Analog & Digital
The Public Network
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.
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.
Voice, video and data can all be transmitted over different kinds of media over the
dedicated lines be it copper, fiber or microwave. With dedicated service, organizations
have the use of the line 24 hours a day, 7 days a week, and a flat monthly fee. This is
effective if they transmit a high volume of voice, video and data transmissions between
locations, since the flat fee is less than paying for multiple pay-per-minute calls to the
The dedicated circuits are not shared, and are for the sole use of the customer who owns
or leases the line. If a call is requested to be placed while dedicated private lines are
busy, the private switch sends the call over the public switched network. This allows
firms to pack a high volume of traffic on dedicated lines, but still have the flexibility
to route overflow during peak traffic onto the public network.
Another feature of dedicated lines is that they are leased or built with a fixed capacity
Most lines are leased with a minimum speed of 56 Kbps due to the small price difference
between 56 Kbps and slower lines. Examples of speeds are 9600 bps, 19200 bps, 56 Kbps, 64
Kbps, T-1; 24 channels at 1.54 megabits per second, T1; 2-12 paths in increments of 56 or
64 Kbps per path, T-3; 672 channels at 44 megabits per second, and fractional T-3; 2-18
T-1s in increments of 1.54 megabits. Speeds of greater T-1 are available for large
organizations with high bandwidth requirements. Carriers and local telephone companies
sell both analog and digital private line.
Most users specify digital due to greater speeds, superior quality, more capacity and
greater reliability with fewer errors as compared to analog lines. Speeds greater than
19200 bps are available only in digital formats. Carriers and local telephone companies
prefer leasing digital lines and since digital lines fail less frequently than analog
lines, maintenance costs are reduced.
There are a number of reasons why organizations prefer dedicated private lines to switched
services. One factor is security, especially if transferring proprietary information or
financial data. Private lines are not as easily tapped, and organizations even have the
option of placing encryption devices on both ends of the dedicated services This scrambles
the information leaving the sending location, and unscrambles it before reaching the
receiving end. They are also more convenient since numbers connected by private networks
are dialed with 4-5 digits instead of the normal 11 digits. On-site dedicated links allow
one set of operators to answer calls for multiple locations which saves the cost and
manpower of hiring operators for each location.
These links also improve customer service. A call to any company location can be easily
transferred to another department without having the customer hang up and dial a different
number. Some applications of dedicated services also include video transmission to
multiple sites or call transfers to multiple sites within an organization. Employees at
multiple sites can also access computer data and communicate via voice, video and mail on
There are 4 private line configurations: point-to-point, multi-point, star configuration
and mesh configuration. The main problem with these lines is the time required to manage
them. A lot of maintenance is involved if a company has a private line with multiple sites
that include back-up telecommunication services. Staff time is required for keeping track
of equipment inventory associated with dedicated lines. Staff expertise and knowledge is
also needed for network maintenance.
Many companies are unwilling to spend money for managing, designing and maintaining of the
private facilities so they contract with a carrier for managed value-added services such
as frame-relay. These services are known as virtual networks. Virtual networks, such as a
frame-relay, are like private networks but without the hassle of maintenance and staff. A
carrier manages them, and customers connect to the carrier via a dial-up or dedicated
line. Due to the cost of a private network in both staff and lines, a number of
organizations have private lines for routes with the highest amount of data traffic, and
virtual networks for routes with less voice and data traffic.
Signaling and Network
Signaling is the process of sending information between two points of a network to
control, route and maintain a telephone call. There are basically 3 types of signals
supervisory signals, alerting signals and addressing signals. Supervisory signals
monitor the condition of a telephone and whether it is busy or idle. These signals are
also used to determine when the telephone handset is lifted, hung-up or in an idle
The bell tones or strobe lights that alert the end user that a call has arrived are the
alerting signals. The addressing signals are dial tones or data pulses that tell the
network where to send a call. Signals can be sent over the same channel as voice or data,
or on a separate channel. Before 1976, they were sent along the same path, which this was
known as in-band signaling. In-band signaling sets up calls slower and is inefficient.
Out-of-band signaling, sends signals on a separate channel, and is more efficient, since
the other channels carry more data instead of just signaling information. ISDN is an
example of a technology that uses out-of-band signaling.
Local telephone companies have 2 types of central offices.
Tandem offices work in tandem (along with) each central office. They connect central
offices to each other or to inter-exchange carrier switches, without direct connections to
end-users. They carry the calls on trunks and are therefore able to handle a greater
volume of calls. In contrast, end-offices provide many POT lines between homes and
telephone company switches. The dialing sequence is sent over the same channel as the user
data or call. This results in less volume for calls on these lines than on trunks.
Network-based computer intelligence changes the public network from POTs to a network
capable of delivering advanced features. This helps to generate huge profits for carriers.
Examples of these advanced features are call forwarding, caller ID and voice mail.
A common channel for inter-office signaling or out-of-band signaling was a data
communications network laid over the switching networks. Although this started as a way to
increase network efficiency, it eventually evolved into the basis for intelligent
networks. Signaling System 7 (SS7) is a layered protocol approved by ITU that enables
carriers to work harmoniously with each other. Signals are sent between central offices,
carriers and end-users. SS7 is based on the common channel signaling architecture. Signals
can be sent both ways providing faster speeds. AT&T specified the faster SS7 with
signaling links of 64000 bps. This was later approved by the CCITT (translated from
French, International Telegraph and Telephone Consultative Committee now known as
the International Telecommunications Union, or ITU).
The implementation of SS7 differed among countries. The US implemented the ANSI (American
National Standards Institute) version of SS7 and Europe implemented the ETSI (European
Telecommunications Standards Institute) version. SS7 played an important part in lowering
costs and increasing the reliability of public networks.
The public network has evolved from one carrying only voice and data calls to a network
with increased intelligence, greater capacity and faster recovery from equipment failures.