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Chapter 15

Chapter 15. Network Properties (Ownership, Service Paradigm, Measures of Performance). Comer, 4e, Ch 15 and Ch 16 Comer 5e, Ch 26. Network Ownership And Service Type. Private Owned by individual or corporation Restricted to owner’s use Typically used by large corporations Public

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Chapter 15

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  1. Chapter 15 Network Properties (Ownership, Service Paradigm, Measures of Performance)

  2. Comer, 4e, Ch 15 and Ch 16 Comer 5e, Ch 26

  3. Network Ownership And Service Type • Private • Owned by individual or corporation • Restricted to owner’s use • Typically used by large corporations • Public • Owned by a common carrier • Individuals or corporations can subscribe • “Public” refers to availability, not data

  4. Advantages and Disadvantages • Private • Complete control • Installation and operation costs • Public • No need for staff to install/operate network • Dependency on carrier • Subscription fee

  5. Public Network Connections • One connection per subscriber • Typical for small corporation or individual • Communicate with another subscriber • Multiple connections per subscriber • Typical for large, multi-site corporation • Communicate among multiple sites as well as with another subscriber

  6. Virtual Private Network • A service • Provided over public network • Interconnects sites of single corporation • Acts like private network • No packets sent to other subscribers • No packets received from other subscribers • Data encrypted

  7. Virtual Private Network

  8. Frame Relay Pricing • Permanent Virtual Circuits (PVCs) • Leased access line must be fast enough to handle all of the PVCs it is multiplexing • Example: if it multiplexes 15 64 kbps PVCs, the access line must be 840 kbps (T1 line needed) PVC Leased Access Line PVC

  9. Network Service Paradigm • Fundamental characteristic of network • Understood by hardware • Visible to applications • Two basic types of networks • Connectionless • Connection-oriented

  10. Connectionless ( CL ) • Sender • Forms packet to be sent • Places address of intended recipient in packet • Transfers packet to network for delivery • Network • Uses destination address to forward packet • Delivers

  11. Characteristics of Connectionless Networks • Packet contains identification of destination • Each packet handled independently • No setup required before transmitting data • No cleanup required after sending data • Think of postcards

  12. Connection-Oriented (CO) • Sender • Requests “connection” to receiver • Waits for network to form connection • Leaves connection in place while sending data • Terminates connection when no longer needed

  13. Connection-Oriented (CO)(continued) • Network • Receives connection request • Forms path to specified destination and informs sender • Transfers data across connection • Removes connection when sender requests • Think of telephone calls

  14. Terminology • In conventional telephone system • Circuit • In CO data network • Virtual Circuit • Virtual Channel

  15. Comparison of CO and CL • CO • More intelligence in network • Can reserve bandwidth • Connection setup overhead • State in packet switches • Well-suited to real-time applications • CL • Less overhead • Permits asynchronous use • Allows broadcast / multicast

  16. Two Connection Types • Permanent Virtual Circuit (PVC) • Entered manually • Survives reboot • Usually persists for months • Switched Virtual Circuit (SVC) • Requested dynamically • Initiated by application • Terminated when application exits

  17. Examples of Service ParadigmVarious Technologies Use

  18. Connection Multiplexing • Typical computer has one physical connection to network • All logical connections multiplexed over physical interconnection • Data transferred must include connection identifier

  19. Connection Identifier • Integer value • One per active VC • Not an address • Allows multiplexing • Computer supplies when sending data • Network supplies when delivering data

  20. Example Connection Identifier (ATM) • 24 bits long (The full address is 160 bits) • Divided into two parts • Virtual Path Identifier • Virtual Channel Identifier • Known as (VPI/VCI) • Different at each end of connection • Mapped by switches

  21. Illustration of ATM VC • Switch maps VPI/VCIs • 17 to 12 • 96 to 8

  22. Two PrimaryPerformance Measures • Delay • Throughput

  23. Delay • Time required for one bit to travel through the network • Three types (causes) • Propagation delay • Switching delay • Queuing Delay • Intuition: “length” of the pipe

  24. Throughput • Number of bits per second that can be transmitted • Capacity • Intuition: “width” of the pipe

  25. Components of Delay • Fixed (nearly constant) • Propagation delay • Switching delay • Variable • Queuing delay • Depends on throughput If delay is changing rapidly, we refer to it as Jitter

  26. Relationship BetweenDelay and Throughput • When network idle • Queuing delay is zero • As load on network increases • Queuing delay rises • Load defined as ration of throughput to capacity • Called utilization

  27. Relationship BetweenDelay and Utilization • Define • D0 to be the propagation and switching delay • U to be the utilization (0  U  1) • D to be the total delay • Then • High utilization known as congestion

  28. Practical Consequence Any network that operates with a utilization approaching 100% of capacity is doomed

  29. Delay-Throughput Product • Delay • Time to cross network • Measured in seconds • Throughput • Capacity • Measured in bits per second • Delay * Throughput • Measured in bits • Gives quantity of data “in transit”

  30. Summary • Network can be • Public • Private • Virtual Private Network • Uses public network • Connects set of private sites • Addressing and routing guarantee isolation

  31. Summary (continued) • Networks are • Connectionless • Connection-oriented • Connection types • Permanent Virtual Circuit • Switched Virtual Circuit • Two performance measures • Delay • Throughput

  32. Summary (continued) • Delay and throughput interact • Queuing delay increases as utilization increases • Delay * Throughput • Measured in bits • Gives total data “in transit”

  33. Chapter 16 Protocols and Protocol Layering

  34. Protocol • Agreement about communication • Specifies • Format of messages (syntax) • Meaning of messages (semantics) • Rules for exchange • Procedure for handling problems

  35. Need for Protocols • Hardware is low level • Many problems can occur • Bits corrupted or destroyed • Entire packet lost • Packet duplicated • Packets delivered out of order

  36. Need for Protocols (continued) • Need mechanisms to distinguish among • Multiple computers on a network • Multiple applications on a computer • Multiple copies of a single application on a computer

  37. Set of Protocols • Work together • Each protocol solves part of communication problem • Known as • Protocol suite • Protocol family • Designed in layers

  38. Plan for Protocol Design • Intended for protocol designers • Divides protocols into layers • Each layer devoted to one subproblem • Example: ISO 7-layer reference model

  39. Illustration of the 7-Layer Model • Defined early • Now somewhat dated • Does not include internet layer! All People Seems To Need Data Processing

  40. ISO Layers • Layer 1: Physical • Underlying hardware (Example: RS-232) • Layer 2: Data Link (media access) • Hardware frame definitions • Layer 3: Network • Packet forwarding • Layer 4: Transport • Reliability

  41. ISO Layers (continued) • Layer 5: Session • Login and passwords • Layer 6: Presentation • Data representation • Layer 7: Application • Services for common applications

  42. TCP/IP protocol suite

  43. Layers and Protocol Software • Protocol software follows layering model • One software module per layer • Modules cooperate • Incoming or outgoing data passes from one module to another • Entire set of modules known as stack

  44. Illustration of Stacks

  45. Layers and Packet Headers • Each layer • Prepends header to outgoing packet • Removes header from incoming packet

  46. Example of encapsulation

  47. Scientific Layering Principle Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source

  48. Illustration of Layering Principle

  49. Protocol Techniques • For bit corruption • Parity • Checksum • CRC • For out-of-order delivery • Sequence numbers • Duplication • Sequence numbers

  50. Protocol Techniques (continued) • For lost packets • Positive acknowledgement and retransmission • For replay (excessive delay) • Unique message ID • For data overrun • Flow control

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