The public switched network was originally created by AT&T, who used Bell Laboratories
standards to ensure that all central office switches and lines that carried calls met
these preset standards. During the period 1893-1907, people needed different telephones to
communicate with other on a different companys network. The standards set by
AT&T enabled everyone to communicate with anyone else regardless of the service
provider since the dialing, ringing, routing and telephone numbering were all uniform.
The public network consists of two services. It is important to understand the concept of
switched and dedicated services since mistakes in configuring telephone networks could
result in extra expense, insufficient capacity and increased maintenance.
Switched calls are all dial-up. To reach another user, users dial a telephone number to
create a temporary connection. When a telephone number is dialed, dialing the correct
telephone number accesses the switched service. Switched services give an address to which
all calls are directed when a telephone number is dialed. The numbers on the telephone are
used to send dual tone multifrequency (DTMF) tones over the network, which are then
decoded to address signals.
The telephone numbers of areas close together would tend to begin with the same first
three digits (called exchanges). By dialing different telephone numbers, data can be
transferred to multiple locations. ISDN, Switched 56, and Plain Old Telephone services
(POTs) are used to carry switched data calls.
Current developments in technology allow voice mail, bank accounts, home shopping, and
other services to be accessed through switched calls. This is possible due to the standard
DTMF signals that were established by the AT&T so that all callers on the network
would have a consistent format for addressing calls. Using touch-tone dialing means that
functionality of the switched services network has expanded from merely addressing
telephone calls to accessing information in computers.
The charges on the networks are based on the amount of time the calls are connected, with
the rates varying according to time of day and location.
Peak hour calls are more expensive to discourage unimportant calls. This helps ensure that
facilities do not have to be built just to accommodate peak traffic. Calls made across
oceans are also more expensive due to the larger distance, and expense of laying cables
under the oceans.
Switched services are available on-demand. When a handset is lifted, or a modem is
instructed to call, it is expected to connect immediately. Natural disasters, human error,
peak traffic or unusual demand impact availability. Most carriers take great efforts to
ensure the immediate telephone service that customers have come to expect.
The Public Network
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It is a common misconception that
all switched services are analog. ISDN is an example of a digital switched service that
transmits information at higher speeds. Digital services are usually more reliable with
fewer errors than analog. More and more customers want high-capacity ISDN for faster
connections due to work-at-home applications, Internet access and medical uses such as
The existing analog services cabling laid by small companies in residential areas are much
slower which is why many telephone companies and competitive access providers are laying
fiber cables. Although cables terminating at households are expensive, in large
office-buildings, with multiple customers, costs are spread over many customers.
The switched service network is a popular marketing tool. Incoming only telephone lines
are mainly used for toll-free numbers, while outgoing only lines are used mainly for sales
applications or surveys. The network is used more efficiently, with 2-way lines reserved
for other applications. The public switched network was originally designed for voice
traffic, which has very different usage patterns than data traffic.
People on the Internet usually spend a longer time on the telephone line as compared to
voice calls. For residential and business lines, there is a great demand for longer, more
frequent calls and additional telephone numbers.
Carriers originally designed the network based on the assumption that not every telephone
user would be on the network at any one time and that most calls would not tie up the
carriers network for long periods of time.
Circuit switching, the method used by the public network to transmit calls, provides a
very inefficient usage of the network. A circuit is a physical path for voice, image or
data transmission. The International Telecommunications Union (ITU) definition of circuit
switching is the "switching of circuits for the exclusive use of the connection for
the duration of the call".
A user dials, initiating a call, while the network sets up a path that is available
exclusively for the duration of the call and is not shared. The fact that the network
capacity is used for the entire duration of the transmission before releasing the circuit
and freeing up the path for another call illustrates this inefficient utilization of
Newer technologies such as ATM do not have this limitation. In ATM, transmissions from
multiple voice and data services share the same path. Pauses in connection are filled by
data from other sources. Network capacity is therefore not reserved for exclusive use of
idle devices (as is the case with circuit switching). The storage and transfer of messages
during off-peak hours is known as message switching, or store and forward switching. With
message switching, stored messages are transferred at off-peak hours to minimize the
network idle time, and network overload during busy times. This method of switching does
not require both the sender and the receiver to be available at the time of transmission
since the network can hold on to the message, retrying multiple times until the receiving
equipment is finally available.
Not all applications require the real-time, immediate transmission of circuit switching.
It therefore saves cost sending information to multiple users via facsimile or modems
pre-programmed to transmit during off-peak hours. This is especially cost-effective when
sending over long distances.