Mobile Communication System

Azizol Bin Abdullahazizol@fsktm.upm.edu.my (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:

BS (BTS and BSC) contained all associated electronics.

Authentication Center(AUC) 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) 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) 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.)

Steps used for mobile phones outside their own subscription areas:

1. 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.

2. The MS locates the nearest BS via user-level processing3. The visiting BS perform user-level processing and determines the

MS, user’s registered home site.4. The home site sends an appropriate authentication response to BS

currently serving the user.5. 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: 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: 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: Signal strength Carrier-to-interference ratio (CIR)

The specific units involved in setting up a call: 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: Hard handoff

break before makeEmploy by FDMA & TDMA

Soft handoffMake before break

Roaming 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: 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 HLR-VLR, 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 Each source of the group maintain shortest path tree, the source

being the root of the tree. Core-based tree

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: Nonoptimal path length Packet duplication Distruption of packet delivery

IETF proposed two approach for providing multicast over mobile IP: The bidirectional tunneling (BT) The remote subscription approach.

Multicasting (cont.)

Bidirectional Tunneling 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: 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 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:

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) 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:

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:

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: 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: 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: 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: 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.