Transcript 09/24/04

COMS 161
Introduction to Computing
Title: Local Area Networks
Date: September 24, 2004
Lecture Number: 13
1
Announcements
• This material is from chapter 4 and 17 in
the book
2
Review
• Connecting to the Digital Domain
3
Outline
• Connecting to the Digital Domain
4
Benefits and Costs of LANs
• Benefits
– Communications
• Direct communications
• Client/server architectures
• Distributed computing
– Management control
•
•
•
•
Centralized mass storage
Centralized backups
Roll-out of upgrades
Software license control
• Costs
– Need for additional
equipment/software
– Maintenance
requirements
• Standardization of
hardware and software
across nodes to avoid
incompatibilities
– Support staff
– Cost-effectiveness
• Resource sharing
• Downsized, distributed
computing
5
Differentiating LANs
• Transmission media
– What are the actual hardware connections
between nodes (computers) made from?
• Topologies
– In what way are the various nodes arranged and
interconnected?
6
Transmission Media
• Bounded media
– Coaxial cable (like a TV cable)
– “Twisted-pair” cable (copper wires)
– Optical fiber cable
• Unbounded media (wireless networking)
– RF (radio frequency)
– IR (infrared)
– Cellular modem
7
Transmission Media
• Bounded media
– Coaxial cable (like a TV cable)
• Original LAN installations were coax
• Now almost never used (cost)
– “Twisted-pair” cable (copper wires)
• Generally limited to about 100 meters max (330 ft)
• Telephone wire (“CAT-3”, or “category 3”)
• CAT-5 wiring (up to 10 Mbps – 10 million bits per
second)
• CAT-5e wiring (up to 100 Mbps)
• CAT-6 wiring (emerging “gigabit” standard – up to 1
Gbps)
8
Transmission Media
• Bounded media (cont’d)
– Optical fiber cable
• Signal is composed of pulses of laser light, not
electricity
• Extremely thin glass strand transmits the light pulse
• Lower error rates and high data bandwidth (>2 Gbps)
• Becoming very cost-effective for high speed data needs
9
Transmission Media
• Unbounded media (wireless networking)
– RF (radio frequency)
• Becoming very common
• Speeds of 11 Mbps now common (“802.11b”)
• Faster speeds becoming available (54 Mbps, 108
Mbps)
• Public access points (“hotspots”) becoming
common
– Various areas on campus
– Downtown areas, such as Manhattan
– Airports, hotels, coffee shops, etc. (free or pay)
10
Transmission Media
• Unbounded media (cont’d)
– IR (infrared)
• An early wireless technique, now mostly unused as
RF wireless has advanced
• Required line-of-sight – works well only within enclosed
spaces
• Still used for some simple ad hoc networking tasks,
such as
–
–
–
–
Laptop-to-PDA
PDA-to-printer
Digital camera uploads
Etc.
11
Transmission Media
• Unbounded media (cont’d)
– Cellular modem
• Wireless connectivity anywhere that there is
appropriate digital cellular service
– Various cellular companies are competing
– Coverage is still spotty, but improving
• Up to 120 Kbps – better than dialup
12
Transmission Media
NETWORKS ARE BUILT ON PHYSICAL MEDIA
Type
Uses
Maximum Operating Principal
Distance (without amplification)
Cost
Twisted pair
Small LANs
300 feet
Low
Coaxial cable
Large LANs
600–2,500 feet
Medium
Fiber optic
Network backbones; WANs
1–25 miles
High
Wireless/infrared
LANs
3–1,000 feet (line of sight)
Medium
Wireless/radio
Connecting things that move
Varies considerably
High
13
LAN Topologies
• Topology
– The logical layout or geometric organization of a
network
– Topology indicates potential paths for
communications between nodes
– Many topologies possible, with pros and cons
•
•
•
•
Point-to-point
Star
Bus
Ring
14
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
• P2P file-sharing programs
(Napster, Kazaa, etc.) use
centralized directory servers
– While this works for the
Internet, it requires too much
overhead for a successful
LAN implementation
15
Star Topology
• All nodes are connected to a single hub
HUB
16
Star Topology
• Advantages
–
–
–
–
Simple to implement
Centralized management
Easy to add new nodes
Network can expand by
‘daisy-chaining’ hubs
– Not subject to failure due
to a single node or cable
failure
• Disadvantages
– 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
17
Bus Topology
• Single transmission medium (‘bus’ or
‘backbone’)
• Nodes connected to the bus by ‘taps’
18
Bus Topology
• Advantages
–
–
–
–
Simple to implement
Shorter cabling
Easy to add new nodes
Not subject to failure due
to a single node failure
• Disadvantages
– Length of backbone
limited
– Failure of the backbone
cable is catastrophic
– Centralized management
difficult
– Cannot expand network
through daisy-chaining
19
Ring Topology
• All nodes connected in a ring (‘token ring’)
• Once heavily promoted by IBM, now not used much
• Nodes have a specified order on the ring
1
6
2
5
3
4
20
Ring Topology
• Advantages
– Originally higher speed
than possible with other
types (first to 10 Mbps
– Exactly predictable delay
rate
• Disadvantages
– Size of ring limited
– Adding or removing
nodes is difficult
– Cannot expand network
through daisy-chaining
– Failure of the backbone
cable is catastrophic
– Failure of any single node
is also catastrophic
– No centralized
management
21
Media Access Control (MAC)
• Determines how nodes make use of the
underlying medium
• Not all nodes can talk at once! A protocol is
needed to act as the ‘traffic cop’
• Two MAC protocols are in common use in
LANs
– Ethernet MAC (“CSMA/CD” or “802.3”)
– Token passing MAC (“802.5”)
• These are both packet-switching protocols
– Data is broken into discrete packets which are
sent individually and reassembled at the
destination
22
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
You address you comments to one person, even though
everyone can hear you
– 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’)
• Ethernet follows this paradigm
• Leads to ‘orderly chaos’ – very efficient for low to
medium load networks
23
Token Passing MAC
• Typically used in a ring topology
• Very methodical protocol
– A ‘token’ (a small data packet) is passed around the
ring continuously (like a baton in a relay race)
– When a node receives the token, it can attach some
data to the token, and then pass it to the next node
– When that 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
• “And ya don’t stop, and ya don’t quit”
24
Layered Network Model
• Networks are created by layers of networking
software and hardware
– Consider two nodes communicating
• Use an application (e.g. send Email, get a file)
• The application uses the network services of the
operating system (provides high-level functions,
e.g. file sharing services)
• The operating system services make use of the
network system (provides basic connectivity tools
– ensuring that the nodes are communicating)
• The network system works through the physical
transport layer (the hardware and software of the
network – Ethernet, token ring, etc. – that actually
moves the data packets from node to node)
Applications
Network
Services
Network
System
Physical
Transport
25
Extending LANs
• LANs are great for local networks
• Need to be able to tie these local networks
together into larger groupings
• Connecting separate LANs (possibly of
different types) is called internetworking
• Eventually, want local networks to be part of
the global network – the Internet
• How do we interconnect these local networks?
26
Metropolitan Area Network (MAN)
•
•
•
•
Link two or more LANs in a city
Extends over a longer distance than a LAN
Each network site is a node on the network
Data is transmitted over common
“superhighways” called the backbone
27
Interconnecting LANs
• Different devices available for connecting
LANs together
– Repeaters
– Routers
– Bridges
– Gateways
28
Repeater
• Amplifies and repeats all signals
• Used to increase the size of a LAN
• Especially useful when the LAN must extend
to a distance longer than a single cable can
handle
HUB
REPEATER
HUB
29
Bridge
• Connects two LANs of similar types
• Only data for the ‘other’ LAN is passed
through
• Lets LANs act together like a larger LAN while
still maintaining their individual autonomy
HUB
BRIDGE
HUB
30
Router
• Can connect LANs of similar or different
types
– Specially designed to manage data flow in connected
networks – knows which route to use to most
effectively get the data to the right destination
HUB
ROUTER
31
Gateway
• Generally used to connect LANs to WANs
• Very effective at routing Internet traffic
BRIDGE
HUB
HUB
BRIDGE
HUB
The
Internet
ROUTER
HUB
GATEWAY
32
Internet(work)
• Collection of
autonomous
networks
• The Internet
• Intranets
33
The Internet
• The Internet
– Really just a very loose collection of networks
– No single entity controls the Internet
– Many kinds of information fly through it constantly
•
•
•
•
•
•
Email, IM (instant messaging)
Web pages
Entertainment – files and streaming media
Commerce and business data
VOIP – Voice over Internet Protocol (telephone)
Etc., etc.
– No one validates this information
– No one directly polices this information
34
History of The Internet
• Originally a US military project from the late
60’s: ARPANET
– Designed to survive a nuclear attack
• Expanded into academics and research in the
70’s
• Separated from MILNET in early 80’s
• Decommissioned in 1990
• Originally only non-commercial uses allowed
– Peer-pressure only, since there is no central
control!
35
History of The Internet
• The ARPANET in 1971 – 18 sites
36
History of The Internet
• The ARPANET in 1980 – about 75 sites
37
History of The Internet
• Original top-level domains:
.edu .gov .org .net .mil .com .int
• International domains came later:
.us .uk .jp .de .tv etc.
38
The Web
• The World Wide Web (WWW)
– Just one part of the Internet
– Consists of all information on the Internet that has
been made available using a particular method
(HTML & HTTP)
– Your computer is part of the Internet whenever you
are connected
– Information on your computer will become part of
the Web during the first few labs
39
Web Browsers
• Web browsers are programs that make it easy for
anyone on the Internet to access information on the
Web
• Many to choose from; they all use common
techniques
–
–
–
–
–
–
–
–
Mozilla (www.mozilla.org)
Netscape Communicator (AOL Time Warner)
Internet Explorer (Microsoft)
Opera (www.opera.com)
MSN TV [formerly WebTV] (Microsoft)
Cell Phones
PDAs (e.g. Palm Pilots)
Lynx (text-only browser)
40
History of The World Wide Web
• Invented by Tim Berners-Lee at CERN (1989)
• Originally designed for
distributing scientific
research results
– Text pages that can be
shared among different
computer systems
– Simple, text-based browsers
• Quickly adopted by other organizations
41
History of The World Wide Web
• First graphical browser (Mosaic) developed in 1993
by an undergrad at University of Illinois (Marc
Andreesen)
– (only 11 years ago!)
– Distributed freely
– Widely used in academics and government
• Mosaic expanded by Netscape
– Internally called “Mozilla”
– Originally still distributed without charge
• Microsoft’s Internet Explorer came later
• Netscape now commercialized (AOL Time/Warner)
• Mozilla still free (split off from Netscape)
42
History of The World Wide Web
• “Ban” on commercial traffic on the Web lifted soon
after Mosaic released
• .com quickly becomes largest domain
• E-commerce explosion starts in the late 90’s
•
•
•
•
Amazon.com founded 1994 by Jeff Bezos
IPO 1997 @ $18/share
Each share bought then now worth $540 – 3000% in 7 yrs
First profit 2001; currently $6,000,000,000/yr sales
• New top-level domains recently added to the Internet
due to increased Web activity:
.biz .info .name etc.
43
Internet Growth and the Web
Year
Internet Nodes1
Web Servers2
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1971
240,000,000
162,129,000
93,048,000
36,739,000
12,881,000
3,212,000
992,000
313,000
56,000
5,089
1,024
235
18
48,000,000
33,083,000
18,170,000
4,279,000
300,000
3,000
50
Sources: (1) www.isc.org/ds; (2) www.netcraft.com/survey
44
Client/Server
• CLIENT--(local)
system requesting
services
• SERVER--(remote)
system that
receives and
handles requests
from many clients
concurrently
45
Web as Client/Server App
• Protocol--a set of rules that govern how
an activity takes place
• HTTP--(HyperText Transfer Protocol)
specifies how Web clients and servers
communicate
46
Web as Client/Server App
• URL--(Uniform Resource Locator)
addressing for Web resources
• HTML--(HyperText Markup Language)
defines content and display of Web
pages
47
URL Example
• URL for the book
• Protocol
• Domain Name
• Resource
location
48
Web Browser (Client)
49
Researching on the Web
• Search service--generates lists of other
Web sites containing information about
supplied topics
• Web directory--a search service
organized as a topical hierarchy and
compiled by (human) editors
• Search engine--search services whose
databases are compiled by automated
Web crawlers
50
Keyword Searching
• A form of (automated) text matching
• Keyword--a word or phrase used as a
text pattern for matching
• Hits--matches with text patterns
• Misses--fails to match some or all of the
text pattern
• False positives--hits that are not related
to the desired topic
51
Boolean Expressions
• search engines
interpret multiple
keywords as Boolean
expressions
• AND--intersection,
i.e., references that
contain both
keywords
52
Boolean Expressions
• OR--union, i.e.,
references that
contain either
keyword
53
Boolean Expressions
• NOT--negation,
i.e., references
that do not contain
the keyword
54
Mathematical Operators
Mathematical operators may be used to
signify Boolean expressions
Boolean Expression
Mathematical Operator
Labrador AND retrievers +Labrador +retrievers
Labrador AND NOT
retrievers
+Labrador -retrievers
55
Hybrid Search Services
• Metasearch engines--consult other
search engines and directories and
summarize query data
• Niche services--commercial services
targeted at a specific audience or topic
• Portals--gateway Web site with
searching capabilities
56