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CIS-532 Lecture 1 Summer of 2004 1 CIS-532 Lecture 1 Summer of 2004 1

Communication Network • Collection of computers that are both autonomous and interconnected • Differs Communication Network • Collection of computers that are both autonomous and interconnected • Differs from distributed computer system – in distributed system, presence of multiple processors is made transparent to user whereas in network knowledge of the multiple processors is fundamental to user 2

Digital Versus Analog • Classification applies to data, signaling, and transmission • Data is Digital Versus Analog • Classification applies to data, signaling, and transmission • Data is “information content” • Examples of analog data – acoustic wave amplitude as function of time for voice – brightness of each pixel as function of time for video 3

Digital Data • Examples of digital data – binary data – sequence of alphanumeric Digital Data • Examples of digital data – binary data – sequence of alphanumeric characters (e-mail) 4

Signaling • Electromagnetic waves used to carry data through channel • Analog signaling varies Signaling • Electromagnetic waves used to carry data through channel • Analog signaling varies continuously with time and takes continuum of values • Digital signaling has fixed waveforms that represent the bits 0 and 1 5

Analog Signaling • Analog signaling can be used for analog data – by using Analog Signaling • Analog signaling can be used for analog data – by using same waveform as data – by modulation • Analog signaling can be used for digital data by modulation (modem) 6

Digital Signaling • Can be used for digital data – by direct encoding (e. Digital Signaling • Can be used for digital data – by direct encoding (e. g. , NRZ code) – by more complicated encoding • Can be used for analog data – sampling – quantization 7

Analog Transmission • Analog transmission means amplifiers used – Amplification of noise is cumulative Analog Transmission • Analog transmission means amplifiers used – Amplification of noise is cumulative – Only used for analog signals – Some degradation of signal must be acceptable • e. g. , local subscriber loop for telephone 8

Digital Transmission • Repeaters used instead of amplifiers – bits are determined and signal Digital Transmission • Repeaters used instead of amplifiers – bits are determined and signal regenerated at each repeater – effect of noise eliminated at each stage unless bit inversions – always used when signaling is digital – can be used for analog signaling provided analog signal encodes digital data 9

Digitization • Nyquist Sampling Theorem – Analog signal is uniquely determined by its samples Digitization • Nyquist Sampling Theorem – Analog signal is uniquely determined by its samples taken at twice its maximum frequency • this “direction” is relevant for digital encoding of analog data • If 2^^N quantization levels, need N bits per sample • Quantization error can be regarded as noise – SNR due to quantization is approx. 6 X N d. B where N is number of bits used to represent each sample 10

Example: Digitized Voice • Voice grade telephone channel transmits frequencies up to 4 k. Example: Digitized Voice • Voice grade telephone channel transmits frequencies up to 4 k. Hz – To convert to digital signal, sampling at 8 k. Hz is needed – For telephone, SNR of 48 d. B is needed. • So 8 bits per sample are required • Generates digital bit stream at 64 kbps (PCM channel) 11

Analog signaling for digital data – The values of an analog signal required to Analog signaling for digital data – The values of an analog signal required to have maximum frequency W may be arbitrarily specified every 1/(2 W) sec-- 2 W samples/sec can be specified • this “direction” relates to analog signaling for digital data • data rate depends on number of bits per sample – In QAM, signal constellation in plane determines number of bits encoded per sample – Shannon’s Theorem gives upper bound on capacity (depends on SNR) 12

Evolution of Telephone Networks • Circuit switching as opposed to dedicated circuits – Switching Evolution of Telephone Networks • Circuit switching as opposed to dedicated circuits – Switching introduces economy of scale since traffic for many source/destination pairs can be routed over high-capacity trunks – Switching originally by operators, then automated mechanical, now electronic 13

Telephone Network Evol. (cont) • Common channel signaling (CCS) – data network used by Telephone Network Evol. (cont) • Common channel signaling (CCS) – data network used by switches to exchange control information – Separates call control from transfer of voice – Together with programmable switches, permits value-added services (e. g. , call waiting, call forwarding) 14

Telephone Network Evol. (cont) • Since 1980’s, transmission changing to SONET (Synchronous Optical Network) Telephone Network Evol. (cont) • Since 1980’s, transmission changing to SONET (Synchronous Optical Network) – Basic STS-1 signal has rate of 51. 840 Mbps • ISDN: digital subscriber loops and service integration. – Basic access is 2 B + D • B channel is full-duplex 64 kbps. Suitable for circuit -switched connection, connection to packet-switched network, or permanent digital connection • D channel is 16 kbps packet-switched 15

Broadband ISDN • Integration of voice, video, data in high speed network – ATM Broadband ISDN • Integration of voice, video, data in high speed network – ATM running over SONET 16

Computer (data) Networks • Organization of data in packets – requires control bits (headers Computer (data) Networks • Organization of data in packets – requires control bits (headers and trailers) • Packet switching (store and forward) – allows link bandwidth shared on as-needed basis – superior to FDM and TDM for bursty traffic 17

Internet Protocol Hierarchy • IP and (TCP/IP) can run over many physical networks (e. Internet Protocol Hierarchy • IP and (TCP/IP) can run over many physical networks (e. g. , Ethernet, Token Ring, ATM) – allows interconnection of heterogeneous networks – presents uniform interface to applications • allows development of applications that are independent of physical network 18

Multiple Access Techniques • Common channel shared by multiple stations • Medium access control Multiple Access Techniques • Common channel shared by multiple stations • Medium access control (MAC) required to transform shared channel into virtual intermittent point-to-point link • LAN standards include Ethernet, Token Bus, Token Ring 19

Multiaccess (cont) • MAN standards – Fiber distributed data interface (FDDI) • similar to Multiaccess (cont) • MAN standards – Fiber distributed data interface (FDDI) • similar to token ring but faster (100 Mbps) • has timed-token mechanism that can transmit realtime traffic (voice or video) with guaranteed delay – Distributed Queue Dual Bus (DQDB) 20

Cable Television Networks • Current CATV uses FDM – 69 analog TV channels, each Cable Television Networks • Current CATV uses FDM – 69 analog TV channels, each 4. 5 MHz wide – Transmission over coaxial cable arranged as unidirectional tree • Fiber to curb (with cable to individual subscriber) can increase BW and decrease attenuation 21

CATV continued • Migration to digital transmission is occurring to increase number of channels CATV continued • Migration to digital transmission is occurring to increase number of channels • Future developments include creating LANs for subscribers to use to send reverse traffic (e. g. , requests for movies) 22

Service Integration • Example: Asynchronous transfer mode (ATM) networks – Runs over a physical Service Integration • Example: Asynchronous transfer mode (ATM) networks – Runs over a physical layer such as SONET – 53 -byte cells transmitted over virtual circuits – Different connections can be allocated different amounts of resources (bandwidth and buffers) – User and network negotiate contract that specifies user traffic characteristics and network guaranteed Qo. S – Accomodates both real-time and nonreal-time traffic 23

Economic Issues • Economies of scale – Due to fixed network management costs as Economic Issues • Economies of scale – Due to fixed network management costs as well as fact that cost increases slower than linearly with data rate, cost per user decreases as number of users increases • Network externalities – Value of network service to user increases as number of users increases 24

Economics cont. • Due to economies of scale and network externalities, there is “critical Economics cont. • Due to economies of scale and network externalities, there is “critical number” of users – below critical number, subsidy is required • Service integration – combining services in single network (e. g. , BISDN) reduces cost of each service due to shared infrastructure 25

Network Structure • Point-to-point versus broadcast – in point-to-point, each link connects pair of Network Structure • Point-to-point versus broadcast – in point-to-point, each link connects pair of nodes – in broadcast, all nodes share common link (channel). All users receive each packet sent but only those for whom it is addressed retain it – more generally, network may support multicasting 26

Local Area Networks (LANs) • Typically use broadcast link(s) – require multiaccess algorithms • Local Area Networks (LANs) • Typically use broadcast link(s) – require multiaccess algorithms • Maximum distance between hosts is limited – implies upper bound for propagation delay-which is important to multiaccess algorithms • Examples: Ethernet, Token ring, Token bus • MANs are similar to LANs but larger area 27

Wide Area Networks (WANs) • End systems (hosts) are interconnected via communication subnet • Wide Area Networks (WANs) • End systems (hosts) are interconnected via communication subnet • Subnet consists of switching nodes (routers) connected by transmission lines (links) • May be packet switched or circuit switched 28

Wireless Networks • Radio, packet radio, microwave, satellite • May or may not involve Wireless Networks • Radio, packet radio, microwave, satellite • May or may not involve host mobility and time-varying network topology – Example: cellular radio systems – Example: wireless LANs 29

Hybrid Networks • Both wireless and wireline components – Example: satellite-fiber networks – Example: Hybrid Networks • Both wireless and wireline components – Example: satellite-fiber networks – Example: wired LAN on aircraft with flying router having wireless connection to terrestrial network 30

Messages • Message is a single unit of communication in sense that it is Messages • Message is a single unit of communication in sense that it is useful to recipient only if completely delivered – Example: file in file transfer system – Example: image in image transfer system – Example: one line of symbols in interactive terminal session 31

Messages, cont. • Concept of messages not very useful for voice or video – Messages, cont. • Concept of messages not very useful for voice or video – flow model corresponding to stream of bits is more appropriate – stream may be CBR or VBR 32

Packets • Messages are broken up into units of manageable size called packets – Packets • Messages are broken up into units of manageable size called packets – packets are transmitted as strings of bits together with additional control bits – control bits may indicate addresses, offsets, etc. – if packets have constant length, then called cells • Before being broken into packets, messages may be transformed for purpose of data compression and/or encryption 33

Connectionless Versus Connection-Oriented Service • Services provided by a layer may be connectionless or Connectionless Versus Connection-Oriented Service • Services provided by a layer may be connectionless or connection-oriented – for transport layer refers to messages – for network layer refers to packet 34

Connectionless Service • Messages (or packets) are independent of each other – analogous to Connectionless Service • Messages (or packets) are independent of each other – analogous to postal service – order of messages (packets) need not be preserved – generally not reliable, but may be made reliable through use of acknowledgements • analogous to certified return-receipt postal service 35

Connection-Oriented Service • Messages (packets) are part of a connection set up (and later Connection-Oriented Service • Messages (packets) are part of a connection set up (and later terminated) between communicating hosts – Messages are delivered in order – Service is generally reliable: no duplication or omission of messages – Analogous to telephone service 36

Characteristics of Traffic • • • Traffic arrival rate and variability Connection duration Distribution Characteristics of Traffic • • • Traffic arrival rate and variability Connection duration Distribution of message length Allowable delay and variability of delay Required reliability 37

Examples of Traffic Types • Interactive terminal-to-computer sessions – low message rate – message Examples of Traffic Types • Interactive terminal-to-computer sessions – low message rate – message length short – delay requirement moderately strict – required reliability high 38

Traffic Examples, cont. • File transfer sessions – message rate low – message length Traffic Examples, cont. • File transfer sessions – message rate low – message length very long – delay requirement very relaxed – required reliability very high 39

Traffic Examples, cont. • Packetized voice – concept of message not applicable – bit Traffic Examples, cont. • Packetized voice – concept of message not applicable – bit arrival rate moderate – delay requirement stringent (especially jitter) – required reliability low 40

Circuit Switching • When session is set up, path is chosen and bandwidth allocated Circuit Switching • When session is set up, path is chosen and bandwidth allocated on each link (by FDM or TDM). – If no path with sufficient BW, call is rejected – Advantage: once call is accepted, BW is guaranteed; no queuing – Disadvantage: inefficient utilization of transmission capacity if traffic is bursty 41

Packet Switching • Store and forward • Statistical multiplexing – No fixed allocation of Packet Switching • Store and forward • Statistical multiplexing – No fixed allocation of BW – Packets from different sessions combined into single queue for each outgoing link – Full transmission capacity of link dedicated to single packet • Advantage: full utilization of link capacity whenever traffic is present 42

Connectionless versus Connection-Oriented Routing • Virtual circuit routing – connection-oriented – fixed path (but Connectionless versus Connection-Oriented Routing • Virtual circuit routing – connection-oriented – fixed path (but not fixed BW) assigned at start of session; all packets follow same path – Example: ATM • Datagram routing – packets in session are routed independently – Example: IP 43