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PSTN Telephone Network

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The automatic circuit switch was invented by an undertaker named Almon B. Strowger. ... The wife of the other undertaker was a telephone operator. ... – PowerPoint PPT presentation

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Title: PSTN Telephone Network


1
PSTN Telephone Network
2
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.

3
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).

4
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.

5
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.

6
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.

7
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.

8
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.

9
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.

10
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.

11
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.

12
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.

13
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.

14
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.

15
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.

16
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.

17
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.

18
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).

19
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.

20
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.

21
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.

22
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.

23
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.

24
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.

25
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).

26
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.

27
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).

28
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.

29
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).

30
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.

31
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.

32
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.

33
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).

34
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.

35
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).

36
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).

37
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.

38
The Switch Hierarchy
39
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).

40
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.

41
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.

42
  • 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.

43
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.

44
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.

45
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

46
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.

47
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.

48
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).
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