Download 01-Internet

CIS 203
01 : Data Networks and
The Internet
Data Networks
• Communication by transmitting data
intermediate switching nodes
• Switching nodes not concerned with content
• End devices referred to as stations
—Computers, terminals, telephones, etc.
• Nodes connected in some topology by
transmission links
• Station attaches to node
• Collection of nodes is a communications network
Figure 1.1
Simple Switching Network
Circuit Switching
• Dedicated path between two stations
—Connected sequence of links between nodes
—E.g telephone network
• Communication involves three phases
—Circuit establishment
—Data transfer
—Circuit disconnect
Circuit Establishment
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Station A to node 4 requesting connection to station E
Circuit from A to 4 usually dedicated line
Node 4 finds next leg to node 6
Based on routing information, availability, cost, node 4
selects circuit to node 5
• Allocates a free channel
— TDM [time-division multiplexing]
— FDM [frequency-division multiplexing]
• Node 4 requests connection to E
• And so on
Data Transfer
• Data may be digital (e.g., terminal to host) or
analog (e.g., voice)
• Signaling and transmission may each be digital
or analog
• Path is A-4 circuit, internal switching through 4,
4-5 channel, internal switching through 5, 5-6
channel, internal switching through 6, 6-E circuit
• Generally, full duplex (data in both directions)
Circuit Disconnect
• Connection terminated
—Usually by one of the stations
• Signals to 4, 5, and 6 to de-allocate resources
Circuit Switching - Notes
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Connection established before data transmission begins
Channel capacity must be available and reserved.
Nodes must have capacity to handle connection
Switches must have intelligence to make allocations and
devise route
• Can be inefficient
— Capacity dedicated for duration of connection
• Even if no data are being transferred
— For voice, utilization high, but still doesn’t approach 100%
— For terminal connection, may be idle most of the time
— Delay prior to data transfer for call establishment
— Once circuit established, network transparent to users
— Data transmitted at fixed rate
• No delay other than propagation
• Delay at node negligible
Packet Switching
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1970
Evolved substantially since
Basic technology fundamentally the same today
One of few effective technologies for long-distance data
communications
Frame relay and ATM (see later) variations
Carries directly to Internet
Advantages: flexibility, resource sharing, robustness,
responsiveness
Cost: elaborate algorithms to cope with time delay and
overhead penalties of network operation
Packet Switching –
Circuit Switching Issues
• Designed for voice
• Resources dedicated to particular call
• For voice, high utilization
—Most of the time, someone is talking
• For data
—Line idle much of the time
—Constant data rate
• Limits interconnection of variety of host computers and
terminals
Packet Switching –
Basic Operation
• Data are transmitted in short blocks, called packets
• Typical upper bound 1000 octets (bytes)
• Longer messages broken up into series of packets
— Each packet contains part (or all for short message) of user's data plus
some control information
— Control information includes network routing
— At each node, packet is received, stored briefly, and passed on to the
next node
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Transmitting computer sends message as sequence of packets
Packet includes control information including destination station
Packets sent to node to which sending station attaches
Node stores packet briefly, determines next leg of route, and
queues packet to go out on that link
• When link is available, packet is transmitted to next node
• All packets eventually work their way through network
Figure 1.2
The Use of Packets
Packet Switching –
Advantages
• Line efficiency greater
— Node-to-node link dynamically shared by many packets
• Data-rate conversion
— Each station connects to its node at its proper data rate
— Nodes act as buffers
• Packets accepted, even under heavy traffic, but delivery
delay increases
— Circuit switching networks would block new connections
• Priorities can be used
Packet Switching –
Disadvantages
• Delay
— Transmission delay equal to length of packet divided by
incoming channel rate
— Variable delay due to processing and queuing
• Packets may vary in length
— May take different routes
— May be subject to varying delays
— Overall packet delay can vary substantially (jitter)
— Not good for real-time applications like voice and real-time video
• Overheads including address of destination, sequencing
information added to packet
— Reduces capacity available for user data
• More processing required at node
Switching Technique –
Datagram
• Datagram: each packet treated independently
— No reference to packets that have gone before
— Each node chooses next node on path
— Packets with same destination address do not follow same route
— May arrive out of sequence
— Exit node or destination restores packets to original order
— Packet may be destroyed in transit
— Either exit node or destination detects loss and recovers
• Call setup avoided
• For an exchange of a few packets, datagram quicker
• More flexible.
— E.g. Routing away from the congestion
— Delivery is inherently more reliable
• If a node fails, subsequent packets may be re-routed
Figure 1.3
Packet
Switching:
Datagram
Approach
Switching Technique –
Virtual Circuit
• Preplanned route established before packets sent
• All packets follow same route
• Similar to circuit in circuit-switching network
— Hence virtual circuit
• Each packet has virtual circuit identifier
— Nodes on route know where to direct packets
— No routing decisions
• Not dedicated path, as in circuit switching
— Packet still buffered at node and queued for output
— Routing decision made oncefor that virtual circuit
• Network may provide services related to virtual circuit
— Sequencing and error control
• Packets should transit more rapidly
• If node fails, all virtual circuits through node lost
Figure 1.4
Packet
Switching:
Virtual-Circuit
Approach
Figure 1.5
Effect of
Packet Size
on
Transmission
Time
Routing
• Adaptive routing
—Routing decisions change as conditions on network
change
• Failure of node or trunk
• Congestion
—Route around congestion
• Requires exchange of network state information
—Tradeoff between quality of information and overhead
Frame Relay
• Considerable overhead in packet-switching schemes to
compensate for errors
— Additional bits added to packet for redundancy
— Additional processing at stations and nodes to detect and
recover from errors
• Modern systems more reliable
— Any remaining errors caught in systems above packet-switching
logic
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Original packet-switching networks data rate 64 kbps
Frame relay networks up to 2 Mbps
Strip out most error control overhead
Uses variable-length packets (frames)
Asynchronous Transfer Mode
ATM or Cell Relay
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Little overhead for error control
Fixed-length packets (cells)
Processing overhead reduced
Designed to work at 10s, 100s of Mbps, and Gbps
Small, fixed-size cells are efficient
— Can offer constant-data-rate channel using packet-switching
• Also evolution from circuit switching
• Allows definition of multiple virtual channels
— Data rates dynamically defined at virtual channel setup
— Extends circuit switching to allow multiple channels with data
rates set on demand
Local Area Networks
LAN
• Small scope
— Typically single building or a cluster of buildings
— Leads to different technical solutions
• Usually LAN owned by same organization that owns
attached devices
— For WANs, a significant fraction of the network assets are not
owned.
— May be a substantial capital investment for both purchase and
maintenance.
— Network management responsibility falls on owner
• Data rates of LANs typically much greater than WANs
• Most common are switched LANs and wireless LANs
— E.g. Ethernet, ATM, Fibre Channel, WiFi (802.11)
• More later
Metropolitan Area Networks
MAN
• Between LANs and WANs
• Traditional point-to-point and switched networks
in WANs inadequate for growing needs of
organizations
• Requirement for high capacity private and public
networks at low costs over a large area
—E.g. Wireless networks, metropolitan extensions to
Ethernet
The Internet History (1) Background
• Evolved from ARPANET, 1969 Advanced Research
Projects Agency (ARPA),U.S. Department of Defense
• First operational packet-switching network
• Began in four locations: UCLA, University of Santa
Barbara, the University of Utah, and SRI (Stanford
Research Institute)
• Today tens of millions of hosts
• Hundreds of millions of users
• Nearly 200 countries
• Number of connections growing exponentially
• Allowed devices from different manufacturers and with
different data rates to communicate
• Used adaptive routing
Figure 1.6
Number of
Internet Hosts
The Internet History (2) Applications
• Telnet provided common denominator terminal
— Software written support “Telnet terminal,”
— Any terminal could interact with any computer
• File Transport Protocol (FTP) offered similar open
functionality
— Transparent transfer of files from one computer to another
— Overcomes different word sizes, different bit orders and
different word formats
• First “killer app” was electronic mail
— Previously all single computer systems
— 1972, Ray Tomlinson of Bolt Beranek and Newman (BBN)
— Distributed mail service across network using multiple
computers
— 1973 three quarters of all ARPANET traffic was e-mail
The Internet History (3) –
TCP/IP
• Packet-switching applied to tactical radio communication (packet
radio) and satellite communication (SATNET)
— Different communication environments
— Certain parameters, e.g. maximum packet size, different
• Vint Cerf and Bob Kahn of ARPA developed protocols for
communicating across arbitrary, multiple, packet-switched networks
(internetting)
• May 1974 Transmission Control Protocol
• Refined by ARPANET community
• Major contributions from participants from European Networks,
such as Cyclades (France), and EIN
• Leading to TCP and IP
• Basis for TCP/IP protocol suite
• 1982-1983, ARPANET switched from NCP to TCP/IP
• Many networks connected using TCP/IP
• Use of ARPANET restricted to ARPA contractors
The Internet History (4) –
National Science Foundation
• Extended support to other computer research groups
— CSNET in 1980-1981
• 1986, extended Internet support to general research
community
— NSFNET backbone
— Originally designed to interconnect six NSF funded
supercomputer centers across USA and to supercomputer users
• Eventually, interconnection through NSF backbone to
regional packet switched networks across USA
• In 1990 ARPANET was shut down
The Internet History (5) –
Acceptable Use Policies
• In many countries (including United States until
1995) national governments subsidized the
Internet backbone
• Acceptable use policies limited commercial
activities
—Research and educational (and of course
government) use only
—The “culture” of the Internet imposed additional
informal limitations on commercial uses
The Internet History (6) –
Internet Interconnection Points
• 1991 almost all commercial TCP/IP service in USA
provided by:
• General Atomics
— Operated CERFnet (a California regional network)
• Performance Systems International
— PSINet (commercial spin-off from New York’s NYSERnet)
• UUNET Technologies
— Commercial Internet service provider that owned Alternet
• Did not use NSF backbone on own networks
— Not subject to Acceptable Use Policy
• Communication between their networks did use NSF
backbone
— Under the policy
The Internet History (7) –
CIX
• Commercial Information Interchange
• Originally mechanism to interchange traffic at a West Coast router
• Each network’s customers access customers on others’ networks at
no extra charge
• 1996, CIX had 147 member networks
• No settlements
— No traffic based fees for use
• Similar interconnection point (1994) in England
— London Internet Exchange (LINX)
— 1996, it had 24 member networks
• 1991, U.S. government said it would no longer subsidize Internet
after 1995
• Mandated network access points
• Now three, New York, Chicago, and San Francisco
• Metropolitan area exchanges, MAE East and MAE West
• U.S. part of Internet opened to commercial activity
Figure 1.7
U.S. Internet Access Points
The Internet History (8) –
The World Wide Web
• Spring 1989, at CERN (the European Laboratory for
Particle Physics)
• Englishman Tim Berners-Lee proposed a distributed
hypermedia technology to exchange research findings
over Internet
• 1991 prototype World Wide Web (WWW or the Web)
developed at CERN using NeXT computer as a platform
• End of 1991, limited release of line-oriented browser or
reader
• Explosive growth came with first graphically oriented
browser, Mosaic, 1993
— NCSA Center, University of Illinois
— Mark Andreasson and others
— Two million copies delivered over Internet
— Now ubiquitous
The Internet History (9) –
What is the Web
• Internationally distributed collection of multimedia files supported
by clients (users) and servers (information providers)
• Each file addressed in consistent manner using its Uniform
Resource Locator (URL)
• Viewed by clients using browsers
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E.g. Netscape Navigator, Microsoft’s Internet Explorer
There are others!
Usually graphical display and support for multimedia
Move from file to file by clicking with mouse highlighted text or image
(link)
— Layout of display controlled by Hypertext Markup Language (HTML)
— Embedded commands in text files
— Specify fonts, colors, images and their placement and links
• Hypertext Transfer Protocol (HTTP)
— Protocol used in TCP/IP networks for fetching WWW files
— More later
The Internet History (10) –
The Internet Today
• Users connect through an Internet service provider (ISP)
• Home users
— Major online services such as America Online and Compuserve
— Connect to ISPs over phone lines using modems at 56.6 kbps
— OK for e-mail but marginal for graphics-intensive Web surfing
— New alternatives include ISDN, ADSL, and cable modem
• Work users
— Workstations or PCs connected to LANs
— LAN connects through trunks to ISP
— T-1 or T-3 connection for large organizations
— Smaller organizations may use 56 kbps or ISDN connections
• ISPs connected by "wholesalers,“
— Network service providers
• They interconnect using Internet connection points
• T-3 rates or ATM connections
The Internet History (11) –
Commercial Use
• Acceptable Use Policy limited early commercial use to
research and educational
— Some informational activities that could be considered
marketing went on
• First commercial applications were mainly informational
— Sales, marketing, public relations
• Electronic data interchange (EDI)
— Intercompany invoices, billing, etc.
— Designed for dedicated WAN
• America Online, bulletin board type services dealing with
technical and usage problems
The Internet History (12) –
Direct Sales
• Initially Internet did not support online transactions well
— No easy to use graphical user interface
• World Wide Web not commonly available until 1993
• Initially little support for submitting information (forms) to server
— No security
— No effective payment systems
— Credit card?
— People uncomfortable sending credit card numbers over Internet
— If information not encrypted it is easy to “listen in”
— Several files of customer's credit card numbers on merchant’s
computers have been compromised
• Privacy concerns
— “data mining,”
• Collecting customer transaction information to improve targeting of
marketing
Figure 1.8
A Network
Configuration
Intranets
• Implementation of Internet technologies within a corporate
organization, rather than for external connection to the global
Internet
• Rapid prototyping and deployment of new services
• Scales effectively
• Virtually no user or developer training required
— Services and user interfaces familiar from Internet
• All platforms with complete interoperability
• Open architecture
— Lots of add-on applications available
• Range of distributed computing architectures
— Few central servers or many distributed servers
• Support of "legacy" information sources
— databases, word processing documents, groupware
• Range of media types (audio, video, interactive applications)
• Inexpensive to start
Intranets and the Web
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Web has become universal information interface
Management-employee communication
Job-related information
Departmental- and project-level information and
services.
— Set up Web pages to disseminate information and maintain
project data
• Easy to develop Web pages for specific needs
• Connect Web service to database
— Ease of administration
— Deployment
— Development speed
— Flexible information presentation
— Limited functionality
— Stateless operation
Figure 1.9
Web/Database Connectivity
Other Intranet Technologies
Electronic Mail
• Electronic Mail
—Most heavily used network application in corporate
world
—Attach documents multimedia to mail messages
—Electronic mailing list
• Alias with multiple destinations
Other Intranet Technologies
Network News or USENET
• Collection of electronic bulletin boards
• Work in similar way to Internet mailing lists
• Subscribe to news group to you receive all messages
posted to that group
• May post message that available to all subscribers
• Mechanics different from e-mail lists
• Distributed network of sites that collect and broadcast
news group entries
• Need access to USENET node
• Messages are archived at news sites
— Organized by subject matter (thread)
• Readily adapted to form an intranet news service
Extranets
• Makes use of TCP/IP protocols and applications
— Especially the Web
• Provides outside clients access to corporate resources
— Suppliers and customers
— Through the Internet or other networks
• Security
— Resources available to outside parties
— Privacy and authentication concerns must be addressed
• Access
— Long-distance dial-up access
— Internet access to intranet with security
• Authentication of users and encryption of communications
— Internet access to an external server
• Duplicates some intranet data
— Internet access to external server that originates database queries to
internal servers
— Virtual private network (VPN)
Required Reading
• Stallings, W. [2003], Computer Networks with
Internet Technology, Prentice Hall, Upper Saddle
River NJ. Chapter 1
• Web site for book: http://WilliamStallings.com/
• Any and all links from this site