PSTN Telephone Network

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PSTN Telephone Network
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The simple way your own cables

• If you wanted to set up a communication channel
  between two sites, you could install your own
  cables.
• The trouble with that idea is that it can be
  prohibitively expensive. Especially if the cable
  has to travel across roads, rivers or across the
  sea.
• There is an existing system that is available for
  anyone to use - the telephone network.

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Public Switched Telephone Network

• A telephone call is established when the number
  is dialed.
• A connection between the caller and the phone be
  called is established when the last digit is
  dialed.
• The connection follows a route through the
  telephone network.
• This network is called the Public Switched
  Telephone Network (PSTN).

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Communications via the PSTN

• However, the PSTN is designed to carry speech,
  not computer data.
• It is worth looking at because it is so widely
  used to carry computer data.
• It is also a communications network that can be
  used to illustrate many of the problems and ideas
  involved in computer networks.

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Early Days of the Telephone

• When Alexander Graham Bell first marketed
  telephones, they were sold in pairs.
• They were connected by stringing a single wire
  between them and using earth for the return
  circuit.
• This system allowed only
• two people to talk.

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Early Telephone Networks

• If the owner of a telephone wanted to talk to n
  other people, then n separate wires needed to be
  installed.
• Of course everyone wanted to talk to everyone
  else - this meant a lot of wires.

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Fully Interconnected Networks

• If we have n people with telephones and they all
  wanted to talk to each other, we could connect
  every telephone to every other telephone.
• However, toconnect n people,
• we need n(n-1)/2 wires.
• It was quickly realized that a different network
  was needed.

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The Centralized Switching Office

• Bell saw the problem and a solution - rather than
  connecting every telephone to every other
  telephone, why not connect them all to one
  switching office?
• Customers cranked thephone to make a ringing
  noise to attract the attention of the operator.
• The operator then manually connected the caller
  to the callee using a jumper cable.

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Two-level Hierarchy

• Soon there were switching offices springing up in
  cities all over America.
• The next problem was how to connect calls to
  different cities. The answer was to route long
  distance calls through a second hierarchy of
  switching offices.
• It then became a matter ofconnecting regions and
  so onuntil the hierarchy grew to five levels.

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The Basic Bell System

• By 1890, the three major parts of the telephone
  system were in place
• the switching offices
• the wires between customers and the switching
  offices (now balanced, insulated, twisted paired
  cables)
• the long-distance connections between the
  switching offices were in place.
• The basic Bell system has essentially remained in
  use for over 100 years.

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The Basic Telephone System

• The present telephone system is organized as a
  highly redundant (i.e. if one part fails, another
  can take over) multilevel hierarchy.
• Each telephone is connected to the nearest end
  office (also called a local exchange) via a
  twisted pair cable.

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The Basic Telephone System

• The distance to the nearest end office is
  typically between 1km and 10km.
• The connection between a customers telephone and
  the end office is known as the local loop.

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Connecting Calls

• Locally When a call is made to another telephone
  attached to the same end office, the switching
  mechanism in the end office sets up a direct
  connection between the two local loops.
• The connection remains in place for call duration
• Outside local area Each end office has a number
  of outgoing lines to one or more nearby switching
  centers called toll offices
• The connection to a toll office is called a toll
  connecting trunk. If both the caller callee
  happen to have toll connecting trunks to the same
  toll office, then connection established within
  toll office.

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Connecting Long Distance Calls

• If the caller and callee do not have a common
  toll office, the connection must be made higher
  up in the hierarchy.
• Above the toll offices are primary, sectional and
  regional exchanges.
• The higher switching offices connect with each
  other using very high bandwidth intertoll trunks.

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Physical Connections between Switching Offices

• All sorts of different media are used between
  switching offices. Coaxial cables, microwaves
  and (increasingly) optical fibers are used.
• There is often great demand on the connections
  between switching offices and FDM and TDM
  techniques are used to in order to establish
  calls with the minimum cost.

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Establishing Calls

• Calls are no longer connected by operators
  sticking jumper cables into switch boards.
• Calls are now established automatically when a
  number is dialled.
• The numbers identify the regional exchange, the
  sectional exchange, the primary exchange, the
  toll office, the end office and finally the line
  number from the end office of the phone being
  called.

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Telephones Designed for Voice Communication

• The telephone system was originally designed for
  voice communication.
• Analogue local loops are still in wide use and
  will be for many years to come although the UK
  through BT will be one of the first countries to
  be completely digital
• Many other parts of the telephone system are
  being replaced by digital technologies.

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Digital Telephone Systems

• In digital systems, signals are transmitted using
  two level signals (say 5V and 0V but it could be
  5V and -5V or 1.5V and -1.5V). Each level
  represents either a 1 or a 0.
• We have already seen that two level signals
  suffer more distortion than analogue waves (this
  can be overcome by placing digital regenerators
  into the line or by sending the digital signals
  using a carrier wave).

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Advantages of Digital Systems

• Digitals advantage is that the signal can be
  reconstructed whereas analogue always suffers
  distortions and cannot be reconstructed exactly
  (i.e. better quality).
• With digital technology, voice, data, fax and
  images can all be interspersed to make more
  efficient use of equipment and we can make better
  use of high bandwidth cables by using TDM.
• Digital equipment is easier to maintain than
  analogue equipment. A bit is either right or
  wrong, making it easier to track down where
  problems are occurring.
• Sophisticated services can be provided using
  digital technology
• when you move, you can now take your telephone
  number with you.
• you can have dial back options or set up an
  automatic wake-up call.

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Switching Techniques

• Two different switching techniques are used
  inside the telephone system packet switching and
  circuit switching.
• In circuit switching, a single path is
  established through the network.

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Circuit Switching

• Once established, the path of the circuit remains
  the same for the duration of the call.
• Originally the circuits were set up manually by
  operators (using jumper cables).
• The automatic circuit switch was invented by an
  undertaker named Almon B. Strowger.

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Invention of Automatic Circuit Switching Equipment

• Almon B Strowger was one of two undertakers in a
  town. The wife of the other undertaker was a
  telephone operator.
• Every time someone died, their relatives would
  phone the operator and ask to be put through to
  the undertaker.
• Strowger decided that either he had to invent
  automatic switching equipment or go out of
  business.

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Setting up a Path

• Before any data can be sent, the path between the
  caller and callee must be established.
• It can easily take 10 seconds to set up the path
  (more if its an international call).
• During this time interval, the switching
  equipment is searching for a copper path
  through the network.

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Advantages of Circuit Switching

• The advantages are
• For the duration of the call, the communicating
  computers have exclusive use of a connection.
• The full bandwidth of the connection can be used.
• Data can be sent at a constant rate (there are
  not unexpected delays and data arrives in the
  order it was sent).
• Circuit switching is also easier to administer,
  charge for and maintain.

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Disadvantages of Circuit Switching

• The disadvantages are
• There is along delay while the circuit is set up
  and acknowledgement sent.
• The connection can be tapped (thus a potential
  security problem).
• No error checking or flow control is done by
  network, the computers must to it themselves.
• Traffic often consists of short bursts of data
  followed by long periods of inactivity (thus line
  utilization is low).

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Message Switching

• An alternative to circuit switching is message
  switching.
• In message switching, all the connections are
  permanently set up.
• Rather than having exclusive use of a connection,
  all the communicating computers share the
  connections.
• It is the data (or messages) that are switched
  around the network.

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Operation of Message Switching

• Each message incorporates a header containing the
  address of the source and destination computer.
  It may also contain routing information.
• Each message is sent to the local switching
  office that stores the message (checking it for
  errors) and then forwards it on to the next
  appropriate switching office (this technique is
  called store-and-forward).

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Advantages of Message Switching

• The advantages are
• Because the connections are permanent, there is
  no waiting for connections be to set up.
• Flow control and error checking can be handled by
  the network.
• Messages can be sent even when the receiving
  computer is not ready (they can be stored until
  it is ready).
• Security is better because successive messages
  may not be sent along the same path.

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Disadvantages of Message Switching

• The disadvantages are
• There is no limit to the length of a message so a
  single message may block a link for a long time.
• If messages are too long, intermediate switching
  offices may not have sufficient memory to store
  them (in which case they cannot be passed on).

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Packet Switching

• Packet switching, like message switching, uses
  permanent connections.
• In packet switching, the messages are broken up
  into smaller messages called packets (typically
  512 bytes long).
• Each packet has a header containing address and
  routing information as well as its position in
  the original message.

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Packet Switching

• Packets are reassembled by the receiving computer
  to form the original message.
• Packet switching has become widespread in many
  computer networks and the internet.

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Advantages of Packet Switching

• The advantages are
• Because packets are smaller than messages, they
  take less time to transmit across links.
• They also take less memory to store and forward.
• Packets from the same message may be transmitted
  along parallel routes (and may actually take less
  time to reach the destination than a single
  message would).
• More secure because line taps will reveal only
  fragments of messages.

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Disadvantages of Packet Switching

• The disadvantages are
• Packets may arrive at their destination
  out-of-order and there may be a long delay while
  a small number of slow packets find their way
  through the network.
• It is not certain how long it will take a packet
  to pass through the network or how long to wait
  before deciding to request its retransmission).
• Packet switching is not ideal for supplying
  streams of data (as required for radio or T.V).

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Virtual Circuits

• Virtual Circuits attempt to combine the
  advantages of an exclusive connection with the
  advantages of packet switching.
• Essentially, a virtual circuit is a fixed path
  through a network that is establish when a call
  starts.
• Data is transmitted as packets.
• The packets follow the fixed path through the
  network.

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Virtual Circuits

• The difference between a virtual circuit and an
  ordinary switched circuit is that packets from
  other sources can share common links.
• The packets are guaranteed to arrive in the
  correct order.
• It is usually left to the receiving computer to
  ask for damaged or missing packets to be
  retransmitted (this reduces the workload of the
  network and allows higher transfer rates in
  general).

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Virtual Circuits

• Virtual circuits cope well with transmitting
  video and speech data (occasional missing or
  damaged packets are ignored).
• It also handles other types of data (such as file
  transfers).
• When a packet is lost, its absence is detected
  immediately (because of the guaranteed order of
  packets).

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The Switch Hierarchy

• In the USA there are 10 regional switching
  offices. They are fully connected to minimize
  the delay for inter-regional calls.
• They are connected by high-bandwidth fiber optic
  trunks.
• Below the regional offices are 67 sectional
  offices.
• Below them are 1,300 toll offices.
• Below them are 19,0000 end offices.

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The Switch Hierarchy
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Direct Trunks

• Calls are generally connected at the lowest
  possible level. With a pure tree structure,
  there is only one route from end office A to end
  office B.
• During years of operation, the telephone
  companies have noticed that some paths are busier
  than others.
• Rather than going all the way up the hierarchy,
  the busiest routes are connected by direct trunks
  (short cuts).

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Effect of Direct Trunks

• When direct trunks are installed, there is no
  longer a single route for connecting end office A
  to end office B.
• There are now multiple routes along which calls
  can be directed.
• Generally, a call is directed along the shortest
  route but if the necessary direct trunks are
  full, an alternative is chosen.
• Complex routing is now possible because of
  computers.

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Crossbar Switches

• Several kinds of switches are (or were) common
  within the telephone system.
• The simplest kind is the crossbar switch
  (sometimes called a crosspoint switch).
• The switch has N inputs and N outputs for N full
  duplex lines.
• There are N2 intersections, called crosspoints.

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• The connection is a direct electrical connection
  (just like the jumper cables that operators used
  to use).
• Every line can be connected to every other line.

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Space Division Switches

• Rather than having one big crossbar switch, we
  could have lots of smaller connected crossbar
  switches. Theses are called space division
  switches.
• For example, if we had 16 lines, we could have
  four crossbar switches each taking 4 lines.
• The output of the crossbar switches can
  themselves be fed into crossbar switches.

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Space Division Switches

• Each stage of the space division switch is fully
  connected to the next stage. This means that an
  electrical connection can be made from any input
  to any output.

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Pros and Cons of Space Division Switches

• Because the space division switches use many
  smaller crossbar switches, if one fails it can be
  easily replaced without disrupting all the calls.
• On the bad side, it is possible for a Space
  Division Switch to be jammed (i.e. a lot of calls
  had to go through one crossbar switch, all its
  input or output lines may be used up).
• Setting k2n-1 will ensure this will not happen
  Clos 1953

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Time Division Switches

• A completely different kind of switch is the time
  division switch.
• With this kind of switch, the n inputs are
  scanned in sequence to build a frame with n
  slots.
• For T1 switches, the slots are 8 bits (including
  1 control bit).
• 8,000 frames are processed every second.

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Time Division Switches

• Each input is mapped (using an n word mapping
  table) to one of the n output lines.
• The slots are reordered so that they are sent to
  the correct output lines.

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Finally.Advantages of Time Division Switches

• Time Division Switches use digital technology.
• The number of switches involved (be they
  electronic gates) grows linearly with the number
  of inputs.
• The Time Division switch must, however, store and
  forward the n inputs within 1/8000 of a
  second(125 ?sec).