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Computer Networks N Amanquah, PhD What is a Network? • A connection between two nodes for the purpose of electronic data exchange • Same medium, independent hosts -Tanenbaum Usage • • • • • • • • Communication Information Access/dissemination Entertainment & games E-commerce Education Social networking Resource sharing Illicit- eg hacking, snooping, gambling etc Types of networks • By Size and Scale – PAN, LAN, WAN, MAN, the internet • Media type – copper, fibre, wireless • Topology – Star, bus, ring, mesh A basic network Wireless: BT, IrAD, Wifi etc ` serial, parallel, twisted pair coax Media: 3 types: copper, fibre, wireless Media • Copper – UTP – Coax • Fibre • Wireless • **Connectors Not so basic ` topologies Topologies Ultimate network • The internet • *pix – cabinet, satellite, dish, microwave links, etc Network nodes • 3 or more nodes: – Hub, Switch • Integrate networks – Routers, bridges, repeaters, gateways • Transmission lines From here B1 & back From here to to Google & back Components of network • Transmission lines – Transmission mechanisms • Store and Forward (excludes satellites) • Circuits & Switching elements – Packet switched vs circuit switched – Subnets – routers Count down to transmission ….….Between two very different hosts • 7. Need a program –an application – Eg FTP, email, Telnet, http(web) • 6. syntax and semantics, presentation – Unix, Mac, Windows are different – Big/little endian IBM: Extended binary coded decimal interchange code – ASCII and EBCDIC, Unicode Count down …II • 5. May want to log the session for future review, or to resume the session. • 4 End-to-end connection management – – – – – – Connection setup Must be able to stream, distinguish traffic sessions Guarantee QoS, Fragmentation, Error free end to end in-order delivery • Also broadcasting, non guaranteed • 3. Route to the destination (next hop), – addressing – Control/avoid congestion, – communication with other networks (heterogeneity) – Datagram segmentation Count down ..III • 2a. Further segmentation and reassembly – Error detection • Need to be able to able to detect errors that may occur on the imperfect transmission line – Flow control • Avoid swamping a slow receiver – Framing • Determine where one frame/chunk of data starts and ends (be able to distinguish 000 from a transmission line gone dead) Count down …IV • 2b Medium access control – With more than two hosts, need some arbitration for transmission. • 1. Electronic Transmission – Convert the data (in 01010) to electrical format and transmit – Modulation etc : electrical, mechanical, timing Reference Model Have enough layers with distinct functionality Well defined functionality Minimize information flow across layers Internet Model Network Architecture • “Set of layers and protocols” • Services – Operations a layer should perform • Protocols – rules/agreements on format of packets • Protocol Stack – Set of protocols used by one system Layers • Communication takes place between peer layers – Virtual comms: eg bt layer 3 and layer 3 – Actual comms: down/up the stack • An interface is between two layers • Layers perform clearly defined task – Are replaceable – Each layer provides a service to the one above – Abstracts layers below Design issues • Broadcast networks – single communication channel shared by all – May use broadcast address every to host – Multicasting to subset • Point to Point network (unicasting) Design issues • Duplexing: – Simplex • One way only – (broadcasting is simplex!) – Half duplex • one way at a time – Full duplex • simultaneously Design issues • Error control: – Error correcting codes – Acknowledgements • *Packet sequencing • *Flow control • Multiplexing & de-multiplexing – Same connection for multiple communications • Connection oriented service – Establish connection, transmit, tear down connection. – Sequencing is usually preserved. • Connectionless – Each packet carries destination information and is routed independently. Reliability & QoS • Acknowledgement – overhead & delay, vs reliability • Negotiation for QoS parameters • **Un/Reliable datagram service Wireless networks • Infrastructure – APs • Infrastructure-less – Peer to peer, – Ad hoc Network standards • • • • • • IEEE 802.3 – Ethernet IEEE 802.4 –token bus IEEE 802.5 –token ring -IBM IEEE 802.11 –wireless LAN IEEE 802.15 –wireless PAN IEEE 802.16-wirleess WAN Example networks • The Internet is a network of networks • ARPANET- Advanced Research Projects Agency Network (DoD) – Precursor to Internet. A resilient network to survive a knock out of network hosts • NSFNET – university research Example Networks • Connection oriented – X.25 – Frame relay, • For interconnecting LANS • in-order delivery, no flow control – ATM • • • • • Used by telcos Move data eg IP over virtual circuits @ high speed 53byte cell, with 5 byte header. Inorder delivery Small overhead. NO error control Example networks • Ethernet – Developed from ALOHANET – Uses coax, or UTP • Wifi Readings • Andrew Tanenbaum – Sections 1.0 to 1.9