Exploring Computer Networks & Protocols

1. CEG-4188Lecture 1:Computer Networks and ProtocolsProf. Gregor v. BochmannSITE, University of OttawaThese course notes are based on slides prepared by Drs. Makrakis and Shirmohammadi CEG 4188

2. Why Computer Networks? Application Type CEG 4188

3. Communications Tasks CEG 4188

4. Types of Communication Networks Classification according to the way the “information flows” are transported to the users • Switching Networks • Broadcast Networks CEG 4188

5. Switching Networks Data are transferred from source to destination through a series of intermediate nodes CEG 4188

6. Broadcast Networks --There are no intermediate switching nodes --All users are connected on the same medium Satellite CEG 4188

7. Classification According to Coverage Area • Local Area Networks (0-2 Km; campus) • Ethernet (10/100/1000 Mbps), Token ring (4, 16 Mbps), • IEEE 802.11(b, g, a, n) • Metropolitan Area Networks (2-50 km; corporate offices, city) • DQDB (Distributed Queue Dual Bus), WiMAX • (IEEE 802.16.a/b/e) • Wide Area Networks (country, continent) • transmission lines, switching elements • Personal Access Networks (PANs) • Bluetooth, IEEE 802.15.3 CEG 4188

8. Local Area Networks (LANs) It expands over small geographic areas (within a building or close-by buildings) It is usually owned by the same organization The internal data rates are typically much greater than those of WANs Typically, they make use of broadcast rather than switching CEG 4188

9. Local Area Networks (LANs) Backbone Single-building LAN Multi-building LAN Examples: home network, wireless-wired campus network CEG 4188

10. Metropolitan Area Networks (MAN) Examples: Ottawa-Carleton Research Institute (OCRI) MAN, National Capital Institute on Telecommunications (NCIT) MAN CEG 4188

11. Wide Area Networks (WAN) Example: Canadian Network for the Advancement of Research, Industry and Education (CANARIE). CEG 4188

12. Wide Area Networks (WAN) • Traditionally, WANs have been implemented using one of two technologies • Circuit Switching • Packet Switching • Datagram • Virtual Circuit CEG 4188

13. Circuit Switching • Uses a dedicated path between two stations • Process consists of three phases • establish • transfer • disconnect • Bandwidth inefficient • channel capacity dedicated for duration of connection • if no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Can provide deterministic performance guarantees CEG 4188

14. Circuit Switching Network Example: Public Service Telephone Network (PSTN) CEG 4188

15. Event Timing for Circuit Switching CEG 4188

16. Packet Switching • circuit switching was designed for voice • packet switching was designed for data • transmitted in “small” packets • packet contains user data and control info • user data may be part of a larger message • control info includes routing (addressing) info • packets are received, stored briefly (buffered) and passed on to the next node CEG 4188

17. Packet Switching • Source breaks long message into “information transporting segments” (packets). • Packets are sent one at a time to the network. • Packets contain user data and control/signaling information. • user data may be part of a larger message • control information includes routing/addressing information • Packets are received, stored “briefly” (buffered) and are passed onto the next node. CEG 4188

18. Characteristics • Line efficiency • single link shared by many packets over time • packets queued and transmitted as fast as possible • Data rate conversion • stations connect to local nodes at their own speed • nodes buffer data if required to equalize rates • Packets are accepted even when the “line” is busy • Priorities can be used to support users’ needs, instead of dedicating resources regardless if they are used or not (becoming wasted if they are not) CEG 4188

19. DatagramPacket Switching 19 CEG 4188

20. Event Timing for Datagram CEG 4188

21. VirtualCircuitPacket Switching CEG 4188

22. Event Timing for the 3 switching methods CEG 4188

23. Virtual Circuits versus Datagram • Virtual circuits • network can provide sequencing • traffic engineering can be applied, enabling more practical provision of quality of service (QoS) support • less reliable in cases of switching node failures • Datagram • no call setup phase • more flexible • more reliable in cases of switching node failures • difficult to control network’s state and provide quality of service CEG 4188

24. Circuit versus Packet Switching:Comparison • Circuit Switching • Dedicated channels/resources • Constant delay • Blocking • Continuous flow • Point-to-Point • Packet Switching • Shared channels • Variable delay • Store-and-forward point-to-point & multipoint CEG 4188

25. Other forms of Packet Switching Services • Frame Relay based Networks • requires high level of network reliability. • took away a lot of overhead, improved efficiency. • Asynchronous Transfer Mode (ATM) • based on the use of fixed size packets (53 bytes, called ATM cells). • first Broadband Integrated Services (ISDN) network . • offered quality of service (QoS) choices. • MPLS (label switching) CEG 4188

26. Internetworking Internetworking is a scheme for interconnecting multiple networks of nonidenticaltechnologies Uses both hardware and software Extra hardware positioned between networks Software on each attached computer System of interconnected networks is called an internetwork or an internet CEG 4188

27. Internetwork (Internet) CEG 4188

28. OSI Reference Modelandthe Link layer – as an example protocol layer CEG 4188

29. Open System Interconnection (OSI) Reference Model • Developed by the International Organization for Standardization (ISO). • Has become the standard model for classifying communication functions. • Has seven layers. • It is a “theoretical” system delivered too late! • It has NOT dominated. TCP/IP is the de facto standard. • Several reasons: • TCP/IP appeared earlier • Internet “won” the game • OSI has a “complex” structure that could result in “heavy processing” CEG 4188

30. Functions of the OSI Layers • Physical • The bits that are transmitted over the communication media. • Deals with network hardware, bit encoding. • Examples: copper, fiber, radio, satellite. • Data Link • Activates, maintains, and deactivates the physical link between two adjacent nodes (node-to-node delivery). • Deals with framing, windowing, flow control, error detection and recovery. • Network • Determines how best to route packets of data from source to destination via intermediate network nodes. • Deals with addressing, routing, fragmentation, and congestion. CEG 4188

31. Functions of OSI Layers (…) • Transport • Provides end-to-end message delivery and error recovery. • Deals with end to end integrity and quality of service. • Session • To establish, manage, and terminate sessions. • Controls the dialogue between two host applications. • Reports exceptions to upper layers. • Presentation • Resolves data representation differences. • To translate, encrypt, and compress data. • Application • Perform functions to implement network applications. • E.g.; e-mail, teleconferencing. CEG 4188

32. Generic Communication Issues • Error control: making a channel more reliable, and handling lost or out of sequence messages. • Flow control: avoid flooding a slower peer entity. • Resource allocation: mediating contention for physical (e.g. buffers) or logical (e.g. data structures) resources • Fragmentation: dividing chunks of data into smaller pieces, and subsequent reassembly • Multiplexing: combining several higher layer sessions • Connection setup: initiating logical communication with peer entity • Addressing / naming: managing identifiers • Compression: reducing data rate • Encryption: provide data security • Timer management: bookkeeping and error recovery CEG 4188

33. Link layer protocols - as an example • Transmission mode - physical link property • Line discipline - Who should send now? • Flow control - How much data should be sent? • Error control - How can errors be corrected? CEG 4188

34. Transmission Mode Simplex Half-duplex Full-duplex CEG 4188

35. Line Discipline ENQ/ACK Point-to-point CEG 4188

36. Line Discipline (…) • Multi-point CEG 4188

37. Select Line Discipline CEG 4188

38. Poll Line Discipline CEG 4188

39. listen idle? no wait random time yes transmit collision? yes jamming signal no CSMA/CD (network access protocol = line discipline) • Carrier Sense Multiple Access with Collision Detection • Used in Ethernet: the most widely-used type of LAN CEG 4188

40. Stop and Wait Flow Control What is the problem with this scheme? CEG 4188

41. Flow Control (…) Sliding Window Sender Sliding Window CEG 4188

42. Error Control: Stop-and-Wait Damaged Frame • What causes an error? CEG 4188

43. Error Control: Stop-and-Wait (…) Lost Frame CEG 4188

44. Error Control: Stop-and-Wait (…) Lost ACK CEG 4188

45. Error Control: Go-Back-N Damaged Frame CEG 4188

46. Error Control: Go-Back-N (…) Lost Frame What seems to be a drawback of Go-Back-N? CEG 4188

47. Error Control: Go-Back-N (…) Lost ACK CEG 4188

48. Error Control: Selective Reject Damaged Frame Seems to save bandwidth compared to Go-Back-N. What’s the cost? CEG 4188

49. What is a protocol ? CEG 4188

50. Architectural structure of a protocol layer user of protocol user of protocol service interface service interface service offered by the protocol protocol entity protocol entity Protocol message encoding message encoding service interfaces of lower layer communication service used by the protocol (offered by the lower layer)