Transcript Skr4800_CHP5

Mobile Communication
System
Azizol Bin Abdullah
[email protected] (A2.04)
Rujukan: Text Book Chapter 9
Introduction
 A wireless system implies support for
subscriber mobility and the overall
communication infrastructure.
 In this chapter we will consider handoff
schemes, allocation of resources, routing in
the backbone network and security.
Cellular System Infrastructure
 A Cellular system – complex infrastructure.
 Consist of:
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BS (BTS and BSC)
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Authentication Center(AUC)
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contained all associated electronics.
provides authentication and encryption parameters that verify
user identity and to ensure the confidentiality of each call.
Protects network operators from different types of frauds and
spoofing
Equipment Identity Register (EIR)
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database that contains information about the identity of mobile
equipment that prevents calls from being stolen, unauthorized or
defective MSs
Cellular System Infrastructure (cont.)
 Home location register (HLR) and visitor
location register (VLR)
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Support mobility and enable the use of the same
telephone number over a wide range.
HLR is located at the Mobile switching Service
Center (MSC) where the MS initially registered
for billing and access information
Diagram of Mobile Communication System: GSM
Registration
 MSs must be registered at one of MSCs for
operation.
 It is maintained for: billing, authentication,
verification, access privileges and call routing.
 Call is route to an appropriate location based on
registration information.
 Call routing is done by exchanging signals known
as “beacons” between the BS and MS.
 Information carried by beacon signal: cellular
network identifier, timestamp, gateway address, ID
of paging area and other BS parameters.
Registration (cont.)
 BSs periodically broadcast beacons signal to
determine and test nearby MSs.
 Each MSs that hear from a new BS, it adds
BS to the active beacon kernel table for
locating the nearest BS as a gateway.
Registration (cont.)
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Steps used for mobile phones outside their own
subscription areas:
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5.
MS listen for new beacons, if it detects one, adds it to the active
kernel table. If it needs to communicate via new BS, kernel
modulation initiates the handoff process.
The MS locates the nearest BS via user-level processing
The visiting BS perform user-level processing and determines the
MS, user’s registered home site.
The home site sends an appropriate authentication response to BS
currently serving the user.
The BS at the visited location approves of disapproves user
access.
Handoff Parameters and Underlying
Support
 Basically, handoff involves change of radio
resources from one cell to another cell.
 Handoff depend on: cell size, boundary
length, signal strength, fading, reflection and
refraction of signal, and by man-made noise.
 Handoff could be due to: Radio link,
Network Management and services issues.
Handoff Parameters and Underlying
Support (cont.)
 Radio link-type handoff due to mobility of MS and
depends on the relative value of the radio link
parameter:
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Number of MSs that are in the cell
Number of MSs that left the cell
Number of call generated in the cell
Number of call transferred to the cell from neighboring
cell by the handoff
Number and duration of calls terminated in the cell
Number of calls handed off to neighboring cell
Cell dwell time
Handoff Parameters and Underlying
Support (cont.)
 Network management may cause handoff if there is
a drastic imbalance of traffic over adjacent cell
 Service-related handoff is due to degradation of
quality of Service.
 The factor that defined to handoff:
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Signal strength
Signal phase
Combination of Signal strength and phase
Bit error rate
Distance
Handoff Parameters and Underlying
Support (cont.)
 The need for handoff is determined in two different
ways:
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Signal strength
Carrier-to-interference ratio (CIR)
 The specific units involved in setting up a call:
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Base station controller (BSC)
Mobile station (MS)
Mobile Switching center (MSC)
Handoff Parameters and Underlying
Support (cont.)
 Handoff can be classified into two different
types:
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Hard handoff
break before make
 Employ by FDMA & TDMA
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Soft handoff
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Make before break
 Roaming
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By using beacons signal and HLR-VLR, MSCs
are allow to roam anywhere as long as the same
service provider.
It is possible to move from one cell another cell
that control by other MSC area.
Roaming Support using System
Backbone
 A number of cell are controlled by an MSC.
 Signal go through the network backbone
(interconnecting MSC with PSTN) through access
points.
 Routing process – moving packet to appropriate
endpoints of the backbone network.
 Paging area:
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the area covered by one or several MSC in order to find
the current location of MS.
Use the internet network routing area concept.
Roaming Support using System
Backbone (cont.)
 In Mobile IP, two important agents are associated
with the routers: home agent (HA) and foreign
agent (FA).
 MS also registered with a router, a router closest to
the home MSC can be selected to serve as its HA.
 Once MS moves from the home network to a
foreign network, FA will assist the MS by
forwarding packets for the MS.
 The functionality of HA-FA is analogous to HLRVLR, except it supports mobility in a much broader
sense.
Multicasting
 A process of transmitting messages from a source to
multiple recipients by using a group address for all
hosts that wish to be members of the group.
 It reduces the number of messages as compared
with multiple unicasting for each member –
enhancing bandwidth utilization.
 Extremely useful in video/audio conferencing,
distance learning and multiparty games.
 Can be performed by building a source-based tree
or by using a core-based tree.
Multicasting (cont.)
 Source-based tree
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Each source of the group maintain shortest path tree, the
source being the root of the tree.
 Core-based tree
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A particular router is chosen as a core and tree is
maintain, with the core being the root.
Every source then forwards the packet to the core router,
then it forwards to the tree and members.
 Tree rebuilding in wireless is more complex than
wired – mobility: MS changing from one MSC to
another MSC.
Multicasting (cont.)
 There is a need to route packets to MS and address
these problems during multicast tree generation:
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Nonoptimal path length
Packet duplication
Distruption of packet delivery
 IETF proposed two approach for providing
multicast over mobile IP:
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The bidirectional tunneling (BT)
The remote subscription approach.
Multicasting (cont.)
 Bidirectional Tunneling
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Whenever an MS moves into a foreign network, HA is
responsible for forwarding the multicast packets to MS
after due registration process.
HA creates a bidirectional tunnel to that FA that currently
serving the MS.
FA encapsulates the packets for the MS, the forward to
MS.
BT prevents data distruption due to movement of MS but
cause problems:
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Packet duplication if several MSs of the same HA, which have
subscribed to same multicast group, move to the same HA
Packet may use nonoptimal paths for delivery
Multicasting (cont.)
 Remote subscription approach
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Whenever an MS moves into foreign network, the FA
sends a tree join request (if it not already a member of
the multicast tree).
MS receives multicast packet through the FA.
Simple and prevents packet duplication & nonoptimal
path delivery.
Problems:
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However, since the FA needs to join the multicast tree, it can
cause data distruption until FA is connected to the tree.
Frequent tree updates when the MSs move frequently
Multicasting (cont.)
 The Mobile Multicast (MoM)
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Attempts to prevent packet duplication by forcing an HA
to forward only one multicast packet for a particular
group.
The FA select a designated multicast service provider
(DMSP) for each group.
DMSP is an HA that is responsible for forwarding a
multicast packet to the FA of the group.
Problems:
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Selecting a DMSP: if MS of the DMSP that subscribed to FA
move out, DMSP may stop forwarding packets to FA – cause data
distruption.
Selecting more that one DMSP: cause data duplication.
Wireless System Security
 Needs to capable of protecting.
 The services of security can be classified as:
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Confidentiality : only the authorized party can access the
information in the system and transmit data.
Nonrepudiation: the sender and receiver cannot deny the
transmission.
Authentication: ensure the sender
Integrity : the content of the message or information can
only be modified by authorized users.
Availability : the resources should be available only to
the authorized users.
Wireless System Security (cont.)
 Security mechanisms can be divided into three
categories:
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Security prevention : enforces security during the
operation of the system to counter security attacks.
Security detection : Detects both attempts to violate
security.
Recovery : use to restore the system to a presecurity
violation state.
Wireless System Security (cont.)
 Threats can be broadly classified in two
types:
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Accidental threats : due to operational failure.
Intentional threats : any action performed by an
entity with an intention to violate security.
Wireless System Security (cont.)
 Intentional threats can be categorized as:
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Interruption : attack by blocking or interrupting system
resources.
Interception : System resources accessed by illegal party
– attack the confidentiality
Modification : Create anomaly in the network, illegal
party transmits spurious message – affect authenticity
Fabrication : unauthorized party transmit counterfeit
object into the system – cause an attack on authenticity
Wireless System Security (cont.)
 Different types of active attack:
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Masquerade: Attacker poses as an authorized party to
make previledged changes in network.
Replay: Playing back previous sent data to create
undeseirable effects.
Modification of data: Original message is tempered to
create inconsistency in the network
Denial of service: Hijacking of network resources
 Passive attack : listen and monitor the
communication between to parties.