# Others

Full names of abbr?

CSMA: Carrier Sense Multiple Access

FDMA: Frequency-division multiple access

WLAN: wireless local area network

WPAN: Wireless personal area network

SDMA: space devision multiple access

VLR: visitor location register

HLR: home location register

LTE: Long-Term Evolution

AP: Access Point

LA: Local Area

DCF: Distributed Coordination Function

PCF: Point

ESS: Extended Service Set

# 1- Mobile Computing Overview

## Do mobile applications mean the same as wireless applications?

Midterm

No. They are different.

Wireless applications can be run on mobile devices as well as from devices in fixed locations, as long as they make use of a wireless communication network.

Mobile applications can be run on a mobile device without accessing a wireless network. But in most cases, wireless applications are a subset of mobile applications, because even though mobile applications do not require wireless connectivity, they can often benefit from it when it is available.

## Please discuss why developing mobile computing applications faces more challenges.

Special characteristics of wireless communications, mobile devices and mobility:

• Different level of mobility
• Constraints on wireless communication (shared wireless transmission medium, low network bandwidth, intermittent network connectivity)
• Constraints on mobile devices (size, limited power, reduced security)

# 2- Wireless Communication

## Calculate network capacity OR bandwidth size for each user

Tut 03-3, Tut 03-4

$Capacity = \frac{1}{2}\times \#cell \times \frac{channel\ capacity}{each\ channel\ bandwidth} \div Freq\ reuse\ factor$

## Why a wireless communication system always needs an analog modulation module?

Analog modulation superimposes the baseband signal on the carrier signal so as to shift the data signal to a higher frequency. Purpose:

• The size of the antenna for transmitting/receiving radio signals is proportional to the wave length of the transmitted signal. Frequency high → antenna size can be small → mobile devices can be small
• Using high frequency radio carrier signal to modulate the baseband signal implements the frequency division multiplexing. → Allow several baseband signals to transmit at the same time at different frequency bands.
• Avoid interfered frequency, make sure quality
• Avoid signal distortion

## Please explain why the Hidden Terminal and Exposed Terminal problems occur when the carrier sense multiple access mechanism CSMA is used in wireless environments.

CSMA requires a node to first monitor the channel before it actually sends a packet. If the channel is free, the node will access the channel directly. If busy, the node will wait for the channel to become clear.

Hidden Terminal Problem happens when two nodes that are outside each other's transmission range are trying to send the packets to the same node at the same time. They cannot use carrier sense of carrier detection mechanism to avoid collision.

Exposed terminal problem happens when two nodes that are within each other's transmission range are trying to send the packets to different nodes that do not collide with each other. Two senders are within each other's range, one may detect anther's transmission and unnecessarily wait for accessing the medium.

## MACA can solve HT and ET problem 👆, why?

RTS keeps all the nodes within sender's range quiet.

CTS keeps all the nodes within receiver's range quiet. → no HT.

If one only get RTS, no CTS, still transmit. → no ET.

# 3- Wireless Networks

## IEEE802.11 why use following mechanisms?

Tut 03 2, Midterm

Carrier Sense

CS allows multiple mobile stations to access the wireless channel in a distributed way. Sense whether the channel is free before using.

Random back-off time

Collision Avoidance mechanism.

After failing, the station wait DIFS+randomized back-off time

Diff stations have diff back-off time

Dynamic contention window

The size of contention window determins the probability that two stations have the same back-off time.

The window can make a trade-off between the collision probability and unnecessary delay.

Residual back-off time

Fairness

Tries to access eailier have higher priority due to the shorter residual back-off time

ACK packet

Send from the receiver to sender

improve the wireless channel utilization

RTS/CTS packets

Reserve a wireless channel

Hidden terminal problem can be avoided

SIFS/PIFS/DIFS

Different priorities to different stations.

Priority: stations that intend to transmit

CTS/ACK/DATA > PCF > RTS

NAV time

Neighbours can know how long the channel is occupied. Will not try to access during the NAV time.

160921_24

What is an extended service set?

A set of Basic SS integrated together.

ESS appears the same to LLC layer as independent BSS

Mobile stations can move from one BSS to another transparently to LLC

What are these?

What is the difference between the DCF and PCF services?

DCF: transmit data packets based on a best effort strategy, asynchronous data service

PCF: based on a periodic polling strategy, time bounded service

Why do the standards specify different inter frame spaces?

Diff priority. CTS/ACK/DATA > PCF > RTS

For what purpose do the standards request the sender and receiver to exchange RTS and CTS packets before the actual data transmission?

Using RTS/CTS packets to reserve a wireless channel between the sender and receiver so that the hidden terminal problem can be avoided.

Three MAC methods used?

DCF CSMA/CA

DCF with RTS/CTS

PCF

Why do the standards set different NAV values in RTS and CTS packets?

RTS NAV time = s+s+s+CTS time + data time + ACK time

CTS NAV time = s+s+data time + ACK time

160921_24, Midterm

What is a piconet? What is master? What is slave?

• Piconet: an ad hoc connection of Bluetooth devices which are synchronized to the same hopping sequence. Devices in the same piconet can use the same frequency hopping sequence for communications.
• Master: the Bluetooth device that initiates a Piconet and determines the hopping sequence of this piconet.
• Slave: a Bluetooth device that joins in a piconet and synchronizes the hopping pattern to the master.
• Scatternet: many piconets with overlapping coverage can exist simultaneously and form a scatternet. All piconets in the same scatternet use a different hopping sequence, always determined by the master of the Piconet.

Why does Bluetooth only support one master and up to seven active slaves in a piconet?

All active device in the piconet are assigned a 3-bit active member address 23 = 1+7

Why does Bluetooth use the frequency hopping technology?

To suppress interference

Why cannot a Bluetooth device become a master/master node in a scatternet?

If a device acts as masters in two Piconets, it would lead to identical behavior as both Piconets would have the same hopping sequence.

Why does the Bluetooth technology become more popular compared with infrared technology?

Bluetooth link does not have the line-of-sight restriction.

Infrared link requires two devices to directly see each other.

Which tech is used in Bluetooth for avoiding interferences? Why need?

To avoid interferences at 2.4GHz ISM frequency spectrum.

Why need: 2.4GHz ISM frequency is free-of-charge, many applications, many WLANs and WPANs use at the same time. So interferences exist.

How does Bluetooth support the voice transfer and data transfer?

SCO(Synchronous connection oriented) for voice transfer.

ACL(Asynchronous connectionless) for data transfer.

# 4- Mobile Computing Models and Architectures

## How to solve the difficulties incurred by directly applying the standard client-server model in mobile computing environment

170704_24, 04-9

Cannot assume continuous network connectivity and powerful client ability

Thin client → Resource limitation on mobile client forces moving client activities back to server side

Smart client → Client cannot be too thin since disconnection and low communication bandwidth of the network require some processing ability on client side for connection operations

Mobility of client causes additional problem for the server → when and where to transmit a reply back to client

How to solve? Extend C/S model to multiple tiers + conduct further optimizations

## Pros and Cons for CAS and CIS Model?

CAS

Pros

Client functions are shifted to agent → better for thin client

Only final result is transmitted to the client

Server can also shift some activities to agent

Agent can cache

Cons

Client app needs to change to communicate with agent

Do not support disconnection for client

CIS

Pros

Better for client with enough computational power and storage

No need to change client-side/service-side code

Diff protocols can be used between client ←> client agent; server ←> server agent

The pairs of agents masks disconnection and optimizes transmission

Cons

Client need more resources

Need to dev intercept agents

## Differences between mobile client/server model and mobile P2P model

P2P model is a generalized C/S model. No center server, no difference between client and server.

All nodes have the dual roles, serving as both S and C.

Nodes can communicate with each other directly

Nodes' operations are completely decentralized & asynchronous

Server can move and suffer from disconnection

## Pro and cons of Thin vs Smart Client Architecture

170704_24, 04

Thin Client

Pro

Extend internet computing applications to mobile env

Need minimal to zero software deployment

Keep fresh data (connect to server all the time)

Provide high data security (all data in server)

Cons

Need persistent wireless connectivity

Performance is highly affected by network conditions

Availability (if server down, then down)

Application testing is difficult

Smart Client

Pros

Offline data access

Performance

Distributed computing → reduce server load

Security → have data both side

Cons

Application deployment ← diff mobile devices

Development complexity ← diff mobile devices with diff OS

Security risk ← mobile OS open to viruses

## Do you think smartphones are suitable client devices for deploying smart client applications? Reasons?

170704_24

Yes. Smart client application software is needed. To run the smart client application, the mobile devices should have following features:

Install a mobile OS

Connections over wireless networks

Persistent data storage

Computing ability

Can sync data with back-end data sources

Smartphones all have.

## Data synchronization server: What is a conflict? How to detect a conflict? Once a conflict has been detected, how to resolve it?

A conflict occurs when two or more users attempt to update the same data field with different values.

Detect

Both original and new values of data being updated must be stored.

When update, sync server check origin value from the client match the db one.

Not match, then conflicts

Resolve

Last-in wins

First-in wins

# 5- Mobility Management

## How the tradeoff between the location update cost and call delivery cost has been made for cellular networks using location area information for the location management?

There are two main factors to affect the performance of location management. They are size of location area and location update methods.

For size of LA, big LA can decrease the location update cost and increase the paging cost. While smaller LA is different.

For the location update methods, static LAs may not match mobility changes and calling patterns. While dynamic LAs combines the use of location updates and paging operations, and make a tradeoff between them.

Three types of dynamic location update schemes: Time/Distance/Movement-based.

## How the cache technique can be used to solve the routing inefficiency problem in the mobile IP protocol?

In routing optimization, the MN(mobile node) inform the CN(correspondent node) of its COA, then CN can send the packet directly to the MN.

Step 1: when the HA received packets for the MN that is not at home, the HA send binding update message to the CN. It has current COA of MN.

Step 2: CN caches COA and future packets are directly send to COA

Step 3: when MN moves to new FA, cached COA becomes stale

Then the CN sends a binding request message to the HA of the MN, HA send new binding update message to CN, CN cache again

## Pros and cons of Parallel Paging and Sequential Paging in Location Management for CN

Parallel paging

Reduce delay, good for paging with delay constraint

More bandwidth

Sequential paging

Save bandwidth, good for paging without bandwidth constraint

More delay

# 6- Location Based Services

## How the TDOA(Time Difference of Arrival) approach can improve its positioning accuracy compared with the TOA(Time of Arrival) approach?

For TOA, the differences among the times must be very accurate. MS clock might shift so that error in distance calculation.

TDOA does not suffer from MS synchronization errors, MS sends out a signal, each station estimate the distance between the MS and the BS. Calculate difference between two distances.

TDOA only requires tight time synchronization among BSs → estimated distances are good approximations of the actual distances

## Give the reason why the TOA positioning requires at least 3 different base stations to receive the signal from a mobile station for determining the location of the mobile station in a two-dimensional space

160921_49

1 circle → at most n points

2 circles → at most 2 points

3 circles → only point