OFDMA, SDMA, PDMA, CSMA-DA, CSMA-CA Difference between GSM CDMA

Orthogonal frequencyvdivision multiplexing (OFDM)

• Access technique used in Long-Term Evolution (LTE) to accommodate multiple users in a given bandwidth

• Modulation method that divides a channel into multiple narrow orthogonal bands that are spaced so they don’t interfere with one another.
• DATA divided into many lower bit streams.
• Time slot bw each subchannel are used to package the data.
• This technique is very spectrally efficient, so it provides very high data rates. It also is less
• affected by multipath propagation effects.
• To implement OFDMA, each user is assigned a group of subchannels and related time slots.

Space division multiple access SDMA

• SDMA uses physical separation methods that permit the sharing of wireless channels.
• single channel may be used simultaneously if the users are spaced far enough from one another to avoid interference. Known as frequency reuse.
• Useful in gsm

Pdma polarization division multiple access (PDMA)

• One unique variation of SDMA, polarization division multiple access (PDMA), separates signals by using different polarizations of the antennas. 
• Two different signals then can use the same frequency, one transmitting a vertically polarized signal and the other transmitting a horizontally polarized signal.

CSMA-CD

• A unique and widely used method of multiple access is carrier sense multiple access with collision detection (CSMA-CD).
• used in Ethernet local-area networks (LANs). 
• It allows multiple users of the network to access the single cable for transmission. 
• All network nodes listen continuously. When they want to send data, they listen first and then transmit if no other signals are on the line. 
• If two or more transmissions occur simultaneously, a collision occurs.
• The network interface circuitry can detect a collision, and then the nodes will wait a random time before retransmitting.

Csma ca

variation of this method is called carrier sense multiple access with collision avoidance (CSMA-CA).

This method is similar to CSMA-CD. However, a special scheduling algorithm is used to determine the appropriate time to transmit over the shared channel.

 While the CSMA-CD technique is most used in

wired networks, CSMA-CA is the preferred method in wireless networks.

Differentiate between GSM and CDMA

GSM

- GSM uses multiple frequencies.

- Uses TDMA and FDMA for accessing

signals.

- The voice rate is 9.6 KBPS.

- Uses less bandwidth.

CDMA

- CDMA uses single frequency as

carrier.

- The voice rate is 14.4 KBPS.

- CDMA power control to access

method is better as Phased Locked

Loops are used.

- Follows soft handoff, hence call

handling is more efficient.

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CDMA - Basic points

Cdma

• Use spread spectrum techniques.
• Every  channel uses full frequency channel.
• Conversation are encoded with pseudo random digital signal.
• Better capacity for data and vioce.
• Common tech in 3g umts and wcdma.
• Also in gps.
•  A spread spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power.
• Code run at much higher speed than data.
• Data is bitwise xor with code.
• The separation of the signals is made by correlating the received signal with the locally generated code.  If the signal matches User code then the correlation function will be high and the system can extract that signal. If the user's code has nothing in common with the signalthe correlation then zero (thus eliminating the signal); this is referred to as cross-correlation .

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FDMA - Basics

Fdma

• Divide channel into multiple bands.
• Users share the same frequency channel But use a single frequency.
• Between these feq glad band is used.
• Used in analog radio and analog cellular system. Can be used in digital too.
• Req high fiters
• Crosstalk may occur.
• No time problem and near far problem as in tdma and cdma.
• Also support demand assigned, freq assigned on temp basis.
• Tv sysyem.

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TDMA-Main Points

Tdma

1. N users share same radio channel
2. But uses only particular slot
3. Only gsm technology
4. Data streams divided in frames which contains N slots.
5. Duplex mode
6. Bw is N times the Bw required to transmit for single user.
7. Less expansive and smaller
8. N down link it is simple - just multiplexing.
9. In unlink - hard as user has to be aligned in time.

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Networking Basics and Types of Computer Networks

Computer networking is basically the way to connect different computer devices by using the different components mainly hardware and software across the large distances. Thus networking makes us to easily access the information, sends the information from one device to other devices and makes our life easier.

Basic benefits of Networking

There could be many benefits and advantages of connecting through networking but if we talk in basics there are some main basic benefits of computer networking which are as follows

Information or file sharing: It is the need of the modern technology that information should be easily accessible and easily transferable. It could be in any type, a signal, a file or a file or folder. If you want to send information from one computer to other then networking is the essential part.

Resource Sharing : In case you want to share your computer resources using computer networking such as printers, fax machines, Storage Devices, Webcam, Scanners, Modem and many more devices can be shared using computer networking.

Program sharing and remote services:
You can easily share and send any program from one computer to the other computer by using different techniques of networking and by using remote services you can access the resources far away from the actual computer

Different types of computer networks:

LAN (Local area network):

LAN is called as Local area network and as name suggests it is used to connect networking devices which are geographically close to each other. This type of networking include the examples such as a floor of a building, a building itself, or a campus environment.

WAN (Wide area networks)

Wide area network is used when the geographical locations are not that much close so that we can bind them together with the help of LAN, so we make use of wide area network in that case.

MAN (metropolitan area network)

A metropolitan area network (MAN) is considered the mixture of LAN and WAN to connect the devices in a particular geographical area.

Intranet:

Intranet is a network which is local to an organization. All the information that will be available is from that network only and to access that information; they would not need to rely on the other network. This network comprises the LAN, MAN and WAN too.

Extranet:

Extranet is bit wider that of intranet where some of the information or services can be accessible by the thirds party, so the data can be share to outside an oragnisation up to some extent.

Internet:

Internet is used when the information and resources are too made available to all the external and outside users.

VPN: Virtual private network

VPN is a kind of secured network that is used to provide a secure connection over internet or any other public network. VPN use the three way secure connection which include authentication, confidentiality and integrity to ensure that data is sent between two devices is not been tempered by others.

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