1. Chapter 9 Network Organization Concepts
Understanding Operating Systems, Fourth Edition

2. 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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3. 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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4. 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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5. 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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6. 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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7. Basic Terminology (continued)
D/OS must provide the following components:
Process or object management
Memory management
File management
Device management
Network management
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8. Basic Terminology (continued)
Figure 9.1: Networked management system
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9. Basic Terminology (continued)
Advantages of D/OS over traditional systems:
Easy and reliable resource sharing
Faster computation
Adequate load balancing
Good reliability
Dependable electronic communications among the network’s users
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10. 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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11. 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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12. 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
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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13. 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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14. 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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15. Star (continued) Figure 9.3: Star topology
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16. 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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17. Ring (continued) Figure 9.4: Ring topology
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18. Ring (continued)
Figure 9.5: Double loop computer network using a ring topology
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19. Ring (continued) Figure 9.6: Multiple rings bridged together
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20. Bus 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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21. 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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22. Tree ( continued) Figure 9.8: Tree Topology
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23. 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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24. Hybrid (continued)
Figure 9.10: Hybrid topology combining a star and a bus
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25. 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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26. 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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27. 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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28. 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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29. 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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30. 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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31. 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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32. Wireless Local Area Network (continued)
Figure 9.11: Wireless Local Area Network
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33. Software Design Issues
Software issues that must be addressed by network designers:
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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34. 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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35. Addressing Conventions (continued)
Domain Name Service (DNS) protocol
The DNS is hierarchical
Domain names are read from right to left
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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36. 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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37. 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:
Addressing
Address resolution
Message format
Error reporting
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38. Routing Strategies (continued)
Message formats allow the protocol to perform its functions, such as
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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39. 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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40. 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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41. 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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42. 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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43. 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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44. Packet Switching (continued)
Table 9.2: Comparison of circuit and packet switching
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45. 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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46. 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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47. 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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48. 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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49. 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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50. 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
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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51. 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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52. 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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53. 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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54. 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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55. 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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56. 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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57. 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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58. DQDB (continued) Figure 9.13: DQDB protocol
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59. 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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60. Transport Protocol Standards
Models intended to address need for universally adopted network architecture:
OSI Reference Model
TCP/IP
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61. 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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62. OSI Reference Model (continued)
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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63. OSI Reference Model (continued)
Figure 9.14: OSI transport protocol model
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64. 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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65. 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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66. 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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67. 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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68. TCP/IP Model (continued)
Figure 9.15: TCP/IP model
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69. 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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70. 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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71. 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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72. 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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73. 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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