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More about Transmission Protocol

various multiple access schemes

(FDMA, TDMA and CDMA), network protocols (AMPS/NAMPS,

IS-54/-I36, IS-95, GSM, DCS 1800, and PDC), etc.

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Frequency Modulation (FM)

Frequency Modulation occurs when a carrier's CENTER frequency is changed

based upon the input signal's

amplitude. Unlike Amplitude Modulation, the carrier signal's amplitude is

UNCHANGED. This makes FM

modulation more immune to noise than AM and improves the overall

signal-to-noise ratio of the communications

system. Power output is also constant, differing from the varying AM power

output.

The amount of analog bandwidth necessary to transmit a FM signal is greater

than the amount necessary for AM, a

limiting constraint for some systems.

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Multiple Access Methods [17]

This section contains a survey of the more common techniques for allowing

many users to access information over a

single channel. It is not always possible to provide a path from source to

destination which is dedicated to only one

user, as this can be very expensive or physically impossible. Instead,

networks have been designed to control the

flow of information for many users through a limited supply of physical

links. In a network it is desirable to serve as

many people as possible with the available channel bandwidth.

Besides maximizing the number of users served by the channel, concern must

be given to the highest data rate at

which each user may operate. There are many ways to approach the problem,

and therefore different techniques

have emerged to deal with it. Three general solutions will be discussed in

this section. They are: frequency division

multiple access (FDMA), time division multiple access (TDMA), and code

division multiple access (CDMA). Each

method works on a different principle and each has its own strengths and

weaknesses.

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Frequency Division Multiple Access

One of the most widely used multiple access techniques is frequency

division. Television, AM and FM radio all rely

on this principle. The idea in its simplest form is to divide the usable

channel bandwidth into smaller partitions which

can be assigned to particular users. A user can operate in the assigned

narrow band of frequencies at any time.

To transmit in the assigned band of frequencies the message signal must be

modulated, in some way, up from

baseband. The exact way in which this is done depends on the particular

scheme being employed. Although

implementation schemes for modulated systems vary in approach and

complexity, a simple way to understand the

theoretical basis is as follows. The message is multiplied by a cosine to

move the message's spectrum into the

allotted frequency band. At the receiver, the composite signal can be

multiplied by another cosine of the same

frequency to recover the message signal. Figure 1 displays both baseband

and modulated signals in the frequency

domain.

Figure 1. Baseband and modulated sognal spectra for

FDMA.

When using this scheme there is a trade-off between the number of users and

the data rate for each user. If a large

number of users is required, the assigned bandwidths will have to be very

small. This limits the data rate each

channel can support. If, on the other hand, a large data rate is required,

the number of users in the total usable

bandwidth will decrease. Additionally, in practice a guardband of unused

frequencies must be allocated between

adjacent spectra, further lowering the total data throughput of the

channel.

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Frequency Division Multiplexing (FDM)

In FDM, multiple channels are combined onto a single aggregate signal for

transmission. The channels are

separated in the aggregate by their FREQUENCY.

There are always some unused frequency spaces between channels, known as

"guard bands". These guard bands

reduce the effects of "bleedover" between adjacent channels, a condition

more commonly referred to as

"crosstalk".

FDM was the first multiplexing scheme to enjoy widescale network

deployment, and such systems are still in use

today. However, Time Division Multiplexing is the preferred approach today,

due to its ability to support native

data I/O (Input/Output) channels.

FDM Data Channel Applications

Data channel FDM multiplexing is usually accomplished by "modem stacking".

In this case, a data channel's modem

is set to a specific operating frequency. Different modems with different

frequencies could be combined over a

single voice line. As the number of these "bridged" modems on a specific

line changes, the individual modem

outputs need adjustment ("tweaking") so that the proper composite level is

maintained. This VF level is known as

the "Composite Data Transmission Level" and is almost universally -13 dBm0.

Although such units supported up to 1200 BPS data modem rates, the most

popular implementation was a

low-speed FDM multiplexer known as the Voice Frequency Carrier Terminal

(VFCT).

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Time Division Multiple Access

In some communication systems a single channel will support many users, but

only one at a time. This is called time

division, and it requires a multiplexer and demultiplexer to function

correctly. If, for example, many computers want

to transmit to a common location and only one link exists, TDMA may be

employed to give access to all the

computers. All the computers waiting for service will connect to the

multiplexer at the transmitter. The output of the

multiplexer is always connected to the link, and its function is to provide

a path between one of its many inputs and

the link. The multiplexer selects only one input at a time, but it is able

to switch from input to input rapidly.

Essentially this connects each computer to the link, but for only short

periods of time. The information sent in this

small period is called a packet, of which many are sent by each computer.

At the receiver, the demultiplexer sorts out the packets sent by each

computer and sends each packet to its

intended destination. The demultiplexer has one input, always connected to

the link, and many outputs. Similarly to

the multiplexer, the demultiplexer provides a path between the input and

one of the outputs, and is able to switch

between outputs. All the destination sites are connected to one output port

of the demultiplexer. A high level

diagram of a TDMA network is shown in Figure 2.

Figure 2. A simple TDMA network.

Since some information must be sent with each packet to indicate its

intended destination, controllers must be

employed to label all the packets at the transmitter. At the receiver,

controllers must read the labels and create a

route to the destination. The hardware and/or software needed to accomplish

this control is often very complex.

As with FDMA, there is a trade-off between the number of users and the data

rate per user. In order to

accommodate more users the packet time allotted to each user will decrease,

thereby increasing the actual time it

takes a user to send a specific number of bits. Notice that it is not

always possible to increase the number of bits

per second transmitted over the channel since the channel bandwidth is

finite.

Time Division Multiplexing

Timeplex is probably the best in the business (IMHO) at Time Division

Multiplexing, as it has 25+ years or

experience. When Timeplex was started by a couple of ex-Western Union guys

in 1969 it was among the first

commercial TDM companies in the United States. In fact, "Timeplex" was

derived from TIME division

multiPLEXing!

In Time Division Multiplexing, channels "share" the common aggregate based

upon time! There are a variety of

TDM schemes, discussed in the following sections:

Conventional Time Division Multiplexing

Statistical Time Division Multiplexing

Cell-Relay/ATM Multiplexing

Conventional Time Division Multiplexing (TDM)

Conventional TDM systems usually employ either Bit-Interleaved or

Byte-Interleaved multiplexing schemes as

discussed in the subsections below.

Clocking (Bit timing) is critical in Conventional TDM. All sources of I/O

and aggregate clock frequencies should be

derived from a central, "traceable" source for the greatest efficiency.

Bit-Interleaved Multiplexing

In Bit-Interleaved TDM, a single data bit from an I/O port is output to the

aggregate channel. This is followed by a

data bit from another I/O port (channel), and so on, and so on, with the

process repeating itself.

A "time slice" is reserved on the aggregate channel for each individual I/O

port. Since these "time slices" for each

I/O port are known to both the transmitter and receiver, the only

requirement is for the transmitter and receiver to

be in-step; that is to say, being at the right place (I/O port) at the

right time. This is accomplished through the use of a synchronization

channel between the two multiplexers. The synchronization channel

transports a fixed pattern that the receiver uses to acquire

synchronization.

Total I/O bandwidth (expressed in Bits Per Second - BPS) cannot exceed that

of the aggregate (minus the

bandwidth requirements for the synchronization channel).

Bit-Interleaved TDM is simple and efficient and requires little or no

buffering of I/O data. A single data bit from

each I/O channel is sampled, then interleaved and output in a high speed

data stream.

Unfortunately, Bit-Interleaved TDM does not fit in well with today's

microprocessor-driven, byte-based

environment!

Byte-Interleaved Multiplexing

In Byte-Interleaved multiplexing, complete words (bytes) from the I/O

channels are placed sequentially, one after

another, onto the high speed aggregate channel. Again, a synchronization

channel is used to synchronize the

multiplexers at each end of the communications facility.

For an I/O payload that consists of synchronous channels only, the total

I/O bandwidth cannot exceed that of the

aggregate (minus the synchronization channel bandwidth). But for

asynchronous I/O channels, the aggregate

bandwidth CAN BE EXCEEDED if the aggregate byte size is LESS than the total

asynchronous I/O character size

(Start + Data + Stop bits). (This has to do with the actual CHARACTER

transmission rate of the asynchronous

data being LESS THAN the synchronous CHARACTER rate serviced by the TDM).

Byte-Interleaved TDMs were heavily deployed from the from the late 1970s to

around 1985. These units could

support up to 256 KBPS aggregates but were usually found in 4.8 KBPS to 56

KBPS DDS and VF-modem

environments. In those days, 56 KBPS DDS pipes were very high speed

circuits. Imagine!

In 1984, with the divestiture of AT&T and the launch of of T1 facilities

and services, many companies jumped into

the private networking market; pioneering a generation of intelligent TDM

networks.

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Code Division Multiple Access

Code division schemes involve processing the message signal with a

deterministic code. At the transmitter, the

message signal is altered in some way that is determined by the code. This

altered signal is sent across the channel.

At the receiver, the code is used again to process the input signal so the

message signal may be recovered. Note

that it is necessary for the code to be known at both the transmitter and

the receiver.

CDMA may be achieved through the direct sequence spread spectrum method. In

this case the code is a

pseudo-random noise (PN) binary sequence. The message is altered by the

code by multiplying the message with a

polar PN signal. Multiplying by the PN signal spreads the energy in the

message signal over a large bandwidth.

As described, the technique makes no obvious provisions for the channel to

be shared by multiple users at the same

time. This may be accomplished, however, through careful selection of the

PN codes. It is possible to spread many

messages over the bandwidth of the channel if the codes are chosen to be

sufficiently different. If each code is

discernable from the others, the receiver may recover a specific message

with minimal interference from the other

spread spectrum users. For this to happen, it is necessary that the PN

codes generated at each of the transmitters

be highly uncorrelated. Although complex CDMA schemes may employ adaptive

power control techniques, a

basic CDMA system requires very little hardware in terms of complex

controllers since all users may connect

directly to the channel at the same time.

The trade-off between number of users and user data rate is more complex in

CDMA than in FDMA or TDMA,

and depends on the specific type of CDMA chosen.