Chapter 2. Bandwidth-Limited Signals (2) The Theoretical Basis for Data Communication

Transcription

1 Chapter 2 The Physical Layer The Theoretical Basis for Data Communication Fourier Analysis Bandwidth-Limited Signals Maximum Data Rate of a Channel Bandwidth-Limited Signals Bandwidth-Limited Signals (2) A binary signal and its root-mean-square Fourier amplitudes. (b) (c) Successive approximations to the original signal. (d) (e) Successive approximations to the original signal.

2 *Bandwidth-Limited Signals (3) a) For a telephone wire may have 1 MHz but telephone companies add a filter restricting each customer to about 3100 Hz. b) No hope at all for binary signals to send at data rates much higher than 38.4kbps even if the transmission facility is completely noiseless. (no Harmonics sent) c) Nyquist s theorem: max. data rate = 2Hlog 2 V bits/sec (H: LPF BW, V: discrete levels) d) Shannon Limit: max. data rate = H log 2 (1+S/N) Relation between data rate and harmonics. Guided Transmission Data *Twisted Pair Magnetic Media Twisted Pair Coaxial Cable Fiber Optics (a) Category 3 UTP. (b) Category 5 UTP.

3 Twisted Pair Coaxial Cable a) Twisting is done because two parallel wires constitute a fine antenna. b) Can run several kms without amplification. For longer distance, TP needs repeaters. c) C5: more twists t per cm, less crosstalk and a better-quality signal over longer distances. d) C3 (16MHz), C5(100MHz), C6(250MHz), C7(600MHz) e) UTP (unshielded TP) A coaxial cable.(1ghz BW) *Fiber Optics Fiber Optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection. a) In the race betw. Computing and communication, the latter won. b) PCs run at 2 GHz ( MHz): a gain of a factor of 20 per decade. c) 56kbps to 1 Gbps: a gain of a factor of 125 per decade; available BW in a fiber: Gbps (50 Tbps);the current practical signaling limit of about 10 Gbps is due to our inability to convert betw. electrical and optical signals any faster. In lab, 100 Gbps has been achieved on a single fiber. d) Single mode ( diameter is reduced to a few wavelengths of light, like a wave guide; the light can propagate only in a straight line)-> 50Gbps 100km without amplification. e) Multi-mode fiber

4 Transmission of Light through Fiber Fiber Cables Attenuation of light through fiber in the infrared region. note: Optical trans. System: light source, transmission medium, detector A pulse of light 1 ; absence of light 0 (a) Side view of a single fiber. (b) End view of a sheath with three fibers. Fiber Cables (2) Fiber Optic Networks A comparison of semiconductor diodes and LEDs as light sources. A fiber optic ring with active repeaters.

5 Fiber Optic Networks (2) Wireless Transmission The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission i A passive star connection in a fiber optics network. The Electromagnetic Spectrum Radio Transmission The electromagnetic spectrum and its uses for communication. (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.

6 Politics of the Electromagnetic Spectrum Lightwave Transmission The ISM (Industrial, Scientific, Medical, unlicensed) bands in the United States. Convection currents can interfere with laser communication systems. A bidirectional system with two lasers is pictured here. Communication Satellites *Communication Satellites Geostationary Satellites Medium-Earth Orbit Satellites Low-Earth Orbit Satellites Satellites versus Fiber Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage Note: 地球被高能的電子及離子所包圍著, 這些高能電子 離子是被地球的磁場給限制住, 現在稱這磁層 (magnetosphere) 為范艾倫帶 (Van Allen Belt)

7 Communication Satellites (2) Communication Satellites (3) The principal satellite bands. VSATs (Very Small Aperture( 孔徑 ) Terminal) using a hub. Low-Earth Orbit Satellites Iridium Globalstar (a) (b) (a) () The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth. (a) Relaying in space. (a) Relaying in space. (b) Relaying on the ground.

8 Public Switched Telephone System Structure of the Telephone System Structure of the Telephone System The Politics of Telephones The Local Loop: Modems, ADSL and Wireless Trunks and Multiplexing Switching (a) Fully-interconnected network. (b) Centralized switch. (c) Two-level hierarchy. Structure of the Telephone System (2) Major Components of the Telephone System Local lloops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another A typical circuit route for a medium-distancedistance call.

9 The Politics of Telephones The Local Loop: Modems, ADSL, and Wireless The relationship of LATAs(Local Access and Transport Areas), LECs(Local Exchange Carrier), and IXCs(IntereXchange Carrier). All the circles are LEC switching offices. Each hexagon belongs to the IXC whose number is on it. The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs. Modems Modems (2) (a) A binary signal (b) Amplitude modulation (c) Frequency modulation (d) Phase modulation (a) QPSK. (b) QAM-16. (c) QAM-64.

10 Modems (3) Digital Subscriber Lines (a) () (b) () (a) V.32 for 9600 bps. (b) V32 bis for 14,400 bps. Bandwidth versus distanced over category 3 UTP for DSL. Digital Subscriber Lines (2) Digital Subscriber Lines (3) Operation of ADSL using discrete multitone modulation. A typical ADSL equipment configuration.

11 Wireless Local Loops (802.16) Frequency Division Multiplexing Architecture of an LMDS (local multipoint distribution service) system. (a) The original bandwidths. (b) The bandwidths raised in frequency. (b) The multiplexed channel. Wavelength Division Multiplexing Time Division Multiplexing Wavelength division multiplexing. The T1 carrier (1.544 Mbps).

12 Time Division Multiplexing g( (2) Time Division Multiplexing g( (3) Delta modulation. Multiplexing T1 streams into higher carriers. Time Division Multiplexing g( (4) Time Division Multiplexing g( (5) Two back-to-back SONET frames. SONET and SDH multiplex rates.

13 Circuit Switching Message Switching (a) Circuit switching. (b) Packet switching. (a) Circuit switching (b) Message switching (c) Packet switching Packet Switching The Mobile Telephone System First-Generation ti Mobile Phones: Analog Voice Second-Generation Mobile Phones: Digital Voice Third-Generation Mobile Phones: Third-Generation Mobile Phones: Digital Voice and Data A comparison of circuit switched and packet-switched networks.

14 Advanced Mobile Phone System Channel Categories (AMPS) The 832 channels are divided into four categories: Control (base to mobile) to manage the system Paging (base to mobile) to alert users to calls for them (a) Frequencies are not reused in adjacent cells. (b) To add more users, smaller cells can be used. Access (bidirectional) for call setup and channel assignment Data (bidirectional) for voice, fax, or data D-AMPS Digital it Advanced d Mobile Phone System GSM Global System for Mobile Communications ()ADAMPS (a) D-AMPS channel with three users. (b) A D-AMPS channel with six users. GSM uses 124 frequency channels each of which GSM uses 124 frequency channels, each of which uses an eight-slot TDM system

15 GSM (2) CDMA Code Division Multiple Access a) Each station is assigned a unique m-bit chip sequence.( ) b) To transmit a 1 bit, sends its chip seq; to transmit a 0 bit, sends one s complement of its chip seq. c) B bits/s m B bits/s ( spread spectrum) d) Walsh codes: all chip seq s are pairwise orthogonal. Two chip seq s ST=0 (two different stations). SS (the same station)=1 and S[S(bar)}=-1 A portion of the GSM framing structure. Slot 12 CTL: control channel: paging, random access and access grant *CDMA Code Division Multiple Access Third-Generation Mobile Phones: Digital Voice and Data Basic services an IMT-2000 network should provide High-quality voice transmission Messaging (replace , fax, SMS, chat, etc.) Multimedia (music, videos, films, TV, etc.) Internet access (web surfing, w/multimedia.) (a) Binary chip sequences for four stations (b) () Bipolar chip sequences (c) Six examples of transmissions (d) Recovery of station C s signal

16 Cable Television Community Antenna Television Community Antenna Television It Internet tover Cable Cbl Spectrum Allocation Cable Modems ADSL versus Cable An early cable television system. Internet over Cable Internet over Cable (2) Cable television The fixed telephone system.

17 Spectrum Allocation Cable Modems Frequency allocation in a typical cable TV system used for Internet access Typical details of the upstream and downstream channels in North America. Chap.2 Homework 13, 18, 28, 51, 53