A ‘Baseband’ Network is one in which the cable or other network medium can carry only a single signal at any one time. A ‘Broadband’ network on the other hand can carry multiple signals simultaneously, (using a discrete part of the cables bandwidth for each signal.) As an example of broadband network, consider the cable television service that you probably have in your home. Although only one cable runs at your TV, it supplies you with dozens of channels of programming at the same time.

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Broad Band Networks
by vikas jagtap

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Broad Band Network
A ‘Baseband’ Network is one in which the cable or
other network medium can carry only a single signal
at any one time.
A ‘Broadband’ network on the other hand can carry
multiple signals simultaneously, (using a discrete part
of the cables bandwidth for each signal.)
As an example of broadband network, consider the
cable television service that you probably have in
your home. Although only one cable runs at your TV,
it supplies you with dozens of channels of
programming at the same time.

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Data Transmission Services
We saw that there are many different types of data
transmission media, which may be used for transmission
of data form one point to another.
Organizations may afford to lay their own
communication channels in a small geographical area,
such as within a building or a campus. However, it
becomes impractical for organizations to do so in larger
geographical area, such as between two cities or two
countries. It also impractical for them to set their own
satellite in orbit.

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Data Transmission Services
Therefore organizations hire the services of data
transmission service providers (also known as ‘carriers’)
for their data communications needs.
VSNL( Videsh Sanchar Niger Limited), BSNL (Bharat
Sanchar Nigam Limited), MTNL (Mahanagar Telephone
Nigam Limited) & a few such common carriers in India.
The various types of services offered by common carriers are
1. Dial-Up line
2. Leased- Line
3. ISDN ( Integrated Service Digital Network)

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1. Dial – UP Line
This is service, which operates in a manner similar to a
telephone call i.e. a user of a computer willing to
communicate with a remote computer first makes a
connection request by dialing up the remote
computer.
A circuit is established between two computer via the
telephone company’s switching system.
The modem attached to the user’s computer then
send and receive data over telephone lines.

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2. Leased Line
Leased Line is a special telephone line, which
directly & permanently connects two computers. It
can be used for both voice & data transmission.

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The ISDN is a telephonic system, which provides digital
(not analog) telephone & data services. It carries data at
much higher transmission rates.
ISDN is a digital point-to-point telephone system that has
been around for many years. It is an alternative technology
for home users who require high-bandwidth N/W
connections for links between business networks.
ISDN is a digital service that can provide a good deal more
bandwidth than standard telephone service, but unlike a
leased line, it is not permanent.

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The ISDN devices dial a number to establish a connection,
so that users can connect to different sites as needed. For
this reason, ISDN is known as a circuit-switching service,
because it creates a temporary point-to-point circuit
between two sites.
More over, with the ISDN, no modem is necessary because
it supports digital transmission of all types of data.

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Broadband ISDN
Based on the transmission & switching capabilities the
ISDNs are currently of two types –
1) Narrowband ISDN
2) Broadband ISDN
‘Narrowband ISDN’ is based on 64 Kbps bit-stream that
are combined into higher capacity ‘trunks’ using time-
division multiplexing. For e.g. 32 64 – Kbps channels
can be combined into one 2 Mbps channel.
The narrowband ISDN, however cannot support the
requirements of several types of data services, especially
those needed for multimedia applications.

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For e.g. – the bandwidth required for full definition digital
video is in the 100 Mbps range.
To handle this sort of traffic as well as bursts of data traffic
from computer ‘broadband ISDN’ (B-ISDN) was
introduced
B-ISDN is based on optical fibers & asynchronous time-
division multiplexing.
The advantage of asynchronous time-division multiplexing
over conventional time-division multiplexing is that it
allows the total bandwidth available to be divided between
different activities in a much more flexible way.

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Before proceeding we have to go through following terms –
1) Congestion
2) Queuing theory
3) Traffic Management
4) ATM Traffic Management
1) Congestion : -
“Congestion occurs when the number of packets being
transmitted through a network begins to approach the
packet-handling capacity of the network. The objective of
congestion control is to maintain the number of packets
within the network below the level at which performance
falls off dramatically.”

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The network implements congestion control techniques in
such a way as to protect the network from congestion
while meeting the traffic contracts.
2) Queuing theory : -
A data network or internet is a queues. At each node
there is a queue of packets for each outgoing channel. If
the rate at which packets arrive and queue up exceeds
the rate at which packets can be transmitted, the queue
size grows without bound and the delay experienced by a
packet goes to infinity.
This growth in queue length means that the delay
experienced by a packet at each node increases.

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3. Traffic Management
Congestion control is concerned with efficient use of a
network at high load. When a node is saturated and must
discard packets, it can apply some simple rule, such as
discard the most recent arrival.
Congestion control technique –
1) Fairness – As congestion develops, flows of packets
between source and destination will experience increased
delays and, with high congestion, packet losses.
A node can maintain a separate queue for each logical
connection or for each source-destination pair. If all of the
queue buffers are of equal length, then the queues with
the highest traffic load will suffer discards more often,
allowing lower-traffic connections a fair share of the
capacity.

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2) Quality of Service – Some applications, such as audio
and video, are delay sensitive but loss insensitive. Others,
such as file transfer & e-mail, are delay insensitive but
loss sensitive. Still others, such as interactive graphics or
interactive computing applications, are delay sensitive and
loss sensitive.
A node might transmit higher-priority packets ahead of
lower-priority packets in the same queue. Or a node might
maintain different queues for different QoS levels and give
preferential treatment to the higher levels.
3) Reservations -
One way to avoid congestion and also to provide assured
service to applications is to use a reservation scheme.

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4. ATM Traffic Management
Because of their high speed and small size, ATM networks
present difficulties in effectively controlling congestion not
found in other types of data network.
ATM Forum have defined a range of traffic management
functions to maintain the quality of service (QoS) of ATM
connections.
ATM traffic management function refers to the set of
actions taken by the network to avoid congestion
conditions or to minimize congestion effects.

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Following are the techniques for traffic management –
Resource management using virtual paths
Connection admission control
Usage parameter control
Selective cell discard
Traffic shaping
1) Resource management using virtual paths -
The essential concept behind network resource
management is to allocate network resources in such a
way as to separate traffic flows according to service
characteristics.

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2) Connection admission control -
Connection admission control is the first line of defense
for the network in protecting itself from excessive loads.
By accepting the connection, the network forms a traffic
contract with the user. Once the connection is accepted,
the network continues to provide the agreed QoS as long
as the user complies (fulfill) with the traffic contract.
3) Usage Parameter Control –
Once a connection has been accepted admission control
function, the usage parameter control (UPC) function of
the network monitors the connection to determine
whether traffic conforms to the traffic contract.
The main purpose of Usage parameter control is to
protect network resources from an overload.

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4) Selective Cell Discard -
Selective cell discard comes into play when the network,
at some point beyond the UPC function, discard cells.
The objective is to discard lower –priority cells during
congestion to protect the performance for higer-priority
cells.
5) Traffic Shaping –
Traffic shaping is used to smooth out a traffic flow and
reduce cell clumping. This can result in a fairer allocation
of resources and a reduced average delay time.

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Communication Satellite –
In the 1950s and early 1960s, people tried to set up
communication systems by bouncing signals off
metallized weather balloons. Unfortunately, the received
signals were too weak to be of any practical use.
The US Navy noticed a kind of permanent weather
balloon in the sky – the moon – and built an operational
system for ship-to-shore communication by bouncing
signals off it.
The key difference between an artificial satellite and a
real one is that the artificial one can amplify the signals
before sending them back, turning a strange curiosity
into a powerful communication system.

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A Communication satellite can be thought of as a big
microwave repeater in the sky. It contains several
transponders, each of which listens to some portion of the
spectrum, amplifies the incoming signal, and then
rebroadcast it at another frequency to avoid interference
with incoming signal.
Geostationary Satellite –
Communication satellite can be grouped into categories
according to the height at which they orbit. The easiest
type to understand are known as geosynchronous or
geostationary satellite.
The name arises because a geosynchronous satellite is
placed in an orbit that is exactly synchronized with the
rotation of the earth..

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A new development in the communication satellite world is
the development of low-cost micro stations, sometimes
called VSATs ( Very Small Aperture Terminals).
These tiny terminals have 1-meter or smaller antennas and
can put out about 1 watt of power.
Direct Broadcast satellite television uses this technology
for one-way transmission.
In many VSAT systems, the micro stations do not have
enough power to communicate directly with one another
(via satellite, of course). Instead a special ground station,
the hub, with a large, high-gain antenna is needed to relay
traffic between VSATs as shown in following fig. -

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Communication
satellite
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Hub
VSAT
Fig. – VSATs using a hub.

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In this mode of operation, either the sender or the receiver
has a large antenna and a powerful amplifier.
VSAT has great potential in rural areas. Stringing
telephone lines to thousands of small villages is far
beyond the budgets of most third world governments, but
installing 1-meter VSAT dishes powered by solar cells is
often feasible. VSAT provide the technology that will wire
the world.
Satellite also have the property that the cost of
transmitting a message is independent of the distance
traversed. A call across the ocean costs no more to
service than a call across the street. Satellite also have
excellent error rates and can be deployed almost instantly,
a major consideration for military communication.

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