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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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) Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Basic Terminology (continued) D/OS must provide the following components: Process or object management Memory management File management Device management Network management Understanding Operating Systems, Fourth Edition

Basic Terminology (continued) Figure 9.1: Networked management system Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Star (continued) Figure 9.3: Star topology Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Ring (continued) Figure 9.4: Ring topology Understanding Operating Systems, Fourth Edition

Ring (continued) Figure 9.5: Double loop computer network using a ring topology Understanding Operating Systems, Fourth Edition

Ring (continued) Figure 9.6: Multiple rings bridged together Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Tree ( continued) Figure 9.8: Tree Topology Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Hybrid (continued) Figure 9.10: Hybrid topology combining a star and a bus Understanding Operating Systems, Fourth Edition

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) Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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. Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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) Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Wireless Local Area Network (continued) Figure 9.11: Wireless Local Area Network Understanding Operating Systems, Fourth Edition

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? Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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) Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Packet Switching (continued) Table 9.2: Comparison of circuit and packet switching Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

DQDB (continued) Figure 9.13: DQDB protocol Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

Transport Protocol Standards Models intended to address need for universally adopted network architecture: OSI Reference Model TCP/IP Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

OSI Reference Model (continued) Figure 9.14: OSI transport protocol model Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

TCP/IP Model (continued) Figure 9.15: TCP/IP model Understanding Operating Systems, Fourth Edition

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) Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition

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 Understanding Operating Systems, Fourth Edition