1Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Mobile Communication Systems
Chapter 9
2Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Outline
Cellular System InfrastructureRegistrationHandoffRoamingMulticastingSecurity and Privacy
3Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Cellular System
MSC
BSC
BSC
BTS
BTS
BTS
BTS
BTS
BTS
……
…
MS
HLR
VLR
EIR
AUC
MSGateway
MSCMSC
…
PSTN/ISDN
Base Station System
Base Station System
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VLR/HLR
VLR contains information about all visiting MSsin 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.
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Redirection of Call to MS at a Visiting Location
Call routed as per called number to MS
Home MSC
HLR
Visiting MSC
VLR
BS
MS
Cell where MS is currently located
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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 previouslythen 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.
7Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Using a Mobile Phone Outside the Subscription Area
Visiting BSMS
Beacon signal exchange
1Request fo
r registration
2Authentication/reject
5
Home BS
3Authentication request
4Authentication response
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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.
9Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Handoff
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 linkNetwork managementService issues
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Handoff (Cont’d)
Radio link handoff is due to mobility of MS. It depends on:
Number of MSs in the cellNumber of MSs that have left the cellNumber of calls generated in the cellNumber of calls transferred from the neighboring cellsNumber of calls terminated in the cellNumber of calls that were handoff to neighboring cellsNumber of active calls in the cellCell populationTotal time spent in the cell by a callArrival time of a call in the celletc
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Handoff (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).
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Time for Handoff
Factors deciding right time for handoff:Signal strengthSignal phaseCombination of above twoBit error rate (BER)Distance
Need for Handoff is determined by:Signal strengthCIR (carrier to interference ratio).
13Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Handoff Initiation
BSi
Signal strength due to BSj
E
X1
Signal strength due to BSi
BSjX3 X4 X2X5 Xth
MS
Pmin
Pi(x) Pj(x)
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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 procedures.
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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 have to change the code of the handoff, if the code is not 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.
16Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Hard Handoff
BS1 BS2MS
BS1 BS2MS
BS1 BS2MS
(a). Before handoff (c). After handoff
(b). During handoff(No connection)
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Soft Handoff (CDMA only)
BS1 BS2MS
BS1 BS2MS
BS1 BS2MS
(a) Before handoff
(b) During handoff
(c) After handoff
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Roaming
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 we have the same service provider, using that particular frequency band.
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Roaming
BS1 BS2MS BS1 BS2MS
Home MSC
Visiting MSC
Home MSC
Visiting MSC
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PSTN
MSC4MSC3MSC2MSC1
Paging Area 1
Paging Area 2
a b c d e
Handoff Scenarios with Different Degree of Mobility
MS
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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.
22Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Information Transmission Path when MS Hands Off from “b” to “c”
MSC1HLR
MSC2VLR
a b c
Information to MS being sent
Initial path of information transfer
Path after handoff
MS
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Illustration of MSC Connections to Backbone Network and Routing/Rerouting
MSCRouter
Paging area 1 (PA1) Paging area 2 (PA2)
MSC 1(a,b)
MSC 2(c)
MSC 3(d) MSC 4
(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)
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Backbone Network
Routing done according to the topology 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.
25Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
26Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
R9R6R4R3Selected router for maintaining its home
agent
MSC4MSC3MSC2MSC1Home MSC
Home MSC and Home Agent (HA) for the Previous Network
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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.
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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 the 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.
29Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
FA
1
1”
1’
Beacon Signal
I am new here
OK, send information
Here is my HA and binding infomation.
3 COA or C-COA created
MSHA
2
(Any one new)
4
4’ Same as step
Here is COA or co-located COA (C-CoA) for this MS
Acknowledge Registration + binding
4
4” Same as step 4
Registration Process Between FA, MS, and HA When the MS Moves to a Paging area
30Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Source To MS Payload DataIncoming message for MS
HA
HA CoA/C-CoA Source To MS Payload Data
Encapsulation
FA
Source To MS Payload Data
Forwarding through intermediate router if CoA used
Decapsulation
Decapsulation done at MS
Forwarding through intermediate router if C-CoA used
MS
Message Forwarding using HA-FA Pair
31Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
32Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
PA1 PA2
PA3PA4
PA5
Router X
Router W
Router Y
Router Z
Network 1
Network 2
MS moves
Illustration of Paging Areas (PAs) and Backbone Router Interconnect
33Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
-5Z5Y5X5
-4Z4Y4X4
-3Z3Y3X3
Y2X2-2X2
Y1X1-1X1
Next hop
Route to PA
Next hop
Route to PA
Next hop
Route to PA
Next hop
Route to PA
Table at routerW
Table at router X
Table at router Y
Table at routerZ
Distributed Routing Table and Location PAs
34Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
35Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
36Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.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.
37Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
38Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
39Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
HA
Multicast packets from the multicast tree
MS1
MS2
MS3
FA
MS 1
MS 2
MS 3
Packet Duplication in BT Tunnel Approach
40Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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
41Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
42Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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
43Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
44Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
45Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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
46Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
(a) Permutation before transmission (b) Permutation after reception
Bit Patterns before Transmission and after Reception using DES
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
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
47Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
48Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Encoding
Transmittedsignal
signalReceived
Decoding
Information
block
Encoded
signal
Encoded
signal
Information
block(Original)
A Generic Process of Encoding and Decoding
at
transmitter receiver
at
49Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Key K1f
f
+
+ f+
Input (64 bits)
Initial Permutation (IP)32 bits 32 bits
Left half: L1 Right half: R1
Left half: L1 = R1 R1 = L1 f(R1, K1)++
Left half: L16 = R15R16 = L16 f(R15, K16)++
Inverse initial permutation (IP –1)
Coded Output
Permutation and Coding of Information
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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)
51Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
System User i(1) Compute Public Key for User i
from its private key.
(2) Send Public Key
Save Public Key
Use public key to generate signature.
(3) ID, Signature
(4) Verify using private key of User i.
(5) Authentication Result
Public/Private Key Authentication Steps
usually done off line
System
System
User i
User i
on-line test
52Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
53Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Base Station
Select p and q as two prime numbers
n = p*q
1 < e < n
Base Station
Compute d from e
(n,d) private key
Receive c
Base Station
Compute cd|mod n = mde|mod n = m
If de = 1
Public Key (n,e)Mobile Station
Save public key (n, e)
Mobile Station
Message m < n
Sent as c = me|mod n
c
AuthenticationMobile station OK
Message Authentication using Public/Private Keys
54Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Base Station Mobile Station
(ID)e|mod n
Authentication
Base Station
(ID)e|mod n
R: Random Number as a Challenge
Mobile StationSend Re|mod n
Authentication
(a) Authentication based on ID
(b) Authentication using a challenge
Authentication of a MS by the BS
55Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Wireless System Security
Basic services of security:ConfidentialityNon-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.
56Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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.
57Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Cost Function of a Secured Wireless System
Expected total cost with violations
Cost
Security LevelOptimal Level
Expected total cost
Cost for Security enhancing mechanisms
100%
58Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
Security Threat Categories (Types of Attacks)
S Source D DestinationSource I Intruder
Interception
Message
Message
Interruption
Message
Modification
Message
Message
Fabrication
Message
S I D
SS
S
II
I
DD
D
59Copyright © 2002, Dr. Dharma P. Agrawal and Dr. Qing-An Zeng. All rights reserved.
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 messageDenial of service (DoS)
Passive Attacks: Goal of intruder is to obtain information (monitoring, eavesdropping on transmission)