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Broad Band Networks

by vikas jagtap

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Broad Band NetworkA ‘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 ManagementCongestion 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

1 3 2 4

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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