Although it is a reliable system, PDH has a number of obvious shortcomings. When
designing the next generation of transmission systems, consideration was given to
overcoming these shortcomings. The Synchronous Digital Hierarchy (SDH) was
developed from the American SONET (Synchronous Optical Network) and is
designed to provide an effective, well-managed, reliable, and efficient system for use
with optical-fiber (high-bandwidth) links. It was developed to be compatible with
existing systems and can therefore carry PDH channels as well as other formats.
Although seen as an expensive option compared to the tried and trusted
PDH alternative, the advantages of SDH are well recognized, and SDH is now
the accepted standard for digital transmission around the world. SDH has many
advantages over PDH, most notably:
It is designed to get the best out of high-capacity fiber-optic cables.
It is compatible with many other accepted standards such as E1 and T1.
It has built-in network performance monitoring and management facilities.
It is compatible with both European and American standards.
SDH can multiplex together a variety of different digital signal types, including
those that are already multiplexed using PDH, or even SDH (Figure 2.53).
These signals are arranged by the system onto a standard frame, called a synchronous
transport module (STM), ready for transmission. The smallest of these is STM-1,
which operates at 155 Mbps. There are larger frames, denoted STM-x. The x merely
implies the number of STM-1 equivalents transmitted (systems can employ STM-
4, STM-16, STM-64, or even higher). The inputs are known as tributaries.
STM-1 is equivalent to 63 × E1 links, or 1890 telephone channels.
The common implementation throughout Europe is a 155.52-Mbps link (carrying
many multiplexed channels) in STM-1 (synchronous transfer module) format,
which can itself be multiplexed into higher capacity levels (mainly STM-4, STM-
16, STM-64). These signals are typically transmitted over optical fiber, although it
is possible to send STM-1 over modest distances using coaxial cable or radio.
SDH Network Operation
Every voice or data channel is identifiable in the STM-x and allows selective demultiplexing.
This has the advantage of eliminating the multiplexer mountains of
PDH and allows new network structures beyond simple point-to-point connections.
This also allows some or all of the channels to be effectively protected in
case of a network failure. The ability to automatically protect traffic is an inherent
feature of SDH.
SDH has inherent management capabilities built into its structure. It is possible
to control and configure an entire network remotely. This has given rise to large
NOCs (network operation centers) where an operator can monitor, identify, and
react to any fault in a network within minutes.
Protection and management systems work best where the fiber optic (or other
medium on which SDH is running) is organized in ring structures to provide alternative
reconfigurable routes, and therefore more reliable connections for the user
Tuesday, July 3, 2007
Plesiochronous Digital Hierarchy (PDH)
The PDH system effectively develops the idea of primary multiplexing using time
division multiplexing (TDM) to generate faster signals. This is done in stages by
first combining (multiplexing) E1 or T1 links into what are known as E2 or T2
links, and if required, going even further by combining (multiplexing) E2 or T2
links, etc.
This multiplexing hierarchy is known as the Plesiochronous Digital Hierarchy
(PDH). Plesiochronous, meaning “almost synchronous,” relates to the inputs that
can be of slightly varying speeds relative to each other and the system’s ability to
cope with the differences.
These groups of signals can be transmitted as an electrical signal over a coaxial
cable, as radio signals, or optically via fiber-optic systems. As such, PDH formed
the backbone of early optical networks.
The aggregate signal can be sent to line at any stage of the hierarchy, using the
appropriate transmission medium and modulation techniques.
PDH Network Operation — PDH network equipment is now quite physically
small, allowing for its deployment in locations other than a telephone exchange.
Network operators are able to house such equipment in street cabinets; enough
equipment to supply 120 telephone lines can be stored in an enclosure measuring
1.5 meter by 1 meter high. This has removed the need for many of the smaller telephone
exchange buildings that we used to see in fixed networks.
PDH systems are generally used only for point-to-point communications systems
because the signals must be fully demultiplexed to access a single information
channel. In addition, proprietary alarm configuration and management means that
equipment at either end of a PDH system must be from the same manufacturer.
PDH advantages include:
Equipment small enough for use in street cabinets
Good for point-to-point connections
Cost-effective support for access networks
PDH disadvantages include:
Manufacturer-specific systems
Multiplexer mountains
No integrated network management
Limited management available
division multiplexing (TDM) to generate faster signals. This is done in stages by
first combining (multiplexing) E1 or T1 links into what are known as E2 or T2
links, and if required, going even further by combining (multiplexing) E2 or T2
links, etc.
This multiplexing hierarchy is known as the Plesiochronous Digital Hierarchy
(PDH). Plesiochronous, meaning “almost synchronous,” relates to the inputs that
can be of slightly varying speeds relative to each other and the system’s ability to
cope with the differences.
These groups of signals can be transmitted as an electrical signal over a coaxial
cable, as radio signals, or optically via fiber-optic systems. As such, PDH formed
the backbone of early optical networks.
The aggregate signal can be sent to line at any stage of the hierarchy, using the
appropriate transmission medium and modulation techniques.
PDH Network Operation — PDH network equipment is now quite physically
small, allowing for its deployment in locations other than a telephone exchange.
Network operators are able to house such equipment in street cabinets; enough
equipment to supply 120 telephone lines can be stored in an enclosure measuring
1.5 meter by 1 meter high. This has removed the need for many of the smaller telephone
exchange buildings that we used to see in fixed networks.
PDH systems are generally used only for point-to-point communications systems
because the signals must be fully demultiplexed to access a single information
channel. In addition, proprietary alarm configuration and management means that
equipment at either end of a PDH system must be from the same manufacturer.
PDH advantages include:
Equipment small enough for use in street cabinets
Good for point-to-point connections
Cost-effective support for access networks
PDH disadvantages include:
Manufacturer-specific systems
Multiplexer mountains
No integrated network management
Limited management available
Propagation, Attenuation, and Noise
Propagation describes how a signal travels through a transmission medium (whether
metallic, radio, or optical). The signal energy will generally be confined within
bounded media (mainly cables), but radio signals can follow a variety of paths from
transmitter to receiver, including direct line of sight, reflected (from buildings or
terrain), or refracted (e.g., in layers of the atmosphere).
Attenuation describes the loss of energy as the signal travels through the medium,
resulting in reduced amplitude, while noise will be picked up from other sources of
electromagnetic energy, such as nearby cables or magnetic coils.
The noise levels picked up are usually very small, but the cumulative effect over
distance, together with the attenuation of the original signal, can quickly degrade
the channel to a point where it becomes unintelligible or large data errors occur.
High-bandwidth (usually high data rate) channels tend to degrade faster. Highfrequency
signals also tend to degrade relatively quickly.
Transmission systems are designed to minimize attenuation and provide good
immunity to noise. Fiber-optic cable is exceptionally good on both these counts
and has become the medium of choice for high data rate (high-bandwidth) channels,
especially in the core network.
Even for copper or radio systems, we can still mitigate the problems of attenuation
and noise on long transmission paths by careful system design, and also by
increasing transmitted power to compensate, or amplifying or regenerating the system
at appropriate points in the transmission link before it becomes unintelligible.
Amplifying an analog signal once it has attenuated and also picked up noise
would result in a larger-amplitude signal that also retains the amplified noise.
Whether this affects the user experience would depend on the proportion of noise
in the overall signal and the modulation scheme used.
As long as a digital signal can be regenerated before the noise makes it difficult
to recognize whether the bits are 1s or a 0s, the noise can be eliminated, and a clean
set of data can be retransmitted. Of course, some mistakes will inevitably occur
because of the random nature of the noise that is picked up, but these low levels
of bit errors can be eliminated or minimized by advanced digital processing techniques
in the receiver.
In general, the “noise immunity” that digital signals experience compared to
analog systems ultimately manifests itself in many ways, including clearer voice
channels, low error rate data channels, higher-capacity radio systems (for the same
infrastructure costs), or longer transmission paths.
metallic, radio, or optical). The signal energy will generally be confined within
bounded media (mainly cables), but radio signals can follow a variety of paths from
transmitter to receiver, including direct line of sight, reflected (from buildings or
terrain), or refracted (e.g., in layers of the atmosphere).
Attenuation describes the loss of energy as the signal travels through the medium,
resulting in reduced amplitude, while noise will be picked up from other sources of
electromagnetic energy, such as nearby cables or magnetic coils.
The noise levels picked up are usually very small, but the cumulative effect over
distance, together with the attenuation of the original signal, can quickly degrade
the channel to a point where it becomes unintelligible or large data errors occur.
High-bandwidth (usually high data rate) channels tend to degrade faster. Highfrequency
signals also tend to degrade relatively quickly.
Transmission systems are designed to minimize attenuation and provide good
immunity to noise. Fiber-optic cable is exceptionally good on both these counts
and has become the medium of choice for high data rate (high-bandwidth) channels,
especially in the core network.
Even for copper or radio systems, we can still mitigate the problems of attenuation
and noise on long transmission paths by careful system design, and also by
increasing transmitted power to compensate, or amplifying or regenerating the system
at appropriate points in the transmission link before it becomes unintelligible.
Amplifying an analog signal once it has attenuated and also picked up noise
would result in a larger-amplitude signal that also retains the amplified noise.
Whether this affects the user experience would depend on the proportion of noise
in the overall signal and the modulation scheme used.
As long as a digital signal can be regenerated before the noise makes it difficult
to recognize whether the bits are 1s or a 0s, the noise can be eliminated, and a clean
set of data can be retransmitted. Of course, some mistakes will inevitably occur
because of the random nature of the noise that is picked up, but these low levels
of bit errors can be eliminated or minimized by advanced digital processing techniques
in the receiver.
In general, the “noise immunity” that digital signals experience compared to
analog systems ultimately manifests itself in many ways, including clearer voice
channels, low error rate data channels, higher-capacity radio systems (for the same
infrastructure costs), or longer transmission paths.
Signaling and Control
Control Requirements
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network,
and between network elements. This control information comes in many different
forms, both simple and complex. The information required in the control information
varies from system to system, and also at different points within the same
system. Control information is required to provide services of many types, not just
end-to-end connections. Control data in telecommunications networks has traditionally
been called signaling.
The Importance of Signaling Systems
Signaling systems are essential to the operation of any telecommunications system.
As switching and transmission systems have evolved, signaling systems have had
to develop continuously to support the services offered in modern telecommunications
networks.
Today, with truly global communications systems in place, signaling plays a
critical role. However, most users are unaware of the signal processing that takes
place even for a simple local telephone call. This is a long way from the early
systems where operators sitting in the local exchange performed most signaling
functions.
Functions of a Signaling System
A signaling system must be able to perform many functions within the network.
One of the most important functions is that of call setup, where the dialed digits
are transmitted across the network and the subsequent routing of the call to its
destination. Should the called party be engaged, this must be notified to the calling
party — who will normally hear this as a tone. When a call is completed, clear
down signals must be transmitted to both parties, and the transmission and switching
systems released.
When a normal telephone number is received by a telephone exchange, it can
use simple look-up tables, held locally, to route the call. However, in the case of
freephone or toll-free numbers, the exchange will have to access a regional or central
database to route the call. This will be achieved over signaling links, able to
interrogate and report back from the databases. This technique forms the basis of
Intelligent Network (IN) technology.
Without the ability to accurately and reliably transmit billing information across
and between networks, few network operators would stay in business for very long.
Another key area for signaling is for operation and maintenance purposes. Should it
be necessary to remove an inter-exchange trunk from service, this must be signaled
to a remote exchange so that it does not attempt to use the out-of-service circuit.
Access Signaling
Loop/Disconnect Signaling —The term “loop/disconnect” refers to a type of
signaling used within the local loop, whereby the telephone, fax machine, modem,
or other device sends signaling information to the local exchange (Figure 2.27). It
is so called because the system relies on connecting and disconnecting a loop across
the line, allowing an electrical current to flow between the two wires when the loop
is made. This current is detectable by circuitry in the telephone exchange.
Although not commonly used anymore for this purpose, loop/disconnect signaling
can be used for the transmission of the dialed digits. Each number is transmitted
as a corresponding number of breaks or disconnections in the local loop.
This form of dialing has a speed of ten pulses per second and consequently it takes
a long time to transmit national or international telephone numbers.
While loop/disconnect dialing is not often used, loop/disconnect signaling is
still used for the basic signaling requirements of the local exchange, where the telephone
signals to the exchange request to make a call (line seizure) and also when
the call is cleared down.
DTMF Signaling — Dual-tone multi-frequency (DTMF) signaling (Figure 2.28)
is now the preferred signaling method for the transmission of dialed digits. This
system works by representing each digit on the keypad with a combination of two
frequencies. These frequencies are audible to the caller. DTMF signaling has the
advantage of being much faster than loop/disconnect dialing. It also has the advantage
that the user, being able to hear the tones, is aware if a button press is missed
or accidentally repeated.
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network,
and between network elements. This control information comes in many different
forms, both simple and complex. The information required in the control information
varies from system to system, and also at different points within the same
system. Control information is required to provide services of many types, not just
end-to-end connections. Control data in telecommunications networks has traditionally
been called signaling.
The Importance of Signaling Systems
Signaling systems are essential to the operation of any telecommunications system.
As switching and transmission systems have evolved, signaling systems have had
to develop continuously to support the services offered in modern telecommunications
networks.
Today, with truly global communications systems in place, signaling plays a
critical role. However, most users are unaware of the signal processing that takes
place even for a simple local telephone call. This is a long way from the early
systems where operators sitting in the local exchange performed most signaling
functions.
Functions of a Signaling System
A signaling system must be able to perform many functions within the network.
One of the most important functions is that of call setup, where the dialed digits
are transmitted across the network and the subsequent routing of the call to its
destination. Should the called party be engaged, this must be notified to the calling
party — who will normally hear this as a tone. When a call is completed, clear
down signals must be transmitted to both parties, and the transmission and switching
systems released.
When a normal telephone number is received by a telephone exchange, it can
use simple look-up tables, held locally, to route the call. However, in the case of
freephone or toll-free numbers, the exchange will have to access a regional or central
database to route the call. This will be achieved over signaling links, able to
interrogate and report back from the databases. This technique forms the basis of
Intelligent Network (IN) technology.
Without the ability to accurately and reliably transmit billing information across
and between networks, few network operators would stay in business for very long.
Another key area for signaling is for operation and maintenance purposes. Should it
be necessary to remove an inter-exchange trunk from service, this must be signaled
to a remote exchange so that it does not attempt to use the out-of-service circuit.
Access Signaling
Loop/Disconnect Signaling —The term “loop/disconnect” refers to a type of
signaling used within the local loop, whereby the telephone, fax machine, modem,
or other device sends signaling information to the local exchange (Figure 2.27). It
is so called because the system relies on connecting and disconnecting a loop across
the line, allowing an electrical current to flow between the two wires when the loop
is made. This current is detectable by circuitry in the telephone exchange.
Although not commonly used anymore for this purpose, loop/disconnect signaling
can be used for the transmission of the dialed digits. Each number is transmitted
as a corresponding number of breaks or disconnections in the local loop.
This form of dialing has a speed of ten pulses per second and consequently it takes
a long time to transmit national or international telephone numbers.
While loop/disconnect dialing is not often used, loop/disconnect signaling is
still used for the basic signaling requirements of the local exchange, where the telephone
signals to the exchange request to make a call (line seizure) and also when
the call is cleared down.
DTMF Signaling — Dual-tone multi-frequency (DTMF) signaling (Figure 2.28)
is now the preferred signaling method for the transmission of dialed digits. This
system works by representing each digit on the keypad with a combination of two
frequencies. These frequencies are audible to the caller. DTMF signaling has the
advantage of being much faster than loop/disconnect dialing. It also has the advantage
that the user, being able to hear the tones, is aware if a button press is missed
or accidentally repeated.
Circuit Switching, Packet Switching, and Message Switching
Circuit Switching — In circuit-switched networks , a dedicated path
is set up between the two parties. This path remains for the exclusive use of both
parties for the duration of the call, and is therefore not available to any other users.
This method has traditionally been used within standard telephone exchanges and
networks since telephony was first developed.
There is a delay involved in setting up circuit-switched calls because each of the
switching nodes has to route the call. However, the actual delays encountered once
the connection has been established are minimal. This makes circuit switching
ideal for voice and other real-time applications.
Circuit switching can be inefficient in its use of network resources. During a
voice conversation there are periods when neither party is talking but the connection
is still tied up and unavailable for other users. Similarly, bursty data, which has
gaps between the data, is not efficiently carried over circuit-switching networks.
Charging for circuit-switched services is generally based on the duration of the
call. The PSTN and ISDN are examples where circuit switching is employed.
Packet Switching — Packet switching involves dividing the data into packets (or
cells or frames) prior to transmission. The length of the packets varies enormously,
depending on the technology employed.
Added to each packet is the destination address, together with other control
information. The packets are then transmitted across the network. This addressing
means there is no requirement to set up a pre-established link. To some extent, each
individual packet can be viewed as being able to find its own way to its destination.
In a packet-switched network (Figure 2.23), the resources are shared between
many users. This leads to more efficient use of these resources than provided for by
circuit-switching techniques. However, with packet switching there is a danger of
congestion occurring. The subsequent delays are both variable and unpredictable.
Much effort has been put into reducing these delays for applications that require
near-real-time transmission such as voice telephony.
When data is divided into packets, it does not follow that all the packets that
contain the original data will follow the same route to the destination. This can
order before delivery to the recipient. Examples of packet-switching technologies
include X25, FR (Frame Relay), ATM (Asynchronous Transfer Mode), and IP
(Internet Protocol).
Because it is possible to charge for data throughput rather than for the duration
of the connection within a packet-switched network, it is more feasible to have permanent
online connections than can be provided for by traditional circuit-switched
networks. Most commentators see packet switching as the future backbone of new
high-bandwidth telecommunications services.
Message Switching — It is not always necessary to establish an end-to-end circuit
for the transmission of data. So-called store and forward techniques can be
applied.
In Figure 2.24, an e-mail message is transmitted between nodes A and D.
Because no circuit is established, the message is carried in stages over the links
between the nodes. At each stage, the message is stored within the node while the
next link is established. This does lead to queuing delays at each stage; but to the
applications utilizing message switching, these small delays are unimportant.
Another example of the use of message switching is for SMS text messaging
within a GSM network (Figure 2.25.). When a user sends a text message, it is
transmitted across the air interface into nodes within the GSM network. The entire
message is stored within the node responsible for SMS messaging. Here, the message
is stored before it can be forwarded to its destination. Should a node be unable
to forward a message for any reason, it will retry until an expiry time or number of
attempts is reached. Depending on the application, a failure message can be transmitted
to the originating subscriber to inform them that the message transmission
has failed.
is set up between the two parties. This path remains for the exclusive use of both
parties for the duration of the call, and is therefore not available to any other users.
This method has traditionally been used within standard telephone exchanges and
networks since telephony was first developed.
There is a delay involved in setting up circuit-switched calls because each of the
switching nodes has to route the call. However, the actual delays encountered once
the connection has been established are minimal. This makes circuit switching
ideal for voice and other real-time applications.
Circuit switching can be inefficient in its use of network resources. During a
voice conversation there are periods when neither party is talking but the connection
is still tied up and unavailable for other users. Similarly, bursty data, which has
gaps between the data, is not efficiently carried over circuit-switching networks.
Charging for circuit-switched services is generally based on the duration of the
call. The PSTN and ISDN are examples where circuit switching is employed.
Packet Switching — Packet switching involves dividing the data into packets (or
cells or frames) prior to transmission. The length of the packets varies enormously,
depending on the technology employed.
Added to each packet is the destination address, together with other control
information. The packets are then transmitted across the network. This addressing
means there is no requirement to set up a pre-established link. To some extent, each
individual packet can be viewed as being able to find its own way to its destination.
In a packet-switched network (Figure 2.23), the resources are shared between
many users. This leads to more efficient use of these resources than provided for by
circuit-switching techniques. However, with packet switching there is a danger of
congestion occurring. The subsequent delays are both variable and unpredictable.
Much effort has been put into reducing these delays for applications that require
near-real-time transmission such as voice telephony.
When data is divided into packets, it does not follow that all the packets that
contain the original data will follow the same route to the destination. This can
order before delivery to the recipient. Examples of packet-switching technologies
include X25, FR (Frame Relay), ATM (Asynchronous Transfer Mode), and IP
(Internet Protocol).
Because it is possible to charge for data throughput rather than for the duration
of the connection within a packet-switched network, it is more feasible to have permanent
online connections than can be provided for by traditional circuit-switched
networks. Most commentators see packet switching as the future backbone of new
high-bandwidth telecommunications services.
Message Switching — It is not always necessary to establish an end-to-end circuit
for the transmission of data. So-called store and forward techniques can be
applied.
In Figure 2.24, an e-mail message is transmitted between nodes A and D.
Because no circuit is established, the message is carried in stages over the links
between the nodes. At each stage, the message is stored within the node while the
next link is established. This does lead to queuing delays at each stage; but to the
applications utilizing message switching, these small delays are unimportant.
Another example of the use of message switching is for SMS text messaging
within a GSM network (Figure 2.25.). When a user sends a text message, it is
transmitted across the air interface into nodes within the GSM network. The entire
message is stored within the node responsible for SMS messaging. Here, the message
is stored before it can be forwarded to its destination. Should a node be unable
to forward a message for any reason, it will retry until an expiry time or number of
attempts is reached. Depending on the application, a failure message can be transmitted
to the originating subscriber to inform them that the message transmission
has failed.
Transmission: Media and Systems
The media needs to carry information, and to represent this information requires
a variation in the electrical or optical signal. This variation can take many different
forms, but is generally referred to as modulation, and can use analog or digital
techniques. Modulation is used to vary the electrical or optical signal to represent
information on transmission media.
A fundamental aspect of the transmission medium is the frequency at which it
is designed to work. This will differ from telecommunications system to telecommunications
system and can include multiple frequencies to provide for multiple
communication channels.
Radio systems such as GSM can have hundreds of frequencies specified for
use within a specified part of the frequency spectrum, whereas copper-based telephony
systems might only have a single frequency band specified for use. GSM
needs multiple frequencies to allow different frequencies to be used in different
geographical parts of the network (to avoid radio interference within the communication
channels), whereas copper wire physically separates the communication
channels. This illustrates the difference between unbounded and bounded media
(respectively).
Transmission systems are designed to organize the information in a way that
allows the equipment at either end of the media to work in unison. They provide a
scheme for coding and decoding the information such that one or more communication
channels can be identified (on the media, and at the specified frequency), and
include extra information so that the equipment can be effectively synchronized
and managed. If problems are experienced within the system, this can be notified
via specific alarm channels, allowing remedial action to commence.
Copper Twisted-Pair Cable
Although they are the oldest of the media types used in telecommunications, copper
cables remain the foundation of most national fixed telecommunications systems,
especially within the local loop between the customer’s premises and the local
telephone exchange.
Copper was chosen for its good conductivity, together with its price and flexibility.
There are metals that have better electrical conductivity properties, but most
are more expensive than copper. The requirement for early telephone circuits was
that the media should be capable of carrying low-bandwidth audio signals. The first
copper cables were paper insulated. Paper worked well as an insulator, but as the
number of cable pairs increased, the cables became extremely stiff due to internal
friction. In the 1950s, plastics were introduced as the insulating material, with lowdensity
polyethylene, high-density polyethylene, and polypropylene all being used.
Crosstalk is a phenomenon where signals intended for transmission on a circuit
are electrically induced into adjacent circuits, causing interference. This was a particular
problem of early cables. To reduce this, twists were introduced along each
pair of cables, with up to 25 unique twists being used within a 25-pair cable group,
each spaced at a different distance. It was assumed in the 1980s and 1990s that copper
was an old technology with a limited future. Telecommunications companies
planned ahead to install fiber and coaxial cable systems in the local loop. Eventually,
the cost of doing so was judged to be too high in most cases.
Today, new techniques have been developed to transmit higher data rates than
had previously been thought possible using copper wires with technologies such as
Asymmetric Digital Subscriber Line (ADSL). This means that telecommunications
companies have the opportunity to bring in revenue from high-speed data services
using the existing cable.
Copper Coaxial Cable
A variation of the use of copper is its application in coaxial cables (Figure 2.14).
Here, an inner conductor is first covered in an insulating material and then surrounded
by a wire mesh or metallic screen. The cable is so named because both the
inner conductor and the outer screen share the same axis. Coaxial cables are far
more tolerant of electrical noise than traditional copper pairs and are able to transmit
higher data rates. The use of coaxial cabling dramatically reduces crosstalk. The
main application for coaxial cables was to serve inter-exchange trunk connections
where a pair of cables is used for carrying multiple voice channels, one each for
the transmit and receive paths. Coaxial cabling is also used for so-called thin-wire
Ethernet connections. However, this system has the disadvantage that any failure
along the cable route will cause the entire network to fail.
Radio
Radio systems can be used to transmit signals from a few meters to several thousand
kilometers and can be used for both point-to-point and broadcasting applications.
Radio signals are a type of electromagnetic radiation, with similar properties
to light but with a much shorter wavelength. As with all electromagnetic radiation,
radio waves have both an electric and a magnetic field that travel at right angles
to each other. As the signal travels outward, it can be compared to a stone being
thrown into a pond. Much like the waves on a pond, radio waves get weaker the
further from the transmitter they are. In radio, this progressive weakening of the
signal is referred to as attenuation or path loss.
The frequency of a radio signal will determine how it can be transmitted. Lowerfrequency
signals, such as those in the very low frequency (VLF), low frequency (LF),
and medium frequency (MF) bands that cover frequencies up to about 2 MHz, can
propagate using surface or ground waves. As currents are induced in the Earth, this
has the effect of slowing down the part of the wave that is closest to the ground,
causing a “bending” of the wavefront around the surface of the Earth. Hence, these
lower frequencies can be transmitted either by line of sight or by surface waves. The
combined effect is termed “ground wave” .
Another method of radio propagation is by the use of sky waves.
Certain frequencies, including those in the high frequency (HF) band have the
property of being refracted by layers in the atmosphere known as the ionosphere
and troposphere. Essentially, when the signal reaches a heavily ionized layer, this
layer reflects the signal toward an area of lower ionization. This makes it possible to
send HF transmissions many thousands of miles around the globe. It is possible for
sky waves to propagate lower frequencies but in most cases these work only under
certain atmospheric conditions or at night.
At yet higher frequencies, such as those in the very high frequency (VHF), ultra
high frequency (UHF), and super high frequency (SHF) bands, most of the radio
signal is transmitted by direct waves. Although there is still some component of
ground- and sky-wave propagation, most of the signal relies on line-of-sight transmission.
As frequency increases, so path loss increases. This necessitates the use of highly
focused, directional antennas for microwave transmission such as that used in satellite
communications. One of the main advantages of radio transmission is that
it removes the need for expensive cable-laying activities, which can account for 50
percent of telecommunications infrastructure costs. Coupled with the recent progress
using the radio spectrum more efficiently has led to radio being heralded as one of the
most promising media types for the next generation of telecommunications services.
Optical Fiber
Although first proposed in 1966 by Kao and Hockham, it was only in 1970 that Maurer,
Keck, and Schultz designed and produced the first optical fiber that had characteristics
that made it suitable for use in telecommunications. Their work enabled the
production of fibers that had very little attenuation (loss of signal strength or intensity
in the cable), and which kept most of the light traveling through the cable.
An optical fiber has a very thin core of glass or silica surrounded by an outer
cladding made of a similar material, but with a lower refractive index.
Transmission Systems
Transmission systems are complex and involve many different aspects of information
transfer and managing that information. They refer fundamentally to the way
in which channels can be identified on the transmission medium, rather than the
way the application data is coded or any higher layer transport protocols/systems
(such as IP technology used to code Internet-type traffic).
Just about in all cases, the application data or transport protocols for any network
will require the final coding and synchronization that will allow the information
to be carried and identified within one or more specific channels on the
transmission medium (often at or around a specified “carrier” frequency). It is this
final coding process (and the additional features provided within the coding) that
defines the transmission system used.
a variation in the electrical or optical signal. This variation can take many different
forms, but is generally referred to as modulation, and can use analog or digital
techniques. Modulation is used to vary the electrical or optical signal to represent
information on transmission media.
A fundamental aspect of the transmission medium is the frequency at which it
is designed to work. This will differ from telecommunications system to telecommunications
system and can include multiple frequencies to provide for multiple
communication channels.
Radio systems such as GSM can have hundreds of frequencies specified for
use within a specified part of the frequency spectrum, whereas copper-based telephony
systems might only have a single frequency band specified for use. GSM
needs multiple frequencies to allow different frequencies to be used in different
geographical parts of the network (to avoid radio interference within the communication
channels), whereas copper wire physically separates the communication
channels. This illustrates the difference between unbounded and bounded media
(respectively).
Transmission systems are designed to organize the information in a way that
allows the equipment at either end of the media to work in unison. They provide a
scheme for coding and decoding the information such that one or more communication
channels can be identified (on the media, and at the specified frequency), and
include extra information so that the equipment can be effectively synchronized
and managed. If problems are experienced within the system, this can be notified
via specific alarm channels, allowing remedial action to commence.
Copper Twisted-Pair Cable
Although they are the oldest of the media types used in telecommunications, copper
cables remain the foundation of most national fixed telecommunications systems,
especially within the local loop between the customer’s premises and the local
telephone exchange.
Copper was chosen for its good conductivity, together with its price and flexibility.
There are metals that have better electrical conductivity properties, but most
are more expensive than copper. The requirement for early telephone circuits was
that the media should be capable of carrying low-bandwidth audio signals. The first
copper cables were paper insulated. Paper worked well as an insulator, but as the
number of cable pairs increased, the cables became extremely stiff due to internal
friction. In the 1950s, plastics were introduced as the insulating material, with lowdensity
polyethylene, high-density polyethylene, and polypropylene all being used.
Crosstalk is a phenomenon where signals intended for transmission on a circuit
are electrically induced into adjacent circuits, causing interference. This was a particular
problem of early cables. To reduce this, twists were introduced along each
pair of cables, with up to 25 unique twists being used within a 25-pair cable group,
each spaced at a different distance. It was assumed in the 1980s and 1990s that copper
was an old technology with a limited future. Telecommunications companies
planned ahead to install fiber and coaxial cable systems in the local loop. Eventually,
the cost of doing so was judged to be too high in most cases.
Today, new techniques have been developed to transmit higher data rates than
had previously been thought possible using copper wires with technologies such as
Asymmetric Digital Subscriber Line (ADSL). This means that telecommunications
companies have the opportunity to bring in revenue from high-speed data services
using the existing cable.
Copper Coaxial Cable
A variation of the use of copper is its application in coaxial cables (Figure 2.14).
Here, an inner conductor is first covered in an insulating material and then surrounded
by a wire mesh or metallic screen. The cable is so named because both the
inner conductor and the outer screen share the same axis. Coaxial cables are far
more tolerant of electrical noise than traditional copper pairs and are able to transmit
higher data rates. The use of coaxial cabling dramatically reduces crosstalk. The
main application for coaxial cables was to serve inter-exchange trunk connections
where a pair of cables is used for carrying multiple voice channels, one each for
the transmit and receive paths. Coaxial cabling is also used for so-called thin-wire
Ethernet connections. However, this system has the disadvantage that any failure
along the cable route will cause the entire network to fail.
Radio
Radio systems can be used to transmit signals from a few meters to several thousand
kilometers and can be used for both point-to-point and broadcasting applications.
Radio signals are a type of electromagnetic radiation, with similar properties
to light but with a much shorter wavelength. As with all electromagnetic radiation,
radio waves have both an electric and a magnetic field that travel at right angles
to each other. As the signal travels outward, it can be compared to a stone being
thrown into a pond. Much like the waves on a pond, radio waves get weaker the
further from the transmitter they are. In radio, this progressive weakening of the
signal is referred to as attenuation or path loss.
The frequency of a radio signal will determine how it can be transmitted. Lowerfrequency
signals, such as those in the very low frequency (VLF), low frequency (LF),
and medium frequency (MF) bands that cover frequencies up to about 2 MHz, can
propagate using surface or ground waves. As currents are induced in the Earth, this
has the effect of slowing down the part of the wave that is closest to the ground,
causing a “bending” of the wavefront around the surface of the Earth. Hence, these
lower frequencies can be transmitted either by line of sight or by surface waves. The
combined effect is termed “ground wave” .
Another method of radio propagation is by the use of sky waves.
Certain frequencies, including those in the high frequency (HF) band have the
property of being refracted by layers in the atmosphere known as the ionosphere
and troposphere. Essentially, when the signal reaches a heavily ionized layer, this
layer reflects the signal toward an area of lower ionization. This makes it possible to
send HF transmissions many thousands of miles around the globe. It is possible for
sky waves to propagate lower frequencies but in most cases these work only under
certain atmospheric conditions or at night.
At yet higher frequencies, such as those in the very high frequency (VHF), ultra
high frequency (UHF), and super high frequency (SHF) bands, most of the radio
signal is transmitted by direct waves. Although there is still some component of
ground- and sky-wave propagation, most of the signal relies on line-of-sight transmission.
As frequency increases, so path loss increases. This necessitates the use of highly
focused, directional antennas for microwave transmission such as that used in satellite
communications. One of the main advantages of radio transmission is that
it removes the need for expensive cable-laying activities, which can account for 50
percent of telecommunications infrastructure costs. Coupled with the recent progress
using the radio spectrum more efficiently has led to radio being heralded as one of the
most promising media types for the next generation of telecommunications services.
Optical Fiber
Although first proposed in 1966 by Kao and Hockham, it was only in 1970 that Maurer,
Keck, and Schultz designed and produced the first optical fiber that had characteristics
that made it suitable for use in telecommunications. Their work enabled the
production of fibers that had very little attenuation (loss of signal strength or intensity
in the cable), and which kept most of the light traveling through the cable.
An optical fiber has a very thin core of glass or silica surrounded by an outer
cladding made of a similar material, but with a lower refractive index.
Transmission Systems
Transmission systems are complex and involve many different aspects of information
transfer and managing that information. They refer fundamentally to the way
in which channels can be identified on the transmission medium, rather than the
way the application data is coded or any higher layer transport protocols/systems
(such as IP technology used to code Internet-type traffic).
Just about in all cases, the application data or transport protocols for any network
will require the final coding and synchronization that will allow the information
to be carried and identified within one or more specific channels on the
transmission medium (often at or around a specified “carrier” frequency). It is this
final coding process (and the additional features provided within the coding) that
defines the transmission system used.
Transmission
The transmission medium can take many different forms, such as (1) copper wire,
which has been used since the early days of communication; (2) optical fiber, which
is a relatively new medium and is increasingly being used; and (3) radio, which also
has been used for many years. Each of these media can be bi-directional, that is,
allowing information to pass in both directions.
Copper may be limited by bandwidth, which limits the amount of information
that can be transmitted in a given time, compared to optical fiber, but its cost is
low. In contrast, optical fiber is relatively expensive but can carry more data in a
given time. Both copper wire and optical fiber cable are more reliable than radio,
but radio has many other advantages.
Radio transmission is a very versatile medium, ideally suited to a mobile environment,
but its bandwidth is extremely limited and therefore its total capacity is
limited. It can also be adversely affected by atmospheric conditions.
Speed of data transfer, error rates, and other key characteristics determine which
transmission medium will be used in particular circumstances. Information can be
represented in many different forms. To ensure compatibility across systems, standard
transmission systems and techniques have been specified in many instances.
Switching
To enable the placement of a call from the originator through the network and
on to its final destination, that call must pass through several switches or routers.
Switches or routers allow great flexibility in connecting transmission
resources. This gives rise to endless possibilities for end-to-end connections between
users. Users may be located nearby or be very remote, but the switches or routers
within the network will ensure that the information passes over the required transmission
resources to provide end-to-end connectivity.
The number of switches or routers needed in an end-to-end connection will
vary, and will depend on many factors, including network topology (the configuration
of the various elements), the geographical distance between the user terminals,
and the capacity of the switch or router and associated transmission media.
Signaling and Control
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network, and
between network elements. This control information comes in many
different forms, both simple and complex.
The information that must be included in the control information varies from
system to system, and also at different points within the same system. Control
information is required to provide services of many types, not just end-to-end connections.
Control data in telecommunications networks has traditionally been
called signaling.
Billing
Increasing network sophistication and a corresponding increase in service choices
and methods of service provision, have led to an increased requirement for flexible
and more complex billing systems. This is confusing for network operators and
customers alike, and a great deal of work is underway to provide choices that are
achievable, simple to implement, and, very importantly, reflect the requirements of
the customer. Service charges that are simple to understand, as well as flexible payment
options, are of increasing importance.
which has been used since the early days of communication; (2) optical fiber, which
is a relatively new medium and is increasingly being used; and (3) radio, which also
has been used for many years. Each of these media can be bi-directional, that is,
allowing information to pass in both directions.
Copper may be limited by bandwidth, which limits the amount of information
that can be transmitted in a given time, compared to optical fiber, but its cost is
low. In contrast, optical fiber is relatively expensive but can carry more data in a
given time. Both copper wire and optical fiber cable are more reliable than radio,
but radio has many other advantages.
Radio transmission is a very versatile medium, ideally suited to a mobile environment,
but its bandwidth is extremely limited and therefore its total capacity is
limited. It can also be adversely affected by atmospheric conditions.
Speed of data transfer, error rates, and other key characteristics determine which
transmission medium will be used in particular circumstances. Information can be
represented in many different forms. To ensure compatibility across systems, standard
transmission systems and techniques have been specified in many instances.
Switching
To enable the placement of a call from the originator through the network and
on to its final destination, that call must pass through several switches or routers.
Switches or routers allow great flexibility in connecting transmission
resources. This gives rise to endless possibilities for end-to-end connections between
users. Users may be located nearby or be very remote, but the switches or routers
within the network will ensure that the information passes over the required transmission
resources to provide end-to-end connectivity.
The number of switches or routers needed in an end-to-end connection will
vary, and will depend on many factors, including network topology (the configuration
of the various elements), the geographical distance between the user terminals,
and the capacity of the switch or router and associated transmission media.
Signaling and Control
To ensure the establishment of end-to-end connections with the required quality
of service, control information must be passed between users and the network, and
between network elements. This control information comes in many
different forms, both simple and complex.
The information that must be included in the control information varies from
system to system, and also at different points within the same system. Control
information is required to provide services of many types, not just end-to-end connections.
Control data in telecommunications networks has traditionally been
called signaling.
Billing
Increasing network sophistication and a corresponding increase in service choices
and methods of service provision, have led to an increased requirement for flexible
and more complex billing systems. This is confusing for network operators and
customers alike, and a great deal of work is underway to provide choices that are
achievable, simple to implement, and, very importantly, reflect the requirements of
the customer. Service charges that are simple to understand, as well as flexible payment
options, are of increasing importance.
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