Copyright 2002, S.D. Personick. All Rights Reserved. Telecommunications Networking I Topic 6 Point-To-Point Digital Communications Dr. Stewart D. Personick Drexel University

Copyright 2002, S.D. Personick. All Rights Reserved. Digital Point to Point Communications In a real digital communication system, one has to be concerned with noise, interference and other effects that can cause errors In the previous discussion, we briefly covered the topic of additive noise, and its impact on errors (misses and false alarms) In metallic cable systems, “intersymbol interference” is a key factor that we must deal with

Copyright 2002, S.D. Personick. All Rights Reserved. Intersymbol Interference Transmitted pulse stream T T Cable output (dispersion)

Copyright 2002, S.D. Personick. All Rights Reserved. Intersymbol Interference: Equalization Cable: H(f) Equalizer: 1/[H(f)]

Copyright 2002, S.D. Personick. All Rights Reserved. Digital Regenerator (Repeater) Cable: H(f) Equalizer & Matched Filter Timing Recovery Clock signal Decision circuit

Copyright 2002, S.D. Personick. All Rights Reserved. Timing recovery (x)**2 Filter or PLL PLL=Phase-locked Loop Pulse stream Clock

Copyright 2002, S.D. Personick. All Rights Reserved. Decision Circuit Comparito r Pulse stream Threshold D Flip Flop Clock

Copyright 2002, S.D. Personick. All Rights Reserved. Eye Diagram

Copyright 2002, S.D. Personick. All Rights Reserved. Eye Diagram (cont’d)

Copyright 2002, S.D. Personick. All Rights Reserved. T-Carrier First introduced by the former Bell System in The first digital transmission system Digital transmission rate is Mbps Works on 24 gauge wire pair cables Repeaters every 6000 ft ~ 2km Max cable loss at 772MHz ~35 dB The signal is a “DS1” (but everyone calls it a “T1”)

Copyright 2002, S.D. Personick. All Rights Reserved. T-Carrier (cont’d) Maximum cable length between repeaters limited by crosstalk: interference from other signals on other pairs in the same cable Transmitted signal format: + or - 3V equals a logical “1”; 0V equals a logical “0”, no more than 7 “zeros” in a row are permitted is mapped to originally; recently mapped to (“B8ZS”) 2-way repeater price: ~$50.00

Copyright 2002, S.D. Personick. All Rights Reserved. Analog-to-Digital Conversion (and vice versa) Sampling Theorem: If we sample an analog signal at twice its highest frequency, we can reproduce it exactly from its samples

Copyright 2002, S.D. Personick. All Rights Reserved. A/D Conversion Example: Voice signals… -Highest frequency is limited (by a filter) to 4kHz -We sample this band limited signal at 8000 samples per second (125 microseconds between samples) -We represent each sample with 1 byte (positive and negative values are both captured by 256 levels) 8000 samples per second x 8 bits = 64 kbps

Copyright 2002, S.D. Personick. All Rights Reserved. D/A Conversion filter Samples reconstructed from the received digital bit stream Reconstructed waveform

Copyright 2002, S.D. Personick. All Rights Reserved. Multiplexing 24 inputs, each at 64 kbps 1 output at Mbps T1 multiplexer; also known as a “D-channel bank” [24 x 64 kbps] + 8kbps = Mbps; 8 kbps = overhead Output signal is a “DS1”, but everyone calls it a “T1”

Copyright 2002, S.D. Personick. All Rights Reserved. D-Channel Bank Frame Format 1st data byte, 2nd data byte, ……, 24th data byte, F F= Framing bit Frame length is (8 x 24) + 1 = 193 bits 8000 frames per second, corresponding to the rate at which voice signals are sampled 193 bits per frame x 8000 frames per second = Mbps 125 microseconds

Copyright 2002, S.D. Personick. All Rights Reserved. Multiplexing Standards In the United states, some common multiplex standards are: (above DS1, they are used with radio or fiber optic transmission systems) -DS1 (called T1) Mbps -DS3 (called T3) 44.7 Mbps (28 DS1 + overhead) -STS1 (SONET-1) Mbps -STS3 (SONET-3) Mbps -STS Mbps -STS Gbps -STS 192 ~10 Gbps

Copyright 2002, S.D. Personick. All Rights Reserved. Other Popular Digital Metallic Cable Transmission Systems Ethernet: 10Base-T, 100Base-T; coax versions Telephone modems: up to 56 kbps; limited by end-to-end switched telephone network ADSL: asymmetric digital subscriber line; e.g., 1.5Mbps downstream, 384 kbps upstream …using only the existing telephone loop (not the switched network) Cable modems: ~ 20 Mbps downstream, shared with other users; upstream depends on the system

Copyright 2002, S.D. Personick. All Rights Reserved. Ethernet: 10Base-T Hub Twisted Pair To other hubs or router NIC Computer NIC= Network Interface Card

Copyright 2002, S.D. Personick. All Rights Reserved. Modem modem Computer (Philadelphia) modem Public Switched Telephone Network (PSTN) Subscriber loop Computer (Los Angeles) A/DD/A

Copyright 2002, S.D. Personick. All Rights Reserved. ADSL PSTN computer ADSL DSLAM Router To the Internet Switched voice Packet data DSLAM=Digital subscriber line access multiplexer Loop pair

Copyright 2002, S.D. Personick. All Rights Reserved. Cable Modem Coaxial Cable Splitter TV Modem Coaxial Cable Twisted pair PC