copyright 2002, s.d. personick. all rights reserved. telecommunications networking i topic 6...

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

Telecommunications Networking I

Topic 6

Point-To-Point Digital CommunicationsDr. Stewart D. Personick

Drexel University


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


Intersymbol Interference

Transmitted pulse stream

T

T

Cable output (dispersion)


Intersymbol Interference: Equalization

Cable: H(f) Equalizer:

1/[H(f)]


Digital Regenerator (Repeater)

Cable: H(f)Equalizer & Matched Filter

Timing Recovery

Clock signal

Decision circuit


Timing recovery

(x)**2 Filter or PLL

PLL=Phase-locked Loop

Pulse stream

Clock


Decision Circuit

Comparitor

Pulse stream

ThresholdD Flip Flop

Clock


Eye Diagram


Eye Diagram (cont’d)


T-Carrier• First introduced by the former Bell System

in 1962. The first digital transmission system

• Digital transmission rate is 1.544 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”)


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

• 00000000 is mapped to 00000001 originally; recently mapped to ++0000-- (“B8ZS”)

• 2-way repeater price: ~$50.00


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


A/D ConversionExample: 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 andnegative values are both captured by 256 levels)

8000 samples per second x 8 bits = 64 kbps


D/A Conversion

filter

Samples reconstructed from the received digital bit stream

Reconstructed waveform


Multiplexing24 inputs, each at 64 kbps 1 output at

1.544 MbpsT1 multiplexer; also known as a “D-channel bank”

[24 x 64 kbps] + 8kbps = 1.544 Mbps; 8 kbps = overheadOutput signal is a “DS1”, but everyone calls it a “T1”


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 = 1.544 Mbps

125 microseconds


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) 1.544 Mbps -DS3 (called T3) 44.7 Mbps (28 DS1 + overhead)-STS1 (SONET-1) 51.84 Mbps-STS3 (SONET-3) 155.52 Mbps-STS 12 622.08 Mbps-STS 48 2.48832 Gbps-STS 192 ~10 Gbps


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


Ethernet: 10Base-T

Hub

Twisted Pair

To other hubs or router

NIC

Computer

NIC= Network Interface Card


Modem

modem

Computer (Philadelphia)

modem

Public Switched Telephone Network (PSTN)

Subscriber loop

Computer(Los Angeles)

A/D D/A


ADSL

PSTN

computer

ADSL DSLAM

RouterTo the Internet

Switched voice

Packet dataDSLAM=Digital subscriber lineaccess multiplexer

Loop pair


Cable Modem

Coaxial Cable

Splitter TV

Modem

Coaxial Cable

Twisted pairPC

copyright 2002, s.d. personick. all rights reserved. telecommunications networking i topic 6...

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