MULTIPLEXING & MULTIPLE ACCESSGROUP MEMBERS HAFIZ ZAIN-UL-ABIDEEN 31 MUHAMMAD SAAD SIDDIQUI 53 KHALIL AHMED 43 ALI RAZA 15 MUHAMMAD UMAIR 57

Contents

Multiplexing TechniquesSpace Division Multiplexing (SDM)Frequency Division Multiplexing (FDM)Time Division Multiplexing (TDM)Wavelength Division Multiplexing(WDM)

Contents

Multiple AccessFrequency Division Multiple Access (FDMA)Time Division Multiple Access (TDMA)Code Division Multiple Access (CDMA)

Multiplexing Multiplexing is a technique by which several low bandwidth channels are combined into one channel with a larger bandwidth. It is a technique(s) that makes efficient use of transmission medium by allowing multiple signals to be carried on a single medium.

Why Multiplexing?In past one transmission path was used for every signal which is inefficient and costly. It is solved through multiplexing by which we can transmit many signals on one transmission path.

Why Multiplexing?

Under the simplest conditions, a medium can carry only one signal at any moment in time.For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth.

Multiplexing HistoryBy the early 1900’s most cities were filled with telephone poles from which were hung hundreds of wires. In 1910 an Army Signal Corporations officer, Major George O. Squier, built an experimental system based on the concept of separating speech signals in frequency and sending them over the same transmission line.

Multiplexing

This approach of separating signals was called multiplexing, and it made possible the extension of telephone service without adding a pair of wires for every additional circuit that was needed.

Multiplexing

Two or more simultaneous transmissions on a single circuit. Transparent to end user.

Multiplexing costs less.

MultiplexorA device that combines n input lines into one single line (many to one).

DemultiplexorA device that separates one single line into n output lines (one to many).

Analogy – highway with several lanes.medium – highwayusers – carsinterference – accidents 

The current techniques that can accomplish this include frequency division multiplexing (FDM) time division multiplexing (TDM)Synchronous vs statisticalwavelength division multiplexing (WDM)code division multiplexing (CDM)

Categories of multiplexing

Space division multiplexing

The earliest multiplexing technique in which separate pair of wires used for the source to destination end is called Space-division multiplexing. In the early telephone days, each conversation was carried on a separate pairs of wires this approach is known as space division multiplexing.

Suppose that if you want to see two television channels like Discovery and ARY using Space-division multiplexing you would need two television set, each connected to a separate wire. On one set you could see Discovery on the other ARY. You cannot see both channels in one one set because space division multiplexing requires a separate physical path for each signal.

ADVANTAGES AND DISADVANTAGES

Space-division multiplexing has three advantages.

simple and straightforward.

Can be very economical over short physical distances, because no physical equipment is needed other than the wires.

Using separate circuits minimizes the interference among signals being carried between the same two points.

On the other hand Space-division multiplexing has two main disadvantages.Cost and waste. It can be very expensive, because lots of wires and poles are required.Maintaining space-division multiplex systems is often costly because of the number and length of circuits.Wasteful of available bandwidth.

Frequency Division Multiplexing (FDM)

Frequency division multiplexing is a analog technique that combines analog signals. It can be applied when the bandwidth of link is greater than the combine bandwidths.

Frequency Division MultiplexingAssignment of non-overlapping frequency ranges to each “user” or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies.

A corresponding multiplexer, or demultiplexer, is on the end of the high-speed line and separates the multiplexed signals.

FDM process

Multiplexing process Each source generates a signal of a

similar frequency range. Inside the multiplexer, these similar signals modulates different frequencies (f1, f2, and f3). The resulting modulated signals are then combined into a composite signal that is sent out over a media link that has enough bandwidth to accommodate it.

FDM demultiplexing example

Demultiplexing

The demultiplexer uses a series of filters to decompose the multiplexed signal. The individual signals are then passed to demodulator that separates them from their carriers and passes them to the output lines.

Applications of FDMA very common application of FDM is AM and FM radio broadcasting. A special band from 530 KHZ to 1700 KHZ is assigned to AM radio. All radio stations share this band.

Another common use of FDM is in television broadcasting. Each TV channel has its own bandwidth of 6 MHZ. The first generation of cellular telephones (AMPS) also uses FDM. Each user is assigned two 30 KHZ channels, one for sending voice and the other for receiving by using FM.

Advantages

The traditional way of allocating a single channel, such as a telephone trunk, among multiple competing users, is to use Frequency Division Multiplexing.

No two users use the same frequency and thus do not interfere with each other.This technique is good if the number of sending computer is small and fixed with each sending computers having a lot of information to send

DisadvantagesThis technique is bad if there are many users interference problem.If a user has nothing to send, no other user can use their frequency.Data traffic tends to be burstyCapacity of the channel is not being fully exploited. However, it is a fair scheme since each user have opportunity to transmit on the shared medium

Time Division MultiplexingSharing of the signal is accomplished by dividing available transmission time on a medium among users.

Digital signaling is used exclusively.

Time division multiplexing comes in two basic forms:

1. Synchronous time division multiplexing, and

2. Statistical, or asynchronous time division multiplexing.

Synchronous Time Division MultiplexingThe original time division multiplexing.

The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern.

T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing.

Synchronous Time Division MultiplexingIf one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream.

If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.

Synchronous time division multiplexing

So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream.

Synchronous Time Division MultiplexingThree types popular today:

•T-1 multiplexing (the classic)

•ISDN multiplexing

•SONET ( Synchronous Optical Network)

The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels.

24 separate 64Kbps channels

The ISDN multiplexor stream is also a continuous stream of frames. Each frame contains various control and sync info.

SONET – massive data rates

Synchronous TDM

Very popularLine will require as much bandwidth as all the bandwidths of the sources

Statistical Time Division MultiplexingA statistical multiplexor transmits only the data from active workstations (or why work when you don’t have to).

If a workstation is not active, no space is wasted on the multiplexed stream.

A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted.

To identify each piece of data, an address is included.

If the data is of variable size, a length is also included.

More precisely, the transmitted frame contains a collection of data groups.

Statistical Time Division MultiplexingA statistical multiplexor does not require a line over as high a speed line as synchronous time division multiplexing since STDM does not assume all sources will transmit all of the time!

Good for low bandwidth lines (used for LANs)

Much more efficient use of bandwidth!

Wavelength Division Multiplexing (WDM)Give each message a different wavelength (frequency)

Easy to do with fiber optics and optical sources

Dense Wavelength Division Multiplexing (DWDM)Dense wavelength division multiplexing is often called just wavelength division multiplexing

Dense wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line.

Different wavelength lasers (called lambdas) transmit the multiple signals.

Each signal carried on the fiber can be transmitted at a different rate from the other signals.

Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber.

Code Division Multiplexing (CDM)

Old but now new method

Also known as code division multiple access (CDMA)

An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes

Used for mobile communications

Code Division MultiplexingAn advanced technique that allows multiple devices to transmit on the same frequencies at the same time.

Each mobile device is assigned a unique 64-bit code (chip spreading code)

To send a binary 1, mobile device transmits the unique code

To send a binary 0, mobile device transmits the inverse of code

Code Division MultiplexingReceiver gets summed signal, multiplies it by receiver code, adds up the resulting values

Interprets as a binary 1 if sum is near +64

Interprets as a binary 0 if sum is near –64

MULTIPLE ACCESSMultiple hosts sharing the same mediumSHARED MEDIAEthernet busRadio channelToken ring networkSingle shared broadcast channel Two or more simultaneous transmissions by nodes: interference Collision if node receives two or more

signals at the same time

COMMON MULTIPLE ACCESS TECHNOLOGIES

FDMA (FREQUENCY DIVISION MULTIPLE ACCESS)TDMA (TIME DIVISION MULTIPLE ACCESS)CDMA (CODE DIVISON MULTIPLE ACCESS)

FREQUENCY DIVISION MULTIPLE ACCESSFrequency Division Multiple Access or FDMA is an channel Access method that is used by radio systems to share a certain radio spectrum between multiple users. The users are individually allocated one or several frequency bands, allowing them to access the radio system without interfering with each other.

It is important to distinguish between FDMA and frequency-division duplexing (FDD). While FDMA allows multiple users simultaneous access a certain system, FDD refers to how the radio channel is shared between the uplink and downlink (for instance, the traffic going back and forth between a mobile-phone and a base-station).

Furthermore, frequency-division multiplexing (FDM) should not be confused with FDMA. The former is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer.

FDMA

FDMA puts each call on a separate frequency Separates spectrum into distinct

channels by splitting it into uniform bands of bandwidth

Mainly used for analog transmission

FDMA Diagram

TDMA (TIME DIVISION MULTIPLE ACCESS)

Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using his own time slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications (GSM).

TDMA

•With TDMA, a narrow band that has a bandwidth of 30 kHz and is 6.7 ms long is split time-wise into three time slots

–Each conversation gets to transmit for 1/3 of the time

–Possible because voice data converted into digital information that is compressed allowing for less transmission time

TDMA Diagram

CDMA (CODE DIVISION MULTIPLE ACCESS)

Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne and CDMA2000 (which are often referred to as simply "CDMA"), that use CDMA as their underlying channel access methods.

•One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of frequencies. This concept is called multiplexing. CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel.

By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated

CDMA Diagram

2G: IS-95A (1995)Standard IS-95, ANSI J-STD-008

Multiple Access CDMA

Uplink Frequency 869-894 MHz

Downlink Frequency

824-849 MHz

Channel Separation

1.25 MHz

Modulation Scheme

BPSK/QPSK

Number of Channel

64

Channel Bit Rate 1.25 Mbps (chip rate)

Speech Rate 8~13 kbps

Data Rate Up to 14.4 kbps

Maximum Tx Power

600 mW

2.5G: IS-95B (1998)

Increased data rate for internet applications

Up to 115 kbps (8 times that of 2G)

Support web browser format language

Wireless Application Protocol (WAP)

3G TechnologyAbility to receive live music,

interactive web sessions, voice and data with multimedia features

Global Standard IMT-2000

CDMA 2000, proposed by TIA

W-CDMA, proposed by ARIB/ETSI

Issued by ITU (International Telecommunication Union)

3G: W-CDMA (2000)

Stands for “wideband” CDMAChannel Bandwidth: 5, 10 or 20 MHzChip rate at 4.096 MbpsModulation Scheme QPSK in downlink BPSK in uplink

Wish you Best of Luck