protocols
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Apple Computers can use AppleTalk
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UNIX workstations use TCP/IP
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Client/Server applications could use Novell
NetWare
 To require one router for each protocol on the LAN is
not efficient
 Multiprotocol routers were invented to handle this
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Arrived around 1986
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Routes not only based on the network IDs but are
able to pass the packet to the correct protocol
processor by examining the Type of packet
Gateways
 Complex devices that provide for a
protocol translation during data
forwarding
 Examples are:
 TCP/IP to SNA
 asynchronous to synchronous serial
stream
 Gateways differ from bridges and routers
 Perform protocol translation of the
incoming packet to match the
outgoing stream
References
From Networking 101
Jim Cabral, Puget Technology Group, Inc. &
Tammy Ruth, Children’s Hospital and Medical Center
www.pugettech.com
cabralje@pugettech.com
truth@chmc.org