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Networks Networks 11 © 2000 Franz Kurfess
Course OverviewPrinciples of Operating Systems
Course OverviewPrinciples of Operating Systems
Introduction Computer System
Structures Operating System
Structures Processes Process Synchronization Deadlocks CPU Scheduling
Memory Management Virtual Memory File Management Security Networks Distributed Systems Case Studies Conclusions
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Networks Networks 22 © 2000 Franz Kurfess
Chapter Overview Networking
Chapter Overview Networking
Motivation Objectives Topology Network Types Communication Design Strategies
Examples Client/Server Model Middleware Important Concepts and
Terms Chapter Summary
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Networks Networks 33 © 2000 Franz Kurfess
MotivationMotivation resource sharing
users can access all resources available on the machines connected to the network
computation speedup computation-intensive tasks can be partitioned into subtasks and
distributed over several computers on the network specialized or more powerful computers can be used instead of or in
addition to the local machine of the user
reliability the overall system can continue to operate even if some of its
components fail
communication fast exchange of information remote procedure calls
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Networks Networks 44 © 2000 Franz Kurfess
ObjectivesObjectives
be aware of benefits and problems of computers connected via networks
know relevant network topologies and network types understand the issues involved in communication of
computers via networks apply networking concepts to the client/server model
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Networks Networks 55 © 2000 Franz Kurfess
TerminologyTerminology site
indicates the location of computer systems
host specific system at a site frequently implies the execution of a program or the
availability of services
local resources all the resources available directly within a specific
computer system system local sometimes also refers to a particular site, not one
single computer system
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Networks Networks 66 © 2000 Franz Kurfess
Terminology cont. Terminology cont. remote resources
mainly for exchange of information sometimes specialized resource types
network operating system users are aware of the individual machines in the network resources are accessible via login or explicit transfer of data
distributed operating system users are unaware of the underlying machines and networks remote resources are accessible in the same way as local
resources
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Networks Networks 77 © 2000 Franz Kurfess
Computer NetworkComputer Network
set of computers linked to each other through some type of network physical link virtual link
indirect via other computers
logical link more abstract level, independent of the physical realization
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Networks Networks 88 © 2000 Franz Kurfess
Computer Network DiagramComputer Network Diagram
Network
logicallink
physicallink
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Networks Networks 99 © 2000 Franz Kurfess
Network TopologyNetwork Topology
describes the interconnection structure of the network fully connected partially connected hierarchical network
tree structure
star network ring network bus network
unless otherwise noted, the topology refers to the physical interconnections
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Networks Networks 1010 © 2000 Franz Kurfess
Comparison CriteriaComparison Criteria
important properties of the configuration site refers to one or several computers with a single access
point to the network link is a connection between two sites
basic cost setup of the communication between two sites
communication cost costs of transferring a message from one site to another time required for the transfer
reliability effects of a failure in a link or site
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Networks Networks 1111 © 2000 Franz Kurfess
Fully ConnectedFully Connected
each site is directly linked with all the others basic cost
high: requires a direct link between every two sites quadratic w.r.t. the number of sites
communication cost low: no intermediate steps
reliability very high if messages can be rerouted
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Networks Networks 1212 © 2000 Franz Kurfess
Fully Connected DiagramFully Connected Diagram
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Networks Networks 1313 © 2000 Franz Kurfess
Partially ConnectedPartially Connected
direct links exist between some pairs of sites basic cost
lower than for the fully connected network
communication cost higher than for the fully connected network
reliability lower than for the fully connected network
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Networks Networks 1414 © 2000 Franz Kurfess
Partially Connected DiagramPartially Connected Diagram
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Networks Networks 1515 © 2000 Franz Kurfess
Hierarchical NetworkHierarchical Network the sites are organized as a tree
often used for corporate networks headquarters -> main offices -> regional offices
basic cost much lower than fully, usually lower than partially conn.
communication cost higher than for the fully connected network acceptable if communication patterns match the hierarchical
structure
reliability medium: the failure of a site or link partitions the network into
disjoint subtrees
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Networks Networks 1616 © 2000 Franz Kurfess
Hierarchical Network DiagramHierarchical Network Diagram
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Networks Networks 1717 © 2000 Franz Kurfess
Star NetworkStar Network all sites in the network are linked to a central hub
the hub may be a special device for communication only, or a regular site
basic cost linear with the number of sites
communication cost very low if there isn’t too much traffic central hub may become a bottleneck
reliability depends on the central hub: if it fails, the network is completely
partitioned the failure of a site or link affects only that site
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Networks Networks 1818 © 2000 Franz Kurfess
Star Network DiagramStar Network Diagram
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Networks Networks 1919 © 2000 Franz Kurfess
Ring NetworkRing Network
each site is connected to its two neighbors links can be uni- or bi-directional
basic cost linear with the number of sites
communication cost linear with the number of sites n-1 transfers for unidirectional, n/2 transfers for bi-
directional links in the worst case
reliability low: the failure of one site or link partitions the network
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Networks Networks 2020 © 2000 Franz Kurfess
Ring Network DiagramRing Network Diagram
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Networks Networks 2121 © 2000 Franz Kurfess
Directional Ring Network Diagram
Directional Ring Network Diagram
unidirectional bi-directional
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Networks Networks 2222 © 2000 Franz Kurfess
Bus NetworkBus Network
single shared link (bus) multiple simultaneous access must be coordinated
all sites are directly connected to the bus basic cost
linear with the number of sites
communication cost low as long as traffic is not too high network contention can become a problem for high traffic
situations
reliability the bus is the critical component
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Networks Networks 2323 © 2000 Franz Kurfess
Bus Network DiagramBus Network Diagram
linear bus ring bus
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Networks Networks 2424 © 2000 Franz Kurfess
Hybrid NetworksHybrid Networks
combination of different network topologies different topologies usually use different protocols
requires routers or bridges to translate between different protocols and routing mechanisms
more or less unavoidable in practice Ethernet LAN hierarchical WAN
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Networks Networks 2525 © 2000 Franz Kurfess
Hybrid Network DiagramHybrid Network Diagrambridge node
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Networks Networks 2626 © 2000 Franz Kurfess
Network TypesNetwork Types
geographical distribution of networks local area networks (LANs)
single or adjacent buildings
wide area networks (WANs) sites distributed over a large geographical area
has a major impact in the design of networks speed, reliability, protocol, security
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Networks Networks 2727 © 2000 Franz Kurfess
Local Area NetworksLocal Area Networks
connect sites within a small geographical area make resources available to all sites
applications, files, special devices
high-speed communication links Mega-Bit to Giga-Bit per second transfer rate wire, optical fiber, infrared requires expensive links
gateways provide access to other networks other LANs WANs Internet
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Networks Networks 2828 © 2000 Franz Kurfess
LAN ExamplesLAN Examples
Ethernet usually wire (shielded or twisted-pair cable) TCP/IP protocol variations
10BaseT 100BaseT
FDDI optical fiber token ring protocol 100 MBit/s
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Networks Networks 2929 © 2000 Franz Kurfess
Wide Area NetworksWide Area Networks
connect geographically distributed sites exchange of information access to special purpose computers
not useful for access to peripheral devices
links must cross long distances speed, reliability, security problems may involve other organizations
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Networks Networks 3030 © 2000 Franz Kurfess
WAN TechnologyWAN Technology
link technology telephone lines microwave links satellite connections
specialized communication processors interface to LANs (protocol, transfer rate) transmission of information routing billing, usage statistics
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Networks Networks 3131 © 2000 Franz Kurfess
WAN ExamplesWAN Examples
Arpanet Internet company networks
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Networks Networks 3232 © 2000 Franz Kurfess
Internet as WAN ExampleInternet as WAN Example
LANs are connected to regional networks regional network are connected with routers and
high-speed links special purpose links telephone connections
connections from LANs to the Internet telephone modems (up to 50 KBit/s) ISDN (up to 128 KBit/s) T1 connections (1.544 MBit/s)
can be combined into multiple links
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Networks Networks 3333 © 2000 Franz Kurfess
CommunicationCommunication
internal workings of networks naming routing packet strategies connection strategies contention
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Networks Networks 3434 © 2000 Franz Kurfess
NamingNaming the entities to exchange information are processes processes must specify the recipient
sometimes also the sender names are more appropriate for human users
must be unique, at least within the horizon of the user computers use numbers internally
hierarchical numbering scheme for unique addresses mapping of names to numbers (addresses)
single file with all names and addresses impractical for large systems
distributed scheme more complicated to implement
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Networks Networks 3535 © 2000 Franz Kurfess
Domain Name ServiceDomain Name Service
naming scheme used in the Internet specifies the naming structure of sites and hosts conversion from names to addresses
logical names consist of several fields separated by periods www.cis.njit.edu
refers to a host named www in the CIS department at NJIT
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Networks Networks 3636 © 2000 Franz Kurfess
Name Resolution Name Resolution
the name is parsed in reverse order edu indicates that the requested host is in an educational
institution, and the name server for the edu domain is contacted for the address of njit.edu refers
the edu name server returns the address of the host acting as name server for njit.edu refers
the njit.edu returns the address of the cis.njit.edu name server, or directly the address of the requested host
this results in an Internet address (IP-address) of the form 128.132.55.116
caching improves access speed and reliability
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Networks Networks 3737 © 2000 Franz Kurfess
RoutingRouting
in WANs there are usually multiple potential connection from host A to host B
a routing table contains information about possible communication paths alternative routes speed, costs
routing schemes fixed virtual dynamic
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Networks Networks 3838 © 2000 Franz Kurfess
Fixed RoutingFixed Routing
the path from host A to host B is specified in advance
it does not change unless severe obstacles are encountered e.g. hardware failure
used for heavily used connections minimization of communication costs or transfer time
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Networks Networks 3939 © 2000 Franz Kurfess
Virtual RoutingVirtual Routing
the path from A to B is fixed for the duration of one session
different paths for different sessions are possible more flexible than fixed routing reasonably easy to administrate
parts of a message take the same path
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Networks Networks 4040 © 2000 Franz Kurfess
Dynamic RoutingDynamic Routing
the path for a message from A to B is chosen at the time of sending the message
different messages from the same session may take different paths composition of messages at the receiver can become
complicated messages may arrive out of order
takes into account the traffic conditions link failures, load changes
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Networks Networks 4141 © 2000 Franz Kurfess
RouterRouter
responsible for routing messages can be a computer, or a special purpose device
has routing tables for the networks it is connected to possibly cached copies of the name files from the name
server
examines the destination IP address and decides where to send the message
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Networks Networks 4242 © 2000 Franz Kurfess
Packet StrategiesPacket Strategies
variable-length messages are commonly implemented through fixed-length packets datagrams, frames are alternative names for packets
connections are established to transfer packets constituting a message to increase reliability, acknowledgment packets can be
sent
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Networks Networks 4343 © 2000 Franz Kurfess
Connection StrategiesConnection Strategies
establishing communication between processes that want to exchange information
circuit switching a connection is established for the whole duration of the
session similar to the telephone system inefficient resource utilization
message switching packet switching
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Networks Networks 4444 © 2000 Franz Kurfess
Circuit SwitchingCircuit Switching
a connection is established for the whole duration of the session similar to the telephone system
inefficient resource utilization the communication line is reserved even if there is no
activity
little overhead for individual messages
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Networks Networks 4545 © 2000 Franz Kurfess
Message SwitchingMessage Switching
a connection is established for the transfer of one particular message similar to letters sent through the postal service
better resource utilization many messages can use the same link no waste due to inactivity of individual processes
more overhead for messages each message must contain administrative information
receiver, sender, etc.
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Networks Networks 4646 © 2000 Franz Kurfess
Packet SwitchingPacket Switching
individual packets from one message are sent separately
very good resource utilization better load balancing efficient treatment of packets since all are of the same
size
overhead for individual packets receiver, sender
packets must be reassembled into messages may not be suitable for time-sensitive information
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Networks Networks 4747 © 2000 Franz Kurfess
ContentionContention
several hosts may want to use a link simultaneously mainly relevant for bus topologies
without coordination, data will be scrambled and becomes useless
solutions collision detection token passing message slots
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Networks Networks 4848 © 2000 Franz Kurfess
Collision DetectionCollision Detection
a host can only use a link if it is free if two hosts start transmitting at the same time, a
collision will occur the collision must be detected the hosts will try again with some random delay
many collisions in high-traffic situations limited number of hosts on a network segment used in the Ethernet protocol
CSMA/CD: carrier sense with multiple access/collision detection
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Networks Networks 4949 © 2000 Franz Kurfess
Token PassingToken Passing
a unique message (token) circulates in the network normally a ring topology
a host may transmit only if it has the token must wait until the token arrives forwards the token after the transmission is finished precautions must be taken for a lost token
constant performance, independent of traffic load worse than CSMA/CD in low traffic, better for high traffic
used in IBM Token Ring networks
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Networks Networks 5050 © 2000 Franz Kurfess
Message SlotsMessage Slots
a number of fixed-length message slots circulate in the system normally a ring topology variable-size messages may have to be partitioned
a host must wait until an empty slot arrives used int the Cambridge Digital Communication Ring
experimental system little experience with performance
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Networks Networks 5151 © 2000 Franz Kurfess
Network DesignNetwork Design
complex problem involving various levels of abstraction names, IP addresses, messages, packets
coordination between different protocols one approach is to partition the problem into several
layers ISO/OSI protocol layers ISO/OSI network model
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Networks Networks 5252 © 2000 Franz Kurfess
OSI ModelOSI Model
Open Systems Interconnection (OSI) reference model
developed by the International Organization for Standardization (ISO)
is commonly used as abstract model TCP/IP is much more widely used in practice
less complex more mature
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Networks Networks 5353 © 2000 Franz Kurfess
OSI LayersOSI Layers
Physical
Data Link
Network
Transport
Session
Presentation
Applicationinteraction with the userfile transfer, remote login, email, distributed data bases
conversion of different formats and data representations;characters (ASCII, ISO), transmission modes
communication protocols between processesremote login, file and mail transfer (at the process level)
transfer of data between processesmessage handling, error recovery, flow control
independence from network technologiesnetwork routing, addressing, connection management
reliable transfer of information on physical linkspacket handling, error detection and recovery on lower level
transmission of a bit stream over the physical mediummechanical and electrical network interface
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Networks Networks 5454 © 2000 Franz Kurfess
Network ExampleNetwork Example
exchange of information between hosts on different Ethernet networks
TCP/IP TCP: transmission control protocol IP: Internet protocol very widely used
available for practically all computer systems
simpler than the ISO/OSI model four layers instead of seven
Ethernet very popular LAN
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Networks Networks 5555 © 2000 Franz Kurfess
TCP/IP ExampleTCP/IP Example
physical
network accessprotocol 1
IP
TCP
application
Network 1
physical
network accessprotocol 1
IP
TCP
application
Network 2
Router
Host A Host B
IP
NAP 1 NAP 2
logical connection
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Networks Networks 5656 © 2000 Franz Kurfess
Client/Server ComputingClient/Server Computing
applications are separated into tasks client tasks
user interface, presentation, some processing
server tasks data management, storage, computation-intensive processing
cooperation between client and server clients request services from servers servers return results to clients
network environment frequently LAN, sometimes WAN
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Networks Networks 5757 © 2000 Franz Kurfess
Client/Server DiagramClient/Server Diagram
[Stallings 98]
ServersClients
Network(LAN, WAN)
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Networks Networks 5858 © 2000 Franz Kurfess
Client/Server ArchitectureClient/Server Architecture
[Stallings 98]
Hardware Platform
Operating System
Communication
Application Logic(Client Side)
PresentationServices
Hardware Platform
Operating System
Communication
Application Logic(Server Side)
ClientServer
Request
Response
Protocol
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Client/Server Example: Data Base
Client/Server Example: Data Base
[Stallings 98]
Hardware Platform
Operating System
Application Logic(Client Side)
Data BaseLogic
Hardware Platform
Operating System
Communication
Application Logic(Server Side)
Client
Server
Request
Response
Protocol
PresentationServices
Communication
Data BaseLogic & DBMS
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Client/Server UsageClient/Server Usage
a data base is a good example for client/server centralized maintenance possibly large storage space requirements computation-intensive operations
sorting, searching, joins
powerful server with large hard disk(s) client provides user interface, smaller computations potential problem: balance between client & server
searching should not be done by the client network traffic to transfer the whole data base computation power required
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Client/Server AdvantagesClient/Server Advantages
lower cost than mainframes better utilization of PCs, workstations distribution or centralization of critical services
possible, as needed
resource sharing load distribution
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Client/Server ProblemsClient/Server Problems
client administration different platforms (hardware, OS, language)
software distribution reliable verifiable
security network information
integration of various systems legacy systems multi-vendor, multi-platform environments
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MiddlewareMiddleware
tools and methods that provide a uniform access mechanism to systems across all platforms standardized interfaces and protocols
examples Common Object Request Broker Architecture (CORBA) Common Object Model (COM), Object Linking and
Embedding (OLE) Java
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Middleware DiagramMiddleware Diagram
Middleware(distributed system services)
Application
Platform
[Stallings 98]
APIs
PlatformInterfaces
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Middleware ExampleMiddleware Example
ObjectRequest Broker
DBMSServlet
OODBMSServlet
CORBAServlet
Server Middleware
WebServer
CustomServer
Servers
Web Browser
CORBAClient
CustomClient
CustomServlet
Data Base ServerClients
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Tiered ArchitecturesTiered Architectures
distinction between various types of networked computer system
drive towards a global shared information space (WWW) dynamic, executable content platform-independent implementation
integration of existing systems
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Tiered Architectures cont.Tiered Architectures cont.
utilization of resources processing power of underutilized computers access to networked resources
total cost of ownership (TCO) basic costs for infrastructure cost per computer system
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Single-Tiered ArchitectureSingle-Tiered Architecture
typical mainframe with directly connected terminals all resources are available through the mainframe advantages and problems
easy to manage high cost low flexibility connectivity
usually star network with mainframe as central hub
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Single-Tier DiagramSingle-Tier Diagram
mainframe
terminals
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Two-Tiered ArchitectureTwo-Tiered Architecture typical client/server model
server provides various services clients have limited processing power
advantages and problems lower cost better use of desktop processing power higher flexibility
access to services on various servers
more difficult to manage and program connectivity
flexible, usually LAN
separation of business logic from client processing
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Two-Tier DiagramTwo-Tier Diagram
servers
PCs, workstations
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Three-Tiered ArchitectureThree-Tiered Architecture integration of existing systems (“legacy” systems)
frequently via middleware reduction of management and program development
problems through platform-independent methods CORBA, Java
advantages and problems reasonable to manage and program
platform-independent applications intermediate components
high flexibility integration of legacy systems
LAN-based connectivity
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Three-Tier DiagramThree-Tier Diagram
PCs, workstations
servers
mainframes
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Important Concepts and TermsImportant Concepts and Terms application layer bus computer system CSMA/CD distributed operating system Ethernet fully connected hierarchical network host ISO/OSI model Internet local machine middleware
network operating system node operating system partially connected physical layer processes, tasks remote resource ring service site star TCP/IP tier topology
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Chapter SummaryChapter Summary
most computer systems are connected to networks local area network (LAN) wide area network (WAN), especially Internet
networking enables communication, collaboration, and access to shared resources
networks require infrastructure interconnections (links) protocols
a client/server model is frequently used in networks middleware integrates various platforms and applications
on networks