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Lecture on Broadband Communications Chapter 16

An insight to digital communication systems and associated phenomenon. Lecture on Broadband Communications Chapter 16. Multiple Access Techniques. Definitions. TDMA – Time Division Multiple Access FDMA – Frequency Division Multiple Access CDMA – Code Division Multiple Access.

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Lecture on Broadband Communications Chapter 16

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  1. An insight to digital communication systems and associated phenomenon Lecture on Broadband CommunicationsChapter 16

  2. Multiple Access Techniques

  3. Definitions • TDMA – Time Division Multiple Access • FDMA – Frequency Division Multiple Access • CDMA – Code Division Multiple Access

  4. General Specification of TDMA • Rx: 869-894MHz Tx: 824-849MHz • 832 Channels spaced 30kHz apart(3 users/channel) • DQPSK modulation scheme • 48.6kbps bit rate • Interim Standard (IS) – 54 • Digital AMPS (Advanced Mobile Phone System) • Uses Time Division Duplexing (TDD) usually

  5. TDMA Details • The incoming data from each source are briefly buffered and scanned to to form a composite digital data stream mc ( t ) . m1 ( t ) U1 Buffer m2 ( t ) Frame Frame U2 Buffer mc ( t ) ... preamble 1 2 N preamble 1 2 N mN ( t ) information UN Time slot Buffer Each slot may be empty or occupied. + has preamble & guard bits Scan operation Kashif Shahzad

  6. Advantages of TDMA • Flexible bit rate • No frequency guard band required • No need for precise narrowband filters • Easy for mobile or base stations to initiate and execute hands off • Extended battery life • TDMA installations offer savings in base station equipment, space and maintenance • The most cost-effective technology for upgrading a current analog system to digital

  7. Disadvantages to using TDMA • Requires network-wide timing synchronization • Requires signal processing fro matched filtering and correlation detection • Demands high peak power on uplink in transient mode • Multipath distortion

  8. General Specification of FDMA • Rx: 869-894MHz Tx: 824-849MHz • 832 Channels spaced 30kHz apart(3 users/channel) • DQPSK modulation scheme • 48.6kbps bit rate • Used in analog cellular phone systems (i.e. AMPS) • Uses Frequency Division Duplexing (FDD) • ISI (Intersymbol Interference) is low

  9. Advantages of FDMA • If channel is not in use, it sits idle • Channel bandwidth is relatively narrow (30kHz) • Simple algorithmically, and from a hardware standpoint • Fairly efficient when the number of stations is small and the traffic is uniformly constant • Capacity increase can be obtained by reducing the information bit rate and using efficient digital code • No need for network timing • No restriction regarding the type of baseband or type of modulation

  10. Disadvantages to using FDMA • The presence of guard bands • Requires right RF filtering to minimize adjacent channel interference • Maximum bit rate per channel is fixed • Small inhibiting flexibility in bit rate capability • Does not differ significantly from analog system

  11. General Specification of CDMA • Rx: 869-894MHz Tx: 824-849MHz • 20 Channels spaced 1250kHz apart(798 users/channel) • QPSK/(Offset) OQPSK modulation scheme • 1.2288Mbps bit rate • IS-95 standard • Operates at both 800 and 1900 MHz frequency bands

  12. CDMA Operation • Spread Spectrum Multiple Access Technologies

  13. Advantages of CDMA • Many users of CDMA use the same frequency, TDD or FDD may be used • Multipath fading may be substantially reduced because of large signal bandwidth • No absolute limit on the number of users • Easy addition of more users • Impossible for hackers to decipher the code sent • Better signal quality • No sense of handoff when changing cells

  14. Disadvantages to using CDMA • As the number of users increases, the overall quality of service decreases • Self-jamming • Near- Far- problem arises

  15. Modes of Communication and RS232

  16. Simplex, Half-Duplex, and Full-Duplex Transmission • A communications channel is classified as one of three types: (depending on the direction of transfer) • Simplex • Full-Duplex • Half-Duplex • Simplex: a simplex mechanism can only transfer data in a single direction • It is analogous to broadcast radio or television • Figure 9.8a illustrates simplex communication • Full-Duplex: allows transmission in two directions simultaneously • It is analogous to a voice telephone conversation • in which a participant can speak even if they are able to hear background music at the other end • Figure 9.8b illustrates the concept 16

  17. 9.12 Simplex, Half-Duplex, and Full-Duplex Transmission 17

  18. What is RS-232 • RS-232 is a popular communications interface for connecting modems and data acquisition devices (i.e. GPS receivers, electronic balances, data loggers, ...) to computers. • RS-232 can be plugged straight into the computer’s serial port (know as COM or Comm port).

  19. (serial port - PC side) RS-232 Signals • Architecturally RS-232 is a bi-directional point to point link. • Two independent channels are established for two-way (full-duplex) communications. • RS-232 can also carry additional signals used for flow control (RTS, CTS) and modem control (DCD, DTR, DSR, RI).

  20. Pin Signal1 PGND Protective Ground2 TXD Transmit Data3 RXD Receive Data4 RTS Ready To Send5 CTS Clear To Send6 DSR Data Set Ready7 SG Signal Ground8 CD Carrier Detect20 DTR Data Terminal Ready22 RI Ring Indicator (serial port - PC side) RS-232 Signals • Common 25 pin D-shell connector pinout used for asynchronous data communications.

  21. RS-232 Speed • How fast can RS-232 be? • The maximum speed, according to the standard, is 20kbit/s.However, modern equipment can operate much faster than this. (i.e. Lynx can reach 115200 baud.) • The length of the cable also plays a part inmaximum speed.The longer the cable and the slower the speed at which you can obtain accurate results.A large wire capacitance and inductance limits the maximum length of the cable and/or the maximum speed; Moreover higher is the capacitance of the cable higer is the interference between two adjacent signal wire.50 feet (15m) @ max baudrate is commonly quoted as the maximum distance.It is not specified in EIA standard but it’s recommended respect these values.

  22. + 25 - 25 RS-232 sw settings • Stop bits = 1 (or 2) • Parity = NONE (or EVEN or ODD) • One byte of async data has: • Start Bit = 1 (always) • Data Bits = 8 (or 7)

  23. Network types

  24. Switching Networks • Long distance transmission is typically done over a network of switched nodes • Nodes not concerned with content of data • End devices are stations • Computer, terminal, phone, etc. • A collection of nodes and connections is a communications network • Data routed by being switched from node to node

  25. Nodes • Nodes may connect to other nodes only, or to stations and other nodes • Node to node links usually multiplexed • Network is usually partially connected • Some redundant connections are desirable for reliability • Two different switching technologies • Circuit switching • Packet switching

  26. Circuit Switching • Dedicated communication path between two stations • Three phases • Establish • Transfer • Disconnect • Must have switching capacity and channel capacity to establish connection • Must have intelligence to work out routing

  27. Circuit Switching - Applications • Inefficient • Channel capacity dedicated for duration of connection • If no data, capacity wasted • Set up (connection) takes time • Once connected, transfer is transparent • Developed for voice traffic (phone)

  28. Public Circuit Switched Network

  29. Circuit Establishment

  30. Circuit Switching Principles revisited • Circuit switching designed for voice • Resources dedicated to a particular call • Much of the time a data connection is idle • Data rate is fixed • Both ends must operate at the same rate

  31. Packet Switching: Basic Operation • Data transmitted in small packets • Longer messages split into series of packets • Each packet contains a portion of user data plus some control info • Control info • Routing (addressing) info • Packets are received, stored briefly (buffered) and past on to the next node • Store and forward

  32. Packet-Switched Network

  33. Use of Packets

  34. DatagramDiagram

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