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 Wireless Networks ppt
Post: #1

Wireless Networks

Cell: Geographical division unit of wireless networks.

• Access Point: Gateway to the network for mobile phones in a cell to contacts.

• Levels of cells in hierarchical cellular network:

– picocell: Each covers up to 100 meters, useful for wireless/cordless applications and devices (e.g. PDAs) in an office or home.
– microcell: Each covers up to 1,000 meters in cities or local areas, e.g. radio access pay-phones on the streets.
– cell: Each has up to 10,000 meters coverage, good for national or continental networks.
– macrocell: World-wide coverage, e.g. satellite phones.

Analog Wireless Networks

1G cellular phones used analog technology and FDMA.
– AMPS (Advanced Mobile Phone System) in North America, operating at 800-900 MHz frequency band.

a) Each of the two-way communication is allocated 25 MHz with (MS Transmit) in the band of 824 to 849 MHz and (BS Transmit) in the band of 869 to 894 MHz.

b) Each of the 25 MHz band is then divided up for two Operator bands, A and B, giving each 12.5 MHz.

c) FDMA further divides each of the 12.5 MHz operator bands into 416 channels – each channel having a bandwidth of 30 KHz.

– TACS (Total Access Communication System) and NMT (Nordic Mobile Telephony) were similar standards in Europe and Asia.

A layout for FDMA cellular system

Each cell in the seven-cell cluster is assigned a unique set of frequency channels, the interference from neighboring cells is negligible.

• The same set of frequency channels (denoted as f1 to f7) will be reused once in each cluster. The so called reuse factor is K = 7.

• In an AMPS system, for example, the maximum number of channels (including control channels) available in each cell is reduced to 416/7 ≈ 59.

TDMA and GSM

TDMA creates multiple channels in multiple time slots while allowing them to share the same carrier frequency.

• GSM was established by CEPT, a standard for a mobile communication network throughout Europe:

– GSM 900: operate in the 900 MHz frequency range.

– GSM 1800: the original GSM standard modified to operate at the 1.8 GHz frequency range.

• In North America:

– GSM 1900: GSM network uses frequencies at the range of 1.9 GHz.

– TIA/EIA IS-54B and the IS-136 standards – the most predominant use of TDMA technology.

– IS-136, superseding IS-54B, operates in the frequencies of 800 MHz and 1.9 GHz (the PCS frequency range).

Spread spectrum: A technology in which the bandwidth of a signal is spread before transmission.

– Distinct advantages of being secure and robust against intentional interference (jamming).

– Applicable to digital as well as analog signals because both can be modulated and “spread”.

– It is the digital applications in particular CDMA that made the technology popular in various wireless data networks.

– Two ways of implementing spread spectrum: frequency hopping and direct sequence.
Post: #2
Wireless Networks

Wireless?

A wireless LAN or WLAN is a wireless local area network that uses radio waves as its carrier.
The last link with the users is wireless, to give a network connection to all users in a building or campus.
The backbone network usually uses cables

How do wireless LANs work?

Wireless LANs operate in almost the same way as
wired LANs, using the same networking protocols
and supporting the most of the same
applications.

Integration With Existing Networks

Wireless Access Points (APs) - a small device that bridges wireless traffic to your network.
Most access points bridge wireless LANs into Ethernet networks.

Hardware

PC Card, either with integral antenna or with external antenna/RF module.
ISA Card with external antenna connected by cable.
Handheld terminals
Access points

Provides higher speeds and higher capacity requirements for applications
Wireless Public Access
Compatible with existing 802.11b standard
Leverages Worldwide spectrum availability in 2.4 GHz
Likely to be less costly than 5 GHz alternatives
Provides easy migration for current users of 802.11b WLANs
Delivers backward support for existing 802.11b products
Provides path to even higher speeds in the future
Post: #3
Wireless Networks

What is Wi-Fi

Wi-Fi is an abbreviation for Wireless Fidelity and a catch all phrase for the several different standards and recommendations that comprise wireless networking.
Wi-Fi enables the user to deploy a computer network without needing to run cable throughout the facility.

Standards

International standard for wireless networking that operates in the 5 GHz frequency range (5.725 GHz to 5.850 GHz) with a maximum 54 Mbps data transfer rate. The 5 GHz frequency band is not as crowded as the 2.4 GHz frequency, because the 802.11a specification offers more radio channels than the 802.11b. These additional channels can help avoid radio and microwave interference.
Cost of 802.11a equipment is approximately twice that of 802.11b and current deployment is limited.

Limitations

300 ft. effective range from access point
1400 ft maximum range
Structural interference
Interference from other devices such as cordless phones.

Applications

Any application currently used on a traditional wired network can be used on a wireless network.
New applications may be available or can be developed to take advantage of wireless, such as Wireless PDA access to a card catalog.

Security

Encryption
Encryption on the pubic network can be used but would create administrative overhead. Encryption keys would have to be changed regularly and anyone using their own laptop would have to be given the key.
Encryption works best in a network that does not allow people to use their own laptops.
Encryption increases privacy, but can be thwarted, either by software, or by gaining access to a PC configured with wireless and writing down the key.

Why Deploy Wireless?

Cost savings over wireline.
Can not run wire to locations needed.
To enable patrons to bring their own laptop, therefore reducing the cost of owning many PCs.
To enable you to easily move PCs.
To be on the leading edge.
Post: #4
Wireless Networking

Class Communication

This class will use class web site to post news, changes, and updates. So please check the class website regularly

Wireless Networking

Wireless networks are everywhere – cellular phone networks, wireless LANs, Bluetooth
This class is intended to cover a very wide spectrum of topics related to wireless networking, including the physical layer, the MAC layer, and the network layer.
After taking this class, you should be able to
Understand basic wireless communication theory (BPSK, CDMA, OFDM, RS code, etc)
Understand the design of wireless networks (802.11 network, cellular phone network, wireless sensor network, etc)

How this course is designed

This class is designed for CS majors who are interested in wireless networks.
There are two groups of people studying wireless networks.
The signal processing approach. Typically focusing on signal processing and deriving the channel capacity. Focusing on physical layer and cellular phone networks.
The computer science approach. Typically treat the physical layer as a black box and focusing on MAC layer and network layer. Wireless LANs, wireless sensor networks.

Physical Layer

Physical layer design goal: send out bits as fast as possible with acceptable low error ratio
Some simple schemes:
There is a wire between A and B. If A wants to send a bit 1’, he connects the wire to the positive end of a battery. Otherwise he disconnects it from the battery.
Or A can hold a radio, if 1’, he sends at frequency f1 and if 0’ he sends at frequency f2.
Or there is an optical fiber between A and B and if 1’ A lit up a light and if `0’ A does nothing.

Wireless communications

The fundamental fact is that if the sender sends a sine wave, the receiver will receive a sine wave at the same frequency. But with
A different phase
A new amplitude
How do you design communication schemes based on that?

BPSK

The simplest transmission scheme is BPSK, which is also widely used.
Convert your information bits to a {-1,+1} square waveform. Let it be I(t). Multiply I(t) with cos(2 \pi ft), and send out.
This is the basic idea. But to make it work, more work has to be done.

The Transmitted Signal

So what you actually send is I(t)cos(2\pi ft), where I(t) is band-limited to BHz.
In 802.11g network, each channel has 22MHz bandwidth. What should B be?
Assume you are given a bandwidth 2BHz centered at fHz. It means that all components higher than (f+B)Hz and all frequency lower than (f-B)Hz will be (or should be) cut-off.

The receiver receives r(t) = AI(t) cos(2 \pi ft + \phi). Here, just for now, assume the receiver somehow magically finds the value of \phi and set it to be 0 (we will talk about this shortly). So he multiplies r(t) with cos(2 \pi ft), and gets AI(t)/2 + AI(t)cos(4 \pi ft)/2.
You apply the LPF again to get rid of the high-frequency components (AI(t)cos(4 \pi ft)/2), and what is left will be proportional to I(t).
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