Overview of Computer Science
CSC 101 — Summer 2011
LANs
I t
Internetworking
t
ki
Lecture 20 — August 2, 2011
Announcements
• Final Exam next Tuesday (8/9/11).
– This Room – 2pm – 5pm
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Final Review on Mondayy – 8/8/11
Quiz #3 on Friday
Lab #7 Today
WA#7 due Today
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Objectives
•
•
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•
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Classifying networks by switching
LAN Topologies
Methods for controlling LAN communications
How LANs are extended and interconnected
Internetworking
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Networks Classified by Switching
• Nodes communicate with each other through a network
using shared, common paths
– Messages pass through intermediate nodes between the
sender and receiver
– Messages must follow a correct route to reach the destination
• The method used for routing is called switching
• There are two major switching techniques
– Circuit switching
– Packet switching
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Circuit Switching
• In circuit switching, a continuous connection or circuit is
created between sender and receiver
• Ordinary telephone connections are made by circuit
switching
sender
receiver
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Packet Switching
• In packet switching, the message is broken up into separate data
packets, each addressed to the destination
• Each packet travels separately over any available connection to
the destination (different packets may take different routes)
• The receiving node reassembles the message at the destination
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LAN Topologies
• Topology
– The layout of a network
• Logical and geometric interconnections between nodes
• Potential paths of communication between nodes
– Many topologies possible, with pros and cons
•
•
•
•
Point-to-point
Star
Bus
Ring
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Point--to
Point
to--Point Topology
• Point-to-point is the simplest topology
– Each node connected to some of its neighbors
– Needs a control mechanism
• The Internet uses TCP/IP (More on that later)
– While this works for the Internet, it requires too much
overhead for a successful
LAN (small scale)
implementation
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Point--to
Point
to--Point Topology
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Point--to
Point
to--Point Topology
• The “One Laptop Per Child” program
(www.laptop.org) is using a point-to-point
topology they call “mesh networking”
– Each laptop wirelessly connects with others in range
– If one laptop is connected to the Internet, there is a
pathway for Internet traffic to all laptops in the mesh
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Star Topology
• All nodes are connected to a single hub
HUB
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Star Topology
• Advantages
• Disadvantages
– Simple to implement
– Easy to add new nodes
– Not subject to failure due
to a single node or cable
failure
– Network can expand by
‘daisy-chaining’ hubs
HUB
– Number of nodes limited to
size of hub
– Cabling must all feed back
to the hub
– Hub failure is catastrophic
– Hub can be a bottleneck for
data throughput
HUB
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Bus Topology
• Single transmission medium (‘bus’ or ‘backbone’)
• Nodes connected to the bus by ‘taps’
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Bus Topology
• Advantages
–
–
–
–
• Disadvantages
Simple to implement
Shorter cabling
Easy to add new nodes
Not subject to failure due
to a single node failure
– Length of backbone limited
– Failure of the backbone
cable is catastrophic
– Cannot expand network
through daisy-chaining
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Ring Topology
• All nodes connected in a ring (‘token ring’)
– Nodes have a specified order on the ring
• Once very common for LANs, now not used as much
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2
5
3
4
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Ring Topology
• Advantages
• Disadvantages
– Originally higher speed
than possible with other
yp
types
• First network type to reach
10 Mbps
– Exactly predictable delay
rate
• Good for high-speed,
demanding requirements
– Size of ring limited
– Cannot expand network
through daisy
daisy-chaining
chaining
– Failure of the backbone
cable is catastrophic
– Failure of any single node is
also catastrophic
• Adding or removing nodes is
difficult
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Media Access Control (MAC)
• Determines how nodes make use of the
underlying medium
• Not all nodes can talk at once! A protocol is
needed
d d tto actt as the
th ‘traffic
‘t ffi cop’’
• Two MAC protocols are in common use in LANs
– Ethernet MAC
– Token passing MAC
• These are both packet-switching protocols
– Data is broken into discrete packets which are sent
individually and reassembled at the destination
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Ethernet MAC
• Commonly used in star and bus topologies
• Much like a conversation at a dinner table
–
–
–
–
Not everyone can talk at once
If someone is talking, politely wait until they’re done
When there is a lull in the conversation, you can speak
If two people happen to try to talk at the exact same time (a
‘collision’), both stop and wait a moment to see if they can talk
(‘random backoff’)
– You address you comments to one person, even though
everyone can hear you
• Ethernet follows this paradigm
• Leads to ‘orderly chaos’ – very efficient for low to
medium load networks
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Token Passing MAC
• Typically used in a ring topology
• Very methodical protocol
– A ‘token’ (a small data packet) is passed around the
ring
i continuously
ti
l (like
(lik a baton
b t in
i a relay
l race))
– When a node receives the token, it can attach some
data to the token, then passes it to the next node
– When a node receives the token, it looks to see if any
attached data is addressed to it; if so, it keeps it
– Any attached data addressed to someone else is
simply passed on with the token to the next node
– And so on, ad infinitum
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MACs Compared
• Ethernet and Token Passing MACs are both packet-switched
– Messages are broken into packets which travel independently
• Ethernet packets are broadcast onto the LAN
– Every node on the LAN sees every packet
– Nodes
N d iignore packets
k t nott addressed
dd
d to
t them
th
– When two nodes send packets simultaneously they interfere with each other
– called a collision
• The packets don’t actually “collide” or “cross” each other; rather they overlap
like two people talking simultaneously
• Token passing MAC avoids collisions, but has more overhead
• Token passing MAC delivers packets within a known delay time,
while Ethernet’s delivery delay is random
– Token passing MAC is thus better for time-critical uses such as high traffic
volume between connected servers
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Extending LANs
• LANs are great for small, local networks
• Need to be able to tie these local networks together into
larger groupings
– Larger
L
LANs,
LAN MAN
MANs, WANs
WAN
• Interconnecting LANs (possibly of different types) is
called internetworking
• Local networks are usually part of larger networks
– An internet
– The Internet
• How do we interconnect these local networks?
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Interconnecting LANs
• A LAN can sometimes be extended by simply
daisy-chaining hubs together
– Only works for star-topology (hub-based) LANs
– Hubs must be physically close together
• When that is not possible, various devices are available
for connecting LANs together
–
–
–
–
Repeaters
Bridges
Routers
Gateways
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Repeater
• Useful for:
– Linking together hubs that are in different locations
– Extending a LAN beyond the maximum cable length
• All data is passed ((“repeated”)
repeated ) between the two LANs
– The extended LAN acts identically to a single LAN
HUB
REPEATER
HUB
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Bridge
• Connects two LANs of compatible type
– Only data for the ‘other’ LAN crosses the bridge
– Lets LANs act together like a larger LAN while still
maintaining some individual character
HUB
BRIDGE
HUB
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Switch
• Can connect several LANs of compatible type
– Switch only sends data to the correct destination LAN
HUB
SWITCH
HUB
HUB
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Router
• Can connect compatible or incompatible LANs
– Translates packets between incompatible network types
– Knows the best route to send packets to the right
destination
HUB
Access
Point
ROUTER
HUB
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Gateway
• Used to connect LANs to WANs and the Internet
– Very effective at routing traffic between
internets
Access
Point
HUB
ROUTER
ROUTER
ROUTER
HUB
BRIDGE
HUB
SWITCH
HUB
ROUTER
HUB
The Internet
HUB
BRIDGE
HUB
GATEWAY
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Firewalls
• Works together with a gateway to protect a
network by filtering data traffic
– Access control policies determine what is permitted or
denied through the gateway
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WFU Network
• The campus network infrastructure was
completely upgraded in 2003-2004
• The campus network as of Spring 2003
– Single gateway to the Internet
– Five main switches – lack of redundancy
• The “Next Generation Network” as of Fall 2004
– Dual Internet gateways
– Extensive redundancy
– Complete campus wireless coverage included
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Internetworking
• LANs are great for covering small areas
• But, in the world there are
–
–
–
–
Manyy types
yp of LANs and WANs
Many types of hardware and software
Many languages
Large distances
• How can all these separate networks be
connected effectively, efficiently, seamlessly,
transparently?
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Internetworking
• Internetworking enables seamless communication
across disparate networks by using six special
techniques:
11. Packet-switched
Packet switched data transport system
2. Special protocols
• TCP – transmission control protocol
• IP – internet protocol
3. Dedicated internet routers send packets in the right
directions
4. DNS – the domain name system
5. Client/server model allows asynchronous transactions
6. A layered architecture
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Packet Switching and Datagrams
• Data is broken up into packets (datagrams)
• Each packet travels via whichever path is best at the time
– Packets from the same message probably travel different paths
• Receiver reassembles the message from packets received
FINISH
START
THE
INTERNET
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Packet Switching and Datagrams
• Data is broken up into packets (datagrams)
• Each packet travels via whichever path is best at the time
– Packets from the same message probably travel different paths
• Receiver reassembles the message from packets received
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Packet Switching and Datagrams
•
Two processes are necessary to ensure that this works
1. Managing the process of breaking the data into packets and
reassembling them at the destination
2. Addressing and routing the packets through an internet
•
TCP (the Transmission Control Protocol)
–
•
–
•
•
Manages creating and reassembling packets
IP (the Internet Protocol)
Manages the addressing and routing
These are usually referred-to together as TCP/IP
They work independently from the underlying network
technologies (hardware)
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TCP – Transmission Control Protocol
• Each packet includes sender’s and receiver’s IP address
• Each packet also includes a sequence number so that
packets can be reassembled the in the correct order
– Packets travel different paths so often arrive out
out-of-order
of order
– Receiver sends back an ACK (acknowledgement) for each
packet correctly received
– Receiver sends back an NAK (negative acknowledgement, or
resend request) for each packet received damaged
• Sender re-sends that packet
– If packet is lost, sender notices that it doesn’t get either an
ACK or a NAK before some timeout period
• Sender resends that packet
• TCP simulator (today’s lab) demonstrates this procedure35
IP – Internet Protocol
• Every node on every network of the Internet is
assigned a unique IP address
– Four 3-digit numbers separated by periods
• E.g. 18.7.22.69
– Our laptops are assigned temporary IP addresses each
time we boot
• DHCP (Dynamic Host Configuration Protocol)
• whoami
• IP not only specifies IP addresses, but also the
routing of data from one IP address to another
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Routing
• Internet routers are specialized gateways that
pass data between the various networks that
make up the Internet
• Routers
R t maintain
i t i routing
ti tables
t bl that
th t help
h l them
th
decide the best direction to send data to get it to
its destination
• These routing tables are constantly updated
between the routers to reflect changing network
conditions
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DNS – Domain Name System
• Hierarchical IP address numbers are very efficient for computers
to use…
• …But, they’re difficult for humans to use
– Hard to remember
p
– Error prone
– Non-intuitive
• The Domain Name System (DNS) translates between IP numbers
and domain names
– Examples:
www.wfu.edu Ù 152.17.48.77
www.amazon.com Ù 72.21.206.5
• Domain name servers keep track of this info
• Can find out information about a domain name from a
whois server such as whois.net or arin.net/whois
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Client/Server Model
• Most internet traffic uses a client/server model
– Client nodes request a resource
– Server nodes provide resources
• Allows for asynchronous communication
– Not everything needs to happen in real time
• What would happen if a large number of users clicked the
‘Buy It!’ button simultaneously?
– Requests can be processed in the order received
• Analogies: Telephone call – synchronous communication
IM – asynchronous communication
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Risks of Digital Information
• “In this age of advanced technology, thick walls
and locked doors cannot guard our privacy or
safeguard our personal freedom.”
– Former
F
P
President
id L
Lyndon
d B.
B Johnson,
J h
F b 77, 1968
Feb.
• “The American government can stop me from
going to the US, but they can’t stop my virus.”
– An anonymous virus creator
• “Every system has vulnerabilities. Every system
can be compromised.”
– Peter G. Neumann, author
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