Fundamentals of Mobile Computing Understanding the PSTN

1
Fundamentals of Mobile ComputingUnderstanding
the PSTN

• Kelvin Hilton
• k.c.hilton_at_staffs.ac.uk

2
Objectives

• Understanding
• Background of PSTN
• Network topography
• Fundamentals of modulation
• Basic modulation techniques
• Introducing codecs
• Circuit switching
• Origins of packet switching

3
Background
4
Why do we need to know about PSTN

• Many of the design decisions that have shaped
  mobile communications have been made because of
  the existing PSTN technology
• Public Switched Telephone Network (PSTN) is at
  the heart of all communications fixed or mobile
• Much of the terminology and concepts used in
  Mobile Comms stem from the wired network
• Aka POTS (Plain Old Telephone System)

5
Origins of the PSTN

• 1876 Alexander Graham Bell patented the telephone
• Initially, telephones came in pairs umbilically
  linked. If you wanted to communicate with
  another telephone you had to run a wire between
  them

B
A
C
E
D
G
F
6
Origins of the PSTN

• Quickly became apparent that this was not going
  to work!
• Bell acknowledged the problem and formed the Bell
  Telephone Company which opened the worlds first
  telephone switching office in Connecticut in 1878
• The company ran a wire to each house/business
  from the office
• To make a call the customer 1st called the
  switching office, an operator would then manually
  connect to the callee using a wire jumper cable

7
The Switch is Born
Switching Office
8
Origins of the PSTN

• This was still a city by city solution
• Soon people wanted to call between cities
• Bell started connecting switching offices
• The spaghetti problem returned, to run a
  connection between every switching office was
  just the same as the original peer-to-peer
  paradigm
• Solution was to introduce a second level of
  switching offices

9
Second Level Switches
Second Level Switching Office
10
Origins of the PSTN

• Eventually, 5 levels of switching offices were
  defined
• By 1890 the three main components of the
  telephone system were in place
• The umbilical connection to the end-user
• The switching offices
• The long distance umbilical connection between
  switching offices
• Original wires which were un-insulated and
  earthed to ground had been replaced by insulated
  twisted pair
• Little change conceptually for nearly 100 years

11
Todays PSTN
12
Conceptual View of Todays PSTN

• Customer is connected to local switch (for
  example called an End Office)
• Typical distance 1 10km
• The physical link between the customer and the
  End Office is called the Local Loop
• The concatenation of area code and the first part
  (in US first 3 digits) of the number identifies
  the End Office
• If the all the worlds local loops were
  concatenated they would stretch to the moon and
  back 1000 times! Thats a lot of copper

13
Making a Call

• If the call is local the End Office the switch
  makes and maintains the connection between the
  two local loops
• If call is not local, the End Office has a fixed
  number connecting lines to one or more nearby
  switching centres (in US called Toll Offices)
• The connection between the End Office and the
  Toll Office is called a toll connecting trunk
• If the caller callee share a common Toll Office
  the connection is made and maintained there

14
Making a Call

• If not then the network continues routing up
  through the hierarchy (primary, sectional and
  regional exchanges)
• These are connected via high bandwidth intertoll
  trunks (or inter office trunks)
• Terminology may vary internationally but the
  concepts are the same

15
Transmission Media

• Local loops still use copper twisted pair
• Between switching offices coaxial, microwave and
  more latterly fibres are used
• Until relatively recently all signalling on the
  network was analogue with voice being transmitted
  as an electrical voltage throughout
• Digital technology has changed (and will continue
  to change) many of the components on the PSTN

16
The Local Loop

• Remain predominantly analogue
• None voice data must be converted to analogue
  (via modem) for transmission to End Office then
  back to digital for transmission over the long
  haul trunks
• Originally, leased line was the only way to go
  digital end-to-end

17
Digitalisation

• Much cheaper (no need to have to reproduce
  accurately an analogue signal)
• More efficient (attenuation, propagation, etc)
• More precise (easier to calculate where repeaters
  will be required, easier to amplify, etc)

18
Overview of Modulation
19
Key Facts About Voice

• Human hearing range
• 0.2 20kHz
• Human speech range
• 0.25 10kHz

• Human Intelligible speech range
• 0.3 3.4kHz
• Therefore for phone to carry voice suitable for
  communication need 3.4kHz bandwidth

20
Key Facts About Voice

• Nyquist theorem states that to capture a spectrum
  B Hz wide sample rate (R) must be
• R gt 2B samples/sec
• Therefore sample rate must be 2 x 3.4kHz
  (minimum) original PSTN engineers settled on 4kHz
• Therefore sample rate 2 x 4kHz 8kHz
• Therefore need to 8000 samples / sec at 125 msec
  per sample

21
Key Facts About Voice

• Therefore, codec takes 8k samples per second
  using 8 bits per sample
• Minimum theoretical bandwidth for voice is
  therefore
• 8000 x 8 64000 bps (64kbps)
• The process forms the basis for Pulse Code
  Modulation (PCM)
• PCM used extensively in modern telecommunications
• However, no international standard on PCM so
  interconnection between different schemes is
  expensive

22
Problems with Analogue

• Covered in detail in Lecture on 1G technologies,
  however overview is
• Attenuation loss of energy as signal propagates
  out from source
• Delay Distortion - components of analogue systems
  run at different speeds, feasible for digital
  data transmitted on fast components to overtake
  data transmitted on slower ones
• Noise impairment in signal due to other
  transmission sources (eg thermal noise, inductive
  coupling of wires causing crosstalk, impulse
  noise)

23
Problems with Analogue

• Hence it is undesirable to have a wide range of
  frequencies (spectrum) in the signal
• Unfortunately, square waves (as with digital
  data) have a wide spectrum hence are subject to
  strong attenuation and delay distortion
• To over come this an AC signal is used ranging
  from 1000 2000 Hz called a sine wave carrier

24
The AC Sine Wave Carrier

• Can be modulated in one of three ways
• Amplitude Modulation two different voltages are
  used to represent 1s and 0s
• Frequency Modulation Aka Frequency Shift
  Keying, two (or more) different tones are used to
  represent 1s and 0s
• Phase Modulation the carrier wave is
  systematically shifted varying degrees (typically
  45, 135, 225, 315) at uniformly spaced intervals.
  Each phase shift transmits 2bits
• Combinations of modulation techniques can be used
  to increase throughput

25
Modulation Schemes
26
Multiplexing

• It costs significant amounts to change a
  telephone system, not least the cost of the
  construction
• Hence the more calls you can pump down a cable
  the more profitable the cable becomes
• Telcos (Telephone companies) have developed
  elaborate multiplexing schemes
• The schemes can be divided into two categories
• Frequency Division Multiplexing (FDM)
• Time Division Multiplexing (TDM)
• Lectures on 1G and 2G will discuss these
  different scheme

27
Switching
28
Switching

• As previously stated, original telephone network
  was a series of copper wires leading to a
  Switching Office
• When a call was made an operator used a jumper
  cable to connect caller and callee
• Thus circuit switching was born
• Eventually a automatic switches replaced the
  human operator
• Despite innovations such to microwave, etc the
  basic circuit switch model remained

29
Circuit Switching
30
Circuit Switching

• Advantages
• Once connection established only delay is form
  signal propagation
• No danger of congestion
• Statically reserves required bandwidth
• Transparency, caller / callee are free to use
  whatever protocols, bit rate, frame format they
  want
• How much traffic travels at any given moment does
  not effect cost

31
Circuit Switching

• Disadvantages
• Call cannot commence until connection established
• Connection delay due to system hunting a
  suitable path (may be as much as 10 seconds)
• Statically reserves required bandwidth
• Unused bandwidth is wasted
• Costing model based on time (paying for silence!)
  distance

32
Packet Switching

• Originated as a replacement for message switching
  as used in telegrams
• These were store and forward message systems
  because the entire message was transmitted in one
  block and had to be received in one prior to
  retransmission

33
Packet Switching

• Packet switching placed a limit on the maximum
  size of a block allowing the first block to be
  transmitted before the second was received

34
Packet Switching

• Advantages
• No dedicated route setup time required
• Packets could be buffered at the routers
• Ensured that trafficked flowed freely by
  preventing monopolisation of resource
• Acquires and released bandwidth as required
• Ideally suited to data
• Pay for volume not time or distance

35
Packet Switching

• Disadvantages
• Saturation at routers
• Danger of congestion on every packet
• No guarantee of ordered delivery of packets
• Carrier determined the bit rate, format, framing,
  etc
• Exceeding volume quotas is very expensive

36
Questions?