data communication in mobiles

8/4/2019 Data Communication in Mobiles

http://slidepdf.com/reader/full/data-communication-in-mobiles 1/16

1

Data Communication in Mobile Phones

Mobile Network

A mobile network is a radio network made up of a number of Mobiles, each

served by at least one fixed-location transceiver known as a Mobile site or base

station. When joined together these Mobiles provide radio coverage over a wide

geographic area. This enables a large number of portable transceivers (mobile

phones, pagers, etc) to communicate with each other and with fixed transceivers

and telephones anywhere in the network, via base stations, even if some of the

transceivers are moving through more than one Mobile during transmission.

Mobile Network Advantages:

Mobile networks offer a number of advantages over alternative solutions:

y Larger coverage area

y R educed interference from other signals

y Increased capacity

y R educed power usage



The Concept of Mobile Network

In a mobile radio system, a land area to be supplied with radio service is divided

into regular shaped Mobiles, which can be hexagonal, square, circular or some

other irregular shapes, although hexagonal Mobiles are conventional. Each of

these Mobiles is assigned multiple frequencies which have corresponding radio

base stations. The group of frequencies can be reused in other Mobiles, provided

that the same frequencies are not reused in adjacent neighboring Mobiles as that

would cause co-channel interference.

Techniques used for the Encoding of Mobile signal:

To distinguish signals from several different transmitters, frequencies division

multiple access (FDMA) and code division multiple access (CDMA) were

developed.

With FDMA, the transmitting and receiving frequencies used in each Mobile are

different from the frequencies used in each neighboring Mobile. In a simple taxi

system, the taxi driver manually tuned to a frequency of a chosen Mobile to obtain

a strong signal and to avoid interference from signals from other Mobiles.



The principle of CDMA is more complex, but achieves the same result; the

distributed transceivers can select one Mobile and listen to it.

Other available methods of multiplexing such as polarization division multiple

access (PDMA) and time division multiple access (TDMA) cannot be used to

separate signals from one Mobile to the next since the effects of both vary with

position and this would make signal separation practically impossible. Time

division multiple access, however, is used in combination with either FDMA or

CDMA in a number of systems to give multiple channels within the coverage area

of a single Mobile.

Directional Antennas

Although the original 2-way-radio Mobile towers were at the centers of the

Mobiles and were Omni-directional, a mobile map can be redrawn with the

mobile telephone towers located at the corners of the hexagons where three

Mobiles converge. Each tower has three sets of directional antennas aimed in

three different directions and receiving or transmitting into three different Mobiles

at different frequencies



Broadcast Messages and Paging

Practically every mobile system has some kind of broadcast mechanism. This can

be used directly for distributing information to multiple mobiles, commonly, for

example in mobile telephony systems, the most important use of broadcast

information are to set up channels for one to one communication between the

mobile transceiver and the base station. This is called paging.

The details of the process of paging vary somewhat from network to network, but

normally we know a limited number of Mobiles where the phone is located (this

group of Mobiles is called a Location Area in the GSM or UMTS system, or

R outing Area if a data packet session is involved). Paging takes place by sending

the broadcast message to all of those Mobiles. Paging messages can be used for

information transfer. This happens in pagers, in CDMA systems for sending SMS

messages, and in the UMTS system where it allows for low downlink latency in

packet-based connections.

The most common example of a mobile network is a mobile phone (Mobile

phone) network. It receives or makes calls through a Mobile site (base station), or

transmitting tower. R adio waves are used to transfer signals to and from the

Mobile phone. Large geographic areas are split into smaller Mobiles to avoid line-

of-sight signal loss and to support a large number of active phones in that area. All

of the Mobile sites are connected to telephone exchanges (or switches) , which in

turn connect to the public telephone network.

Elements of a MOBILE NETWORK

y Base Station (BS): The covered area of a mobile network is divided into

smaller areas called Mobiles. Each Mobile has a base station which

communicates simultaneously with all mobiles within the Mobile, and



passes traffic to the Mobile Switching Centre. The base station is connected

to the mobile phone via a radio interface.

y Mobile Station (MS) : This is basically the mobile phone

y Mobile Switching Centre (MSC): This controls a number of Mobiles (or

cluster), arranges base stations and channels for the mobiles and handles

connections.

y National Carrier Exchange: This is the gateway to the national fixed

public switched telephone network (PSTN). It handles connections on

behalf of the national communication systems, and is usually integrated

with the MSC.

Operation of the MOBILE NETWORK

When the mobile unit is active (i.e. when a mobile phone is switched on), it

registers with the appropriate BS, depending on its location, and its Mobile

position is stored at the responsible MSC. When a call is set-up (when a user

makes a call), the base station monitors the quality of the signal for the duration of

the call, and reports that to the controlling MSC, which in turn makes

Decision is concerning the routing of the call.

When a mobile phone moves from one Mobile to the other, the BS will detect this

from the signal power and inform the MSC of that. The MSC will then switch the

control of the call to the BS of the new Mobile, where the phone is located. This is

called handover. It normally takes up to 400ms, which is not noticeable for voice

transmission.

A mobile phone user can only use his/her mobile within the covered area of the



network. R oaming is the capacity of a mobile phone, registered on one system, to

be able to enter and use other systems. Those other systems must be compatible to

enable roaming i.e. they must have the same type of networks). In Europe, the

standard mobile network is called GSM (Global System for Mobile

Communication). Incoming calls to GSM users are routed to them, irrespective of

where they are, as long as they are within Europe.

To connect and place a call, mobile phones send a radio wave through the air.

These radio transmissions are picked up by a large radio tower. However, there is

a limit to how far each radio tower can transmit its signal. The radio tower and its

transmission area is called a 'Mobile', and it only covers a circular (or, technically,

spherical) area of just a few miles. The radio tower transmits your signal to the

radio tower next door, which then passes it on to the next radio tower, and so on.

Your call travels through several radios tower Mobiles in order to place a call²

hence the term "mobile phone" When your Mobile phone tells you "no service",

this means that you have moved outside of the range of ANY radio tower.

Hardware Installation used in Mobile Networks

A simple view of the mobile mobile-radio network consists of the following:

A network of R adio base stations forming the Base station subsystem.

The core circuit switched network for handling voice calls and text

A packet switched network for handling mobile data

The Public switched telephone network to connect subscribers to the wider

telephony network

This network is the foundation of the GSM system network. There are many

functions that are performed by this network in order to make sure customers get

the desired service including mobility management, registration, call set up, and

handover.



Any phone connects to the network via an R BS in the corresponding Mobile

which in turn connects to the MSC. The MSC allows the onward connection to the

PSTN. The link from a phone to the R BS is called an uplink while the other way

is termed downlink.

R adio channels effectively use the transmission medium through the use of the

following multiplexing schemes: frequency division multiplex (FDM), time

division multiple (TDM), code division multiple (CDM), and space division

multiplex (SDM). Corresponding to these multiplexing schemes are the following

access techniques: frequency division multiple access (FDMA), time division

multiple access (TDMA), code division multiple access (CDMA), and space

division multiple access (SDMA)

Mobile Handover in Mobile Networks

As the phone user moves from one Mobile area to another Mobile whilst a call is

in progress, the mobile station will search for a new channel to attach to in order

not to drop the call. Once a new channel is found, the network will command the

mobile unit to switch to the new channel and at the same time switch the call onto

the new channel.

With CDMA, multiple CDMA handsets share a specific radio channel. The

signals are separated by using a pseudo noise code (PN code) specific to each

phone. As the user moves from one Mobile to another, the handset sets up radio

links with multiple Mobile sites (or sectors of the same site) simultaneously. This

is known as "soft handoff" because, unlike with traditional mobile technology,

there is no one defined point where the phone switches to the new Mobile.

Mobile Frequency Choice in Mobile Phone Networks

The effect of frequency on Mobile coverage means that different frequencies

serve better for different uses. Low frequencies, such as 450 MHz NMT, serve



very well for countryside coverage. GSM 900 (900 MHz) is a suitable solution for

light urban coverage. GSM 1800 (1.8 GHz) starts to be limited by structural walls.

UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800.

Higher frequencies are a disadvantage when it comes to coverage, but it is a

decided advantage when it comes to capacity. Pico Mobiles, covering e.g. one

floor of a building, become possible, and the same frequency can be used for

Mobiles which are practically neighbors.

Mobile service area may also vary due to interference from transmitting systems,

both within and around that Mobile. This is true especially in CDMA based

systems. The receiver requires a certain signal-to-noise ratio. As the receiver

moves away from the transmitter, the power transmitted is reduced. As the

interference (noise) rises above the received power from the transmitter, and the

power of the transmitter cannot be increased any more, the signal becomes

corrupted and eventually unusable. In CDMA-based systems, the effect of

interference from other mobile transmitters in the same Mobile on coverage area

is very marked and has a special name, Mobile breathing

Utilization Of The Spectrum In MOBILE NETWORKS

In any radio network, the number of simultaneous calls that may occur is

governed largely by the available frequency spectrum and the number of channels

that can be supported by the available bandwidth.

In a conventional radio system (the previous modes of mobile communications) ,

groups (or areas) are allocated dedicated radio frequencies. In order to ensure that

those channels are not affected by transmissions from other users operating at the

same frequency, sufficient separation between the transmitters must be allowed.

In a mobile system, frequency re-use is achieved by assigning a subset of the total

number of channels available to each base station, and controlling the power



output of the transmitters. In this way mobile networks increase capacity.

Adjacent Mobiles are not allowed to operate at the same frequency since this

causes interference between the Mobiles.

From the above argument, it would seem that increasing the number of Mobiles in

the covered area i.e. by decreasing the Mobile size would increase the capacity.

But by doing so, a number of difficulties arise:

Interference: decreasing the Mobile size, especially with a low repeat

factor increases the problems of interference between Mobiles which are

using the same frequency.

Handovers: Decreasing the Mobile size increases the frequency of

handovers, since a moving mobile phone would be changing Mobiles more

often. Since the MSC needs time to switch (for handovers), increasing the

handovers will increase that time delay.

Micro mobile Systems

It has been pointed out that decreasing the Mobile size increases capacity

but causes other problems such as increased interference and time to handle

handovers. However having an intelligent Mobile, which is able to monitor

where exactly the mobile unit is and find a way to deliver confined power

to that mobile unit, will increase channel capacity without causing these

problems

In a micro mobile system, each Mobile is divided into a number of micro

Mobiles; each micro mobile (or zone) has a zone site and the mobile itself has one base station. It is necessary to note that all the micro Mobiles,

within a mobile, use the same frequency used by that Mobile; that is no

handovers occur between micro Mobiles.



An active mobile unit sends a signal to all zone sites, which in turn send a

signal to the BS. A zone selector at the BS uses that signal to select a

suitable zone to serve the mobile unit - choosing the zone with the strongest

signal.

Base Station Signals

When a call is made to a mobile phone, the system already knows the Mobile

location of that phone. The base station of that Mobile knows in which zone,

within that Mobile, the mobile phone is located. Therefore when it receives the

signal, the base station transmits it to the suitable zone site. The zone site receives

the mobile signal from the base station and transmits that signal to the mobile

phone after amplification. By confining the power

transmitted to the mobile phone, co-channel

interference is reduced between the zones and the

capacity of system is increased.

The benefits of Micro mobile Systems:

Interference reduced (compared to decreasing

the Mobile size)

Handovers reduced (also compared to decreasing the Mobile size) since the

micro mobiles within the Mobile operate at the same frequency; no

handover occurs when the mobile unit moves between the micro Mobiles

Size of the zone apparatus. The zone site equipment is small, so they can be

mounted on the side of a building of on poles.

Increased system capacity. The micro mobile is an intelligent mobile. The

new micro mobile knows where to locate the mobile unit in a particular

zone of the mobile and deliver the power to that zone. Since the signal



power is reduced, the micro mobiles can be closer and therefore increase

capacity

Limitations of Micro mobile system

However, in micro mobile system, the transmitted power to a mobile phone within

a micro mobile has to be precise; too much power results in interference between

micro mobiles, while too little power and the signal might not reach the mobile

phone. This is a drawback of micro mobile systems, since a change in the

surrounding a new building, say, within a micro Mobile will require a change of

the transmission power.

DIGITAL DATA MOBILE NETWORK FOR DATA

TRANSMISSION:

As explained before, CMPS is a network which uses circuit-switched technology

that allocates dedicated line between two users. In order to establish this line, a

call setup procedure has to be followed.

In order to transmit data, a packet switched network is needed and Digital Data

Mobile Network is a network of this form. Here, the MS would transmit and

receive data packets only when asked by the base station (more detail in section

2.2.2). Therefore there is no permanent connection between the MS and the BS.

Only a virtual connection exists between them.

So no call setup is needed for the MS to send and receive data from the BS.

In order to illustrate data transmission in this form of mobile networks, a

conceptual Digital Data Mobile network model. This conceptual model would be



referred to as MCN and it is flexible enough to be adapted to any other Digital

Data.

As this model is already known to be better in transmitting data than the CMPS,

this model would be discussed in detail. The performance of the mobile network

in transmitting data would be based on this mobile network model. But before the

performance issue is considered, three important aspects of this network is

explained in order to give sufficient background on how this network works.

These three important aspects are:

Modification to the mobile network architecture described in section 1 to

form the MCN architecture

Protocols used in MCN

Buffer Administration techniques used in MCN

Although these topics are about the MCN network, they are applicable to

any other Digital Data Mobile Network with slight variations.

Medium Access Protocols

In mobile networks, a set of protocols, which describes the data that is to be

transmitted and to be expected by both mobile station and base station, is defined.

This allows these components within the mobile network to know what and when

to listen, transmit and receive the relevant data. This data could be either control

information or the information itself.

The Protocol

The protocol described here is a combination of TDMA and Polling. The right to

access each channel is controlled by the BS. In the channels, data packets are

transmitted at discrete time intervals known as time slots.

There are three kinds of channels. They are control, uplink data transfer and

downlink data transfer channels.



The uplink data transfer channel is used to transmit packets from the MS to the BS

while the downlink data transfer channel is used to transmit packets in the

opposite direction.

The control channel transmits system packets from the BS to all the MSs in the

Mobile. The most important contents of these packets are the Header and the Data

Assignment Table (DAT). The Header contains information regarding number of

channels available and its corresponding frequency. Meanwhile, the DAT acts to

inform which MSs are allowed to transmit and receive packets in the next time

slot.

In the next time slot, the MS informed to transmit uses the appropriate upward

data transfer channel to transmit packets if it has message, otherwise a null reply

is sent.

This shows that the BS is effectively polling the MSs to transmit packets in the

channel.

Buffer Administration Technique:

The MS and the BS receive and transmit data packets. If the MS has only one

receiver, it can only tune into one channel at a time and receive a single packet

from this channel time slot. So, the packets that arrive at the base station from the

packet switch exchange meant for several MSs, have to be queued and forwarded

one at a time to the appropriate transmitting channel (channel which the MS is

waiting to receive the packets from). Transmitting more than a single packet to a

mobile station would result in packet overlap and thus packet loss.

There are three Buffer Administration Techniques:-

y Cyclic Polling

y Channel Splitting

y R eservation



Cyclic Polling

In this technique there are an arbitrary number of queues of fixed capacity. Each

MS has its queue that is not shared with other MS. There is several numbers of

transmitters. This system would consist of a single server (channel controller)

Channel Split Technique

This is the same as the Cyclic Polling Technique but it has several channel

controllers which operate in parallel. Each channel controller is dedicated to a

group which consists of several queues. Each group of queues also has its own

dedicated transmitter. Packets queued in a group can only be transmitted using

this group's transmitter.

Reservation Technique

In this technique, similar to the Channel Split Technique, the transmitters are

dedicated to groups. But now, in each group there is only a single queue. The

incoming stream of packets which would have been put into different queues

within a group in the Channel Splitting Technique would be put into this single

queue. Therefore the queue capacity in this technique is normally multiples of the

queue capacity in both Channel and Cyclic Polling Techniques.

Performance of the DIGITAL DATA MOBILE NETWORK

y Congestion

C ongestion is said to occur at a BS when it does not have enough space in its

queues to put the new arriving packets. These new packets would then be lost.

Congestion leads to the packets already in the queue to wait the longest time

before being transmitted.

So, congestion introduces unacceptable packet delay

This congestion problem cannot be completely avoided but it can be minimized by



Choosing the correct buffer administration technique. This reduces

congestion at the BS transmit buffer.

Increasing the number of channels per Mobile. Doing this reduces

congestion at both receiver and transmit buffer of the BS.

R educing the rate at which BS informs MS to transmit data. This reduces

congestion at the BS receive buffer

y Choosing the Best Buffer Administration Technique:

The best technique should be able to support a large rate of packet arrival without

any queue overflow. It should also cause minimal congestion when both large and

moderate amount of packets arrive (high and moderate incoming traffic).Therefore in order to identify the best technique, the different techniques

discussed are compared with respect to the

I. Maximum arrival rate it supports

II. Its performance at moderate and high incoming traffic levels

Conclusion & Comments:

Currently, there are different standards of mobile systems in different parts of the

world; the major ones are the GSM in Europe, and the PCS in North America.

R oaming is not possible between these two systems. The next step would be to

have dual-mode phones which could operate in the two different systems at the

touch of a button. Having one global mobile communication system is ideal, but

will take a while, since it will require altering one system's hardware (which costs

a lot of money) Since we are in the "information age" and due to the rapid growth

of the mobile system, one could predict that in the very near future, everyone will

have a portable communication terminal, which is small in size, fast in accessing



the internet and transmit or receive data, cheaper, and could virtually be used from

anywhere in the system.

Finally it will soon be replaced by the spread spectrum technology this move has

already begun in some parts of the world and the rest will sure to follow.

References:

y Bibliography

³SMS and MMS interworking in mobile networks´ by A. Henry-Labordere,

Vincent Jonack

³Mobile telecommunications protocols for data networks´

by Anna Hac

y Other References

http://en.wikipedia.org/wiki/Mobile_network_operator

http://en.wikipedia.org/wiki/Wireless

http://www.theiet.org/publishing/books/telecom/19215.cfm

http://en.wikipedia.org/wiki/Cellular_network

http://mobnet.epfl.ch/

http://www.springer.com/engineering/signals/journal/11036

data communication in mobiles

Documents