Physical layer Public Switched Telephone Network PSTN Public

Physical layer: Public Switched Telephone Network (PSTN)

Public Switched Telephone Network n n Structure The Local Loop Trunks and Multiplexing Switching Sharif University of Technology

Network Structure n n minimize number of wires add multiple levels Sharif University of Technology

Typical Circuit n n n local loops trunks switching offices Sharif University of Technology

The Local Loop n n n Modems (A)DSL Wireless Sharif University of Technology

Modems n n n Analog and digital transmission Sine wave carrier Baud Phase shift keying Limits Sharif University of Technology

Analog and Digital Transmission n n modem – modulator, demodulator codec – coder, decoder Sharif University of Technology

Modems – Sine Wave Carrier Sharif University of Technology

Baud and Symbols n n Baud rate is the sampling rate Baud is the time to read one symbol When the number of symbols is 2, the baud rate is the bit rate Modern modems use large sets of symbols Sharif University of Technology

Quadrature Phase Shift Keying n n Constellation diagrams Amplitude (distance from origin) Phase QAM: Quadrature Amplitude Modulation Sharif University of Technology

Trellis Coded Modulation n add bits for error correction V. 32: 32 constellation points, 4 data bits, 1 parity bit V. 32 bis: 6 data bits, 1 parity bit Sharif University of Technology

Limits n Base sampling rate – 2400 baud n Variations q q n handshake to determine line quality compression 35 kbps is the Shannon limit, 56 kbps? q q q eliminate one local loop V. 90 56 -kbps down stream, 33 -kbps upstream V. 92 48 -kbps down stream, 48 -kbps upstream Sharif University of Technology

DSL: Digital Subscriber Line n Remove the filters Bandwidth n Goals n q q use existing Cat-3 lines not interfere with current phone uses always on much better than 56 kbps Sharif University of Technology

Techniques n n POTS (Plain Old Telephone Service) – Frequency Multiplexing DMT (Discrete Multi. Tone) Sharif University of Technology

DSL Equipment n n n NID: Network Interface Device ADSL Modem: 250 QAM modems DSLAM: Digital Subscriber Line Access Multiplexer Sharif University of Technology

Trunks and Multiplexing n n n Frequency division multiplexing Wavelength division multiplexing Time division multiplexing q n aside on compression SONET Sharif University of Technology

Frequency Division Multiplexing Sharif University of Technology

Wavelength Division Multiplexing n FDM for optical Sharif University of Technology

Time Division Multiplexing (T 1) n n n n codec 8000 samples / second (4000 Hz signals) 125 µsec / sample PCM (Pulse Coded Modulation) codec is multiplexed between 24 analog lines each analog line inserts 8 bits every 125 µsec 7*8000=56000 bps / channel 1. 544 Mbps aggregate Sharif University of Technology

Framing n n n Framing bit is used to recover the sender’s clock The framing bit is 010101010. . I. e. , 4000 Hz Filtered for analog customer (3100 Hz filter) Not likely for digital customers Sharif University of Technology

Multiple T 1 s n n n Bitwise multiplexing 4 to 1; 7 to 1; and 6 to 1 Overhead added at each step for framing and recovery T 2 and T 4 are only used inside the phone company Sharif University of Technology

Compression n n differential modulation (send change to value, rather than value) delta modulation (shown) +1 or -1 predictive encoding: extrapolate from earlier values and then send the change to this extrapolation Sharif University of Technology

SONET (Synchronous Optical NETwork) n n STS-1 Synchronous Transport Signal-1 SPE – Synchronous Payload Envelope Sharif University of Technology

SONET Rates n n n Gross – signalling rate SPE – excludes line and selection overhead User – excludes path overhead Sharif University of Technology