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Computer Networks

Computer Networks. Chapter 1 Introduction Prof. Jerry Breecher CSCI 280 Spring 2002. The Weeks Ahead. Date. Lecture. Lab/Project. Jan 14 - 16. Introduction. Start Proj. 0. Project 0 Due. Jan 21 - 23. The Physical Layer. Start Project 1. Project 1 Checkin. Jan 28 - 30.

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Computer Networks

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  1. Computer Networks Chapter 1 Introduction Prof. Jerry Breecher CSCI 280 Spring 2002

  2. The Weeks Ahead Date Lecture Lab/Project Jan 14 - 16 Introduction Start Proj. 0 Project 0 Due Jan 21 - 23 The Physical Layer Start Project 1 Project 1 Checkin Jan 28 - 30 Medium Access Layer Feb 4 - 6 Medium Access Layer Project 1 Checkin Chap. 1 - Introduction

  3. Chapter Overview 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization Chap. 1 - Introduction

  4. What You Will Learn • Terminology • Communication basics • Media and signals • Binary numbering system • Data transmission characteristics • asynchronous and synchronous communication • serial and parallel transmission • bandwidth, throughput and noise • multiplexing Chap. 1 - Introduction

  5. What You Will Learn • Networking and Network Technologies • Packet Switching, Circuit Switching • Protocols and Layering • Network Addressing • Interconnection (bridges, switches, routers) • Topologies (star, ring, bus, mesh) • Routing • Flow, Error and Congestion Control Chap. 1 - Introduction

  6. The Big Picture: Where are We Now? Introduction 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization Some of the overall issues we’ll be dealing with in this course. Chap. 1 - Introduction

  7. Introduction Motivation for Networks • Information Access • Sharing of Resources • Facilitate Communications What A Network Includes • Transmission hardware • Special-purpose hardware devices • interconnect transmission media • control transmission • run protocol software • Protocol software • encodes and formats data • detects and corrects problems Chap. 1 - Introduction

  8. Introduction What A Network Does • Provides communication that is • Reliable • Fair • Efficient • From one application to another • Automatically detects and corrects • Data corruption • Data loss • Duplication • Out-of-order delivery • Automatically finds optimal path from source to destination Chap. 1 - Introduction

  9. Lingo Introduction Where are we going: This chapter is basically an overview of the course. Before we get mired down in details, we want to get a general idea of what this whole course is about. Here are the kind of issues we’ll be dealing with. Broadcast Networks - Single channel with multiple machines connected to it. Point-to-point - Individual connections between pairs of machines. Mostly EE issues. Packets - Messages - the "chunk" of data transmitted from one machine to the next. Addressing - + One to one: Packet contains specific target address. + Broadcasting: All machines on the network receive and process the packet. + Multicasting: A subset of machines receive and process the packet. Chap. 1 - Introduction

  10. Lingo Introduction Can also view messaging in terms of the distance of transmission: 0.1 meter Single processor 1 meter Multiprocessor 10 meter Room 100 meter Building -- Local area network 1 kilometer Campus 10 kilometer City -- Metropolitan area network 100 kilometer Country 1K kilometer Continent -- Wide area network 10K kilometer Planet -- Internet Serial: Parallel: Chap. 1 - Introduction

  11. Dedicated fixed bandwidth route fixed at setup idle capacity wasted network state Lingo Introduction Packet Switching Circuit Switching • Best Effort • end-to-end control • multiplexing technique • re-route capability • congestion problems Chap. 1 - Introduction

  12. The Big Picture: Where are We Now? Network Hardware 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization The various possible hardware structures that are possible. LANs, MANs, WANs and all that. Chap. 1 - Introduction

  13. Network Hardware LOCAL AREA NETWORKS • LANs are: • Privately owned. Can be up to several kilometers long; • Restricted so worst case transmission time can be contained. • Run at speeds of 10, 100, or more Mbps. • Low delay. High reliability. • Requires collision arbitration. • Ethernet: • IEEE 802.3. • Bus based broadcast network with decentralized control at 10 or 100 Mbps. • Token Ring: • IEEE 802.5 • Ring based broadcast network with token arbitration at 4 or 16 Mbps. Chap. 1 - Introduction

  14. Network Hardware METROPOLITAN AREA NETWORKS: MANS are: Larger version of LAN ("city" wide). Public or private / data or voice. Broadcast - no switches. Can be distinguished from LANs based on wiring mechanism. Chap. 1 - Introduction

  15. Network Hardware WIDE AREA NETWORKS WANS are: Networks spanning large distances. Hosts or End Systems - Machines running user applications. (Communication) Subnet - Connections between hosts - transmission lines + switches. A "locality" understanding each other's addresses. Circuits/Channels/Trunks - Transmission lines move the bits. Packet switching nodes/Intermediate systems/Data switching exchanges Specialized computers moving data between several inputs to several outputs. Chap. 1 - Introduction

  16. Network Hardware WIDE AREA NETWORKS WANS are: Networks spanning large distances. Router - Generic term for switching computers. Point-to-point/Store-and-forward/Packet-switched - Moving through a series of routers, packets are received at a router, stored there, then forwarded to the next router. Topologies - Chap. 1 - Introduction

  17. Network Hardware Media Wireline String Garden Hose Copper Twisted Pair Coax Optical Fiber Wireless Sound Light and mirrors Infrared RF Microwave Chap. 1 - Introduction

  18. Network Hardware WIRELESS NETWORKS • What are they: Used where computer is mobile or far away from wires. • Only 1 - 2 Mbps / higher error rates / interference. • Wireless Mobile Applications • NO NO Stationary workstations in offices. • NO YES Using portable in hotel. • YES NO LANs in unwired buildings. • YES YES Portable office; PDA. • INTERNETWORKS: • ( or internet ) • Collection of interconnected networks. • internet ( lower case i ) is generic term. • Internet ( upper case I ) is worldwide connection to all kinds of machines. Chap. 1 - Introduction

  19. The Big Picture: Where are We Now? Network Software 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization This covers the way the software is put together. It talks about the philosophy of connecting together two entities. “Layering” is the key word. Chap. 1 - Introduction

  20. PROTOCOL HIERARCHIES Network Software Layers - The concept that network software is organized functionally into levels. A level on one host talks to the same level on another host (its peer). Protocol - The protocol is the convention or standard that a layer uses to talk to the other layer. An agreement or standard on the conversation. Physical Medium - Underneath the layers is the wire or fiber or whatever. Interface - Defines the services that one layer offers another (either up or down.) Important to keep this simple and clean. Important that each layer perform specific actions. Chap. 1 - Introduction

  21. PROTOCOL HIERARCHIES Network Software • Network architecture - • A set of layers and protocols. It contains details on what happens in the layer and what the layers says to its peer. • Functional interfaces and implementation details are not part of the spec, since that's not visible outside the machine. • Protocol stack - • A list of protocols used by a system, one protocol per layer. • Information flow - • "Send_to_peer" rather than "call_next_layer_down". Chap. 1 - Introduction

  22. DESIGN ISSUES FOR THE LAYERS Network Software Both Directions Simultaneous Simplex No No Half duplex Yes No Full duplex Yes Yes o Number of logical channels per connection (for priority purposes) o Error control. (garbled or missing.) o Preservation of message ordering. o Flow control. o Breaking up messages into a smaller chunks (and reassembly.) o Multiplexing messages on same connection. o Routing - how to get from one host to another. That word “Multiplexing”: Chap. 1 - Introduction

  23. Network Software INTERFACES AND SERVICES Purpose of each layer is to provide services to the layer above it. Entities / Peer entities - Active element in each layer (process/IO chip). Peer entity = layer N entity <--> layer N entity. Layer N entity provides service for layer N + 1. Service providers and users - Layer N is a provider for user N + 1. SAPs (Service Access Points) - Entry points in N that layer N + 1 can access. Has an address that uniquely identifies it. IDUs (Interface Data Unit) - The information from N + 1 provided at the SAP. Made up of SDU + control information. Chap. 1 - Introduction

  24. Network Software INTERFACES AND SERVICES Purpose of each layer is to provide services to the layer above it. SDUs (Service Data Unit) - The portion of the IDU that will be passed up to the peer entity. PDUs (Protocol Data Unit) - The SDU may be broken up into PDUs, that being the chunk size for further transmission. Rolled up in this figure: Chap. 1 - Introduction

  25. Network Software CONNECTION-ORIENTED / CONNECTIONLESS SERVICES: • Connection oriented service - • Like the phone system. The system establishes a connection, uses it, and closes it. Acts like a tube. Data comes out the other end in the same order as it goes in. • Connection Setup • Data Transfer • Connection Termination • Connectionless service - • Like the post office. Each message has the entire address on it. Each • message may follow a different route to its destination. Ordering not • maintained. • Data Transfer Chap. 1 - Introduction

  26. Network Software CONNECTION-ORIENTED / CONNECTIONLESS SERVICES: Quality of service - Will the message arrive?? A reliable connection-oriented service guarantees success. o Message sequence - message boundaries and order are maintained. o Byte streams - messages are broken up or combined; flow is bytes. Can pair mechanism with upper-layer requirements. Chap. 1 - Introduction

  27. Network Software CONNECTION-ORIENTED / CONNECTIONLESS SERVICES: Datagram Service - Like junk mail. It's not worth the cost to determine if it actually arrived. Needs a high probability of arrival, but 100% not required. Connectionless, no acknowledgment. Acknowledged datagram service - As above, but improved reliability via acknowledgment. Request-reply service - Acknowledgment is in the form of a reply. Summarized in this Table. Chap. 1 - Introduction

  28. Network Software SERVICE PRIMATIVES: Primitives are: The operations available to an entity. Possibilities include: Request -- An entity want some work done. Indication -- An entity is told about an event. Response -- An entity wants to respond to an event. Confirm -- Response to earlier request has come back. Example of Connectionless Protocol: Unitdata.request |--------->| Unitdata.indication Unitdata.indication |<---------| Unitdata.request Example of Connection-oriented Protocol: Connect.request |--------->| Connect.indication Connect.confirm |<---------| Data.request |--------->| Data.indication Data.indication |<---------| Data.request Disconnect.request |--------->| Disconnect.indication Disconnect.confirm |<---------| Chap. 1 - Introduction

  29. Network Software SERVICE PRIMATIVES THE RELATIONSHIP OF SERVICES TO PROTOCOLS: Services are primitives that a layer provides for the layer above it. Protocols are rules governing the meaning of frames/packets/messages exchanged with the peer entity. Chap. 1 - Introduction

  30. The Big Picture: Where are We Now? Reference Models 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization There are two competing models for how the software is layered. These are the OSI and the TCP models. We talk about each of these here. Chap. 1 - Introduction

  31. Layering Reference Models Chap. 1 - Introduction

  32. Headers, Data, and Trailers Reference Models Encapsulation Chap. 1 - Introduction

  33. THE OSI REFERENCE MODEL Reference Models OSI == Open Systems Interconnection Developed by ISO == International Standards Organization Principles used to develop OSI Layering: 1. Need a layer for each different level of abstraction. 2. Each layer performs a well defined function. 3. Each layer should be standard-izable. 4. Layer boundaries should minimize data flow across those boundaries. 5. The right number of layers - don't put too many functions together, but not too many layers either. Chap. 1 - Introduction

  34. THE OSI REFERENCE MODEL Reference Models Physical Layer - Purpose -- Transmits raw bits across a medium. Electrical -- Concerns are voltage, timing, duplexing, connectors, etc. Where Taught -- Differentiates an EE course from a CS course. Data Link Layer - Framing -- Breaks apart messages into frames. Reassembles frames into messages. Error handling -- solves damaged, lost, and duplicate frames. Flow control -- keeps a fast transmitter from flooding a slow receiver. Gaining Access -- if many hosts have usage of the medium, how is access arbitrated. Chap. 1 - Introduction

  35. THE OSI REFERENCE MODEL Reference Models Network Layer - Routing -- What path is followed by packets from source to destination. Can be based on a static table, when the connection is created, or when each packet is sent. Congestion -- Controls the number packets in the subnet. Accounting -- Counts packets/bytes for billing purposes. Heterogeneity -- Interfacing so one type of network can talk to another. Chap. 1 - Introduction

  36. THE OSI REFERENCE MODEL Reference Models Transport Layer - Reliability -- Ensures that packets arrive at their destination. Reassembles out of order messages. Hides network -- Allows details of the network to be hidden from higher level layers. Service Decisions -- What type of service to provide; error-free point to point, datagram, etc. Mapping -- Determines which messages belong to which connections. Naming -- "Send to node xyzzy" must be translated into an internal address and route. Flow control -- keeps a fast transmitter from flooding a slow receiver. Chap. 1 - Introduction

  37. THE OSI REFERENCE MODEL Reference Models Session Layer - Sessions -- Provides services that span a particular message. For instance, a login session could be logged. Synchronization-- Provide way to subdivide a long mechanism for reliability. Presentation Layer - Prettiness -- Syntax and semantics of information transmitted. Understands the nature of the data being transmitted. Converts ASCII/EBCDIC, big endian/little endian Application Layer - Interfacing -- Terminal type translation. File transfer -- Programs able to understand directory structures and naming conventions and map them onto various systems. Chap. 1 - Introduction

  38. THE OSI REFERENCE MODEL Reference Models Data Transmission in the OSI Model - Chap. 1 - Introduction

  39. THE TCP/IP REFERENCE MODEL Reference Models Used in the Arpanet and in the Internet. Common mechanism that is gaining on/surpassing the OSI Model. Host to Network Layer - This lowest level is not defined in this model. Various mechanisms abound. Internet Layer - Connector -- Provides packet switched connectionless service. Routing -- The IP (Internet Protocol) does delivery and congestion control. Chap. 1 - Introduction

  40. THE TCP/IP REFERENCE MODEL Reference Models Transport Layer - Allows peer entities to communicate. TCP -- Transmission Control Protocol provides a reliable connection oriented protocol that delivers a byte stream from one node to another. Guarantees delivery and provides flow control. UDP -- User Datagram Protocol provides an unreliable connection-less protocol for applications that provide their own. Application Layer - Terminal -- Telnet File transfer -- FTP The Web -- HTTP Chap. 1 - Introduction

  41. COMPARISON OF REFERENCE MODELS Reference Models OSI has good definition of service, interface, and protocol as discussed before. Fits well with object oriented programming concepts. Protocols are better hidden. With TCP, the protocols came first; model was just a description of the protocols. But then the model isn't good for any other protocols. LAYERS Transport Network OSI Connectionless Connection-Oriented Connection-Oriented TCP Connectionless Connectionless Connection-Oriented Allows connectionless applications! Chap. 1 - Introduction

  42. THE TCP/IP REFERENCE MODEL Reference Models • A CRITIQUE OF OSI: • Bad Timing - • TCP already in use by the time OSI came along. • Bad Technology - • Layers don't match reality . Chosen because IBM's SNA has seven layers. • Dominated by phone company mentality. • Bad Implementation - • Huge, unwieldy, slow. • A CRITIQUE OF TCP/IP: • Doesn't separate spec from implementation. • Model is only good for describing TCP. • Doesn't specify physical and data link layers. • THE HYBRID REFERENCE MODEL: • 5 Application • 4 Transport • 3 Network • 2 Data Link • 1 Physical Chap. 1 - Introduction

  43. The Big Picture: Where are We Now? Example Networks 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization This section talks about some real networks. Chap. 1 - Introduction

  44. Example Networks • NOVELL NETWARE: • Heavily used in PC world. • Proprietary protocol stack • Network layer -- IPX - unreliable connectionless • Transport layer -- NCP (Network Core Protocol) • - connection oriented • - other services • Naming and Addressing -- • SAP (Service Advertising Protocol) - Servers advertise their address to router machines. Clients, when booted, ask for location of nearest server. • THE ARPANET: • 1968 Originally intended as reliable network, with multiple routing. • Used TCP/IP precursor, which got built into early UNIX. Chap. 1 - Introduction

  45. Example Networks NSFNET: Late 1970s - Many other folks wanted to get on the net, but Arpanet was essentially limited to military contractors. NSF set up another network to handle this need. Started at 448 Kbps and by 80's upgraded to 1.5 Mbps. 1990 Formed ANS (Advanced Networks and Services) -- MERIT, MCI, IBM took over from the government running at 45 Mbps. 1995 ANSNET sold to AOL, who now runs it. THE INTERNET: Growing exponentially. All nodes run TCP/IP. Means that all nodes have an IP address by which they can be contacted. Services provided include: o e-mail o news o remote login o file transfer o the web GIGABIT TESTBEDS: Testing higher speed long distance nets. Typically 622 Mbps. While throughput improves, latency remains limited by the speed of light. Chap. 1 - Introduction

  46. Example Data Communications 1.1 Introduction 1.2 Network Hardware 1.3 Network Software 1.4 Reference Models 1.5 Example Networks 1.6 Example Data Communication Services 1.7 Network Standardization These are public networks owned by the phone companies and offered to the public - thus called public networks. Chap. 1 - Introduction

  47. Public Networks Example Data Communications These are subnets, often owned by the phone companies, offered to subscribers. This is a public network. SMDS - SWITCHED MULTIMEGABIT DATA SERVICE Interconnects the LANs operating within a single company. Connects diverse branches. Chap. 1 - Introduction

  48. Public Networks Example Data Communications • X.25 NETWORKS • Developed during 70's. Interface between public packet-switched networks and customers. • Operate at 64 Kpbs, so are very slow and becoming outdated. However there are still many of them in operation. • Connection oriented. Uses: • Switched Virtual Circuit - established when the first packet is sent. Circuit remains for duration of session providing in-order delivery, and flow control. • Permanent Virtual Circuit - established by agreement between the customer and the carrier. Like a leased line. • PAD ( Packet Assembler Disassembler ) allows non-X.25-speaking terminals to connect to X.25. Chap. 1 - Introduction

  49. Public Networks Example Data Communications • FRAME RELAY: • Takes advantage of modern high-speed reliable digital phone lines. • This allows simple protocols with work done by user computers rather than by the network. Runs at 1.5 Mbps with few features. • Customer leases a permanent virtual circuit between two points. This "virtual leased line" means that the wire is shared with other users at a great price reduction. Each is allowed some bursti-ness, but doesn't get the line all day. Chap. 1 - Introduction

  50. Public Networks Example Data Communications BROADBAND ISDN AND ATM: The above networks, together with a multitude of old versions, mean a major headache. Need a way to provide all of these, and many new services with a single high speed network. ISDN(Integrated Services Digital Network) will offer cable, video on demand, e-mail, etc. etc. etc. ATM (Asynchronous Transfer Mode) is underlying mechanism. Transmits in small fixed-size cells. 5 48 +---------+---------------------------------------+ | Header | User Data | +---------+---------------------------------------+ Chap. 1 - Introduction

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