Copyright 2002, S.D. Personick. All Rights Reserved. Telecommunications Networking I Topic 6...
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Transcript of 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
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
Comparitor
Pulse stream
ThresholdD 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 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”)
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
• 00000000 is mapped to 00000001 originally; recently mapped to ++0000-- (“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 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
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.
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”
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 = 1.544 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) 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
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/D D/A
Copyright 2002, S.D. Personick. All Rights Reserved.
ADSL
PSTN
computer
ADSL DSLAM
RouterTo the Internet
Switched voice
Packet dataDSLAM=Digital subscriber lineaccess multiplexer
Loop pair
Copyright 2002, S.D. Personick. All Rights Reserved.
Cable Modem
Coaxial Cable
Splitter TV
Modem
Coaxial Cable
Twisted pairPC