ECE 5600 Introduction to Computer Networking Lecture 9

ECE 5600 Introduction to Computer Networking Lecture 9 –PSTN Prof. Rose Qingyang Hu Electrical and Computer Engineering Department Utah State University

READING Book: Andrew S. Tanenbaum, Computer Networks, Fifth Edition, Prentice Hall Chapters 2. 6. 1 – 2. 6. 4

STRUCTURE OF THE TELEPHONE SYSTEM (1) a) Fully interconnected network. b) Centralized switch. c) Two-level hierarchy.

STRUCTURE OF THE TELEPHONE SYSTEM (2) A typical circuit route for a long-distance call.

MAJOR COMPONENTS OF THE TELEPHONE SYSTEM v Local loops v Analog twisted pairs going to houses and businesses v Trunks v Digital fiber optics connecting the switching offices v Switching offices v Where calls are moved from one trunk to another

LOCAL LOOP v Modem v DSL: Digital Subscriber Loop v Fiber

MODEMS v Use of both analog and digital transmission for computer -tocomputer call. Conversion done by modems and codecs. v Modem: modulator demodulator v Telephone modem, DSL modem, cable modem.

SHANNON LIMIT ON TELEPHONE v Telephone Bandwidth and SNR v 3400 Hz – 300 Hz = 3100 Hz v SNR ≈ 35 d. B

HOW THEN ARE 56 KBPS MODEMS POSSIBLE? v Normally, phone-to-phone connections have 2 local loops, each has about a 68 d. B SNR. v Two local loops have a combined SNR of about 34 d. B v If the ISP is connected via a digital trunk, the SNR increases to 68 d. B.

DIGITAL SUBSCRIBER LINE (DSL) v Makes use of wasted local loop bandwidth. v POTS (Plain old telephone system) filters out frequencies above 3400 Hz. 50 Mbps 30 Mbps 1 km 2 km 3 km 4 km 5 km

ADSL v Breaks frequency spectrum into 4. 3125 k. Hz channels. v Channel 0: POTS v Channel 1– 5 Empty v Channel 6 -31 Upstream (typical) v Channel 32 -255 Downstream (again, typical) v OFDM (orthogonal FDM) is used v Called DMT (Discrete Multi. Tone) in the context of ADSL

DSL CONFIGURATION Network interface device DSLAM: Digital Subscriber Line Access Multiplexer

PULSE CODE MODULATION (PCM) Codec: the device that digitizes the analog signal in the end office. PCM § Telephone channel bandwidth is 4 KHz. § The codec makes 8000 samples per second (125 usec/sample) based on Nyquist theorem. § Each sample of the amplitude is quantized to an 8 -bit number

PSTN TRUNK v PSTN trunk TDM formats: v T 1, T 2, T 3, T 4 – US & Japan v E 1, E 2, E 3, E 4 – CCITT Standard v SONET – Usually used on fiber v Data compression – delta modulation, prediction

T 1 v. One frame every 125 usec (8000 Hz) v 24 channels, 193 bits per frame.

v Question: What is the total throughput of a T 1 line?

MULTIPLEXING T 1 STREAMS INTO HIGHER CARRIERS

TDM FORMATS v US and Japan: v T 1: 1. 544 Mbps, 24 channels v T 2 = 4 T 1 (6. 176 Mbps) v T 3 = 7 T 2 (43. 23 Mbps) v T 4 = 6 T 3 (259. 4 Mbps) v CCITT: v E 1: 2. 048 Mbps, 32 channels v E 2 = 4 E 1 (8. 192 Mbps) v E 3 = 4 E 2 (32. 77 Mbps) v E 4 = 4 E 3 (131. 1 Mbps)