Network Organization Concepts

Chapter 9Network Organization

Concepts

Understanding Operating Systems, Fourth Edition

College of Information Technology - UNO-R 2

ObjectivesObjectives

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


Objectives (continued)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


Basic TerminologyBasic 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)


Basic TerminologyBasic Terminology (continued) (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



• 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



• D/OS must provide the following components:– Process or object management– Memory management– File management– Device management– Network management



Figure 9.1: Networked management system



• 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



• 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



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


Network TopologiesNetwork 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


Network TopologiesNetwork Topologies (continued) (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


StarStar

• 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


StarStar (continued) (continued)

Figure 9.3: Star topology


RingRing

• 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


RingRing (continued) (continued)

Figure 9.4: Ring topology



Figure 9.5: Double loop computer network using a ring topology



Figure 9.6: Multiple rings bridged together


BusBus

• 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


TreeTree

• 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


Tree ( continued)Tree ( continued)

Figure 9.8: Tree Topology


HybridHybrid

Figure 9.9: Hybrid topology combining a star and a ring using a bridge

Selects among the strong points of each topology and combines them to meet that system’s communications requirements most effectively


Hybrid (continued)Hybrid (continued)

Figure 9.10: Hybrid topology combining a star and a bus


Network TypesNetwork Types

• Grouping of networks according to physical distances they cover

• Network types:– Local area networks (LAN)– Metropolitan area networks (MAN)– Wide area networks (WAN)


Local Area NetworkLocal 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


Local Area Network Local Area Network (continued) (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


Local Area NetworkLocal Area Network (continued) (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.


Metropolitan Area NetworkMetropolitan 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


Wide Area NetworkWide 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)


Wireless Local Area NetworkWireless 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

Table 9.1: IEEE standards for wireless networks

WiMax (802.16) would enable wireless broadband connections over much greater ranges (up to 10 miles)


Wireless Local Area NetworkWireless Local Area Network (continued)(continued)

Figure 9.11: Wireless Local Area Network


Software Design IssuesSoftware 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?


Addressing ConventionsAddressing 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


Addressing Conventions (continued)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


Routing StrategiesRouting 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


Routing Strategies Routing Strategies (continued)(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


Routing Strategies Routing Strategies (continued)(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)


Routing Information ProtocolRouting 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


Open Shortest Path FirstOpen 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


Connection ModelsConnection 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


Packet SwitchingPacket 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


Packet SwitchingPacket Switching (continued) (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



Table 9.2: Comparison of circuit and packet switching



• 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



• 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



• 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


Conflict ResolutionConflict 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


Access Control TechniquesAccess 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


Access Control TechniquesAccess Control Techniques (continued)(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


CSMACSMA

• 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


CSMA (continued)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


CSMA (continued)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


Token PassingToken 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


Token PassingToken Passing (continued) (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


Token PassingToken Passing (continued) (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


DQDBDQDB

• 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


DQDB (continued)DQDB (continued)

Figure 9.13: DQDB protocol


DQDB (continued)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


Transport Protocol StandardsTransport Protocol Standards

• Models intended to address need for universally adopted network architecture:– OSI Reference Model– TCP/IP


OSI Reference ModelOSI 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


OSI Reference ModelOSI Reference Model (continued) (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


OSI Reference ModelOSI Reference Model (continued) (continued)

Figure 9.14: OSI transport protocol model


OSI Reference Model (continued)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



• 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



• 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


TCP/IP ModelTCP/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


TCP/IP Model TCP/IP Model (continued)(continued)

Figure 9.15: TCP/IP model


TCP/IP ModelTCP/IP Model (continued) (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)


TCP/IP ModelTCP/IP Model (continued) (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


SummarySummary

• 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


Summary (continued)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


Summary (continued)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

Documents

Network Organization Concepts