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Public Switched Telephone
Network (PSTN)
Overview
Figure 5.19
Modulation/demodulation
Conversions
• Computer (binary data) to analog signals – done by modems – scheme
is TCM: modulation schemes like QPSK, QAMs -- Local loop I.e.
computer/modem to codec
• Analog to Digital – Codecs – scheme is PCM – done thru sampling
(result in noise) -- codec to telephone net to codec
• Digital to Analog – Codecs – Inverse PCM – Codec to modem
• Analog to binary – by modem – reverse TCM -- to computer
The Local Loop
• Modems
• ADSL
• Wireless
Telephone line bandwidth
• 300 Hz – 3300 Hz
• For voice the entire range is used because
some distortion and noise can be tolerated
• But for data, for integrity of data, edges of
this range are not used. The range used for
data is 600 Hz – 3000 Hz = 2400 Hz bw.
Figure 5.18
Telephone line bandwidth
Modem : Modulator-demodulator
• Modulator : converts the binary data into
band-pass analog signal.
• Demodulator : recovers the binary data
from the modulated signal
• To convert binary data into analog signals,
A sine wave is used and one of the
characteristics (amplitude, phase or
frequency) is modulated to carry the binary
information. The sine wave is called the
carrier wave.
Modems
(a) A binary signal
(b) Amplitude modulation
(c) Frequency modulation
(d) Phase modulation
• Fig b uses 2 values : 0 amplitude to
represent 0 and non-zero amp. To rep. 1
• Fig c uses 2 values : f1 to rep. 0 and f2 to
rep. 1
• Fig d uses phase : phase shift represents
change in value and no phase shift rep. No
change in value
Limitations of ASK and FSK
• Limitations of ASK ; very much
susceptible to noise
• Limitations of FSK ; bandwidth
requirement
• For detailed study : refer to Forouzan.
Modems (2)
(a) QPSK.
(b) QAM-16.
(c) QAM-64.
Trellis Coded Modulation
• With a dense constellation diagram, a small
amount of noise in amplitude or phase can
result in an error. To reduce the chance of an
error, higher speed modems do error
correction by adding extra bit(/s) to each
sample. This scheme is called TCM.
Few terms
• Bit rate : number of bits transmitted per
second.
• Baud rate : number of signal units (or
samples) transmitted per second.
• Bit rate = Baud rate x number of data bits
per sample.
Some traditional modem standards
Earlier modems used the QAM schemes. For QAM schemes
the baud rate is equal to the bandwidth (to be shown by
…or refer to Forouzan pg117 to 126). Hence for a
telephone line of 2400 Hz bw, baud rate of 2400 was used.
• V.32 : 4 data bits + 1 error bit : 2400 x 4 = 9600 bps
• V.32bis : 6 data bits + 1 error bit : 2400 x 6 = 14,400 bps
• V.34 : 12 data bits : 2400 x 12 = 28,800 bps
• V.34bis : 14 data bits : 2400 x 14 = 33,600 bps
Limitations of traditional modems
When the analog signal is digitized at the telephone
company switching station (using codecs) noise is
introduced in the signal. Hence the data rate is
limited according to the Shannon’s capacity.
• In traditional modems, data exchange is between
two computers A and B,( thru the digital telephone
network.), Hence this sampling exists in both the
directions.
• Thus the maximum data rate is 33.6 kbps in either
direction.
Figure 5.22
Traditional modems
Some faster modems
• V.90 offers 56kbps download and 33.6 kbps
upload speeds.
• This is possible because communication today is
via ISPs (Internet Service Providers). We still use
modem to upload and download. But, in
uploading, the analog signal must be sampled at
the switching stations which means the data rate
for uploading is limited to 33.6 as earlier. But,
there is no sampling in the downloading, hence no
noise , hence no Shannon’s limit (theoretically at
least).
Figure 5.23
56K modems
The 56Kbps speed for downloading
in V.90
• The telephone companies (at their switching
offices) generate 8000 samples per second
with 8 bits per sample. One bit is for control
giving a data rate of 8000 x 7 = 56Kbps
The V.92
• Adjusts its speed and depending upon the
noise present can even upload at 48kbps.
• Download is at 56kpbs.
ADSL: Asymmetric Digital
Subscriber Line
• ADSL uses a frequency spectrum of 1.1 MHz. Divides it
into 256 channels each of size roughly 4312.5 Hz.
• Channel 0 : POTS
• Channels 1-5 ; guard band between voice and data
• Two for control channels, one for downstream and one for
upstream
• Remaining are partitioned between upstream and
downstream : depends on the service provider; usually it is
asymmetric giving 80-90% for download and remaining
for upstream – hence the word Asymmetric
Digital Subscriber Lines (2)
Operation of ADSL using discrete multitone
modulation.
ADSL contd
• Within each channel, modulation scheme similar
to V.34 is used ;
• QAM with 15 bits per baud
• 4000 baud instead of 2400
• With 224 downstream channels, download speed
13.44 Mbps is theoretically possible
• In practice, S/N ratio is never good enough to
achieve this rate, but 8 Mbps is possible on short
runs over high quality local loops
Installation requirement of ADSL
A typical ADSL equipment configuration.
Cable broadband Vs DSL
• Cable Broadband is a public network and is
shared by several users, hence
– Bandwidth reduces as more users log in, and
– Less secure
• ADSL is a private network ..works on
leased lines from old PSTN, hence
– Dedicated bandwidth, and
– More secure
Cable broadband Vs DSL :
Speeds
• Can’t distinguish on the basis of speeds
• Different companies offer different packages
• Cable modem speeds vary widely. While cable
modem technology can theoretically support up to
about 30 Mbps, most providers offer service with
between 1 Mbps and 6 Mbps bandwidth for
downloads, and bandwidth between 128 Kbps and
768 Kbps for uploads.
• Both take flat monthly or yearly rents
Cable Vs DSL :speeds
• Very recent announcements from two companies (Dec’05)
• Cable : Vietnam Power Telecom (VP Telecom) and Vietnam
Cable Television (VCTV) on Monday officially launched a
service that allows users get broadband Internet access via
cable television.
The service offers web browsers a chance to download at
speeds of 56 megabits per second and upload at a maximum
rate of 30 Mbps.
• AT&T DSL Service : Under its Expert Plus S-package, the
telecom giant offers a 6mbps DSL service for customers that
want to host their own Web site and have a static IP address.
Wireless Local Loops
• MMDS(Multichannel Multipoint Distribution
Service) - Uses microwaves 198 MHz band at 2.1
GHz frequency range
– Range of about 50km
– Penetrate vegetation and rain moderately well
– Advantage
• Technology is well established and equipment readily available
– Disadv : bandwidth available is not much and must be
shared by several users.
WLL - LMDS
• The acronym LMDS is derived from the following:
• L (local)?denotes that propagation characteristics of signals in this
frequency range limit the potential coverage area of a single cell site;
ongoing field trials conducted in metropolitan centers place the range
of an LMDS transmitter at up to 5 miles
• M (multipoint)?indicates that signals are transmitted in a point-tomultipoint or broadcast method; the wireless return path, from
subscriber to the base station, is a point-to-point transmission
• D (distribution)?refers to the distribution of signals, which may
consist of simultaneous voice, data, Internet, and video traffic
• S (service)?implies the subscriber nature of the relationship between
the operator and the customer; the services offered through an LMDS
network are entirely dependent on the operator's choice of business
Wireless Local Loops
• LMDS(Local Multipoint Distribution Service) :
uses Millimeter waves (because of low bw of
MMDS)
• 28-31 GHz band in US and 40GHz band in
Europe (both MM wave bands) were not allocated
because it was difficult to build silicon integrated
circuits that operate so fast. With the invention of
Gallium arsenide ICs the speed became achievable
and hence people started thinking of using MM
waves for communication.
Problems with MM waves
• Highly directional : hence there must be a
clear line of sight between the roof top
antennas and the tower.
• Rain and trees absorb them
Wireless Local Loops
Architecture of an LMDS system.
Long-Haul Trunks
• The next thing now is to combine the signals
received in the end office(switching offices of the
telephone co.s) from various local loops into one
signal that is transmitted on the long-haul trunk.
This is done with the help of various multiplexing
schemes :
• FDM
• WDM
• TDM
Frequency Division Multiplexing
(a) The original bandwidths.
(b) The bandwidths raised in frequency.
(b) The multiplexed channel.
WDM : Wavelength Division
Multiplexing
• In optical fibers, the scheme used is WDM
instead of FDM.
• As more and more wavelengths are being
discovered in a single fiber WDM is getting
denser and now the name DWDM (dense
WDM) is being used when the number of
channels is vary large in a single fiber.
Growth of WDM
• 1990: 8 wavelengths X 2.5 Gbps  20Gbps
• 1998: 40 X 2.5 Gbps  100Gbps
• 2001: 96 X 10 Gbps  100Gbps :
enough to transmit 30 full-length movies
per second.
Wavelength Division
Multiplexing
Wavelength division multiplexing.
TDM
• WDM : applicable only on optical fiber and
not on copper, but a lot of copper is there on
the last mile, also analog.
• FDM : used on copper and microwave but
requires analog circuitry and cannot be done
by a computer,
• Solution : TDM : unfortunately can be used
only for digital data. So,
Digital Trunks
• What we need is to convert the analog
signals received in the end office(switching
offices of the telephone co.s) from various
local loops into digital signals and combine
them into one signal that is transmitted on
the digital trunk. This is done with the help
of TDM.
CODEC : PCM (Pulse Code
Modulation)
• The codec makes 8000 samples per sec or one
sample per 125 microsec. This is because Nyquist
theorem says that this is sufficient to capture all
the information from the 4KHz ( remember? bit
rate = #samples x log L => sample rate = 2B from
Nyquist theorem). This technique is called PCM.
• All the time intervals (a pulse) within the
telephone system are multiples of 125 microsec.
Time Division Multiplexing : T1
Carrier
• T1 carrier is used on long-haul trunks.
• Supports Codec with 24 Local Loops I.e. 24
channels
• Codec picks signals from these 24 channels
on a Round Robin basis to insert 8 bits (7
data + 1 error) for each sample( I.e. for each
channel)
T1 Carrier
The T1 carrier (1.544 Mbps).
193 X 8000 = 1.544 Mbps
T1 Carrier
• 193rd bit is used for frame synchronization :
a pattern of 010101… is looked for --analog nodes cannot generate this pattern,
digital users can but the chances are less.
•
Signaling(control) information
in T1
• Notice : 8000 bps signaling information : too
much : two possible approaches to reduce this :
– Common channel signaling : use of 193rd bit for
signaling in alternate frames say odd frames and for
data in even frames.
– Channel-associated signaling : each channel has its own
private signaling subchannel – one of the eight user bits
in every sixth frame is used for signaling
E1 Carrier
• 32 channels : 30 for data + 2 for signaling
• Each group of four frames provides 64 bits
of signaling : half for channel specific +
half for frame sync
• Capacity : 32 X 8 X 8000 = 2.04 Mbps
Differential Pulse Code
Modulation
• Instead of digitized amplitude, difference is
kept and digitized
• Jumps of the magnitude of more than +-16
are rare in 128 levels. So 5 instead of 8 bits
are sufficient.
Delta Modulation
Delta modulation.
Predictive Encoding
• Extrapolate the previous few values to
predict the next value.
• Encode the difference between actual and
the predicted signal
Time Division Multiplexing (3)
Multiplexing T1 streams into higher carriers.
TDM on optical fiber (for digital
data)
Two back-to-back SONET frames.
Time Division Multiplexing (5)
SONET and SDH multiplex rates.
PSTN contd…
• Can be viewed to have two types of
componenets:
– External (communication medium…last mile,
long haul trunks etc) and,
– Internal (Switching Offices)
Switching Offices
• Two types of switching is used:
– Circuit Switching (PSTN)
– Packet Switching (Internet)
Circuit Switching
(a) Circuit switching.
(b) Packet switching.
Message Switching
(a) Circuit switching (b) Message switching (c) Packet
Topics for presentation
• Satellite Networks (2 people) : Explain user to
user, where and how they are used etc.
• Mobile Networks (3-5 people), take book from me
for reference, rest from net, talk about GSM,
GPRS, EDGE, CDMA, their 2nd gen, 3rd gen etc,
difference between “use of data card to connect to
internet wirelessly anywhere anytime” and
GPRS/EDGE enabled mobile phone etc.
Cover in detail: which frequency range, call setup,
their switching offices etc, technology used etc
Packet Switching
A comparison of circuit switched and packet-switched
networks.
I Acknowledge
Help from the following site
http://www.cs.vu.nl/~ast/
In preparing this lecture.