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Chapter 1 - Foundation

Chapter 1 - Foundation. Outline Basic building blocks of a Network Statistical Multiplexing Inter-Process Communication Performance Metrics Network Architecture. Computer Network. A computer network is a network of computers for exchanging data (bits) among themselves over a single medium

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Chapter 1 - Foundation

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  1. Chapter 1 - Foundation Outline Basic building blocks of a Network Statistical Multiplexing Inter-Process Communication Performance Metrics Network Architecture

  2. Computer Network • A computer network is a network of computers for exchanging data (bits) among themselves over a single medium • Information access (global) – almost immediate access to information • Interaction among cooperative application programs – distributed processing/client-server programming • Resource sharing – sharing hardware and software resources

  3. Popular Applications • E-mail • File transfer/access (ftp) • Web browsing (http, html) • Remote login/execution (telnet) • The Internet (global access to information)

  4. (a) (b) Building Blocks of a Network • Nodes: PC, special-purpose hardware… • hosts • switches • Links: coax cable, optical fiber… point-to-point multiple access

  5. Switched Networks • A network can be defined recursively as... • two or more nodes connected by a link, or • two or more networks connected by two or more nodes Switch Router/Gateway

  6. Why Switched Network? • Multiple-access links are limited in size; cannot support large geographical area (except satellites) • Multiple-access links cannot support large number of nodes • Connecting all nodes directly to each other using point-to-point links is unmanageable and very expensive for large number of nodes covering a large geographic area • A switched network solves all preceding problems

  7. Switching Strategies • Circuit switching: carry bit streams • Example: original telephone network • A dedicated circuit (or channel) is established across a sequence of links. • Source node sends a stream of bits across the circuit • Packet switching: store-and-forward messages (packets) • Example: Internet • Each intermediate node receives and stores a complete packet and then forwards to the next node

  8. What is a packet? • A discrete block of data • Part of a piece of an email, an html page, or an image

  9. Identifying Nodes • How to identify a packet’s sender? • How to identify a packet’s receiver? • How to determine to which link a packet should be forwarded in a switched network?

  10. Addressing and Routing • Address: byte string that identifies a node • usually unique • sender and receiver addresses are always included in a packet • Routing: process of forwarding messages/packets to the destination node based on its address • Routing is done by a device called a router or a gateway • A router connects two or more networks • Address Types • unicast: node-specific • broadcast: all nodes on the network • multicast: some subset of nodes on the network

  11. Resource Sharing • How do all the hosts that want to communicate share the same network? • How do several hosts share the same link when they all want to use it at the same time?

  12. Cost-Effective Resource Sharing: Multiplexing • Time-Division Multiplexing (TDM) • Frequency-Division Multiplexing (FDM)

  13. STDM and FDM • Synchronous time-division multiplexing (STDM) • Each host is allowed to transmit during a designated slot of time in round-robin fashion. • Disadvantage: • Inefficient if not all hosts have anything to transmit • Frequency-division multiplexing (FDM) • Different frequencies (channels) are used for transmit each data flow • Same as the way TV programs are transmitted over the same cable or medium • Inefficient if not all channels are busy all the time • Both STDM and FDM are static methods

  14. ■ ■ ■ Statistical Multiplexing • On-demand time-division • Schedule link on a per-packet basis • Packets from different sources interleaved on link • Buffer packets that are contending for the link • Buffer (queue) overflow is called congestion

  15. Support for Common Services • Hiding complexity of the network from the application • Providing logical channels over which application processes on different hosts can communicate with each other

  16. Inter-Process Communication • Turn host-to-host connectivity into process-to-process communication. • Fill gap between what applications expect and what the underlying technology provides.

  17. Common Communication Patterns: IPC Abstractions • Request/Reply • distributed file systems • digital libraries (web) • Stream-Based • video: sequence of frames • 1/4 NTSC = 352x240 pixels • (352 x 240 x 24)/8=247.5KB • 30 fps = 7500KBps = 60Mbps • video applications • on-demand video • video conferencing

  18. Request/Reply & Stream-Based • Request/Reply Characteristics • Small amount of data sent as request • Large amount of data sent as reply • Not very time sensitive • Very error sensitive • Stream-Based Charecteristics • Time or delay sensitive • Can tolerate errors to some extent

  19. Reliability • What Goes Wrong in the Network? • Bit-level errors (electrical interference) • Packet-level errors (congestion) • Link and node failures • Messages are delayed • Messages are delivered out-of-order • Third parties eavesdrop

  20. Network Architecture • Requirements for good network design • Must provide general, cost-effective, fair, robust, and high-performance connectivity among a large number of computers • Must evolve to accommodate future changes and growths • Two important widely referenced architectures • OSI architecture and Internet architecture

  21. Layering • Use abstractions to hide complexity • Abstractions naturally lead to layering • Alternative abstractions at each layer

  22. Protocol • A set of rules for communications • Implemented in software and hardware • Building blocks of a network architecture • Each protocol object has two different interfaces • service interface: operations on this protocol • peer-to-peer interface: messages exchanged with peer • Term “protocol” is overloaded • specification of peer-to-peer interface • module that implements this interface

  23. Interfaces

  24. Host 1 Host 2 Digital Digital Video Video File File library library application application application application application application Protocol Machinery • Protocol Graph • most peer-to-peer communication is indirect • peer-to-peer is direct only at hardware level RRP: Request/Reply Protocol MSP: Message Stream Protocol HHP: Host to Host Protocol

  25. Host Host Application Application Application Application program program program program Data Data RRP RRP RRP Data RRP Data HHP HHP HHP RRP Data Protocol Machinery (contd..) • Multiplexing and Demultiplexing (demux key) • Encapsulation (header/body)

  26. OSI Architecture

  27. OSI Architecture (contd..) • Application – supports user applications e.g., email, ftp • Presentation – supports formatting or transformation data needed by an application • Session – supports dialog and managing connections between cooperating applications • Transport – supports reliable end-to-end transmission of data • Network – address assignment; routing • Data Link – supports frame construction, flow control, error detection and correction • Physical – deals with mechanical, electrical, functional, and procedural characteristics of the physical medium

  28. FTP HTTP NV TFTP UDP TCP IP NET NET NET ■ ■ ■ 1 2 n Internet Architecture • Defined by Internet Engineering Task Force (IETF) • Hourglass Design • Application vs Application Protocol (FTP, HTTP) • NET1, NET2, . . ., NETn are network protocols, usually implemented by a combination of hardware (e.g., a network adapter) and software (e.g., a device driver).

  29. Alternative View of Internet Architecture

  30. Performance Metrics:Communication Bandwidth • Number of bits that can be transmitted over the network in a certain period of time • Generally means the available bandwidth or the link bandwidth • Effective bit transfer rate is always less than the available bandwidth (commonly referred as throughput or end-to-end bandwidth) • Notation • bps, Kbps, Mbps, Gbps

  31. Performance Metrics: Latency • The time taken for a message to travel from one end of a network to the other • Also called delay • Components Latency = Propagation + Transmit + Queue Propagation = Distance / speed Transmit = Size / Bandwidth

  32. Performance Metrics (contd..) • Propagation Speed • 3.0 x 108 meters/second in a vacuum • 2.3 x 108 meters/second in a cable • 2.0 x 108 meters/second in a fiber • Notes • no queuing delays in direct link • bandwidth not relevant if Size = 1 bit • process-to-process latency includes software overhead • software overhead can dominate when Distance is small

  33. Round-Trip Time (RTT) • Time it takes to send a message from one end of a network to the other and back

  34. Bandwidth versus Latency • Relative importance • 1-byte: 1ms vs 100ms dominates 1Mbps vs 100Mbps • 25MB: 1Mbps vs 100Mbps dominates 1ms vs 100ms • Infinite bandwidth • RTT dominates • Throughput = TransferSize / TransferTime • TransferTime = RTT + 1/Bandwidth x TransferSize • 1-MB file to 1-Gbps link as 1-KB packet to 1-Mbps link

  35. Delay x Bandwidth Product • Amount of data “in flight” or “in the pipe” • Example: 100ms x 45Mbps = 4.5Mbits=562KB

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