Public Switched Telephone Network PSTN

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Public Switched Telephone Network (PSTN)
2
Overview
3
Figure 5.19 Modulation/demodulation
4
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
• 
  

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The Local Loop

• Modems
• ADSL
• Wireless

6
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.

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Figure 5.18 Telephone line bandwidth
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Modem Modulator-demodulator

• Modulator converts the binary data into
  band-pass analog signal.
• Demodulator recovers the binary data from the
  modulated signal

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• 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.

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Modems

• (a) A binary signal
• (b) Amplitude modulation

• (c) Frequency modulation
• (d) Phase modulation

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• 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

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Limitations of ASK and FSK

• Limitations of ASK very much susceptible to
  noise
• Limitations of FSK bandwidth requirement
• For detailed study refer to Forouzan.

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Modems (2)

• (a) QPSK.
• (b) QAM-16.
• (c) QAM-64.

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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.

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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.

16
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

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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
  Shannons 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.

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Figure 5.22 Traditional modems
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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 Shannons limit (theoretically
  at least).

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Figure 5.23 56K modems
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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

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The V.92

• Adjusts its speed and depending upon the noise
  present can even upload at 48kbps.
• Download is at 56kpbs.

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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

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Digital Subscriber Lines (2)

• Operation of ADSL using discrete multitone
  modulation.

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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

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Installation requirement of ADSL

• A typical ADSL equipment configuration.

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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

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Cable broadband Vs DSL Speeds

• Cant 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

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Cable Vs DSL speeds

• Very recent announcements from two companies
  (Dec05)
• 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.
• ATT 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.

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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.

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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-to-multipoint 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

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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.

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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

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Wireless Local Loops

• Architecture of an LMDS system.

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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

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Frequency Division Multiplexing

• (a) The original bandwidths.
• (b) The bandwidths raised in frequency.
• (b) The multiplexed channel.

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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.

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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.

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Wavelength Division Multiplexing

• Wavelength division multiplexing.

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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,

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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.

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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 gt 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.

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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)

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T1 Carrier

• The T1 carrier (1.544 Mbps).

193 X 8000 1.544 Mbps
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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.

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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

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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

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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.

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Delta Modulation

• Delta modulation.

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Predictive Encoding

• Extrapolate the previous few values to predict
  the next value.
• Encode the difference between actual and the
  predicted signal

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Time Division Multiplexing (3)

• Multiplexing T1 streams into higher carriers.

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TDM on optical fiber (for digital data)

• Two back-to-back SONET frames.

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Time Division Multiplexing (5)

• SONET and SDH multiplex rates.

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PSTN contd

• Can be viewed to have two types of componenets
• External (communication mediumlast mile, long
  haul trunks etc) and,
• Internal (Switching Offices)

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Switching Offices

• Two types of switching is used
• Circuit Switching (PSTN)
• Packet Switching (Internet)

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Circuit Switching

• (a) Circuit switching.
• (b) Packet switching.

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Message Switching

• (a) Circuit switching (b) Message switching
  (c) Packet switching

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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

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Packet Switching

• A comparison of circuit switched and
  packet-switched networks.

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I Acknowledge

• Help from the following site
• http//www.cs.vu.nl/ast/
• In preparing this lecture.