1. ETM 555 1 Networks

2. ETM 555 2 Computer Networks “An interconnected collection of autonomous end-systems” End Systems: hosts, PCs, dedicated computers, network components Connection can be via various means: copper wire, fiber optic cables, microwaves, satellites, ham radio. Distributed Systems: – Existence of multiple autonomous computers is transparent (not visible), machines managed automatically Network: – Users explicitly log onto a machine, machines managed personally

3. ETM 555 3 Network Hardware Broadly speaking : two types of transmission technology 1) Broadcast Networks 2) Point-to-Point Networks Broadcast (multiaccess) Point-to-point

4. ETM 555 4 Broadcast Networks Single communication channel that is shared by all the machines on the network Share wire, frequency etc. Packets (small messages) sent by one mechanism is received by the others. Each machine checks the address field in the provided message. If the address is the same as receiving machine’s address, packet is processed, otherwise it is ignored. Some arbitration mechanism is needed when more than one machine want to send message at the same time. Multicasting: transmission to a subset of machines.

5. ETM 555 5 Point-to-Point Networks Consists of many connections between individual pair of machines. To go from source machine to destination machine, packet may have to visit intermediate machines. Various routes (multiple paths) possible  Routing algorithms are required.

6. ETM 555 6 Connecting Networks - Repeaters - Copies bits from one network to another Does not look at (interpret) bits Allows the extension of a network beyond physical length limitations REPEATER

7. ETM 555 7 Connecting Networks - Bridges - Copies frames from one network to another Can operate selectively – does not copy all forms Extends the network beyond physical length limitations

8. ETM 555 8 Connecting Networks - Routers - Switching Elements, packet switching nodes, intermediate systems, data switching exchanges Copies packets from one network to another Makes decisions about what route (path) a packet should take If two routers not connected by direct link want to communicate, they do so by using intermediate routers Subnets in which intermediate routers can store a packet and forward it is called point-to-point or store-and-forward, packet switched.

9. ETM 555 9 Subnet LAN host LAN Router SUBNET

10. ETM 555 10 Connecting Networks - Gateways - Operates as a router A gateway is a host that is connected to two or more physical networks simultaneously and is configured to switch packets between them Can perform – Data conversions – Translation: connect different application protocols – encryption

11. ETM 555 11 Hardware vs. Software Repeaters are typically hardware devices Bridges can be implemented in hardware or software Routers and gateways are typically implemented in software so that they can be extended to handle new protocols Many workstations can operate as routers or gateways

12. ETM 555 12 Local Area Networks (LANs) LANs connect computers that are physically close together LANs are restricted in size Worst case transmisson time is bounded and known in advance High speed Transmission technology: multi-access (broadcast) Speeds – Ethernet: 10 Mbps – Token Ring: 16 Mbps – FDDI: 100 Mbps – Fast Ethernet: 100 Mbps – Gigabit Ethernet: 1 Gbps

13. ETM 555 13 Classification of Networks Smaller, geographically localized networks tend to use broadcasting. Larger networks are usually point-to-point. Interprocessor DistanceProcessors located in the same:Example 1msystemmulticomputer 10m 100m 1km Room Building Campus LAN 10kmCity Metropolitan Area Network (MAN) 100km 1000km Country Continent Wide Area Network (WAN) 10000kmPlanetThe Internet

14. ETM 555 14 Transmission Media Can be classified as: – Guided : waves are guided along a solid medium such as copper wire, optical fiber etc. – Unguided : wireless transmission, atmosphere, outer space, transmit electromagnetic signals, radio, laser etc. Design issues in transmission systems: – Data rate, distance, bandwidth, transmission impairments, interference, number of receivers etc.

15. ETM 555 15 Twisted Pair (guided trans. media) Oldest, least expensive, most widely used Telephones connected to telcos by twisted pair (subscriber loops) Can transmit both analog and digital signals: – For analog: amplifiers needed every 5 to 6 km – For digital: repeaters required every 2 to 3 km Limited distance, bandwidth compared to others

16. ETM 555 16 Twisted Pair (guided trans media) 2 kinds: 1) Unshielded (UTP, 10 Base T) is ordinary telephone wire, least expensive transmission media for LANs 2) Shielded: shield twisted pair with a metalic braid/sheating to reduce interference, more expensive UTP kinds: – Category 3: 3 or 4 twists per foot, (data rates: 16 Mbps) – Category 5: 3 to 4 twists per inch (data rates: 100 Mbps)

17. ETM 555 17 Coaxial Cable (guided trans. media) Can be used over longer distances and supports more stations on a shared line than twisted pair. Bandwidth possible depends on cable length. For 1 km cables 1-2 Gbps feasible. Widespread use: television distribution, long distance telephone transmission, short-run computer system, local area networks Varieties: thin, thick coaxial

18. ETM 555 18 Fiber Optics (guided trans. media) Physiscs: light ray is refracted (bent) when passing from one medium to another) Above a critical angle of incidence, light is refracted back into silica (hence trapped inside fiber) and can travel for many kilometers

19. ETM 555 19 Fiber Optics (guided trans. media) Lower attenuation Fibers thinner and lighter weight (considerably) Data Rates: 1 Gbs, 100 Gbps, 1 Tbps

20. ETM 555 20 Fiber Optics (guided trans. media) Electromagnetic isolation (i.e. not affected by external electromagnetic fields) no interference Difficult to tap, therefore better security Enables greater spacing between repeaters Multimode fiber: allows many different rays incident at different angles to travel Singlemode fiber: employs fiber core which has small distance (of the order of a wavelength) and allows only a single ray to pass. Singlemode fiber is more expensive but can be used for longer distances.

21. ETM 555 21 Global Wireless Network Standards IEEE 802.15 (Bluetooth) for the Personal Area Network (PAN) IEEE 802.11 (Wi-Fi) for the Local Area Network (LAN) IEEE 802.16 (WiMax) for the Metropolitan Area Network (MAN) IEEE 802.20 standard for the Mobile Broadband Wireless Access (MBWA)

22. ETM 555 22 Bluetooth Can link up to 8 devices in 10-m area Can transmit up to 722 Kbps in the 2.4 GhZ band Low power requirements Radio based communication

23. ETM 555 23 Bluetooth

24. ETM 555 24 Wi-Fi Three standards: – 802.11a: up to 54 Mbps (10 – 30 meters) – 802.11b : up to 11 Mbps (30 – 100 meters) – 802.11g : up to 54 Mbps (30 – 100 meters) Infrastructure mode: Devices use access point to communicate with wired network Ad-hoc mode (peer-to-peer): Wireless devices communicate directly with each other

25. ETM 555 25 Wireless LAN

26. ETM 555 26 The Telephone System Telephone was patented by Alexander Graham Bell, 1876. Evolution of the structure of telephone system: (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy.

27. ETM 555 27 The Telephone System Basic Bell system model remained essentially intact up to now. Presently, we have highly redundant multilevel hierarchy. Hierarchy: Home, End Office, Toll Office, Primary Office, Sectional Office, Regional Office Transmission media used: – Local loops consist of twisted pair – Between switching offices: coaxial cables, microwaves, fiber optics Transmission: – Local loops: analog signaling – Trunks: mostly digital

28. ETM 555 28 Structure of Telephone System

29. ETM 555 29 Local Loop Between home and the end office Modems can be used to connect computer at home Baud Rate: Signalling speed – the number of times per second that signal changes its value (i.e. its voltage) – b-baud line does not necessarily transmit b bits/sec – each signal may convey several bits – Example: if voltages 0,1,2,3,4,5,6,7 are used, each signal can convey 3 bits. So bit rate: 3 X baud rate DSL (Digital subscriber lines) : Use existing telephone lines to transmit signals. 500Kbps-1.5Mbps possible. – ADSL is asymmetric DSL : download and upload speeds are different. Upload speed is slower than download speed.

30. ETM 555 30 Trunks and Multiplexing Cost of installing and maintaining both high-bandwidth and low bandwidth trunk the same (essentially) Therefore, choose high-bandwidth trunk installation To efficiently use high bandwidth, multiplex many conversations over a single trunk Multiplexing schemes: – Frequency Division Multiplexing – Wavelength Division Multiplexing – Time Division Multiplexing

31. ETM 555 31 Example of Time Division Multiplexing T1 Carrier (1.544 Mbps): – 24 voice channels multiplexed together – Analog signals are sampled on a round robin basis

32. ETM 555 32 Multiplexing T1 streams into higher carriers

33. ETM 555 33 Layered Network Software Architecture Model

34. ETM 555 34 Protocol Hierarchies Design issues for the layers: addressing, error control, flow control, multiplexing, routing

35. ETM 555 35 OSI Reference Model Network architecture model based on a proposal developed by International Standards Organization (ISO) OSI: Open Systems Interconnection Standardized protocols for network layers OSI has 7 layers

36. ETM 555 36 OSI Layers LayerResponsibility 1) Physical Layer Transmission of raw bits over a communication channel 2) Data Link Layer Data Link Control MAC Sublayer - Provide an error free communication link. Issues: framing, addressing - Medium Access Control needed by multiaccess communications. MAC provides DLC with virtual wires on multiaccess networks 3) Network Layer - Path selection between end-systems (routing) - Subnet flow control - Translation between different network types 4) Transport layer - Provides virtual end-to-end links between peer processes. Fragmentation and reassembly. End-to-end flow control 5) Session Layer - Establishes, manages and terminates sessions between applications. Service location lookup. 6) Presentation Layer- Data encryption, data compression, data conversion 7) Application Layer- Anything not provided by other layers

37. ETM 555 37 TCP/IP Reference Model First used in ARPANET (Advanced Research Projects Agency) sponsored by US Department of Defense Now used in worldwide Internet Goal: ability to connect multiple networks in a transparent way Requirements: Connections should remain intact as long as source and destination machines are functioning even if some of the intermediate machines, links are out of operation Internetwork Layer (Internet) : Packet switching network based on a connectionless internetwork layer.

38. ETM 555 38 TCP/IP Reference Model

39. ETM 555 39 Protocols and networks in the TCP/IP model

40. ETM 555 40 Why OSI Model/Protocols were not implemented Bad timing Bad technology Bad implementations Bad politics