Computer Network Basics

1. Computer Network Basics An overview of computer networking which introduces many key concepts and terminology. Sets the stage for future topics.

2. Components of any Computer Keyboard, Mouse Computer Processor (active) Devices Memory (passive) (where programs, data live when running) Input Control (“brain”) Disk, Network Output Datapath (“brawn”) Display, Printer

3. Communication Devices • Synchronous communication uses a clock signal separate from the data signal- communication can only happen during the ‘tick’ of the timing cycle • Asynchronous communication does not use a clock signal- rather, it employs a start and stop bit to begin and end the irregular transmission of data

4. Connecting to Networks (and Other I/O) • Bus - shared medium of communication that can connect to many devices • Hierarchy of Buses in a PC

5. Operating systems

6. Operating Systems Developed for Portable Devices

7. network edge: applications and hosts network core: routers network of networks A closer look at network structure:

8. General Architecture of Computer Networks

9. mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete “chunks” The Network Core

10. Connection of Networks

11. a) b) c) d) Network Topology a) bus, b) star, c) ring, d) tree structure

12. Classification of the networks according to the connection establishing • Line switched network • Packet switched network • Radiating/data disseminating systems • Point-to-point connected networks

13. Wired media • Telephone line • Thin Coax • Thick Coax • Unshielded Twisted Pair (UTP) • Shielded Twisted Pair (STP) • Fibre

14. (Data) Reliability • A network service is (data) reliable if the sender application can rely on the error-free and ordered delivery of the data to the destination • In the Internet the reliability can obtained mainly by acknowledgements and retransmission • In such a way the losses in the underlying layers can be retrieved

15. Flow-control and Congestion Prevention • Flow-control: to protect the receiver against the overload • I.e.: the sender (source) sends more data than the receiver can process • it is mainly necessary in link and transport level • Congestion prevention: to prevent the intermediate nodes against the overload • it is mainly necessary in network level

16. Overload and Congestion • Overload: Too many packets occur in a subnetwork in the same time, which prevent each other and in such a way the throughput decreases • Congestion: the queues in the routers are too long, the buffers are full. • As a consequence some packages are dropped if the buffers of the routers are overloaded • In extreme case: grid-lock, lock-up

17. Deadlock • Deadlock: the most serious situation of the congestion, the routers wait for each other • Direct store and forward deadlock: the buffers of two neighbouring routers are full with the packets to be sent to the other router • Indirect store and forward deadlock: the deadlock occurred not between two neighbouring routers but in a subnetwork, where any of the routers has not free buffer space for accepting packets

18. Review: Networking Definitions • Network: physical connection that allows two computers to communicate • Packet: unit of transfer, bits carried over the network • Network carries packets from on CPU to another • Destination gets interrupt when packet arrives • Protocol: agreement between two parties as to how information is to be transmitted • Broadcast Network: Shared Communication Medium • Delivery: How does a receiver know who packet is for? • Put header on front of packet: [ Destination | Packet ] • Everyone gets packet, discards if not the target • Arbitration: Act of negotiating use of shared medium • Point-to-point network: a network in which every physical wire is connected to only two computers • Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network • Router: a device that acts as a junction between two networks to transfer data packets among them

19. The Need for a Protocol Architecture • Procedures to exchange data between devices can be complex • High degree of cooperation required between communicating systems • destination addressing, path • readiness to receive • file formats, structure of data • how commands are sent/received and acknowledged • etc.

20. Layered Protocol Architecture • Modules arranged in a vertical stack • Each layer in stack: • Performs related functions • Relies on lower layer for more primitive functions • Provides services to next higher layer • Communicates with corresponding peer layer of neighboring system using a protocol

21. Network Layering • Layering: building complex services from simpler ones • Each layer provides services needed by higher layers by utilizing services provided by lower layers • The physical/link layer is pretty limited • Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) • Routing is limited to within a physical link (wire) or perhaps through a switch • Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere:

22. Key Features of a Protocol • Set of rules or conventions to exchange blocks of formatted data • Syntax: data format • Semantics: control information (coordination, error handling) • Timing: speed matching, sequencing • Actions: what happens when an event occurs

23. Operation of Protocols

24. The OSI Model • Physical Layer • (Data) Link Layer • Network Layer • Transport Layer • Session Layer • Presentation Layer • Application Layer

25. Physical Layer • Transmission of energy onto the medium • Collection of energy from the medium • This layer is concerned with the physical transmission of raw bits • This bits are transmitted through mechanical, electrical, and procedural interfaces which include • interface card standard • modem standards • certain portions of the ISDN and LAN MAN standards

26. (Data) Link Layer • Transmission of frames over one link or network • Often subdivided into the MAC and LLC • It receives bits from the physical layer, converting bits to frames • frame boundaries • Using protocols (e.g. HDLC), this layer corrects errors that might have occurred during transmission across a link • In addition this layer provides an “error-free” transmission channel to the next layer known as the network layer: error control • ARQ • duplicates • Flow control

27. Network Layer I • The previous two layers were concerned with getting error-free data across a link • The network layer establishes connections between nodes, routes data packets through the network, and accounts for them • End-to-end transmission of packets (possibly over multiple links) • Controls the operation of the subnet • Routing • static • dynamic • Congestion control • At this stage, there may be congestion due to many packets waiting to be routed • Some packets may be lost during congestion

28. Network Layer II • Accounting • packets • bytes • etc. • Internetworking • This layer is also concerned with internetworking where there is ‘talking’ between technologies, such as the traditional Internet connected to ATM • segmentation • addressing • sequencing • accounting • Broadcast subnets: thin network layer

29. Transport Layer I • This layer presumes the ability to pass through a network and provides additional services to end-users, such as and-to-and packet reliability • End-to-end delivery of a complete message (end-to-end communication path, usually reliable) • Isolation from “hardware” • Multiplexing/demultiplexing • Divide message into packets • Reassemble (possibly out of order packets) into the original message of the distant end

30. Transport Layer II • End-to-end flow control • Acknowledgments • Types of service • error-free, point-to-point, in sequence, flow controlled • no correctness guarantees • no sequencing • Establishing/terminating connections • naming/addressing • intra-host addressing (process, ports)

31. Session Layer • This layer enables users to establish sessions across a network between machines • In addition, it offers session management services • Set up and management of end-to-end conversation • Establish and terminate sessions • superset of connections • Assignment of logical ports • Dialogue control • Token management • for critical operations • Synchronization • checkpoints/restarts

32. Presentation Layer • This layer is concerned with the syntax and semantics of messages, code conversions between machines, and other data conversion services • Some of these services are data compression and data encryption • Interface between lower layers and application • Formatting • Syntax & semantics of messages • Data encoding (e.g.: ASCII to EBCDIC) • Compression • Encryption/Decryption • Authentication

33. Application Layer • This layer provides support for the user's network applications • Some application layer services have been standardized, e.g.: • File Transfer and Management (FTAM) • Message Handling Services for electronic mail (X.400) • Directory Services (X.500) • Electronic Data Interchange (EDI) • Program you’re running,applications • file transfer, access & management • e-mail • virtual terminals • WWW

34. The OSI Protocol Stack

35. Operation of the model

36. Names of the Nodes, Connections and Data Units

37. Communication among the layers • Connection oriented network service (virtual circuits, eg. ATM) • Reliable transport service • Unreliable transport service • Connectionless network service (datagram service, eg. IP) • Reliable transport service (eg. TCP) • Unreliable transport service (eg. UDP)

38. Network Tools • Repeater: connects network segments logically to one network • Hub: multiport repeater • Bridge: datalink level connection of two networks • Switch: multiport bridge • Router: connects networks that are compatible in transport level • subnetworks are connected to the interfaces of the repeater • Gateway (proxy server): router between two individual network. The “Way Out”

39. Physical Layer Devices • Repeater • Hub • “dumb” • level-1 hub • multi-port repeater

40. Data Link Layer Devices • Bridge • Cascaded vs. Backbone • Single • Multiple • Switch (switched hub)

41. Routers • Provide link between networks • Accommodate network differences: • Addressing schemes • Maximum packet sizes • Hardware and software interfaces • Network reliability • Congestion/Traffic Management

42. Devices of the Network Connection

43. Architectural Implementation of the LANs • Ethernet (IEEE 802.3) • FDDI • Gigabit Ethernet • Token Bus (IEEE 802.4) • Token Ring (IEEE 802.5)

44. Characteristics of High-Speed LANs

45. Wide Area Network Connections • Solutions for connecting LANs to the Internet • Ethernet (ring or star topology) • Managed Leased Line Network (MLLN) • ATM (Asynchronous Transfer Mode) • Switched line • ISDN line

46. Soft and Hard States • State: the data collection, which are necessary for keeping the connection between two protocol entities • Hard state • If the connection is established once, it is never timed out, even if it is not in usage • To cancel the connection one of the participants of the connection must explicitly close it • The history of the state is stored • Soft state • To keep the connection the participants must send occasionally keep-alive messages, since without keep-alive message the state information is timed out after a certain period • The state is called as “soft” since in the ordinary operation the state can change easily • The history of the state is not stored

47. Great for bursty data resource sharing no call setup (less start-up delay) However… Packets can experience delays, so not for “real-time” applications excessive congestion leads to packet delay and loss protocols (like TCP) are needed for reliable data transfer, and congestion control Is packet switching best in every case? Packet switching versus circuit switching

48. Router Router LW1 LR1 LW2 LR2 Lw3 Performance Considerations • Before continue, need some performance metrics • Overhead: CPU time to put packet on wire • Throughput: Maximum number of bytes per second • Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers • Latency: time until first bit of packet arrives at receiver • Raw transfer time + overhead at each routing hop • Contributions to Latency • Wire latency: depends on speed of light on wire • about 1–1.5 ns/foot • Router latency: depends on internals of router • Could be < 1 ms (for a good router)

49. packets experience delay on end-to-end path foursources of delay at each hop Nodal processing: check bit errors determine output link Queueing: time waiting at output link for transmission depends on congestion level of router transmission A propagation B nodal processing queueing Delay in packet-switched networks

50. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s transmission A propagation B nodal processing queueing Delay in packet-switched networks