Network Hardware (II) Networks and Protocols Prepared by: TGK First Prepared on: Last Modified on:...

Network Hardware (II)

Networks and Protocols

Prepared by: TGK First Prepared on: Last Modified on:

Quality checked by: Copyright 2009 Asia Pacific Institute of Information Technology



Slide 2 of 27

Topic & Structure of the lesson

WAN technologies (Cellular Network, Satellite Network

and Wireless Network)



Slide 3 of 27

Learning Outcomes

At the end of this module, YOU should be able to:

• Explain the WAN technologies – Cellular Network, Satellite Network, and Wireless Network



Slide 4 of 27

Key Terms you must be able to use

If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:

• Cellular Network

• Satellite Network

• Wireless Network



Slide 5 of 27

Main Teaching Points

• Large coverage area• Frequency reuse• Base stations• Radio spectrum• Center-excited cells• Edge-excited cells• Line-of-sight transmissions• Geosynchronous orbit• Centrifugal force• Ad-hoc communication• Design goals• Infrastructure wireless networks• Ad-hoc wireless networks



Slide 6 of 27

Cellular Network

• Concepts of Cellular Communications

• The design objective of early mobile radio systems was to achieve a large coverage area

• Using a single, high powered transmitter with antenna mounted on a tall tower

• While this approach achieved very good coverage

• It was impossible to reuse those same frequencies throughout the system

•Mainly adopted for handheld devices and ad-hoc communications

• For example, the Bell mobile system in New York City in the 1970s could only support a maximum of twelve simultaneous calls over a thousand square miles



Slide 7 of 27

Cellular Network

•The cellular concept was a major breakthrough in solving the problem of spectral congestion and user capacity

• It offered very high capacity in a limited spectrum allocation without any major technological changes

• The cellular concept is a system-level idea which calls for replacing a single, high power transmitter (large cell) with many low power transmitters (small cells)

• Each base station is allocated a portion of the total number of channels available to the entire system



Slide 8 of 27

Cellular Network

• Nearby base stations are assigned different groups of channels so that all the available channels are assigned to a relatively small number of neighboring base stations

• By systematically spacing base stations and their channel groups throughout a market

• The available channels are distributed throughout the geographic region and may be reused as many times as necessary

• As the demand for service increases, the number of base stations may be increased

• Along with a corresponding decrease in transmitter power to avoid added interference



Slide 9 of 27

Cellular Network

Thereby providing additional radio capacity with no additional increase in radio spectrum

This fundamental principle is the foundation for all modern wireless communication systems

The cellular concept allows every piece of subscriber equipment within a country or continent to be manufactured with the same set of channel

Frequency Reuse

Cellular radio systems rely on an intelligent allocation and reuse of channels throughout a coverage region

Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell



Slide 10 of 27

Cellular Network

The base station antennas are designed to achieve the desired coverage within the particular cell

The design process of selecting and allocating channel groups for all the cellular base stations within a system is called frequency reuse or frequency planning

The following figure illustrates the concept of cellular frequency reuse, where cells labeled with the same letter use the same group of channels:



Slide 11 of 27

The figure is conceptual and a simplistic model of the radio coverage for each base station, but it has been universally adopted since hexagon permits easy analysis of a cellular system

The actual radio coverage of a cell is known as the footprint and is determined from field measurements or propagation prediction models

A cell must be designed to serve the weakest mobiles within the footprint, and these are typically located at the edge of the cell

By using the hexagon geometry, the fewest number of cells can cover a geographic region

Cellular Network



Slide 12 of 27

When using hexagons to model coverage areas, base station transmitters are depicted as either being in the center of the cell (center-excited cells)

Or on three of the six cell vertices (edge-excited cells)

Normally, omni-directional antennas are used in center-excited cells and sectored directional antennas are used in corner-excited cells

Practical considerations usually do not allow base stations to be placed exactly as they appear in the hexagonal layout

Most system designs permit a base station to be positioned up to one-fourth the cell radius away from the ideal location

Cellular Network



Slide 13 of 27

Satellite Network

• Introduction• Satellite technology evolved from a limitation of RF behavior: line-of-sight transmission

• If the earth were flat, this line-of-sight behavior would not be an issue

• A signal transmitted in any direction which has sufficient power will get to its intended receiver, and over (relatively) short distances, this is exactly what happens

• The problem arises when an RF signal is transmitted over long distances (greater than 25 miles) and the receiver is obscured by the curvature of the Earth as shown in the following figure:



Slide 14 of 27

Satellite Network

• The left side of figure demonstrates what happens to RF

signals which have sufficient power to travel over 25 miles

• They head toward outer space

• This behavior, while a limitation, is also a benefit

• Because of this behavior, the FCC can allocate the same

frequency to different parties in different geographical

locations



Slide 15 of 27

Satellite Network

• After about 25 miles, the signal is no longer Earthbound

and it therefore no longer interferes with other signals

which are at the same frequency

• An interesting thing to note is that there is an exception to

the behavior depicted on the left side of the figure

• At low RF frequencies (less than 30 MHz), the RF energy

gets reflected off the ionosphere and "bent" around the

earth

• Satellites are not needed for signals at these frequencies



Slide 16 of 27

• The right side of figure shows all you need to know

about the utility of satellites

• Satellites allow RF signals to overcome the curvature

of the Earth and still obey their line-of-sight behavior

• Given the tremendous expense involved in launching

and maintaining satellites, they are only used for long

distance communications

Satellite Network



Slide 17 of 27

Satellite Network

• How Satellites Work• Geosynchronous Orbit

• The satellite systems used for intercontinental telecommunications and television re-transmission work for only one reason: the satellite does not move with respect to the Earth

• This is a pretty good trick since the Earth is rotating at about 1000 miles per hour at the equator

• It just so happens that there is one specific orbit around the Earth, located 22,000 miles up from the equator, called geosynchronous orbit (GEO)

• It is in this orbit where a satellite can rotate around the Earth at the same rotational speed as the Earth



Slide 18 of 27

• A satellite rotating around the Earth in geosynchronous orbit appears to remain stationary when viewed from a point on the equator

• If the satellite did not remain fixed, the direction of the transmitted signal from the ground would have to be continually altered

• How does geosynchronous orbit work?

• there are two forces pulling the satellite in different directions

• the GEO these two forces cancel each other out

• one of these forces is the centrifugal force

• the other force acting on the satellite is gravity, which tries to pull the satellite inward (toward Earth)

Satellite Network



Slide 19 of 27

• at 22,000 miles above the equator, these two forces cancel each other out and the satellite does not move with respect to the Earth

• Because the satellite is so high above the Earth, it is a good news/bad news situation

• The bad news is that because the satellite is so high up, it takes a lot of RF power to reach it from the Earth

• The good news is that signals transmitted down from the satellite can reach almost an entire hemisphere

• Therefore, a satellite used to forward telephone calls from the United States to Great Britain is located halfway

Satellite Network



Slide 20 of 27

Satellite Network

• The following diagram shows how high up the GEO satellites is positioned:



Slide 21 of 27

Wireless Network

• Introduction• The global goal of WLANs is to replace office cabling, to enable connectionless access to the internet and to introduce a higher flexibility for ad-hoc communication

• Some advantages of WLANs are:

• Flexibility

Within radio coverage, nodes can communicate without further restriction. Radio waves can penetrate walls, senders and receivers can be placed anywhere

• Planning

Only wireless ad-hoc networks allow for communication without previous planning, any wired network needs wiring plans



Slide 22 of 27

• Design

Wireless networks allow for the design of small, independent devices which can for example be put into a pocket. Cables not only restrict users but also designers of small PDAs, notepads etc

• Robustness

Wireless networks can survive disasters or users pulling a plug. Networks requiring a wired infrastructure will usually break down completely

• Cost

After providing wireless access to the infrastructure via an access point for the first user, adding additional users to a wireless network will not increase the cost

Wireless Network



Slide 23 of 27

Wireless Network

• However, WLANs also have several disadvantages:

• Quality of service

WLANs typically offer lower quality than their wired counterparts. The main reasons for this are the lower bandwidth due to limitations in radio transmission, higher error rates due to interference, and higher delay/delay variation due to extensive error correction and detection mechanisms

• Proprietary solutions

Due to slow standardization procedures, many companies have come up with proprietary solutions offering standardized functionality plus many enhanced features



Slide 24 of 27

• Restrictions

All wireless products have to comply with national regulations. Several government and non-government institutions worldwide regulate the operation and restrict frequencies to minimize interference

• Safety and security

Using radio waves for data transmission might interfere with high-tech equipment in, e.g., hospitals. Special precautions have to be taken to prevent safety hazards

• Many different, and sometime competing, design goals have to be taken into account for WLANs to ensure their commercial success:

Wireless Network



Slide 25 of 27

Wireless Network

• Global operation

WLAN products should sell in all countries so, national and international frequency regulations have to be considered

• Low power

Devices communicating via a WLAN are typically also wireless devices running on battery power. The LAN design should take this into account and implement special power-saving modes and power management savings

• License-free operation

LAN operators do not want to apply for a special license to be able to use the product. The equipment must operate in a license-free band, such as the 2.4 GHz ISM band



Slide 26 of 27

Wireless Network

• Robust transmission technology

Compared to their wired counterparts, WLANs operate under difficult conditions. If they use radio transmission, many other electrical devices can interfere with them (vacuum cleaners, hairdryers, etc.)

• Simplified spontaneous cooperation

To be useful in practice, WLANs should not require complicated setup routines but should operate spontaneously after power-up. These LANs would not be useful for supporting, ad-hoc meetings

• Easy to use

In contrast to huge and complex wireless WANs, wireless LANs are made for simple use. They should work on plug-and-play



Slide 27 of 27

Wireless Network

• Protection of investment

A lot of money has already been invested into wired LANs. The WLANs should protect this investment by being interoperable with the existing networks

• Safety and security

Wireless LANs should be safe to operate, especially regarding low radiation if used e.g., in hospitals. Users cannot keep safety distances to antennas

• Transparency for application

Existing applications should continue to run over WLANs, the only difference being higher delay and lower bandwidth



Slide 28 of 27

• Infrastructure and ad-hoc wireless networks

• Many WLANs of today need an infrastructure network

• Infrastructure networks not only provide access to other networks, but also include forwarding functions, medium access control, etc

• Communication typically takes place only between the wireless nodes and the access points but not directly between the wireless nodes as shown below:

AP: Access Point

wired network

AP

APAP

Wireless Network



Slide 29 of 27

Wireless Network

• The access point does not only control medium access, but also act as a bridge to other wireless or wired networks

• Several wireless networks may form one logical wireless network

• The access points together with the fixed network in between can connect several wireless networks to form a larger network beyond actual radio coverage

• Typically, the design of infrastructure-based wireless networks is simpler because most of the network functionality lies within the access point, whereas the wireless clients can remain quite simple



Slide 30 of 27

• Collisions may occur if medium access of the wireless nodes and the access point is not coordinated. However, if only the access point controls medium access, no collisions are possible

• The access point may poll the single wireless nodes to ensure the data rate

• Ad hoc wireless networks, however, do not need infrastructure to work

• Each node can communicate directly with other nodes, so no access point controlling medium access is necessary

• The following diagram shows two ad-hoc networks with three nodes:

Wireless Network



Slide 31 of 27

Wireless Network

• Nodes within an ad-hoc network can only communicate if they can reach each other physically, i.e., if they are within each other’s radio range or if other nodes can forward the message

• Nodes from the two networks cannot communicate with each other if they are not within the same radio range

• The complexity of each node is higher because every node has to implement medium access mechanisms, mechanisms to handle hidden or exposed terminal problems, and perhaps priority mechanisms, to provide a certain quality of service



Slide 32 of 27

Quick Review Question

1. Explain how cellular networks are established and operates.

2. Discuss a scenario where Geosynchronous satellites would be most appropriate to be utilised.

3. Explain the differences between Infrastructure and Ad-hoc wireless networks.



Slide 33 of 27

Follow Up Assignment

Conduct a research to determine the general components necessary to establish and operate a Cellular-based or Wireless-LAN infrastructure communications.



Slide 34 of 27

Summary of Main Teaching Points



Slide 35 of 27

Q & A

Question and Answer Session



Slide 36 of 27

Topic and Structure of next session

• LAN Topologies and Standards

LAN Topologies (Bus, Star, Ring, Mesh, and Tree) Characteristics of Topologies LAN Standards (IEEE 802.x Series)

Next Session

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