copyright © 2006, dr. dharma p. agrawal and dr. qing-an zeng. all rights reserved. 1 mobile...

1Copyright © 2006, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.

Mobile Communication Systems

Chapter 9


Outline Cellular System Infrastructure Registration Handoff Parameters and Underlying Support Roaming Support Multicasting Security and Privacy Firewalls and System Security


Cellular System Infrastructure

MSC…

HLR

VLR

EIR

AUC

Gateway MSC

MSC

…

PSTN/ISDN

BSCBTS

BTS

BTS

MS

Base Station System

BSCBTS

BTS

BTS

…

MS

Base Station System

…


VLR/HLR/AUC/EIR VLR contains information about all visiting

MSs in that particular area of MSC. VLR has pointers to the HLR’s of visiting MS. VLR helps in billing and access permission to

the visiting MS. AUC provides authentication and encryption

parameters. EIR contains identity of equipments that

prevents service to unauthorized MSs.


Classical Mail Forwarding by Mail Service

Post OfficeNew Post

Office

Mail from the world


PSTN

MS

HomeMobile

Switching Center

HLR Home network

Visitingarea

Caller

VisitingMobile

Switching Center

VLR

MS

1

Location update request Using Becon Signals

Update location Info. sent to HLR

2

Automatic Location Update


PSTN

MS

homeMobile

Switching Center

HLR Home Network

VisitingArea

Caller

Mobile Switching

Center

VLR

Automatic Call Forwarding using HLR-VLR

1 Call sent to home location

2Home MSC checksHLR; gets current location of MSin visiting area

3

Home MSC forwards call to visiting MSC

4

MSC in visiting area sendscall to BS and connects MS


Redirection of Call to MS at a Visiting Location

BS

MS

Cell where MS is currently located

Visiting MSC

VLR

Another MSC

Through backbone

HLR

Home MSC

Call routed as per called number to MS

Home MSC


Registration Wireless system needs to know whether MS is currently

located in its home area or some other area (routing of incoming calls).

This is done by periodically exchanging signals between BS and MS known as Beacons.

BS periodically broadcasts beacon signal (1 signal per second) to determine and test the MSs around.

Each MS listens to the beacon, if it has not heard it previously then it adds it to the active beacon kernel table.

This information is used by the MS to locate the nearest BS. Information carried by beacon signal: cellular network

identifier, timestamp, gateway address ID of the paging area, etc.


Steps for Registration

MS listens to a new beacon, if it’s a new one, MS adds it to the active beacon kernel table.

If MS decides that it has to communicate through a new BS, kernel modulation initiates handoff process.

MS locates the nearest BS via user level processing. The visiting BS performs user level processing and

decides: Who the user is? What are its access permissions? Keeping track of billing.

Home site sends appropriate authentication response to the current serving BS.

The BS approves/disapproves the user access.


Using a Mobile Phone Outside the Subscription Area

Visiting BS(Visiting MSC)

MSBeacon sig

nal exchange

1Request f

or registration

2

Authentication/rejected

5

Home BS(Home MSC)

3Authentication request

4Authentication response

Through backbone


Applications and Characteristics of Beacon Signals

Application Frequency band Information carried

Cellular networks 824-849 MHz (AMPS/CDPD), 1,850-1,910 MHz (GSM)

Cellular IP network identifier,Gateway IP address, Paging area ID,

Timestamp

Wireless LANs

(discussed in Chapter 14)

902-928 MHz (industrial, scientific, and medical band for analog and mixed signals) 2.4-2.5GHz (ISM band for digital signals)

Traffic indication map

Ad hoc networks(discussed in

Chapter 13)

902-928 MHz (ISM band for analog and mixed signals) 2.4-2.5 GHz (ISM band for digital signals)

Network node identify

GPS (discussed in Chapter 11)

1575.42 MHz Timestamped orbital map and astronomical information

Search and rescue 406 and 121.5 MHz Registration country and ID of vessel or aircraft in distress

Mobile robotics 100 KHz - 1 MHz Position of pallet or payload

Location tracking 300 GHz - 810 THz (infrared) Digitally encoded signal to identify user's location

Aid to the impaired 176 MHz Digitally coded signal uniquely identifying physical locations


Handoff Parameters and Underlying Support

Change of radio resources from one cell to an adjacent one.

Handoff depends on cell size, boundary length, signal strength, fading, reflection, etc.

Handoff can be initiated by MS or BS and could be due to Radio link Network management Service issues


Handoff Parameters (Cont’d)

Radio link handoff is due to mobility of MS.

It depends on: Number of MSs in the cell Number of MSs that have left the cell Number of calls generated in the cell Number of calls transferred from the

neighboring cells Number and duration of calls terminated in

the cell Number of calls that were handoff to

neighboring cells Cell dwell time


Handoff Parameters (Cont’d)

Network management may cause handoff if there is drastic imbalance of traffic in adjacent cells and optimal balance of resources is required.

Service related handoff is due to the degradation of QoS (quality of service).


Time for Handoff

Factors deciding right time for handoff: Signal strength Signal phase Combination of above two Bit error rate (BER) Distance

Need for Handoff is determined by: Signal strength CIR (carrier to interference ratio).


Handoff Region

BSi

Signal strength due to BSi

X2

MSX4

Pmin

Pi(x)

E

Signal strength due to BSj

X1 X3X5 Xth

BSj

Pj(x)

By looking at the variation of signal strength from either base station it is possible to decide on the optimum area where handoff can take place.


Handoff initiation (Cont’d)

Region X3-X4 indicates the handoff area, where depending on other factors, the handoff needs to be performed.

One option is to do handoff at X5 where the two signal strengths are equal.

If MS moves back and forth around X5, it will result in too frequent handoffs (ping-pong effect).

Therefore MS is allowed to continue with the existing BS till the signal strength decreases by a threshold value E.

Different cellular systems follow different handoff procedure.


Types of Handoff

Hard Handoff (break before make) Releasing current resources from the prior BS before

acquiring resources from the next BS. FDMA,TDMA follow this type of handoff.

Soft Handoff (make before break) In CDMA, since the same channel is used, we can use

the same if orthogonal to the codes in the next BS. Therefore, it is possible for the MS to communicate

simultaneously with the prior BS as well as the new BS.


Hard Handoff

BS1 BS2MS

(a). Before handoff

BS1 BS2MS

(b). During handoff (No connection)

BS1 BS2MS

(c). After handoff


Soft Handoff (CDMA only)

BS1 BS2MS

(b). During handoff

BS1 BS2MS

BS1 BS2MS

(c). After handoff(a). Before handoff


Roaming Support

To move from a cell controlled by one MSC area to a cell connected to another MSC.

Beacon signals and the use of HLR-VLR allow the MS to roam anywhere provided the same service provider using that particular frequency band, is there in that region.


Roaming Support

BS1 BS2MS

Home MSC

Visiting MSC

BS1 BS2MS

Home MSC

Visiting MSC

MS moves


Handoff Scenarios with Different Degree of Mobility

PSTN

Paging Area 1

MSC2

c

MSC3

d

MSC4

Paging Area 2

e

MS

MSC1

a b


Possible Handoff Situations

Assume MSC1 to be the home of the MS for registration, billing, authentication, etc.

When handoff is from position “a” to “b”, the routing can be done by MSC1 itself.

When handoff is from position “b” to “c” , then bi-directional pointers are set up to link the HLR of MSC1 to VLR of MSC2.

When handoff occurs at “d” or “e”, routing of information using HLR-VLR may not be adequate (“d” is in a different paging area).

Concept of Backbone network.


Information Transmission Path when MS Hands Off from “b” to “c”

Connection Path after handoff

MSC1 HLR

MSC2 VLR

a b c

Information to MS being sent

Initial path of information transfer

MS


Illustration of MSC Connections to Backbone Network and Routing/Rerouting

MSC

Router

Paging area 1 (PA1) Paging area 2 (PA2)

MSC1

(a,b)MSC2

(c)MSC3

(d) MSC4

(e)

(a,b,c,d,e)

(a,b)

(a,b,c,d)

(d)R3

R4 R6

R2

R5

R9

R1

R7

R10

R12

R8

R11 R13

From rest of the backbone

(c) (e)

R: Routers


Backbone Network

Routing done according to the topology and connectivity of the backbone network.

The dotted lines show the possible paths for a call headed for different MS locations.

One option is to find a router along the original path, from where a new path needs to start to reach the MSC along the shortest path.


Home Agents (HA), Foreign Agents (FA) and Mobile IP

Two important software modules are associated with routers, home agent (HA) and foreign agent (FA).

MS is registered with a router, mostly a router closest to the home MSC can be used to maintain its HA.

A router other than closest one could also serve as an HA. Once a MS moves from the home network, a software

module in the new network FA assists MS by forwarding packets for the MS.

This functionality is somewhat similar to HLR-VLR.


Home MSC MSC1 MSC2 MSC3 MSC4

Selected router for maintaining its

home agent

R3 R4 R6 R9

Home MSC and Home Agent (HA) for the Previous Network


Call Establishment using HA-FA

Whenever a MS moves to a new network, it still retains its initial HA.

The MS detects the FA of the new network, by sensing the periodic beacon signals which FA transmits.

MS can also itself send agent solicitation messages to which FA responds.

When FA detects a new MS, it allocates a CoA (care of address) to the MS, using dynamic host configuration protocol (DHCP).

Once MS receives CoA, it registers its CoA with its HA and the time limit binding for its validity.

Such registration is initiated either directly by MS to the HA of the home router or indirectly through FA.


Call Establishment (Cont’d)

HA confirms its binding through a reply to the MS. A message sent from an arbitrary source to the MS at the

home address is received by the HA. Binding is checked, the CoA of the MS is encapsulated in

the packet and forwarded to the network. If CoA of the FA is used, then packet reaches FA, it

decapsulates packet and passes to MS at the link layer. In an internet environment, it is called Mobile IP. After binding time, if MS still wants to have packets

forwarded through HA, it needs to renew its registration. When MS returns to its home network, it intimates its HA.


FA

3 CoA or C-CoA created

MSHA

Here is my HA and binding information

2

OK, send information

1

1”

1’

Beacon Signal

I am new here

(Any one new)

Acknowledge Registration + binding

4

4’ Same as step

Here is CoA or co-located CoA (C-CoA) for this MS

4

4” Same as step 4

Registration Process Between FA, MS, and HA When the MS Moves to a Paging area


Source To MS Payload DataIncoming message for MS

HA

HA CoA/C-CoA Source To MS Payload Data

Encapsulation

FA

Forwarding through intermediate router if CoA used Forwarding

through intermediate router if C-CoA used

Source To MS Payload Data

Decapsulation done at MSMS

Message Forwarding using HA-FA Pair


Routing in Backbone Routers

How FA finds HA of the MS ? One approach is to have a global table at each

router of each MSC so that the route from FA to HA for that MS can be determined.

Disadvantages: Information too large, one network might not like to give out information about all its routers to any external network (only gateways information is provided).

Use of Distributed Routing Scheme.


PA1 PA2

PA3

PA4

PA5

Router X

Router W

Router Z

Network 1

Network 2

MS moves

Illustration of Paging Areas (PAs) and Backbone Router Interconnect

PA1 PA2

PA3

PA4

PA5

Router Y

Network 1

Network 2


Route to PA

Next hop

Route to PA

Next hop

Route to PA

Next hop

Route to PA

Next hop

1 X 1 - 1 X 1 Y

2 X 2 - 2 X 2 Y

3 X 3 Y 3 Z 3 -

4 X 4 Y 4 Z 4 -

5 X 5 Y 5 Z 5 -

Table at routerW

Table at router X

Table at router Y

Table at routerZ

Distributed Routing Table and Location PAs


Multicasting

Process of transmitting messages from a source to multiple recipients by using a group address for all hosts that wish to be the members of the group.

Reduces number of messages to be transmitted as compared to multiple unicasting.

Useful in video/audio conferencing, multi party games.


Multicasting

Multicasting can be performed either by building a source based tree or core based tree.

In source based tree , for each source of the group a shortest path is maintained, encompassing all the members of the group, with the source being the root of the tree.

In core based tree, a particular router is chosen as a core and a tree is maintained with the core being the root.

-- Every source forwards the packet to a core router, which then forwards it on the tree to reach all members of the multicast group.


Multicasting

Bi-directional Tunneling (BT) and Remote Subscription approaches have been proposed by IETF for providing multicast over Mobile IP.

In BT approach, whenever a MS moves to a foreign network, HA is responsible for forwarding the multicast packets to the MS via FA.

In Remote Subscription protocol, whenever a MS moves to a foreign network, the FA (if not already a member of multicast group) sends a tree join request.


Multicasting

Remote Subscription based approach is simple and prevents packet duplication and non optimal path delivery.

It can cause data interruption till the FA is connected to the tree.

It results in a number of tree join and tree leave requests when MS are in continuous motion.

In contrast, in the BT approach, the HA creates a bi-directional tunnel to FA and encapsulates the packets for MS.

FA then forwards the packets to the MS.


Multicasting

BT approach prevents data disruption due to the movement of MS.

But causes packet duplication if several MSs of the same HA, that have subscribed to the same multicast group move to same FA.

Also causes Tunnel Convergence Problem, where one FA may have several MSs subscribed to the same group, belonging to different HAs and each HA may forward a packet for its MSs to the same FA.


HA

Multicast packets from the multicast tree

MS1

MS2

MS3

FA

MS 1

MS 2

MS 3

Packet Duplication in BT Tunnel Approach



HA 1

HA 2

HA 3

CoA (MS1)

CoA (MS2)

CoA (MS3)

CoA (MS4)

MS 1

MS 2

MS 3

MS 4

FA

Tunnel Convergence Problem


Multicasting

To overcome Tunnel Convergence Problem, MoM protocol is proposed wherein the FA selects one of the HAs, called the Designated Multicast Service Provider (DMSP), from the HA List for a particular group.

The remaining HAs do not forward packets to FA.



HA 1

HA 2

HA 3

CoA (MS1)

CoA (MS2)

CoA (MS3)

MS 1

MS 2

MS 3

MS 4

Stop

Stop

Forward

DMSP Selection

FA

CoA (MS4)

Illustration of MoM Protocol


Security and Privacy

Transfer through an open air medium makes messages vulnerable to various attacks.

One such problem is “Jamming” by a very powerful transmitting antenna.

Can be overcome by using frequency hopping. Many encryption techniques used so that

unauthorized users cannot interpret the signals.


Encryption Techniques

Permuting the bits in a pre specified manner before transmitting them.

Such permuted information can be reconstructed by using reverse operation.

This is called “Data Encryption Standard (DES)” on input bits.


Input Output

Simple Permutation Function

1

2

3

4

5

6

7

8

1

5

2

6

3

7

4

8

W

I

R

E

L

E

S

S

W

L

I

E

R

S

E

S


Initial Bit Patterns and effect of before Transmission and after Reception using DES

(b) Permutation of information sequence before transmission

57 49 41 33 25 17 9 1

61 53 45 37 29 21 13 5

58 50 42 34 26 18 10 2

62 54 46 38 30 22 14 6

59 51 43 35 27 19 11 3

63 55 47 39 31 23 15 7

60 52 44 36 28 20 12 4

64 56 48 40 32 24 16 8

(c) Permutation to be performed on received information sequence

8 24 40 56 16 32 48 64

7 23 39 55 15 31 47 63

6 22 38 54 14 30 46 62

5 21 37 53 13 29 45 61

4 20 36 52 12 28 44 60

3 19 35 51 11 27 43 59

2 18 34 50 10 26 42 58

1 17 33 49 9 25 41 57

1 2 3 4 5 6 7 8

9 10 11 12 13 14 15 16

17 18 19 20 21 22 23 24

25 26 27 28 29 30 31 32

33 34 35 36 37 38 39 40

41 42 43 44 45 46 47 48

49 50 51 52 53 54 55 56

57 58 59 60 61 62 63 64

(a) Information sequence to be transmitted


Encryption Techniques

A complex encryption scheme involves transforming input blocks to some coded form.

Encoded information is uniquely mapped back to useful information.

Simplest transformation involves logical or arithmetic or both operations.


A Generic Process of Encoding and Decoding

Information

block

Transmitted signal

Encoded

signal

Encoding

at

transmitter

Information

block

Received signal

DecodingEncoded

signal

(Original)receiver

at


A Generic Process of Encoding and Decoding

Information

block

Encoding

Transmitted signal

Received signal

DecodingEncoded

signal

Encoded

signal

Informationblock

(Original)

at

transmitter receiver

at

Operations done at the transmitting MS

1

0

1

0

1

1

1

0

Initial pattern

1

1

1

1

0

0

0

0

EX-OR bits

0

1

0

1

1

1

1

0

Bits after EX-OR Shuffle

0

1

1

1

0

1

1

0

Transmitted bits

0

1

1

1

0

1

1

0

Received bits

Inverse Shuffle

0

1

0

1

1

1

1

0

Bits after shuffle

1

1

1

1

0

0

0

0

EX-ORbits

1

0

1

0

1

1

1

0

Bits afterEX-OR

Air

Operations done at the receiving MS


Key K1

f+

Input (64 bits)

Initial Permutation (IP)

32 bits 32 bits

Left half: L1 Right half: R1

Inverse initial permutation (IP –1)

Coded Output

Permutation and Coding of Information

f+

Left half: L1 = R1 R1 = L1 f(R1, K1)+

R16 = L16 f(R15, K16)+ Left half: L16 = R15


Authentication

Making sure user is genuine. Using a Hash Function from an associated

unique identification of the user (not full proof) Another approach is to use two different

interrelated keys. One known only to system generating the key

(private key) , other used for sending to outside world (public key).

RSA algorithm (best known public key system)


Public/Private Key Authentication Steps

System User i

(1) Compute Public Key for User i from its private key

usually done off line

(2) Send Public Key

Save Public Key

(4) Verify using private key of User i

(5) Authentication ResultSystem User i

Use public key to generate signature.

(3) ID, Signature

System User ion-line test

(1) Compute Public Key for User i from its private key

usually done off line

(2) Send Public Key

Save Public Key


Authentication (RSA Algorithm)

Let us take p=3 and q=11, giving n=pq=33

Assume e=7, gives (n,e) as public key of (33,7)

For message m=4, c= me| mod n = 47 mod 33 = 16

d is computed such that ed mod (p-1)(q-1) = ed mod 20= 1, thus, d=3, giving private key of (33,3)

After receiving c=16, compute cd mod 30 = 16 3 mod 33 =4


Authentication (RSA Algorithm)• In RSA method 2 large prime numbers (p,q) are selected.• n = p*q,• A number e is selected to use (n,e) as the public key and is

transmitted to the user,• User stores this, whenever a message m< n needs to be

transmitted, user computes c = me| mod n and sends to the system.• After receiving c, the system computes cd|mod n where d is computed

using the private key (n,e) • cd|mod n = (me|mod n) d |mod n = (me)d |mod n

= m ed|mod n

• To make this equal to m, ed should be equal to 1.• This means e and d need to be multiplicative inverse using mod n

(or mod p*q)• This can be satisfied if e is prime with respect to (p-1)*(q-1) • Using this restriction original message is reconstructed.


Base Station

Select p and q as two prime numbers

n = p*q

1 < e < n

Public Key (n,e) Mobile Station

Save public key (n, e)

Base Station

Compute d from e

(n,d) private key

Receive c

Mobile Station

Message m < n

Sent as c = me|mod n

c

Base Station

Compute cd|mod n = mde|mod n = m

If de = 1

AuthenticationMobile station OK

Message Authentication using Public/Private Keys


Base StationMobile Station

(ID)e|mod n

Authentication

(a) Authentication based on ID

Base Station

(ID)e|mod n

R: Random Number as a Challenge Mobile

StationSend Re|mod n

Authentication

(b) Authentication using a challenge

Authentication of a MS by the BS


Wireless System Security

Basic services of security: Confidentiality Non-repudiation: sender and receiver

cannot deny the transmission. Authentication: sender of the information is

correctly identified. Integrity: content of the message can only be

modified by authorized user. Availability: resources available only to

authorized users.


Wireless System Security

Security Mechanisms: Security Prevention: Enforces security

during the operation of the system. Security Detection: Detects attempts to

violate security. Recovery: Restore the system to pre-

security violation state.


Cost Function of a Secured Wireless System

Expected total cost with violations

Cost

Security Level

100%

Expected total cost

Cost for Security enhancing mechanisms

Optimal Level


Security Threat Categories

S Source D DestinationSource I Intruder

Interruption

Message

S I D

Fabrication

MessageS

I

D

Modification

Message

MessageS

I

D

Interception

Message

MessageS

I

D


Wireless Security

Active Attacks: When data modification or false data transmission takes place. Masquerade: one entity pretends to be a

different entity. Replay: information captured and retransmitted

to produce unauthorized effect. Modification of message Denial of service (DoS)

Passive Attacks: Goal of intruder is to obtain information (monitoring, eavesdropping on transmission)


Firewalls and System Security

Firewall carries out traffic filtering, web authentication, and other security mechanisms.

Filtering can be configured by fixing: Source IP Destination IP Source TCP/UDP port Destination TCP/UDP port Arrival interface Destination interface IP protocol

Firewall resides at wireless access point to carry out authentication

copyright © 2006, dr. dharma p. agrawal and dr. qing-an zeng. all rights reserved. 1 mobile...

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