Wireless Local Loop

Kent Williams (00058444)

Research Assignment
48740 - Communications Networks
University of Technology, Sydney

Statement of Originality

The work contained in this assignment, other than that specifically attributed to another source, is that of the authors. It is recognised that, should this declaration be found to be false, disciplinary action could be taken and the assignments of all students involved will be given zero marks.

Table of Contents

1. Introduction
2. Background Information
      2.1 Increasing Demand
      2.2 Local Loop Technologies
            2.2.1 Introduction to the Local Loop
            2.2.2 Wired Local Loop
            2.2.3 Wireless Local Loop
3. Wireless Local Loop Technology
      3.1 Radio Propagation
            3.1.1 Attenuation Due to Distance
            3.1.2 Line of Sight
            3.1.3 Diffraction
            3.1.4 Reflection
            3.1.5 Fresnel Zones
            3.1.6 Multipath
      3.2 Netowork Design
            3.2.1 Basic Network Design
            3.2.2 Factors to consider
                  3.2.2.1 Subscriber Density
                  3.2.2.2 Terrain and Obstructions
                  3.2.2.3 Rainfall and Snow
      3.3 Relevant Standards
            3.3.1 802.16-2001
            3.3.2 GSM
            3.3.3 CDMA
                  3.3.3.1 CDMA2000 1X
                  3.3.3.2 CDMA2000 1xEV-DO
            3.3.4 LMDS
            3.3.5 MMDS
      3.4 Manufactures of Wireless Local Loop Equipment
            3.4.1 Customer Premises Equipment
                  3.4.1.1 Nortel Networks
                  3.4.1.2 Lucent
            3.4.2 Base Station Equipment
                  3.4.2.1 Nortel Networks
                  3.4.2.2 Nokia
4. Deployment of Wireless Local Loop Technologies
      4.1 Wireless Local Loop in Australia
      4.2 Wireless Local Loop in Other Developed Countries
      4.3 Wireless Local Loop in Developing Countries and Areas
5. The Future of Wireless Local Loop
6. References
7. Quiz Questions
8. Key Learning Points

1. Introduction

In recent times there has been an increase in demand for the bandwidth required by telecommunication services provided to subscribers. This demand has driven developments in technologies that can be used in the local loop. Once such branch of local loop technologies employs the use of radio waves to transmit data wirelessly, hence the term Wireless Local Loop (WLL). WLL technologies have many benefits and few costs, many of which shall be explored throughout this document.

2. Background Information

2.1 Increasing Demand

In developed countries, the advent of new consumer telecommunications services such as video conferencing and increased usage of existing services such as email and web access, has led to an increase in the bandwidth required by each subscriber. The following table makes estimates as to the current daily usage and compares it to estimates of usage in 2005. [14]

As can be seen in the above table, bandwidth usage can be expected to increase by roughly 200% by the year 2005. These figures do not include values for Video on Demand that will result in an even more dramatic increase in daily usage.
Another point that should be noted is that whilst daily usage increases, the bit rate required by many services is decreasing due to new developments in compression technologies, for example MPEG 4 compression has resulted in the ability to compress mpeg 2 streams up to 8 times further. [2]

2.2 Local Loop Technologies

2.2.1 Introduction to the Local Loop

Almost every home is connected to a telecommunications network. These networks consist of end users, or subscribers, communications carriers and content providers. Content providers transmit information required by subscribers over a communications channel provided by a carrier. For example, a subscriber could request a web page that is transported from the content provider over a telephone line that has been provided by Telstra.
Another example could be of a telephone conversation between two subscribers separated by a vast distance. The call will travel from one subscriber to the local exchange, from there it may travel through a complex network before arriving at the local exchange of the second subscriber.
This link between the end user and the local exchange is called the Local Loop. Typically this is enabled by using twisted pair copper cables that were laid during development. Whilst the traditional system has proven to be sufficient to carry voice calls as well as small amounts of data, it has far from adequate capacity required by the applications outlined above. This demand for greater bandwidth has driven the development of new technologies for the local loop. These technologies can be broken down into two broad categories, wired and wireless.

2.2.2 Wired Local Loop

Wired Local Loop employs the use of cable to complete the local loop. These new broadband technologies may look at news ways of using the existing twisted pair cable, such as xDSL, or they may use the co-axial cable that has been laid by Telstra and Optus for cable television. These solutions may be useful under some circumstances, however both have drawbacks that may required another approach to be taken, for example, not all areas are currently covered by co-axial cable or a subscriber may be to far away from the local exchange for xDSL to operate. Under these circumstances and others, the use of wireless communications may prove to be a better option.

2.2.3 Wireless Local Loop

As the heading suggests, Wireless Local Loop employs the use of electromagnetic radiation to connect subscribers to the local exchange without the use of wires. Recent developments in technology have made broadband WLL services a viable solution, through standards such as 802.16.
However, using a broadband WLL solution to replace an existing twisted pair system in the case where xDSL is not available is not the only case where WLL may be useful.
Remote country areas, or lesser developed nations that do not have existing telecommunications infrastructure that require only a network capable of voice communications may use a wireless narrowband technology to save the cost of laying extensive runs of cable.

3. Wireless Local Loop Technology

3.1 Radio Propagation

For a WLL system to be successful, an adequate Signal to Noise Ratio (SNR) must be achieved.
An adequate SNR means that the level of signal at the receiver is high enough so that the data can be distinguish from the noise generated by the environment in which the system is operating. The required SNR varies depending on the particular piece of radio equipment.
As WLL is an unguided medium that use air for the transmission medium, there are a number of factors that must be taken into consideration which will have an effect on the SNR. Many of these do not effect cable based systems. A few of these shall be discussed below. [14]

3.1.1 Attenuation Due to Distance

As a the distance between a receiver and transmitter increases in a wireless system, the strength of the signal a the receiving end decreases, even if there are no obstacles in the way. The connection between signal strength and distance can described as an inverse square relationship. For example, a receiver is spaced distance 'D' from a transmitter and receives a signal of strength 'S'. If the distance 'D' is doubled to 2 x 'D' then the signal received will be 1/4 'S'. This is illustrated in the diagram to the left.
This phenomena can be likened to a conversation between two people in open space. As the two people move further apart, the voice (signal) of the other person becomes quieter.

3.1.2 Line of Sight

Line of sight (LOS) refers to a clear path between a transmitter and receiver. If a transmitter can be seen from the receiver then LOS can usually be assumed. For maximum SNR, WLL systems should be designed so that LOS is more or less achieved. The reason behind this is that obstacles such as trees and buildings tend to typically block signals, making a link impossible.
The extent that an obstacle blocks a signal can be related to the carrier frequency. A frequency increases, the greater the blocking effect of an obstacle.
However, in instances where LOS is not achievable, the phenomena of diffraction and reflection off tress and buildings may be used to an advantage.

3.1.3 Diffraction

Diffraction is the result of the way that electromagnetic waves propagate. Each point on a wave front radiates outwards as a hemisphere. As a result, a signal may be strong enough despite a receiver being a few degrees below the LOS path due to an obstruction. This can be illustrated more clearly in the picture to the left.

3.1.4 Reflection

Like mirrors can be used by us to see objects that are blocked by an obstruction, many objects reflect radio waves. The extent to which a signal is reflected is dependant upon properties of the surface. A rougher surface would result in a greater spread of reflections, however the signal would be weaker. In practise, concrete and glass buildings make good reflectors.

3.1.5 Fresnel Zones

If a signal path simply grazes the top of an obstruction, there is still a significant loss in signal. This is the result of the Fresnel zone being disrupted. The Fresnel zone can be imagined to be an ellipse between the transmitter and receiver. The radius of the ellipse is dependant upon both wavelength and distance. Fresnel zones are caused by the way in which wave front propagate, however a more detailed analysis is out side the scope of this report.

3.1.6 Multipath

As a result of radio propagation effects such as diffraction and reflection described above, a signal travelling from a transmitter may take alternative paths on its way to the receiver. These paths may cause delays in the received signal due to the extra distance travelled. This is known as multipath. The overall effect of this is lower throughput as the receiver may confuse adjacent signals resulting in an error and the information having to be retransmitted.
For example, if a receiver is situated between a transmitter and another building, a signal may be received along the LOS path and another signal may also be received that has been reflected off the other building. This second signal will be delayed due to the extra distance, and may interfere with the next signal received along the LOS path.

3.2.1 Basic Network Design

Typically, a single transmitter atop of the local exchange will not provide adequate coverage or bandwidth for a successful WLL system. Instead, for each local exchange, multiple transmitters will be required, each covering a smaller area called a 'cell'. A typical implementation of a WLL system is illustrated in the following diagram:

Typical WLL system
Source: Nortel Networks
http://www.nortelnetworks.com/products/01/fwa/index.html#

The placement of transmitters must be carefully considered during design to ensure adequate coverage and bandwidth, whilst minimising infrastructure costs.
For example, a single cell for a relatively large, open area with a low subscriber density may be sufficient. However, this cell may need to be broken up into smaller cells if:

• The subscriber density increased so that there was insufficient bandwidth to meet demand
• The terrain prevents a satisfactory signal strength at the receiver due to LOS issues.

The cost of fixing the second case is often lower than that of the first as a less expensive repeater maybe used to increase the range.

3.2.2 Factors to Consider

As noted in section 3.1, there many factors that must be considered during the design of a WLL system that do not effect the wired alternative. Aside from these, other factors include:

3.2.2.1 Subscriber Density

The bandwidth provided by a single cell in a WLL system must be shared by all subscribers connected to that cell. Problems will arise if the number of subscribers reaches a level where demand for bandwidth exceeds the capability of the system. Careful planning should prevent this from occurring. This may involve studying typical subscriber usage patterns throughout the day and week.
Solutions include splitting the cell into a number of smaller cells or if the technology in use allows for it, using multiple channels for subscribers in a single cell, effectively reducing the number of subscribers sharing an allocated amount of bandwidth.

3.2.2.2 Terrain and Obstructions

Terrain is a key factor to consider when planning the deployment of a WLL system. Placement of towers must be chosen so that the appropriate number of subscribers will be able to receive the signal. The use of existing structures, such as tall buildings may also be taken into consideration to save costs. For example, building a tower at a proposed location that would cover a certain number of subscribers may cost more than the use of two existing buildings that may be able to cover half of the the subscribers each.
Future obstructions must also be taken into consideration. For example, in developing areas a proposed building that is to be constructed after the WLL system is in operation may block the signal to a number of subscribers. If this occurs, additional repeaters may need to be installed or the effected subscribers may need to be connected to a different cell.

3.2.2.3 Rainfall and Snow

Moisture tends to attenuate signals to an extent that is dependent on frequency and distance. As the distance or carrier frequency increases, attenuation due to moisture also increases. This typically only tends to significantly effect signals greater than 5GHz.
Attenuation due to rainfall has to be taken into consideration when developing systems that are to be located in areas subjected to high levels of rainfall or monsoon seasons.

3.3 Relevant Standards

The idea of a Wireless Local Loop is relatively new and as such, there are a limited number of relevant standards. Those that currently do exist are relatively new. This section will look at what is currently available.

3.3.1 802.16-2001

This IEEE standard looks to be one of the most promising for broadband WLL. As defined in the introduction to the standard 802.16 "... specifies the air interface of a fixed (stationary) point-to-multipoint broadband wireless access system providing multiple services in a wireless metropolitan area network (MAN)."
From section 1.2 of the standard, it can be seen that the standard is aimed at frequencies between 10 and 66 GHz, with channels typically 28MHz allowing bandwidth in excess of 120Mb/s.
At these frequencies, direct LOS is typically required with the effects of reflection being minimal. Currently members of the IEEE are working on an extension of the standard, 802.16a, which allows the use of frequencies between 2 and 11 GHz.

3.3.2 GSM

GSM is perhaps currently the most successful digital wireless communications standard, providing services to more that 721 million customers world wide. [4] GSM is an open standard based on TDMA technology and has been deployed throughout more than 120 countries.
Each GSM channel is 200 KHz wide and modulated by using Gaussian minimum shift keying (GMSK). Each channel can carry 8 full rate voice conversations. In idea situations, GSM can provide a range of up to 30 KM.
Currently a third generation of GSM is evolving that is commonly known as '3GSM', however deployment has not been as successful as first hoped. [8]
3GSM offers the following data rates:

• 144 Kbps for users travelling at speeds greater than 120 Km/h
• 384 Kbps for users travelling at speeds less than 120 Km/h
• 2 Mbps for users stationary or travelling at speeds of less than 10 Km/h

Obviously in a WLL system, subscriber equipment would be in a fixed position. As a result, 3GSM networks would allow a number of voice calls and moderate amounts of data, however there is insufficient bandwidth for services such as multiple video streams (however, a single stream should be possible).
One major issue that restricts the use of GSM for use as a WLL technology is that the frequencies used by GSM networks are already being used to provide services for mobile phone customers.

3.3.3 CDMA

CDMA (Code Division Multiple Access) is a technology developed by QUALCOMM. After being introduced in 1995, it now has a world wide subscriber base of over 100 million. This original CDMA standard (known as cdmaOne) supports bit rates of 9.6 Kbps.
Just like GSM, CDMA has evolved to allow higher data rates required by 3G networks. Two standards have been devised. [10]

3.3.3.1 CDMA2000 1X

CDMA2000 1X uses a single channel to provide voice and data services. Currently CDMA2000 1X supports data rates of up to 153 kbps, with rates up to 307 kbps in the future. This technology is also backward compatible with earlier CDMA technology, hence providing an easy upgrade path where CDMA systems have already been deployed.

3.3.3.2 CDMA2000 1xEV-DO

CDMA2000 1xEV-DO provides peak data rates of up to 2 Mbps, and an average throughput of about 700 Kbps using a 1.25 MHz channel resulting in speeds comparable to 3GSM.
In the second quater of 2001, CDMA2000 1xEV-DO was accepted as an ITU standard as part of IMT-2000. Deployment of a network using this technology is already underway in Korea.
An advantage of CDMA2000 1xEV-DO is that it has the ability to operate in any band. [9]

3.3.4 LMDS

Local Multipoint Distribution Service (LMDS) is not a standard as such. Instead it is a term that can be used to describe a variety of wireless networks that use microwave frequencies greater than 25 GHz along with any number of modulation techniques to provide very high data rates in the order of hundreds of Mbps. [3]

3.3.5 MMDS

Multichannel Multipoint Distribution Service (MMDS) is similar to LMDS with the following exceptions: [3]

• MMDS operates in the 2.1 GHz - 2.7 GHz and the 3.4 GHz - 3.6 GHz bands
• MMDS offers a greater range than LMDS systems (in excess of 55 KM)
• MMDS offers data rates in the order of tens of Mbps

3.4 Manufactures of Wireless Local Loop Equipment

There are many manufacture that make equipment that is to be use in a WLL network. These include companies such as Nortel Networks, Nokia and Lucent. There is a searchable database of WLL equipment vendors available at Intelecon's WLL database.

3.4.1 Customer Premises Equipment

Customer Premises Equipment (CPE) refers to equipment that is located at the subscribers premises. Such equipment may include a directional antenna to be mounted on the roof of the household and a distribution box containing a transmitter, receiver and various circuitry to provide internet, television and / or telephony services.

3.4.1.1 Nortel Networks

Nortel Networks provides a set of CPE that includes a 30 cm antenna, remote power and connection unit, and a remote data adaptor for internet access. This system uses the 3.4 GHz - 3.6 GHz band to provide subscribers with internet access and up to two telephone lines. One advantage of this unit is that it allows subscribers to use traditional telephony equipment rather than having to switch over to an IP based phone. More information on this system is available at: http://www.nortelnetworks.com/products/01/fwa/techspec/premise.html

3.4.1.2 Lucent

Lucent began offering a solution called Airloop in 1996. This system uses CDMA technology to transparently provide POTS or ISDN services. More information can be found at: http://www.lucent.com/press/1095/951004.nsb.html
Another solution offered by lucent is its SWING series. This systems operates in the 1910 - 1930 MHz band and offers ISDN data rates of up to 128 Kbps.

3.4.2 Base Station Equipment

Base station equipment typically consists of a sector or omni directional antenna, along with an transceiver that provides an interface to a wired or wireless back bone.

3.4.2.1 Nortel Networks

The Internet FWA is a system offered by Nortel Networks that can act as a transparent extension to circuit switched voice networks or packet switched data networks. It allows for easy migration to add wireless capabilities for locations that currently house wired voice and data systems. Nortel have a webpage for this product at: http://www.nortelnetworks.com/products/01/fwa/techspec/basestation.html

3.4.2.2 Nokia

Nokia offers a system called the DAXnode 5000 WLL. This system uses existing GSM technology to provide subscribers with a service similar to that of POTS. It has the capacity to support 5000 subscribers. More information can be found at: http://press.nokia.com/PR/199706/775779_5.html

4. Deployment of Wireless Local Loop Technologies

WLL systems may prove to be useful in areas where an increase in demand for telecommunication services cannot be fulfilled in a timely fashion due to the time required to lay cable, or in instances where the laying of new cable is impractical. In a developed nation that has existing wired infrastructure WLL systems may allow new providers to enter the market and offer solutions to subscribers at a cheaper rate. In a developing nation, WLL may allow the deployment of an entirely new telecommunications infrastructure quickly with a minimum cost.
Even though broadband technologies exist, narrowband technologies still play an important role as they are often cheaper and suit applications where only a voice service is required.
Because of the large variety of circumstances that exist throughout the world, many different WLL systems have been deployed to varying extents.

4.1 Wireless Local Loop in Australia

One of the key roles of WLL in Australia is to provide narrow band voice services to rural communities. In December 2000, a six month trial was begun that was to provide voice and data services comparable to those available via POTS on King Island. Existing cable runs were suffering from the harsh conditions, long runs and noise generated by electric fences.
Telstra are currently looking at using WLL as an alternative to upgrading cabling in remote areas, as well as in new developing areas throughout greater western Sydney. [13]
A company called AirNet currently offer fixed wireless access in Adelaide. They use 802.11b technology to offer internet access at rates of 256 Kbps to 2 Mbps. Currently they do not offer any voice or video services.
OzEmail is about to begin trials to provide speeds of up to 1 Mbps using 5 MHz bandwidth in the 3G specturm recently auctioned off in Australia and hopes to have a product available for subscribers by the second quater of 2003. [12]
As stated on the web page 'Wireless Broadband Terms of Reference', the Australian government is currently looking in to broadband WLL solutions and their future role in Australia.

4.2 Wireless Local Loop in Other Developed Countries

Sony has committed itself to providing a broadband WLL system in Japan. It plans to offer a solution that provides voice, video and data services.
WLL have been deployed throughout many parts of the developed world to provide broadband solutions as well as to compliment existing infrastructure.

4.3 Wireless Local Loop in Developing Countries and Areas

In a press release by Qualcomm, it has been stated that Qualcomm had been selected to provide a CDMA WLL system to fulfil a demand by 250,000 subscribers in Krasnodar City and its surrounding region in Russia.
This illustrates the role WLL can play in rapidly creating telecommunications infrastructure where existing cabling doesn't exist.
A similar contract was secured by Qualcomm in Southern Luzon located in the Philippines. This CDMA WLL system was designed to serve 50,000 subscribers that previously didn't have access to any telecommunications infrastructure.
These are just two examples of many systems being deployed in developing nations. It is interesting to note that systems being deployed in these areas are typically narrowband solutions that only have capacity to carry voice calls and minimal amounts of data.
 

5. The Future of Wireless Local Loop

As technology improves, prices for WLL equipment will fall, while speeds will continue to increase, meeting demands of next generation applications. The basic consensus is that the individual voice, television and data networks will all converge into a single packet switched network where each subscriber is connected via a broadband link. In many circumstances it likely that this like would be most suitably provided by the Wireless Local Loop.

6. Referecnces

[1] Bing, Benny (2000) "High-Speed Wireless ATM and LANs", Norwood: Artech House
[2] DivXNetworks (2002) "About DivX", accessed 21/8/02
[3] Garg, Vijay and Smolik Kenneth (1997) "Applications of CDMA in Wireless / Personal Communications", Upper Saddle River: Prentice-Hall
[4] Geier, J. (2000) "LMDS and MMDS defined", http://www.wireless-nets.com/column/column_LMDS_MMDS_wireless.htm, accessed: 7/9/02
[5] "GSM World", http://www.gsmworld.com/index.shtml, accessed: 3/9/02
[6] IEEE Standards (2002) "IEEE Standard for Local and Metropolitan area networks, Part 16: Interface for Fixed Broadband Wireless Access Systems"
[7] Nokia (1997) "Nokia Easywave Access - A Wireless Local Loop Solution Linking GSM to the Local Exchange", http://press.nokia.com/PR/199706/775779_5.html, accessed: 7/9/02
[8] Nortel Networks (2002) "Nortel Networks: Internet Fixed Wireless Access System", http://www.nortelnetworks.com/products/01/fwa/index.html, accessed: 3/9/02
[9] Pawsey, C., Green, J., Dineen, R. and Mendz-Villamil, M. (2002) "3G: The future is not what it used to be", http://www.ovum.com/go/product/LatestResearch/008376.htm, accessed: 3/9/02
[10] QUALCOMM (2002) "QUALCOMM > 1xEV-DO Frequently Asked Questions", http://www.qualcomm.com/cdma/1xEV/faq.html, accessed: 7/9/02
[11] QUALCOMM (2002) "QUALCOMM > About CDMA", http://www.qualcomm.com/cdma/, accessed: 7/902
[12] Riley, J. (2002) " OzEmail preps OzEwireless", http://www.itnews.com.au/story.cfm?ID=10722, accessed 7/9/02
[13] Sainsbury, M. (2001) "Telstra's Ready to ramp CDMA", http://www.itnews.com.au/story.cfm?ID=8478, accessed: 7/9/02
[14] William, Webb (2000) "Introduction To Wireless Local Loop: Broadband and Narrowband Systems", 2nd Ed., Norwood: Artech House
[15] "Wireless Communications Association International", http://www.wcai.com/, accessed: 7/9/02

7. Questions

7.1.1 Is the bit rate required by many furture services expected to

a. Increase
b. Decrease
c. Remain the same

7.1.2 Which of the following is part of the Local Loop

a. Subscriber
b. Local exchange
c. Trunk
d. a & b
e. b & c

7.1.3 Which of the following is false

a. xDSL requires relatively short distances
b. xDSL provides a narrowband solution
c. xDSL uses twisted pair cable
d. none of the above

7.1.4 Which of the following is false

a. WLL employs the use of sounds waves
b. 802.16 is a standard that may be used for WLL
c. WLL systems can only be used to replace service provided by existing vioce systems
d. a & c
e. a, b, c

7.1.5 SNR stands for

a. Signal to Nosie Ratio
b. Signal Notation Rate
c. Simple Network Repeater
d. none of the above

7.1.6 Attenuation due to distance

a. decreases as distance increases
b. increases linearly with distance
c. decreases inversly with distance
d. none of the above

7.1.7 LOS refers to

a. Loss of signal
b. Line of sight
c. Line of signal

7.1.8 Diffraction

a. is a result of wave propogation
b. sometimes allows adquate SNR despite and onbstruction
c. all of the above
d. none of the above

7.1.9 A reflected signal

a. will be more spread but weaker from a rough surface
b. will be more focusses but stronger from a rough surface
c. will be more spread but weaker from a smooth surface
d. none of the above

7.1.10 The shape of a Fresnel zone looks like a

a. square
b. circle
c. ellipse
d. rectangle

7.1.11 Multipath can be cause by

a. Reflections
b. Refractions
c. All of the above
d. None of the above

7.1.12 Subscriber density

a. isn't a problem
b. must be considered
c. is a problem that cannot be solved

7.1.13 Terrain

a. is always an insignificant factor during network design
b. may make it difficult to deploy a cost effective network
c. none of the above

7.1.14 Rainfall and snow

a. has an insignificant effect on frequencies above 5 GHz
b. has a significant effect on frequencies below 2.5 GHz
c. has a significant effect on frequencies aboce 5 Ghz
d. none of the above

7.1.15 In relation to WLL standards

a. there are many standards
b. there are few standards
c. there are no current WLL standards

7.1.16 802.16-2001 is

a. An Australian standard
b. an ISO standard
c. an IEEE standard
d. is not a standard

7.1.17 GSM is based on

a. TDMA technology
b. FDMA technology
c. CDMA technology
d. KDMA technology

7.1.18 3GSM will allow

a. no broadcast quality streaming video
b. possibly a single stream of broadcast quality video
c. many streams of broadcast quality videp

7.1.19 CDMA was developed by

a. Qualcomm
b. Erikson
c. Nokia
d. Lucent

7.1.20 MMDS

a. offers a higher data rate than LMDS
b. operates at a higher frequency than LMDS
c. operates at a lower freqency than LMDS
d. offers the same data rate as LMDS

7.2 Answers

b,d,b,d,a,d,b,c,a,c,c,b,b,c,b,c,a,b,a,c

8. Key learning points

• It is predicted that data requirements of subscribers will increase as services converge.
• The local loop is the link between subscribers and the local exchange.
• There are many instances where a wired local loop may be impractical to provide a broadband solution, such as a subscriber being located too far away from a local exchange for xDSL.
• There are both broadband and narrowband wireless local loop technologies.
• Propagation of radio waves is something that must be considered during the development of wireless local loop systems. This does not typically effect wired systems.
• There is an inverse square relationship between distance and signal strength due to free space loss.
• As frequency increases, so does the tendency of objects to block a signal.
• Diffraction sometimes allows the reception of a signal despite no path with direct LOS.
• Reflection can also be used to obtain a signal when there is no direct LOS path.
• Fresnel zones take the shape of an ellipse. If an obstruction is within the Fresnel zone, there will be a significant drop in signal strength
• Multipath is often a issue at lower frequencies due to the tendency of these signals to reflect and refract off objects.
• A single transmitter may not always result in the most cost effective network design.
• Subscriber density is a key factor to consider during network design.
• Terrain and other obstructions often make it more difficult to design a network.
• Moisture in the air tends to significantly attenuate signals greater than 5 GHz.
• The lack of WLL standards can be attributed to the fact that it is a relatively new technology.
• GSM is the most popular standard for mobile wireless systems with over 700 million subscribers
• LMDS employs the use of high frequencies to provide very high bit rates.
• Many of the popular telecommunications equipment manufacturers provide WLL products.
• WLL systems in the more developed countries tend to provide broadband services, while systems for lesser developed countries tend to be concerned with providing voice services.