Lecture 1: Introduction to
Networks
CS 5516
Computer Network
Architectures
VA
VA Tech
Tech
Prof. Roy M. Wnek
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Introduction to Networks
• Networks are collections of independent
computers which can communicate over a
communications medium
• Overcome limitations
of individual machines
– Share applications
– Share resources
• printers, modems, and CD-ROMs
– Transfer data
• “Real-time” data linking
• File transfer
– Provide personal communication
• e-mail
• Voice
• Video Teleconferencing
– Gang processors
• Provide fault tolerance
• Distribute workload
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Introduction to Networks
• Communications connectivity
– Cables
• Copper
• Optical Fiber
–
–
–
–
Telephone lines
Radio waves
Satellites
IR light beams
• Three basic types of networks
– Local Area Network (LAN)
– Metropolitan Area Network (MAN)
– Wide Area Network (WAN)
• Characteristics of networks
– Topology
– Protocols
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Data Network Milestones
• Unique era
1940: Bell Labs Model 1 Complex
Number Calculator uses
remote data terminals
• Special-purpose era
1958: First SAGE air defense
control center
• Expensive era
1971: NASDAQ System for overthe-counter stocks
(National Association of
Securities Dealers Automated
Quotations)
• Routine era
1996: Number of Internet Host
exceeds 10,000,000
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Local Area Networks (LANs)
• Confined to a relatively small
geographic area
– Lab, office, or building
– Usually less than a mile apart
• Generally used by one organization
or suborganization
• Share common resources
– File server
– Printers
– Modems
• Various possible architectures
– Generally uses only one topology and
protocol set
• Ethernet
• Token Ring
• FDDI
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Metropolitan Area Networks (MANs)
• Used by larger enterprises to
interconnect multiple local offices
– Government agencies
– Private industries
– Educational institutions
• Share larger centralized resources (e.g.)
mainframe
• Covers larger geographic areas
– Cities or districts.
– Interconnecting smaller networks
– Ease dissemination of information
• Often uses commercial carrier for
communications links
– Dedicated leased circuits may be used
• Interconnects LANs or individual nodes
– Interconnected LANs may be of different
architectures
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Wide Area Networks (WANs)
• Connect larger geographic areas
– states, countries, or the world
• Often uses commercial
carrier for communications link
– Dedicated leased circuits may be used
– Large enterprise may use its own transmission
assets
– Satellite circuits may be used
• Interconnects LANs or individual nodes
– Interconnected LANs may be of different
architectures
• From users perspective, WANs do not
appear much different than a LAN or a MAN
• Global Area Networks (GANs) face multiple
national regulatory agencies and
communications
companies
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Advantages of a Network
•
Speed
– Rapid method for sharing and transferring
files
– As opposed to manual or off-line transfer,
“sneaker-net”
• Cost
– Network licenses for applications software
can be far cheaper than multiple individual
user licenses
– Easier to upgrade shared programs once on
a file server
• Security
– Files and programs on a network can be
designated as "copy inhibit," to prevent
about illegal copying
– Specific directories can be passwordrestricted
• Centralized Software Management
eases user support
– No need to recover from damaged or
missing files at terminal locations
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Advantages of a Network
• Resource Sharing
– Laser printers, fax machines, modems,
scanners, and CD-ROM
– Cheaper & Better resource utilization
• Electronic Mail
– Easy, rapid written record communications
– Allows time-shifting
•
Flexible Access.
– Users need not be local to resources
• Workgroup Computing
– Collaborative work
– Integrates specialized efforts
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Disadvantages of a Network
• Expensive to Install
– Initial costs of installation can be prohibitive
– Technical services
• Requires Administrative Time
– Proper maintenance of a network required
– Considerable time devoted to user support
• Central node failure
– Network Hub - impacts network traffic
– File Server - impacts network & potentially
all applications
• Cables May Break
– Risk is dependent on the cables and path
way
– Impact is dependent on the topology
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Communications
Architectures 1
• Basic approaches to organizing
communications
• Circuit Switching
– Traditional telephone systems
– Use of entire path during duration of connection
– Simple processing requirements
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CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
T9
Communications
Architectures 2
• Message Switching
– Traditional telegraph systems
– Use of individual links only during
transmission
– Requires storage capability at each node
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CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
T9
Communications
Architectures 3
• Packet Switching
– Basis for most modern computer networks
– Use of individual links only during
transmission
– Requires more data within packets and
intelligent processing at nodes
– Paths may vary for individual packets
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CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
T9
Topology
• Two types of topology
– physical
– logical
• Physical topology of a network refers
to the configuration network nodes
• Logical topology is the method used
to pass data between network nodes
• Main Types of Physical Topologies
– Point-to-point
– Linear Bus (Multipoint or common
bus)
– Star
– Star-wired Ring
– Tree
– Meshed
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Network Topologies
• Point-to-Point
– Simplest topology
• Single link between two nodes
• Can be composed of multiple physical and
logical circuits
– Multiplexing
• Most common method of circuit
connectivity
• Point-to-point, dial-up lines, and private
lines,
• Most end-user access
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Multipoint or Common Bus
Topologies
• All nodes are physically and logically
connected to a common bus structure,
often local to the bus
– A transmitted message is heard by all nodes
– Avoid delay and overhead associated with
retransmission through a central node
• Each node is passive
• Single node will not affect the bus or the
other nodes
– As long as it presents a high impedance to the
network
– Does not fail with the transmitter on
• IEEE 802.4 Token Bus and IEEE 802.3
Ethernet
• Control strategies
– Contention
– Token passing
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Linear Bus Topology
• Consists of a main run of cable with a
terminator at each end
• All nodes (file server, workstations, and
peripherals) are connected to the
linear cable
• Ethernet and LocalTalk networks use a
linear bus topology
Terminator
Backbone Cable
Terminator
Linear Bus Topology
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Contention strategy in Bus
Networks
• Listen While Talk-Carrier Sense, Multiple
Access (LWT-CSMA)
– Good use of the available channel capacity
– As much as 85% of the channel capacity can be
used
– When a node has a message to transmit, it
listens to see if the channel is available
– If the channel is available, the node begins to
transmit
– If the channel isn't available, the node waits until
it is available and then transmits
– The node listens on the downstream channel
– If the received message matches the transmitted
message, then the transmission was completed
error free
– If the received message does not match the
transmitted message, then the station waits a
random amount of time and retransmits the
same message
• Perhaps two transmitters were trying to send
simultaneously
• Noise on the line
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Linear Bus Topology
• Advantages
– Easy to connect a computer or peripheral
to a linear bus
– Requires less cable length than a star
topology
• Disadvantages
– Entire network shuts down if there is a
break in the main cable
– Terminators are required at both ends of
the backbone cable
– Difficult to identify the problem if the entire
network shuts down
– Not meant to be used as a stand-alone
solution in a large building
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Star Topologies
• Star
– Developed to share expensive central
resource (e.g. mainframes, telephone
switches)
– All devices in network are connected
to the central node
• Central node usually performs the
processing
• Nodes communicate with each other
through point-to-point or multidrop links off
of the central node
– "Dumb" terminals accessing a
central processor
– LAN wiring hubs
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Star Topology
• Terminal nodes connect directly to a
central network hub or concentrator
• All data passes through the hub or
concentrator
– Hub manages and controls all functions of
the network
– Hub acts as a repeater for the data flow
Star Topology
Concentrator
(Hub)
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Star Topology
• Common with twisted pair cable
– Can also be used with coaxial cable or fiber
optic cable
• Protocols usually used with star
topology
– Ethernet or LocalTalk
– Token Ring uses a similar topology, called
the star-wired ring
• Advantages
– Easy to install and wire
– No disruptions to the network when
connecting or removing devices
– Easy to detect faults and to remove parts
• Disadvantages
– Requires more cable length than a linear
topology
– If the hub or concentrator fails, nodes
attached are disabled
– More expensive than linear bus topology
• Cost of the concentrators
• Cost of cabling
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Star-wired ring topology
• Externally similar to a star topology
• Internal and control strategy differences
– Token passing or Slotted Access
• Uses a Multistation Access Unit (MAU)
as the central node
– MAU allows data to pass from one device to
another in a circle (ring)
• Used by Token Ring and protocols
Star-wired ring topology
MAU
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Ring Topologies
• Loop (Ring)
– Each node is connected to two other nodes.
– Used in unidirectional data flow networks
– Each device passes information along the
direction of the ring
– IEEE 802.5 Token Ring
• Each node must be able to recognize its
own address and must be able to
retransmit messages to the next node
• Fairness doctrine so that all stations
have an equal opportunity to send
messages
– Improperly done or network saturation may
cause “starvation” of certain nodes
• Tokenpassing is a common method
– Token is a special control wire or a special
message or bit pattern
– When a node receives a special token and
the node has a message to send, the node
keeps the token and sends the message
– After the message is sent, the token is
passed to the next node
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Ring Topologies
• Slotted access is an alternative
control scheme
– A sequence of message slots is transmitted
around the ring .
– A slot is a series of bits large enough to hold
a message
– When an empty slot comes to a node that
has a message to send, the message is
inserted and the slot is marked full.
– The generation of the empty slots is a
centralized function
– The decision as to when to send a message
is decentralized
•
Fairness doctrine is not as well enforced
– The probability of finding an empty slot
changes as the nodal distance from the
empty slot generator increases
– A break in the ring configuration will cause
the network to fail
– Ring networks are designed so they
automatically go into a bypass or a
reconfiguration mode when a component
failure is detected (e.g. regenerate token)
– Increase the complexity of the individual node
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Tree Topology
• Combines characteristics of linear bus
and star topologies
• Groups of star-configured workstations
connected to a linear bus backbone
cable
Terminator
• Easier to configure a network to meet
needs
Tree Topology
Backbone Cable
Concentrators
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Terminator
• Allows for the expansion of an existing
network
Tree Topology
• Advantages
– Point-to-point wiring for individual
segments.
– Supported by several hardware and
software venders.
• Disadvantages
– Overall length of each segment is limited
by the type of cabling
– If the backbone line breaks, the entire
segment goes down
– More difficult to configure and wire than
other topologies
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Mesh Topologies
• Almost every large-scale network uses
a meshed topology
– Provide alternate routes for backup and
traffic loads
• Semi- or Partially Meshed
– Multiple links between nodes
– Every node does not connect
to every other
• Fully Meshed
– Each node has a point-to-point circuit to
every other node
– Rarely implemented due to cost
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Considerations When
Choosing a Topology
•
Money
– A linear bus network may be the least
expensive way to install
– Do not need a hub node
• Length of cable needed
– Linear bus uses shorter lengths of cable
– Star topologies may use excessive cable if
the hub is remote from terminal nodes
• Future growth
– Linear bus is limited by number of numbers
& length of bus
– Expanding a star topology may require
additional hubs
• Cable type
– Particular cable types are usually
associated with each topology
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Topology Summary:
Physical
Topology
Common
Media
Common
Protocol
Point-to-Point Twisted Pair
PPP
Linear Bus
Coax Cable
Ethernet
Star
Twisted Pair
Optical Fiber
Ethernet
Star-wired
Ring
Twisted Pair
Token Ring
FDDI
Tree
Coax Cable
Twisted Pair
Optical Fiber
Ethernet
Mesh
Twisted Pair
Optical Fiber
RF
ATM
TCP/IP
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Network Protocols
• Common relationships that allow
computers to communicate
– Defined standards
– De-Facto standards
• Typical protocol defines:
– Data format used for transmission
– Node identity on a network
– Procedures for handling lost or damaged
transmissions or "packets"
– Processing of data once it reaches its final
destination
• Examples of network protocols
– Ethernet
– TCP/IP
– AppleTalk
• "Protocol Independence,"
– All protocols use the physical cabling in exactly
the same manner
– Commonality allows protocols to peacefully
coexist regardless of individual implementation
– Allows use of common communications
equipment
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Open Systems Interconnection
(OSI) Reference Model
- Selection of appropriate service
of applications
7
Application
6
Presentation - Code conversion and data
formatting
5
Session
4
Transport
- Coordination of interactions
between end-application
processes
- End-to-end data integrity
3
Network
2
Data Link
- Transfer of data units to the
other end of the physical link
1
Physical
- Transmission of bit stream to the
physical media
- Switching and routing of data
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
OSI Reference Model
• Protocols define relationship between equal
layers, “peers”
• Data flows follow a path through the
communications media
Protocol
Application
7
6
Presentation
Protocol
Presentation
6
5
Session
Session
5
4
Transport
3
Network
2
Data Link
Data Link
2
1
Physical
Physical
1
.
.
.
Transport
Network
Data flow
Application
Data flow
7
Communications Media
Layer n+1
Service Requests
Layer n
Service Provided
Layer n-1
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
4
3
Network Operating Systems
• Network operating systems (NOS)
coordinate the activities of multiple
computers across a network
–
–
–
–
Identification of network resources
Location of network resources
Access to network resources
Coordination of resource use
• The two major types of network
operating systems are:
– Peer-to-Peer
– Client/Server
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Peer-to-Peer
• All node are potential clients and servers
– All nodes may have resources
– All nodes may be allowed access
• No centralized
management source
Peer
to
Peer
• Designed primarily for
small to medium LANs
• May also be found in MANs and WANs
• Advantages
– Less initial expense - No dedicated server
– Setup - An operating system (such as
Windows 95) already in place may only need
to be reconfigured for peer-to-peer operations
• Disadvantages
– Decentralized - No central repository for files
and applications
– Security - Does not provide the security
available on a client-server network
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Client/Server
• Allows the network to centralize
functions and applications in dedicated
servers
• Servers contain network resources
– Provide security
– Can not access client nodes
• Client nodes access to the resources
• Allow multiple users to simultaneously
share the same resources
• Novell Netware and Windows NT Server
Server
are examples
Clients
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Client/Server
• Advantages
– Centralized control & security
– Scalability - Any or all elements can be
replaced individually as needs increase
– Flexibility - New technology can be easily
integrated into system.
– Interoperability - All components (client /
network / server) work together.
– Accessibility - Server can be accessed
remotely and across multiple platforms.
• Disadvantages
– Expense - Requires initial investment in
dedicated server
– Maintenance - Larger networks will require
a dedicated staff to ensure efficient
operation
– Dependence upon server operability
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Introduction to Standards
• Standards are documented agreements
– Containing technical specifications
– Other precise criteria
– Used consistently as rules, guidelines, or
definitions of characteristics
• Meant to ensure that materials, products,
processes and services are fit for their
purpose
• Example: Format of common credit
cards, phone cards, and "smart" cards
– Derived from an ISO International Standard
– Standard defines optimal thickness (0,76
mm)
– Other features
– Allows cards to be used worldwide
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Introduction to Standards 2
• Regulatory Standards
– Established by Governments
under edict of law
– May be related to licensing
– Extended internationally by
treaty or international law
• Formally Recognized
Standards
– Recognized by Government or
private groups
– May or may not be deliberately
established
– Voluntary in nature
– National, Regional and
International levels
• De-facto Standards
– Established by common use or
market dominance
– Complicated by proprietary rights
– Greatest variation in compliance
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Standardization Bodies
• International
– Standards Organizations
– Standardizing Bodies
• Regional
– Multi-national organizations
– ETSI - European
Telecommunications
Standards Institute
• National
– Standards Bodies
– Standards Developers
• Consortia and Forums
– Not officially sanctioned
standards bodies
– Provide publicly available
specifications
– Often a motivator for changing
standards
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
Relationship between
Standards Organizations
ETSI
ITU
CCITT IEC
ISO
JTC1
IEC
International
CEPT
IEEE
Regional
T1
ANSI
U.S.
CCITT
National
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
International Standards
Organizations
• Bodies having recognized
activities in standardization
• Principal functions
– Preparation, approval or adoption of
standards
– Public dissemination
• Membership is open to the
relevant national organizations
• Major International Organizations
– ISO - International Organization for
Standardization
– IEC - International Electrotechnical
Commission
– ITU-T - International
Telecommunication Union
Telecommunication
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
International Standardizing
Bodies
• Bodies that have recognized activities in
standardization at
the international level
• Principal functions
– Preparation, approval or adoption of
standards
– Public dissemination
• May not have national level counterparts
• Major Bodies
– CIE - International Commission on
Illumination
– IAEA - International Atomic Energy Agency
– ILO - International Labour Office
– OIE - International Office of Epizootics
– UNESCO - United Nations Educational,
Scientific, and Cultural Organization
– WHO - World Health Organization
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.
National Standards Bodies
• Bodies having recognized
activities in standardization
• Recognized at the national level
– Private and Government-sanctioned
– Government Offices
• Principal functions
– Preparation, approval or adoption of standards
– Public dissemination
– Representation at corresponding international
and regional standards organizations.
• Major Organizations
– Australia Standards Australia (SAA)
– Canada Standards Council of Canada (SCC)
– France Association française de normalisation
(AFNOR)
– Germany Deutsches Institut für Normung (DIN)
– Japan
Japanese Industrial Standards Committee
(JISC)
– Sweden Standardiseringen i Sverige (SIS)
– USA
American National Standards Institute
(ANSI)
CS 5516 - Computer Networks
Copyright 2001 ©Roy M. Wnek. All rights reserved.