Computer Communications and Networks: Introduction - PDF

1502346 - Computer Communications and Networks Dr. Mohamed Saad Department of Computer Engineering University of Sharjah [email protected] Chapter 1: Introduction ...

1502346 - Computer Communications and Networks Dr. Mohamed Saad Department of Computer Engineering University of Sharjah [email protected] Chapter 1: Introduction  M. Saad Introduction 20th and 21st centuries: dominated by the technology of information gathering, processing and distribution. Rapid technological progress: fields of computers and communications. Computers: – Previously: centralized, large computers, serving many users (mainframe). – Now: computers are more powerful and much smaller (Moore’s law: # of transistors on an IC doubles every 18 months at same cost per IC, or cost per transistor on an IC reduces by a factor of 2 every 18 months). 1502346 - Computer Communications & Networks 1  M. Saad Introduction (contd.) Communications: – Handling data, voice and video, from analog to digital. – High speed communication links (optical fibers), wireless communications (modulation, detection, estimation) 1502346 - Computer Communications & Networks 2  M. Saad Computer Networks Result of the merging of computers and communications. Old model of a single computer serving all users of an organization (e.g., company, university) replaced by one in which a large number of separate but interconnected computers do the job. Computer networks (throughout the course): collection of autonomous computers interconnected by a communication medium. 1502346 - Computer Communications & Networks 3  M. Saad Use of computer Networks (Applications) Business applications (resource sharing: programs, data bases, files, printers,... etc, available to anyone on the network.) Client Server Network In the above example, servers may store S/W (programs, files, records) or may be connected to H/W (printers, scanners, CD burners) that can be used by all clients on the network (client-server model). 1502346 - Computer Communications & Networks 4  M. Saad Applications (contd.) Home applications: e-mails, instant messaging (like Twitter), streaming multimedia, gaming, peer-to-peer communications (for file sharing, like Gnutella and BitTorrent), social network applications (like Facebook), e-commerce (online banking, online shopping) (peer-to-peer network example) 1502346 - Computer Communications & Networks 5  M. Saad Network Hardware Two types of transmission technology: – broadcast links: single communication channel that is shared by multiple machines. – point-to-point links: communication channel connecting only two machines. This leads to the classification of network based on their topology (structure)... 1502346 - Computer Communications & Networks 6  M. Saad Network Classification based on Topology Broadcast networks: – Have a single communication channel that is shared by all the machines on the network. – Example topologies: bus - ring. Computer Cable Computer (a) (b) – “Messages” sent by any machine are received by all others. – Some broadcast system support the transmission from one machine to a subset of the other machines → multicasting 1502346 - Computer Communications & Networks 7  M. Saad Network Classification based on Topology (contd.) Point-to-point networks: – Consists of many point-to-point links that connect individual pairs of machines. – Not every pair of machines is necessarily connected by a point-to-point link. – To go from the source to the destination, a “message” may need to visit one or more intermediate machines. Router Subnet Sending host Receiving host B D A E C Packet Router C makes a Sending process Receiving process choice to forward packets to E and not to D – Example topologies: star - mesh- fully connected. 1502346 - Computer Communications & Networks 8  M. Saad Network Classification based on Scale Local Area Networks (LAN): – Covers a localized area, e.g., office building, university campus, up to a few km. – Usually (not always) LANs have broadcast topologies. Computer Cable Computer (a) (b) – Main issue: how to handle multiple access/avoid transmission conflicts – Static channel allocation: e.g., divide time into slots and use round robin to allow each machine to transmit in its slot only (TDMA) (capacity-inefficient when a machine has nothing to send during its allocated slot). – Dynamic channel allocation: centralized (central controller decides who transmits next) or decentralized (each machine decides when to transmit) 1502346 - Computer Communications & Networks 9  M. Saad LAN Examples Classic Ethernet: developed by Xerox researcher (Bob Metcalfe) in 1976, has a bus topology Transceiver Interface cable Ether 1502346 - Computer Communications & Networks 10  M. Saad LAN Examples (contd.) Wireless LAN (using IEEE 802.11) and switched Ethernet, respectively. – Wireless LANs have a broadcast topology. – Switched Ethernet is a wired LAN built from point-to-point links. 1502346 - Computer Communications & Networks 11  M. Saad Network Classification based on Scale (contd.) Metropolitan area networks (MAN): – Covers a city (10’s of km’s). – Best-known example is the cable TV network available in many cities: Junction box Antenna Head end Internet 1502346 - Computer Communications & Networks 12  M. Saad Network Classification based on Scale (contd.) Wide area networks (WAN): – Covers a country or continent (100’s or 1000’s of km’s). – A WAN that connects hosts in Perth, Melbourne and Brisbane (Australia). 1502346 - Computer Communications & Networks 13  M. Saad WANs (contd.) Hosts: users of the network that run the application programs. (Communication) subnet: responsible for delivering “messages” from host to host. Switching elements: specialized computers in the subnet (also called routers; the name router is now most commonly used). Transmission lines/links: move information between machines. 1502346 - Computer Communications & Networks 14  M. Saad Internetworks Many networks exist with different H/W and S/W. People connected to one network may want to communicate with people attached to a different one. Solution → connect different, possibly incompatible networks by means of machines called gateways. Example: multiple LANs connected through a WAN. A collection of interconnected networks is known as internetwork, example: the Internet. 1502346 - Computer Communications & Networks 15  M. Saad Switching Circuit switching (originally for telephone networks): – to establish a communication session (e.g., phone call), a circuit (path) is established and remains dedicated for the session throughout the session. Packet switching: – communication “messages” are divided into smaller units (called packets) – packets are transmitted individually along (possibly) different routes in the network (no dedicated/reserved paths) – when a packet arrives at a switching element, it is stored then forwarded along a certain route. – subnets using packet switching are called store-and-forward subnets. 1502346 - Computer Communications & Networks 16  M. Saad Switching (contd.) Router Subnet Sending host Receiving host B D A E C Packet Router C makes a Sending process Receiving process choice to forward packets to E and not to D Note: in packet switching packets belonging to the same session may take different paths (example: future packets may take path A-C-D-E) Message switching: same as packet switching, communication units may bigger than packets (called messages). 1502346 - Computer Communications & Networks 17  M. Saad Network Software Host 1 Host 2 Layer 5 protocol Layer 5 Layer 5 Layer 4/5 interface Layer 4 protocol Layer 4 Layer 4 Layer 3/4 interface Layer 3 protocol Layer 3 Layer 3 Layer 2/3 interface Layer 2 protocol Layer 2 Layer 2 Layer 1/2 interface Layer 1 protocol Layer 1 Layer 1 Physical medium 1502346 - Computer Communications & Networks 18  M. Saad Layered Network Architecture Network functionality is organized as a stack of layers. Each layer performs a certain communication service to the higher layers (shielding those other layers from how the offered services are actually implemented). Protocol: set of rules that govern the exchange of information between peer layers (i.e., layer n on one machine and layer n on another machine). Interface: facilitates communication between successive layers on the same machine (facilitates one layer to provide a service to the next layer). Set of layers, protocols and interfaces is known as network architecture. 1502346 - Computer Communications & Networks 19  M. Saad Layer Example Layer 5 protocol 5 M M Layer 4 protocol 4 H4 M H4 M Layer 3 protocol 3 H3 H4 M1 H3 M2 H3 H4 M1 H3 M2 Layer 2 protocol 2 H2 H3 H4 M1 T2 H2 H3 M2 T2 H2 H3 H4 M1 T2 H2 H3 M2 T2 1 Source machine Destination machine Message M is generated by an application running in layer 5 of the source machine. M given to layer 4 for transmission. Layer 4 adds a header (that may may contain message sequence information) and passes the result to layer 3. Layer 3 breaks message M into smaller packets, and adds a layer 3 header (that may contain a packet sequence information) to each packet. 1502346 - Computer Communications & Networks 20  M. Saad Layer Example Layer 5 protocol 5 M M Layer 4 protocol 4 H4 M H4 M Layer 3 protocol 3 H3 H4 M1 H3 M2 H3 H4 M1 H3 M2 Layer 2 protocol 2 H2 H3 H4 M1 T2 H2 H3 M2 T2 H2 H3 H4 M1 T2 H2 H3 M2 T2 1 Source machine Destination machine Layer 3 decides the outgoing link for each packet and passes the packets to layer 2. Layer 2 adds a header and a trailer to each packet and gives the resulting unit to layer 1 for physical transmission. At the receiving machine, the message moves upwards from layer to layer with headers (and trailers) stripped off as it progresses. 1502346 - Computer Communications & Networks 21  M. Saad Connection-Oriented and Connectionless services Layers may provide two types of services: connection-oriented and connectionless services. Connection-oriented service: – service user (1) establishes a connection, (2) uses the connection then (3) releases the connection (similar to telephone system). – Negotiation between sender and receiver may be required before connection establishment. – Usually messages/packets arrive in order. Connectionless service: – each mesage/packet carries full destination address information and is routed through the network independent of all others. – Messages/packets may arrive out of order. 1502346 - Computer Communications & Networks 22  M. Saad The OSI Reference Model Layer Name of unit exchanged Application protocol 7 Application Application APDU Interface Presentation protocol 6 Presentation Presentation PPDU Session protocol 5 Session Session SPDU Transport protocol 4 Transport Transport TPDU Communication subnet boundary Internal subnet protocol 3 Network Network Network Network Packet 2 Data link Data link Data link Data link Frame 1 Physical Physical Physical Physical Bit Host A Router Router Host B Network layer host-router protocol Data link layer host-router protocol Physical layer host-router protocol Open Systems Interconnections (OSI) model developed by the International Standards Organization (ISO) 1502346 - Computer Communications & Networks 23  M. Saad Physical Layer Is concerned with transmitting raw bits over a communication channel. Design issues: transmitted 1 is received as a 1 and not a 0 (modulation, demodulation,... etc.). 1502346 - Computer Communications & Networks 24  M. Saad Data Link Layer Converts an unreliable raw bit pipe into a communication channel for transmitting frames error free (reliably) – Performs error detection, and error correction. – May perform flow control (to avoid a fast transmitter from overwhelming a slow receiver) – In broadcast networks: a special sublayer of the data link layer (called the medium access control (MAC) sublayer) handles the issue of multiple access. 1502346 - Computer Communications & Networks 25  M. Saad Network Layer Performs routing as to provide a virtual channel for end-to-end packets (static routing - dynamic routing). Congestion control to avoid link bottle necks. In broadcast networks routing is simple or non-existing. So, the network layer is often thin or non-existing. 1502346 - Computer Communications & Networks 26  M. Saad Transport Layer Breaks larger “messages” into smaller packets at T x side, and assembles packets into larger “messages” at the Rx side. Corrects out of order delivery and lost packets. Performs end-to-end flow and congestion control. May perform end-to-end error control. 1502346 - Computer Communications & Networks 27  M. Saad Session Layer Allows users on different machines to establish sessions between them 1502346 - Computer Communications & Networks 28  M. Saad Presentation Layer Concerned with the syntax and semantics of the information transmitted. Performs: – Encryption – data compression – File format transfer 1502346 - Computer Communications & Networks 29  M. Saad Application Layer The application program of the end user 1502346 - Computer Communications & Networks 30  M. Saad The TCP/IP Reference Model Used in the grandparent of all computer networks, the ARPANET (Advanced Research Projects Agency) & it successor the Internet. 1502346 - Computer Communications & Networks 31  M. Saad The Internet Layer Connection-less Allows hosts to inject packets into the network. Packets are routed independently to their destination. Packets may arrive out of order. Internet layer defines the packet format, and a protocol called the Internet Protocol (IP). Internet layer is very similar to the network layer in the OSI model. 1502346 - Computer Communications & Networks 32  M. Saad The Transport Layer Allows source and destination hosts to carry on a conversation (just as the OSI transport layer). Is an end-to-end layer. Two end-to-end protocols have been defined: 1. TCP (Transport Control Protocol): is a reliable connection-oriented protocol (fragments byte streams into discrete messages, passes each message to the Internet layer, handles flow/congestion control). 2. UDP (User Datagram Protocol): is an unreliable, connectionless protocol for applications that do not wish for TCP’s sequencing or flow control, is useful for applications in which prompt delivery is more important that accurate delivery (e.g., voice and video). 1502346 - Computer Communications & Networks 33  M. Saad The Application Layer Contains all the higher-level protocols: – File transfer (FTP) – E-mail (SMTP, Simple Mail Transfer Protocol) – Domain Name System (DNS) for mapping host names onto their network addresses – HTTP (the protocol for creating pages on the World Wide Web) 1502346 - Computer Communications & Networks 34  M. Saad The Link Layer Describes what links such as serial lines and Ethernet must do to meet the needs of the connectionless Internet layer. It is not really a layer at all, in the normal sense of the term, but rather an interface between hosts and transmission links (allows hosts to inject packets into the network). Early material on the TCP/IP model has little to say about it. 1502346 - Computer Communications & Networks 35  M. Saad Example Network: ARPANET In the late 50’s US military communications used the public telephone network. US Dept. of Defence wanted a network that could survive failures. Switching office Toll office (a) (b) ARPA (Advanced Research Projects Agency) was created to solve the issue. 1502346 - Computer Communications & Networks 36  M. Saad Original ARPANET Design Host-host protocol Host Host-IMP protocol ol n IMP protoc to destinatio Source IMP -IMP IMP-IMP protocol IMP col o prot Subnet IMP The subnet consists of IMP’s (Interface Message Processors) IMP’s connected by 56 Kbps transmission lines (leased lines from telephone companies). Any IMP connected to at least 2 other IMP’s (for reliability). 1502346 - Computer Communications & Networks 37  M. Saad Growth of the ARPANET SRI UTAH SRI UTAH MIT SRI UTAH ILLINOIS MIT LINCOLN CASE UCSB UCSB SDC UCSB SDC CARN STAN UCLA UCLA RAND BBN UCLA RAND BBN HARVARD BURROUGHS (a) (b) (c) SRI LBL MCCLELLAN UTAH ILLINOIS MIT CCA MCCLELLAN AMES TIP BBN SRI UTAH NCAR GWC LINCOLN CASE HARVARD AMES IMP LINC X-PARC ABERDEEN RADC ILLINOIS STANFORD NBS CARN AMES USC LINC ETAC UCSB MITRE FNWC RAND MIT TINKER STAN SDC ARPA ETAC MITRE RADC UCSB UCSD SAAC UCLA RAND TINKER BBN HARVARD NBS BELVOIR CMU UCLA SDC USC NOAA GWC CASE (d) (e) (a) Dec. 1969 (b) July 1970 (c) March 1971 (d) Sept. 1972 UCLA, UCSB, SRI (Stanford Research Institute) and the University of Utah had a large number of ARPA contracts. 1502346 - Computer Communications & Networks 38  M. Saad NSFNET By the late 1970s, NSF (the U.S. national Science Foundation) saw the enormous impact of the ARPANET was having on university research. However, to to get on the ARPANET, a university had to have a research contract with the Dept. of Defense. NSF’s initial response was to fund the Computer Science Network (CSNET) in 1981. It connected Computer Science departments and industrial research labs to the ARPANET. In the late 1980s, the NSF went further and decided to design a successor to the ARPANET: the NSFNET. The NSFNET consisted of a backbone and various regional networks, which allowed users at thousands of universities, research labs, libraries and museums to be connected. 1502346 - Computer Communications & Networks 39  M. Saad The NSFNET Backbone in 1988 NSF Supercomputer center NSF Midlevel network Both 1502346 - Computer Communications & Networks 40  M. Saad The Internet Today Regional ISP Backbone POP NAP Client Telephone Server farm system Corporate LAN Router 1502346 - Computer Communications & Networks 41  M. Saad The Internet Today (contd.) Client at home calls his/her ISP over a dial-up telephone line. Modem → card within the PC that converts the digital signals (produced by the PC) into analog signals that can be sent over the telephone system. These signals are transferred to the ISP’s Point of Presence (POP), where they are removed from the telephone system and injected in the ISP’s regional network. ISP’s regional network → interconnected routers in the various cities the ISP serves. – If the packet is destined for a host served directly by the ISP, the packet is delivered to the host. Otherwise, the packet is handed to the ISP’s backbone operator. Backbone operators → companies like AT&T and Sprint. They operate large international backbone networks with thousands of routers connected by high-bandwidth optical fibers. Different backbones are connected by Network Access Points (NAPs). NAP → typically a room full of routers (at least one per backbone). A LAN connects these routers, so that a packet can be forwarded from any backbone to any other backbone. 1502346 - Computer Communications & Networks 42  M. Saad Thanks 1502346 - Computer Communications & Networks 43