mobile computing paper

8/14/2019 Mobile Computing paper

http://slidepdf.com/reader/full/mobile-computing-paper 1/13



7. A Cellular IP network comprises a gateway router that connects the Cellular

IP network to the Internet as well as several Cellular IP nodes that are

Internet, Mobile IP Interdomain handover Wireless access network

Micromobility Intradomain handovers Gateway foreign agent responsible

for the Cellular IP routing and mobile hosts which support the Cellular IP

protocol.

8. In Cellular IP, none of the nodes know the exact location of a mobile host.

Packets addressed to a mobile host are routed to its current base station on a

hop-by-hop basis.

9. Mappings are created and updated based on the packets transmitted by

mobile hosts. Its uses two parallel structures of mappings through Paging

Caches(PC) and Routing Caches(RC). PC maintains mappings for mobile

hosts and have large timeout interval, in the order of seconds or minutes. RC

maintains mappings for mobile host currently receiving data or expecting to

receive data and timeout are in the Packet timescale.10. Mobile hosts that are not actively transmitting or receiving data (idle state)

but want to stay reachable, have the opportunity to maintain paging cache

entries. A mobile host with installed route cache entries is said to be in

active stat e.

11. On Cellular IP nodes, where both a route and a paging cache are maintained,

packet forwarding in downlink direction is done in the same way for routing

and paging with priority to the route cache entries. If a Cellular IP node, that

does not maintain a paging cache, receives a downlink packet for a mobile

host for which it has no routing entry in its route cache, it broadcasts the packet to all its downlink neighbours. By this mechanism groups of several,

usually adjacent base stations are built in which idle mobile hosts are

searched when a packet has to be delivered to them. Those groups of base

stations are called paging areas.

12.Cellular IP provides two handover mechanisms: A hard handover and a

semi-soft handover mechanism. For a hard handover, the wireless interface

of a mobile host changes from one base station to another at once. For the

semi-soft handover the mobile host switches to the new base station,

transmits a route update message with a flag indicating the semi-soft

handover and returns immediately to the old base station in order to listenfor packets destined to it.

13.The route update message reconfigures the route caches on the way to the

gateway router as usually, except for the route cache on the cross-over node,

where the new path branches off from the old path. In that node downlink

packets are duplicated and sent along both paths until a new route update

message.



Location 1 1 2 3 4 Service Area

Location 1 Location 2 Location 3

Mobile IP

1. It is not suitable for fast moving

mobile hosts.

2. Smooth Handoff is not possible.3. There is no suppression between

active and idle user.

4. It has low latency.

5. It has low battery life

6. It is based on macromobility.

7. It was introduced in 1996.

8. Handoff is initiated by foreignagent.

9. It is coarse grained.

10. Mapping is done by mobility

binding and digital lists.

11. Concept of discovery, registration

and tunneling is used.

Cellular IP

1. It is was introduced for fast

moving mobile hosts.

2. Smooth Handoff is possible.3. There is suppression between

active and idle user.

4. It has high latency

5. It has high battery life.

6. It is based on micromobility.

7. It was introduced in 1998.

8. Handoff is initiated by mobile

host.

9. It is fine grained.

10. Mapping is done by Pagingcaches and Routing caches.

11.No concept of discovery,

registration and tunneling is used.

Paging Caches Routing

N N N



Q 2: Explain predictive location management scheme in detail.

Ans.

1. In the operation of wireless personal communication service (PCS) networks,

mobility management deals with the tracking, storage, maintenance, and retrieval

of mobile location information. Two commonly used standards, the EIA/TIAInterim Standard 41 in North America and the Global System for Mobile

Communications in Europe , partition their coverage areas into a number of

location areas(LA), each consisting of a group of cells.

2. When a mobile enters an LA, it reports to the network the information about its

current new location (location update).

3. When an incoming call arrives, the network simultaneously pages the mobile

(terminal paging ) in all cells within the LA where the mobile currently resides. Inthese standards, the LA coverage is fixed for all users.

4. Although dynamic LA management is possible , LA-based schemes, in general,

are not flexible enough to adapt to different and differing user traffic and mobility

patterns.

5. In order to reduce the paging costs, mobiles inform the n/w of their locations

from time to time ; this is called location updating.

6. There is a tradeoff between location update cost and paging cost. If a mobileexpands power and bandwidth to update its location more often, it can reduce the

area that needs to be paged when a call arrives. On the other hand, if updates are

performed less frequently, more power and bandwidth will be expanded on paging,

since larger areas need to be paged.

7. The cost of mobility management over any given time period is the sum of the

cost of the location updates and the cost of paging.

8. Dynamic mobility management schemes discard the notion of LA borders. Amobile in these schemes updates its location based on either elapsed time, number

of crossed cell borders, or traveled distance.

9. In particular, in the distance-based scheme, a mobile performs location update

whenever it is some threshold distance away from the location where it last



Such asymmetry can result from several factors, including:

Network Asymmetry — In many networks the bandwidth of the communication

channel from the server to the clients is much greater than the bandwidth in the

opposite direction. An environment in which clients have no backchannel is an

extreme example of network asymmetry. Less extreme examples include wirelessnetworks with high-speed downlinks but slow (e.g., cellular) uplinks, and cable

television networks, where bandwidth into the home is on the order of megabits

per second, while a small number of slow channels (on the order of tens of kilobits

per second) connect a neighborhood to the cable head.

Client to Server Ratio — A system with a small number of servers and a much

larger number of clients (as is the case in dissemination-based applications) also

results in asymmetry because the server message and data processing capacity

must be divided among all of the clients. Thus, clients must be careful to avoid“swamping” the server with too many messages and/or too much data.

Data Volume — A third way that asymmetry arises is due to the volume of data that

is transmitted in each direction. Information retrieval applications typically involve

a small request message containing a few query terms or a URL (i.e., the “mouse

and key clicks”), and result in the transfer of a much larger object or set of objects

in response. For such applications, the downstream bandwidth requirements of

each client are much higher than the upstream bandwidth requirements.

Updates and New Information — Finally, asymmetry can also arise in anenvironment where newly created items or updates to existing data items must be

disseminated to clients. In such cases, there is a natural (asymmetric) flow of data

in the downstream direction.

There are four types of broadcast techniques :

1. Static Broadcast

2. Dynamic Broadcast

3. Selective Broadcast

4. Optimized Broadcast

Static Broadcast : A single sender transmits information simultaneously to many

receivers. The transmitter and all the receivers are synchronously connected along

all communication time. A main interesting result says that it is possible to



transmit at a higher rate than that is achieved by time-sharing the channel between

the receivers.

In a typical situation, there is a common information that is needed to be

transmitted to all receivers.

Dynamic Broadcast : Video-on-demand (VOD) will one day allow customers to

select any given video from a large on-line video library and watch it on their

televisions without any further delay.

This situation has resulted in many proposals aimed at reducing the bandwidth

requirements of video-on-demand services. Despite all their differences, all these

proposals are based on the same idea, namely, sharing as many data as possible

among overlapping requests for the same video.

Selective Broadcast : Servers broadcast selected information only. The overall

system is divided into two overlapping groups:

1. Publication Groups :- In this it is decided what data is to be broadcasted

depending upon its importance.

2. On Demand :- In this server choose the next item based on their request.

The frequently requested items are called “ Hot Spots “ For eg. Stock Info,airlines schedule etc. are demanded frequently , so they are broadcasted

regularly after some time interval.

Optimized Broadcast : It works by letting the server be active and send updated

data only as soon as the new piece of data is available on the network and server

resources are only used when change in information contents has happened and

the clients gets the updated data as soon as possible after the change of data has

took place.

Q 4: Explain energy efficient indexing for push based data delivery model.

Ans. It is distinguished between two fundamental modes of providing users with

information:



1. Data Broadcasting: Accessing broadcasted data does not require any uplink

transmission and is listen only. Querying involves simple filtering of the

incoming data stream according to a user specified filter.

2. Interactive/On demand : The client request a piece of data on the uplink

channel and the server responds by sending this data to the client.

In Practice , a mixture of the above two is used. The most frequently demanded

item, the so called “ Hot Spots” will be broadcasted creating a storage on the air.

For example , stock info.,airline schedules etc.

Data 1 Data M

Index 1 Index 2 index m

The constraints of limited available energy is expected to drive all solutions to

mobile computing on palmtops.

The power consumption in doze mode is only 50 uw and in active mode is only

250 mw { The ratio of power consumption in active mode to doze mode is 5000}.

When palmtop is listening to the channel , CPU must be in active mode for examining data packets.

CPU is a much more significant energy consumer than the receiver itself and since

it has to active to examine the incoming packets it may lead to waste of energy .

Therefore it is beneficial if palmtops can slip into doze mode most of the time and

come in active mode only when data of interest is expected to arrive.

Energy Efficient solutions are important due to the following reasons

1. It make it possible to use smaller and less powerful batteries to run the

same set of applications for the same amount of time.

2. Smaller batteries are important from the portability poit of view, as

palmtops can be compact and weight less.

3. With the same batteries , the unit can run for a very long time without the

problem of changing battery or recharging.

4. Every battery which Is disposed is an environmental hazard.

Previous

Next



Due to the above reasons the directory of the file is broadcasted in the

form of an index in the broadcasted channel. The index we consider is a

multilevel index.

For a file being broadcasted the two parameters are:

1. Access Time: Avg. time elapsed from the moment a client wants a record by a

primary key , to the point when the record is downloaded. The access time can

be determined by two parameters.

Probe wait: When an initial probe is made into the broadcast channel the

client gets the info. About occurrence of the next nearest index information

relevant to the required data .

The avg. duration for getting to this nearest index info Is called probe wait.

Broadcast wait: the avg, duration from the point of the index info relevantto the required data is encountered , to the point when the required record is

downloaded is called broadcast wait.

2. Tuning Time : Amount of time spent by a client listening to the channel.

(1,m) indexing : It is a allocation method in which the index is broadcasted m

times during the broadcast of one version of a file. All buckets have an offset to the

beginning of the next index segment.The access protocol for record with key is as follows:

1. Tune into the current bucket in the broadcast channel.

2. Read the offset to determine the address of the next nearest index segement.

3. Go into doze mode and tune in at the broadcast of the index segment.

4. From the index segment determine when the data bucket contained the

record with primary key is broadcasted. This is accomplished by successive

probes, by following the pointers in the multi level index.

5. Tune in again when the bucket containing the record with primary key is

broadcasted and download the record.

Q 5: Design the coda file system and explain the different states of VENUS,

draw the state transition diagram also.



Ans.

1. Coda was designed to be a scalable, secure, and highly available distributedfile system. An important goal was to achieve a high degree of naming and

location transparency so that the system would appear to its users very similar to a

pure local file system. By also taking high availability into account, the designers

of Coda have also tried to reach a high degree of failure transparency.

2. Coda is a descendant of version 2 of the Andrew File System (AFS), which

was also developed at CMU , and inherits many of its architectural features from

AFS. AFS was designed to support the entire CMU community, which implied that

approximately 10,000 workstations would need to have access to the system. Tomeet this requirement, AFS nodes are partitioned into two groups. One group

consists of a relatively small number of dedicated Vice file servers, which are

centrally administered. The other group consists of a very much larger collection of

Virtue workstations that give users and processes access to the file system.



3. Coda follows the same organization as AFS. Every Virtue workstation hosts

a user-level process called Venus, whose role is similar to that of an NFS

client. A Venus process is responsible for providing access to the files that

are maintained by the Vice file servers. In Coda, Venus is also responsible

for allowing the client to continue operation even if access to the file servers

is (temporarily) impossible.

4. There is a separate Virtual File System (VFS) layer that intercepts all calls

from client applications, and forwards these calls either to the local file

system or to Venus. This organization with VFS is the same as in NFS.

Venus, in turn, communicates with Vice file servers using a user-level RPC

system. The RPC system is constructed on top of UDP datagrams and

provides at-most-once semantics.

There are three different server-side processes. The great majority of the work is

done by the actual Vice file servers, which are responsible for maintaining a local

collection of files.

Communication

Interprocess communication in Coda is performed using RPCs. However, the

RPC2 system for Coda is much more sophisticated than traditional RPC systems

such as ONC RPC, which is used by NFS.

RPC2 offers reliable RPCs on top of the (unreliable) UDP protocol. Each time

a remote procedure is called, the RPC2 client code starts a new thread that sends

an invocation request to the server and subsequently blocks until it receives an

answer.

An interesting aspect of RPC2 is its support for side effects. A side effect is a

mechanism by which the client and server can communicate using an

application-specific protocol.

RPC2 allows the client and the server to set up a separate connection for

transferring the video data to the client on time. Connection setup is done as a side

effect of an RPC call to the server.

Processes



Coda maintains a clear distinction between client and server processes.

Clients are represented by Venus processes; servers appear as Vice processes.

Both type of processes are internally organized as a collection of concurrent

threads. Threads in Coda are nonpreemptive and operate entirely in user space.

Naming

Coda maintains a naming system analogous to that of UNIX. Files are grouped into

units referred to as volumes. Usually a volume corresponds to a collection of files

associated with a user. Examples of volumes include collections of shared binary

or source files, and so on. Like disk partitions, volumes can be mounted.

Volumes are important for two reasons :

First, they form the basic unit by which the entire name space is constructed. This

construction takes place by mounting volumes at mount points. A mount point inCoda is a leaf node of a volume that refers to the root node of another volume.

The second reason why volumes are important, is that they form the unit for

server-side replication.

State Transition diagram

Disconnection Reintegration completed

Disconnection

Reconnection

Hoarding

Emulation Reintegration



Filling the cache in advance with the appropriate files is called hoarding.

Normally, a client will be in the HOARDING state. In this state, the client is

connected to (at least) one server that contains a copy of the volume. While in this

state, the client can contact the server and issue file requests to perform its work.

Simultaneously, it will also attempt to keep its cache filled with useful data (e.g.,

files, file attributes, and directories).

At a certain point, the number of servers in the client’s AVSG will drop to zero,

bringing it into an EMULATION state in which the behavior of a server for

the volume will have to be emulated on the client’s machine. In practice, this

means that all file requests will be directly serviced using the locally cached copy

of the file. Note that while a client is in its EMULATION state, it may still be able

to contact servers that manage other volumes. In such cases, disconnection willgenerally have been caused by a server failure rather than that the client has been

disconnected from the network.

Finally, when reconnection occurs, the client enters the REINTEGRATION

state in which it transfers updates to the server in order to make them permanent.

It is during reintegration that conflicts are detected and, where possible,

automatically resolved. As shown in Figure, it is possible that during reintegration

the connection with the server is lost again, bringing the client back into the

EMULATION state.

mobile computing paper

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