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Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 1
Mobile Communication Systems
Chapter 10
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 2
Outline
Cellular System Infrastructure
Registration
Handoff Parameters and Underlying
Support
Roaming Support
Multicasting
Security and Privacy
Firewalls and System Security
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 3
Cellular System Infrastructure
MSC …
HLR
VLR
EIR
AUC
Gateway
MSC
MSC
…
PSTN/ISDN
BSC BTS
BTS
BTS
MS
Base Station System
BSC BTS
BTS
BTS
…
MS
Base Station System
…
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 4
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Classical Mail Forwarding Technique?
Post Office Cincinnati
Post Office Washington, DC
Mail from the world
Cincinnati Washington, DC
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 6
PSTN
MS
Home
Mobile
Switching
Center
HLR Home
network
Visiting
area
Caller
Visiting
Mobile
Switching
Center
VLR
MS
1
Location update request
Using Becon Signals
Update location
Info. sent to HLR
2
Automatic Location Update
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 7
PSTN
MS
home
Mobile
Switching
Center
HLR Home
Network
Visiting
Area
Caller
Mobile
Switching
Center
VLR
Automatic Call Forwarding using HLR-VLR
1 Call sent to
home location
2 Home MSC checks
HLR; gets current
location of MS
in visiting area
3
Home MSC forwards
call to visiting MSC
4
MSC in visiting area sends
call to BS and connects MS
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 8
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 9
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.
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 10
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 11
Using a Mobile Phone Outside the Subscription Area
Visiting BS
(Visiting MSC)
MS 1
2
5
Home BS
(Home MSC)
3 Authentication request
4 Authentication response
Through backbone
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 12
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
15)
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
14)
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 12)
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 13
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 14
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 15
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)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 16
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)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 17
Handoff Region
BSi
Signal strength
due to BSi
X2
MS X4
Pmin
Pi(x)
E
Signal strength
due to BSj
X1 X3 X5 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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 18
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 19
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 20
Hard Handoff
BS1 BS2 MS
(a). Before handoff
BS1 BS2 MS
(b). During handoff (No connection)
BS1 BS2 MS
(c). After handoff
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 21
Soft Handoff (CDMA only)
BS1 BS2 MS
(b). During handoff
BS1 BS2 MS
BS1 BS2 MS
(c). After handoff (a). Before handoff
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 22
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 23
Roaming Support
BS1 BS2 MS
Home
MSC
Visiting
MSC
BS1 BS2 MS
Home
MSC
Visiting
MSC
MS moves
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 24
Handoff Scenarios with Different
Degree of Mobility
PSTN
Paging Area 1
MSC2
c
MSC3
d
MSC4
Paging Area 2
e
MS
MSC1
a b
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 25
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 26
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 27
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 28
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 29
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 30
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 31
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 32
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 33
FA
3 CoA or C-CoA
created
MS HA
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 34
Source To MS Payload Data Incoming 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 MS MS
Message Forwarding using HA-FA Pair
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 35
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 36
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 37
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 router
W
Table at router
X
Table at router
Y
Table at router
Z
Distributed Routing Table and Location PAs
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 38
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 39
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 40
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 41
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 42
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 43
HA
Multicast
packets from the
multicast tree
MS1
MS2
MS3
FA
MS 1
MS 2
MS 3
Packet Duplication in BT Tunnel Approach
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 44
Multicast packets
from the multicast tree
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 45
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 46
Multicast packets from
the multicast tree
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 47
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 48
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 49
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 50
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 51
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 52
A Generic Process of Encoding and Decoding
Information
block
Transmitted
signal Encoded
signal
Encoding
at
transmitter
Information
block
Received
signal
Decoding Encoded
signal
(Original) receiver
at
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Information
block
53
A Generic Process of Encoding and Decoding
Encoding
Transmitted
signal
Received
signal Decoding
Encoded
signal
Encoded
signal
Information
block
(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-OR
bits
1
0
1
0
1
1
1
0
Bits after
EX-OR
Air
Operations done at the receiving MS
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 54
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
+
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 55
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)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 56
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 Result System User i
Use public key to
generate signature.
(3) ID, Signature
System User i on-
line
test
(1) Compute Public Key for User i
from its private key
usually
done
off line
(2) Send Public Key
Save Public Key
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 57
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 58
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.
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 59
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
Authentication Mobile station OK
Message Authentication using Public/Private Keys
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 60
Base Station Mobile
Station
(ID)e|mod n
Authentication
(a) Authentication based on ID
Base Station
(ID)e|mod n
R: Random Number as a Challenge Mobile
Station Send Re|mod n
Authentication
(b) Authentication using a challenge
Authentication of a MS by the BS
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 61
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
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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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 63
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
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 64
Security Threat Categories
S Source D Destination Source I Intruder
Interruption
Message
S I D
Fabrication
Message S
I
D
Modification
Message
Message
S
I
D
Interception
Message
Message
S
I
D
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 65
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)
Copyright © 2011, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved. 66
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