Understanding Communication Networks in Digital Era

1. CIS-532 Lecture 1 Summer of 2004

2. 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

3. 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

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

5. 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

6. 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)

7. 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

8. 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

9. 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

10. 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 dB where N is number of bits used to represent each sample

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

12. Analog signaling for digital data • The values of an analog signal required to have maximum frequency W may be arbitrarily specified every 1/(2W) sec-- 2W 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)

13. 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

14. 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)

15. 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 2B + 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

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

17. 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

18. 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

19. 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

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

21. 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

22. 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)

23. 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 QoS • Accomodates both real-time and nonreal-time traffic

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

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

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

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

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

41. 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

42. 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

43. 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