Transcript ch09

Chapter 9
Network Organization
Concepts
Understanding Operating Systems,
Fourth Edition
Objectives
You will be able to describe:
• Several different network topologies—including the
star, ring, bus, tree, and hybrid
• Three types of networks: LAN, MAN, and WAN
• The difference between circuit switching and
packet switching
• Conflict resolution procedures that allow a network
to share common transmission hardware and
software effectively
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Objectives (continued)
You will be able to describe:
• The two transport protocol models (OSI and
TCP/IP) and how the layers of each one compare
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Basic Terminology
• Network: Collection of loosely coupled processors
interconnected by communication links using
cables, wireless technology, or both
– Goal: To provide a convenient way to share
resources (hardware and software) while controlling
users’ access to them
• General configurations for OS for networks:
– Network operating system (NOS)
– Distributed operating system (D/OS)
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Basic Terminology (continued)
• Network operating system (NOS): Networking
capability added to single-user operating system
– Users aware of specific computers and resources in
the network
– Access via logon to remote host or by data transfer
from remote host
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Basic Terminology (continued)
• Distributed operating system (D/OS): Users can
access remote resources as if local resources
– Good control for distributed computing systems
– Allows resources to be accessed in a unified way
– Represents total view across multiple computer
systems for controlling and managing resources
without local dependencies
– Management is a cooperative process
– Comprised of four managers with a wider scope
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Basic Terminology (continued)
• D/OS must provide the following components:
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Process or object management
Memory management
File management
Device management
Network management
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Basic Terminology (continued)
Figure 9.1: Networked management system
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Basic Terminology (continued)
• Advantages of D/OS over traditional systems:
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Easy and reliable resource sharing
Faster computation
Adequate load balancing
Good reliability
Dependable electronic communications among the
network’s users
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Basic Terminology (continued)
• In distributed system each processor classifies
other processors and resources as remote and
considers its own resources local
• Site: Indicates a specific location in a network with
one or more computers
• Host: Specific computer system found at a site
whose services and resources can be used from
remote locations
• Node: Refers to the name assigned to a computer
system connected to network to identify it to other
computers in network
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Basic Terminology (continued)
Figure 9.2: Clients request data or services from the host
server and wait for the response. If the client host has
resources needed by the server host, the roles can be
reversed
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Network Topologies
• Sites in any networked system can be physically or
logically connected in a variety of topologies
• Common topologies: star, ring, bus, tree, hybrid
• In each topology there are tradeoffs between
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Need for fast communication among all sites
Tolerance of failure at a site or communication link
Cost of long communication lines
Difficulty of connecting one site to a large number of
other sites
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Network Topologies (continued)
• Four basic criteria :
– Basic cost: Expense required to link the various
sites in the system
– Communications cost: Time required to send a
message from one site to another
– Reliability: Assurance that many sites can still
communicate with each other if a link or site fails
– User’s environment: Critical parameters that
network must meet to be a successful business
investment
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Star
• All transmitted data must pass through a central
controller when going from a sender to a receiver
• Advantages:
– Permits easy routing
– Easy access control to the network
• Challenges:
– Central site must be extremely reliable and able to
handle all network traffic, no matter how heavy
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Star (continued)
Figure 9.3: Star topology
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Ring
• All sites are connected in a closed loop with the
first site connected to the last
• Network can be connected to other networks via a
bridge (same protocols) or gateway (different
protocols)
• Data is transmitted in packets with source and
destination address fields
• Each packet is passed from node to node in one
direction only
• Every node must be functional, or failed node
needs to be bypassed for proper operation
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Ring (continued)
Figure 9.4: Ring topology
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Ring (continued)
Figure 9.5: Double loop computer network using a ring
topology
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Ring (continued)
Figure 9.6: Multiple rings bridged together
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Bus
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All sites connected to a single communication line
Messages from any site circulate in both directions
Only one site can successfully send messages at one time
Needs control mechanism to prevent collision
Data may pass directly from one device to another, or it may
be routed to an end point controller at the end of the line
Figure 9.7: Bus Topology
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Tree
• Tree: A collection of busses connected by a
branching cable with no closed loops
– Allows users to create networks using bridges
– Message from any site can be received by all other
sites, until it reaches an end point
– End point controller absorbs a message if it reaches
end point controller without being accepted by a host
– Advantage: Message traffic can still flow through
the network even if a single node fails
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Tree ( continued)
Figure 9.8: Tree Topology
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Hybrid
Selects among the strong points of each topology and
combines them to meet that system’s communications
requirements most effectively
Figure 9.9: Hybrid topology combining a star and a ring
using a bridge
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Hybrid (continued)
Figure 9.10: Hybrid topology combining a star and a bus
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Network Types
• Grouping of networks according to physical
distances they cover
• Network types:
– Local area networks (LAN)
– Metropolitan area networks (MAN)
– Wide area networks (WAN)
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Local Area Network
• A configuration found within a single office building,
campus, or similarly enclosed environment
• Owned, used, and operated by single organization
• Allows computers to communicate directly through
a common communication line
• Communications aren’t limited to well-defined local
area only
– LAN can be a component of larger communication
network
– Provides easy access to outside through bridge or
gateway
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Local Area Network (continued)
• Bridge: Connects two or more geographically
distant LANs with same protocols
– e.g., simple bridge used to connect 2 Ethernet LANs
• Gateway: Connects two or more LANs or systems
that use different protocols
– Translates one network’s protocol into another,
resolving hardware and software incompatibilities
– e.g., SNA gateway can connect microcomputer
network to mainframe host
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Local Area Network (continued)
• Data rates in LAN vary from 100 Mbps to more
than 40 Gbps
• Close physical proximity allows very high-speed
transmission
• Star, ring, bus, tree, and hybrid are normally used
to construct local area networks
• Transmission medium used may vary from one
topology to another
– Factors determining transmission medium include
cost, data rate, reliability, number of devices that can
be supported, distance between units etc.
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Metropolitan Area Network
• Configuration spanning an area larger than a LAN
– Ranging from several blocks of buildings to an entire
city but not exceeding a circumference of 100 km
• Owned and operated by a single organization
– Usually used by many individuals & organizations
– May be owned and operated as public utilities
providing means for internetworking several LANs
• MAN: high-speed network often configured as a
logical ring
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Wide Area Network
• A configuration that interconnects communication
facilities in different parts of a country or the world,
or that is operated as part of public utility
• Uses communications lines of common carriers
(e.g., telephone companies)
• Uses broad range of communication media (e.g.,
satellite, microwaves)
• WANs are generally slower than LANs
• Examples: ARPAnet (first WAN), Internet (most
widely recognized WAN)
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Wireless Local Area Network
• LAN that uses wireless technology to connect computers or
workstations located within the range of the network
• WLAN typically poses security vulnerabilities
WiMax (802.16) would enable wireless broadband connections over
much greater ranges (up to 10 miles)
Table 9.1: IEEE standards for wireless networks
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Wireless Local Area Network
(continued)
Figure 9.11: Wireless Local Area Network
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Software Design Issues
• Software issues that must be addressed by
network designers:
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How do sites use addresses to locate other sites?
How are messages routed and how are they sent?
How do processes communicate with each other?
How are conflicting demands for resources
resolved?
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Addressing Conventions
• Addressing protocols are closely related to network
topology and geographic location of each site
– Local name: Name by which a unit is known within
its own system
– Global name: Name by which a unit is known
outside its own system
• Must follow standard name lengths, formats, and other
global conventions
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Addressing Conventions (continued)
• Domain Name Service (DNS) protocol
• The DNS is hierarchical
• Domain names are read from right to left
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Rightmost portion is the top-level domain
Next level is the domain name
Next is one or more subdomain names
Leftmost portion is the host
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Routing Strategies
• Router: Internetworking device, primarily software
driven, which directs traffic
– Between two different types of LANs, or
– Between two network segments with different
protocol addresses
• Operates at Network Layer
• Role of routers changes as network designs
change
• Used extensively for connecting sites to each other
and to Internet
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Routing Strategies (continued)
• Router functions include:
– Securing information generated in predefined areas
– Choosing the fastest route from one point to another
– Providing redundant network connections
• Routing protocols must consider following:
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Addressing
Address resolution
Message format
Error reporting
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Routing Strategies (continued)
• Message formats allow the protocol to perform its
functions, such as
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Finding new nodes on a network
Testing to determine whether they’re working
Reporting error conditions
Exchanging routing information
Establishing connections, and transmitting data
• Most widely used routing protocols on Internet:
– Routing information protocol (RIP)
– Open shortest path first (OSPF)
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Routing Information Protocol
• Selection of a path based on immediate number of
nodes, or hops, between source and destination
– Path with smallest number of hops chosen always
• Advantages:
– Easy to implement
• Disadvantages:
– Does not take into consideration bandwidth, data
priority, or type of network
– Updating and reissuing of routing table every 30
seconds
– Tables propagate from one router to another
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Open Shortest Path First
• Selection of a transmission path only after the state
of a network has been determined
• Routing update messages sent only when changes
in routing environment occur
– Reduces number of messages in internetwork
– Reduces size of messages by not sending entire
routing table
• Disadvantages:
– Increased memory usage
– Bandwidth savings offset by higher CPU usage for
shortest path calculation
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Connection Models
• Types of switching:
– Circuit switching
– Packet switching
• Circuit Switching: Communication model in which
dedicated communication path is established
between two hosts before data transmission begins
– Example: Telephone system
– Disadvantage: Delay before signal transfer begins
while the connection is set up
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Packet Switching
• A store-and-forward technique in which a message
is divided into multiple equal-sized units (packets)
before sending to destination
– At destination, packets are reassembled into their
original long format
– A header containing pertinent information about the
packet is attached to each packet before transmission
• Advantages:
– More flexible and more reliable than circuit switching
– Provides greater line efficiency
– Allows users to allocate priorities to their messages
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Packet Switching (continued)
Figure 9.12 : Packet switching; (a) divide the data into
addressed packets; (b) send each packet toward its
destination; (c) reassemble the data at the destination
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Packet Switching (continued)
Table 9.2: Comparison of circuit and packet switching
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Packet Switching (continued)
• Methods of selecting the path:
– Datagrams
– Virtual circuits
• Datagrams: Destination and sequence number of
packet added to information, uniquely identifying
message to which packet belongs
– Each packet handled independently and route is
selected as each packet is accepted into network
– At destination, all packets of same message are
reassembled
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Packet Switching (continued)
• Datagrams: (continued)
– Message can’t be delivered until all packets are
accounted for
– Receiving node requests retransmission of lost or
damaged packets
– Advantages:
• Helps diminish congestion by sending incoming
packets through less heavily used paths
• Provides more reliability, because alternate paths may
be set up when one node fails
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Packet Switching (continued)
• Virtual Circuit: Complete path from sender to
receiver established before transmission starts
– All packets belonging to a message use same route
– Any node can have several virtual circuits to any
other node
• Advantage: Routing decision made once for all
packets belonging to same message – speeds up
transmission
• Disadvantages:
– If node fails, all virtual circuits using that node
become unavailable
– Congestion is difficult to resolve when heavy traffic
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Conflict Resolution
• Some method to control access is necessary to
facilitate equal and fair access to network
• Access control techniques:
– Round robin
– Reservation
– Contention
• Medium access control protocols:
– Carrier sense multiple access (CSMA)
– Token passing
– Distributed-queue, dual bus
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Access Control Techniques
• Round Robin: A node is given certain amount of
time to complete transmission, at end of which
opportunity is passed to next node
– Efficient when many nodes transmitting over long
periods
– Substantial overhead when few nodes transmit over
long periods of time
• Reservation: Access time on medium is divided
into slots and node can reserve future time slots
– Well suited for lengthy and continuous traffic
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Access Control Techniques
(continued)
• Reservation: (continued)
– Good for a configuration with several terminals
connected to host computer through single I/O port
• Contention: No attempt is made to determine
whose turn it is to transmit; nodes compete for
access to medium
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Major advantage: Easy to implement
Better for short and intermittent traffic
Works well under light to moderate traffic
Performance tends to break down under heavy
loads
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CSMA
• Carrier sense multiple access (CSMA):
Contention-based protocol that is easy to
implement
– Carrier sense means that a node will listen to, or
test, communication medium before transmitting any
messages
• Prevents a collision with another node that’s currently
transmitting
– Multiple access means that several nodes are
connected to same communication line as peers, on
the same level, and with equal privileges
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CSMA (continued)
• Disadvantages of CSMA:
– Collision if two or more nodes transmit at same
instant
– Probability of collisions increases if nodes are farther
apart
• CSMA less appealing access protocol for large or
complex networks
• CSMA/CD: CSMA algorithm modified to include
collision detection, e.g., Ethernet
– Collisions not completely eliminated but reduced
– Reduces wasted transmission capacity
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CSMA (continued)
CSMA/CD:
• Access method prevents multiple nodes from
colliding during transmission
– e.g., Implemented in LocalTalk, Apple’s cabling
system
• If collisions occur, involve only a small packet, not
actual data (in case of Apple CSMA/CA)
• Protocol does not guarantee data will reach its
destination, but ensures that any data that’s
delivered will be error free
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Token Passing
• Special electronic message (token) is generated
and passed along from node to node
• Only node with the token allowed to transmit, and
after it has done so, it must pass token on to
another node
• Fast access; collisions are nonexistent
• Typical topologies:
– Bus
– Ring
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Token Passing (continued)
• Token-bus: Token is passed to each node in turn,
which upon receipt, attaches data to it and sends to
destination
• Receiving node copies data, adds acknowledgment,
and returns packet to sending node
• Sending node passes token on to next node in
logical sequence
• Initial node order determined by cooperative
decentralized algorithm
– Once network is running, turns determined by priority
based on node activity
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Token Passing (continued)
• Token-bus: (continued)
– Higher overhead at each node than CSMA/CD
– Nodes may have long waits under certain conditions
before receiving token
• Token-ring: Token moves between the nodes in
turn and in one direction only
– If a node wants to send a message it must wait for
the free token to come by
– Receiving node copies the message in the packet
and sets the copied bit to indicate it was successfully
received
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DQDB
• Distributed-queue, dual bus (DQDB): Intended
for use with a dual-bus configuration, where each
bus transports data in only one direction
• Transmission on each bus consists of a steady
stream of fixed-size slots
• Slots generated at end of each bus marked free
and sent downstream, where they’re marked busy
and written to by nodes ready to transmit
• Nodes read and copy data from slots, which then
continue to travel toward end of bus, where they
dissipate
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DQDB (continued)
Figure 9.13: DQDB protocol
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DQDB (continued)
• Advantages of DQDB:
– Provides negligible delays under light loads and
predictable queuing under heavy loads
– Suitable for MANs that manage large file transfers
– Able to satisfy the needs of interactive users
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Transport Protocol Standards
• Models intended to address need for universally
adopted network architecture:
– OSI Reference Model
– TCP/IP
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OSI Reference Model
• Provides basis for connecting open systems for
distributed applications processing
– “Open” means that any two systems that conform to
reference model and related standards can be
connected, regardless of vendor
• Similar functions collected together into seven
logical clusters (layers)
– Possible to redesign a layer without affecting the
adjacent layers
• Handles data transmission from one terminal or
application program to another
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OSI Reference Model (continued)
OSI Reference Model
• At every layer of the sending unit, a new header is
attached to the previous packet before it’s passed
on to the next lower layer
• At the data link layer, a link trailer (LT) is added,
completing the frame, which is passed to the
physical layer for transmission
• Receiving unit removes each header or trailer until
it delivers the data to the application program at
Layer 7
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OSI Reference Model (continued)
Figure 9.14: OSI transport protocol model
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OSI Reference Model (continued)
• Layer 1—The Physical Layer: Describes all
mechanical, electrical, and functional specifications
for connecting a device to a particular network
– e.g., 100Base-T, RS449, and CCITT V.35
• Layer 2—The Data Link Layer:
– Establishes and controls the physical path of
communications on one side
– Checks for transmission errors and resolves
problems on the other side
– Typical data link level protocols are HDLC and SDLC
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OSI Reference Model (continued)
• Layer 3—The Network Layer: Provides services
such as addressing and routing that move data
through network to its destination
• Layer 4—The Transport Layer: Maintains reliable
data transmission between end users
– Example: Transmission Control Protocol (TCP)
• Layer 5—The Session Layer: Responsible for
– Providing a user-oriented connection service
– Transferring data over communication lines
– Example: TCP/IP
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OSI Reference Model (continued)
• Layer 6—The Presentation Layer: Responsible
for data manipulation functions common to many
applications, such as formatting, compression, and
encryption.
• Layer 7—The Application Layer: Application
programs, terminals, and computers access the
network at this layer
– Provides interface to users and responsible for
formatting user data before passing to lower layers
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TCP/IP Model
• Transmission Control Protocol/Internet Protocol
(TCP/IP):
– Oldest transport protocol standard and the basis for
Internet communications
– File-transfer protocol to send large files error free
– TCP/IP emphasizes internetworking and providing
connectionless services
– Organizes a communication system with three main
components: processes, hosts, and networks
– TCP/IP model is arranged into four layers
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TCP/IP Model (continued)
Figure 9.15: TCP/IP model
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TCP/IP Model (continued)
• Network Access Layer: Protocols at this layer
provide access to a communication network
– Flow control, error control between hosts, security,
and priority implementation are performed at this
layer
• Internet Layer: Equivalent to portion of network
layer of OSI model that performs routing functions
– Implemented within gateways and hosts
– Example: Internet protocol (IP)
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TCP/IP Model (continued)
• Host-Host Layer: Supports mechanisms to transfer
data between two processes on different host
computers
– Services include error checking, flow control, and an
ability to manipulate connection control signals
– e.g., Transmission Control Protocol (TCP)
• Process/Application Layer: Includes protocols for
computer-to-computer resource sharing and
terminal-to-computer remote access
– e.g., FTP, SMTP, and Telnet
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Summary
• Operating systems for networks necessarily include
the functions of Memory Manager, Processor
Manager, Device Manager, and File Manager
• Network’s operating system must meet the
reliability requirements of its owners
• Distributed operating systems allows resources to
be accessed in a unified way
• Sites in any networked system can be physically or
logically connected to one another in a variety of
topologies: star, ring, bus, tree, and hybrid
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Summary (continued)
• Hybrid topology combines the strong points of each
topology to meet communications requirements
most effectively
• Networks are grouped according to physical
distances they cover: LAN, MAN and WAN
• Operating system must detect a failure, change
routing instructions to avoid that node, and make
sure every lost message is retransmitted until it is
successfully received
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Summary (continued)
• Packet switching provides greater line efficiency
than circuit switching
• CSMA/CD prevents multiple nodes from colliding
during transmission
• OSI reference model provides basis for connecting
open systems for distributed applications
processing
• TCP/IP is the oldest transport protocol standard
and the basis for Internet communications
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