Mobility Management in Wireless Communication

Mobility Management in Wireless Communication – Chapter 12

By
Donavon M. Norwood
CS286
Dr. Moh

Introduction - 12.1

Mobility is very important in mobile communication, and can be classified as radio mobility and
network mobility. Radio mobility is mainly concerned with the handoff process, whereas
network mobility deals with mobile location management (i.e., location and updating). The public
land mobile network (PLMN) is a cellular network that provides wireless services to mobile
subscribers to other networks and network services, which include other PLMN's. The PLMN is
divided into regions called LA's, (Location Areas), which are made up of one or more cell areas.
A mobile station (MS) registers with the visitor location register (VLR) each time it enters a
new LA, and is free to move inside the LA without registration. Each cell has a unique
identification number called a cell global identification (CGI), which contains location area
identification (LAI) and the cell identification (CI). The LAI consists of mobile country code
(MCC), mobile network code (MNC), and location area code (LAC). Mobility management in
wireless networks is the primary set of functions important to a network in order to enable
subscriber mobility, which includes enabling the network to keep track of a subscribers status
and location in order to deliver calls to the subscriber. The key component to mobility
management is the subscribers service profile, which is a database record in the network that
contains information about each subscriber. The data in the record is dynamic such as current
location and status of subscriber and permanent data such as service profile, international mobile
subscriber identity (IMSI), etc., of the subscriber.

2

Figure 12.1 Wireless PLMN

3

Figure 12.2 Cell global identification

4

Mobility Management Functions - 12.2

Mobility management generally deals with automatic roaming, authentication, and
intersystem handoff. Automatic roaming includes a set of network functions that allow
the subscriber obtain service outside the home service provider area. These functions
are automatic and do not require special subscriber actions. Automatic roaming functions
are divided into:

I. Mobile station (MS) service qualification
II. MS location management
III. MS state management
IV. Home location register (HLR) and VLR fault recovery

The authentication process requires that the end users of the system are authenticated.
Handoff is one of the essential features that guarantee the subscriber mobility in a mobile
network where the subscriber can move around. The handoff function allows the moving
subscriber to maintain a connection. In simple terms the handoff functions works when a
subscriber moves into a new cell a new connection has to be established and the cell in
which the subscriber left from has to be disconnected.

5

Mobile Location Management - 12.3

Location management schemes are based on subscribers mobility and
incoming call rate characteristics. The location update (LU) is used by the
network to keep track of the mobile subscribers location to direct a incoming
call. A paging process transmits paging messages to all the cells were
mobile terminals are located. If a LU cost is high, the the paging cost is low
and paging is performed over a small area. When LU cost is low, then
paging cost is high and paging is performed over a wider area. Location
management uses either the periodic LU or LU-on-LA-crossing. The VLR
stores the LAI, and the HLR keeps the VLR identifier. Using the periodic LU
method, the MS periodically sends its identity to the network, but the
drawback to using this method is resource consumption, and an example
would be if a MS does not move in a new LA for several hours. In the LU-on-
LA-crossing method, every base station will periodically broadcast the identity
of the LA. The MS is required to listen to network broadcast information and
store the current LA identity if the one stored in the MS is different. The LU
procedure is is automatically done by the MS. The advantage of this method
is that LU's are only generated when the MS moves.
6

Mobility Model - 12.3.1

A mobility model describes the occurrence of procedures such as LU and handoff, and
several models have been proposed.

Fluid model: This model assumes that the traffic flow to be like the flow of fluid. The
model suggests that the amount of traffic flowing out of the area is proportional to the
population density of the area, average velocity of movement and the length of the area
boundary. For an area the average number of crossings per unit time is given as:

 L
=

=number of crossings per unit of time , =average population density ,

=average movement velocity of the area , L= perimeter of the area

7

Mobility Model – 12.3.1 (Continued)


Markovian model: Known as the random walk model, this model describes individual
movement. Also in this model a mobile subscriber remains within a region or moves to an
adjacent region according to a transition probability distribution. One of the limitations of
this model is that there is no concept of trips.

Gravity model: Different types of this model have been employed in transportation
research to model human movement behavior. This model also has been applied to
regions of varying sizes, from city models to national and international models. This
model is difficult because many parameters are required in the calculations, therefore it is
difficult to model geography with many regions.

8

Mobile Registration - 12.4

The detection of a subscriber in a new serving system is an example of a registration
event, which is a subset of automatic roaming functions, which consists of MS service
qualification and MS location management. These two functions allow the subscriber
to register with the network, that is to indicate to the network location register
functional entities of the location and status of the MS. The registration functions need
to be performed before the network can provide service to the subscriber, with the
exception of emergency services like 911. Registration may be initiated by the MS or
the network, or may be implied during the MS access. Upon receiving the registration
request from a MS, the base station (BS) constructs the Registration Update
Request message and sends it to the MSC (Mobile Switching Center), which contains
the MS's identification and location information and may contain authorization
parameters. The MSC may respond with a request for authorization (optional) and
finally with a Registration Update Response message. The MSC sends the
Registration Update Response to the BS when the registration procedure has been
successfully completed, and this message also indicates whether the MS's registration
has been accepted or rejected. The three possible results in the registration request
are: successful registration, unsuccessful registration, and cancellation of
registration.

9

Mobile Registration – 12.4 (Continued)
Registration Types


Distance-based registration: when the distance between the current cell and the cell
where the mobile last registered exceeds a threshold.

Geographic-based registration: whenever a mobile station enters a new area of the
same system.

Parameter change registration: when specific operating parameters in the mobile are
changed.

Periodic registration: when the system sets parameters on the forward control
channel to indicate that all or some of the mobile stations must register.

Power-down registration: when the mobile is switched off. This allows the network to
deregister a mobile immediately upon its power-down.

Power-up registration: when power is applied to the mobile, and used to notify the
network that the mobile is now active and ready to place or receive calls.

Timer-based registration: when a timer expires in the mobile.

10

Figure 12.3 Call flows for MS registration of all mobiles listening to a
control channel (ANSI-41 Standard)

11

GSM Token-Based Registration - 12.4.1

When an MS registers with the new network, it sends its TMSI and LAI. The LAI informs the system
where to find the old VLR. The network then queries the old VLR for data and uses the data to
authenticate the MS. The new VLR then communicates with the HLR to update the location of the MS.
The HLR sends a registration cancellation message to the old VLR. The operation of the token-based
system is slightly different from the ANSI-41 system in that the segmentation of call processing between
the BS, MSC and VLR is defined differently. Here are the steps for Token-Based registration:

1. MS sends registration message to the visiting system with its old TMSI and old LAI.
2. Visiting system queries the old VLR for data.
3. Old VLR returns security-related information.
4. Visiting system issues a challenge to the MS.
5. MS responds to challenge.
6. Visiting system assigns new TMSI.
7. Visiting system sends a message to the HLR with location update information
8. HLR updates its its location database with new location of the MS.
9. HLR acknowledges messages and may send other security related information.
10. HLR sends a registration cancellation to old system.
11. Visiting system sends an encrypted message to the MS with new TMSI.
12. MS acknowledges the message.

12

Figure 12.4 Token-based registration

13

IMSI Attach and ISMSI Detach
(Registration and Deregistration) in GSM - 12.4.2

In GSM a mobile power-up implies a IMSI attach which causes a mobile
registration. See figure 12.5.

14

Figure 12.5
IMSI attach process in GSM

15

IMSI Attach and ISMSI Detach
(Registration and Deregistration) in GSM – 12.4.2
(Continued)

In GSM a mobile power-down implies a IMSI detach
which causes a mobile deregistration. See figure 12.6.

16

Figure 12.6
IMSI detach process in GSM

17

Paging in GSM - 12.4.3

In this scenario we assume that the mobile is already
registered with the system and has acquired a TMSI. It is
also assumed that the mobile is located in its home network.
A land subscriber dials the number of the mobile subscriber.
See Figure 12.7.

18

Figure 12.7
Mobile-terminated call in GSM

19

Handoff – 12.5

As a mobile moves from one cell to another, an active call must
undergo a switch from one channel to another. This process is
called handover and handoff. In TDMA systems handoff usually
involves both a change of channel carrier frequency and time slot.
It also may require a reconfiguration of the wireline facilities by
dropping the connection to the serving (old) BSS and switching to
a connection to the new (target) BSS as shown in figure 12.8. In
this example the mobile tunes from carrier frequency 6 on time
slot 3 served by the old BSS to carrier frequency 9 on time slot 7
by the new BSS (Interfrequency handoff).

20

Figure 12.8
Inter-MSC, Inter-BSS handoff

21

Handoff – 12.5 (Continued)

Handoff is initiated because of a variety of reasons. Signal strength deterioration is the most
common reason for handoff at the edge of a cell. Other reasons could be due to load
balancing where the handoff is network initiated to relieve traffic congestion by shifting calls in
a highly congested cell to a lightly loaded cell. The handoff could be synchronous or
asynchronous. Handoffs are initiated by the BSC based upon radio subsystem criteria such
as RF level (RXLEV), signal quality (RXQUAL) or distance, or they are a result of network
traffic loading. Appropriate decisions are made by a handoff algorithm. The measurements
performed by the MS and the BTS which are collated by the BSC include:

• MS – RXLEV and RXQUAL (for serving downlink and adjacent BS's)
• BTS – RXQUAL and RXLEV (uplink for serving BS)

There are two types of handoff: internal and external. If the serving and target base station are
located in the same BSS, the BSC for the BSS performs the handoff process without the help
of a MSC. This type of handoff is known as a intra-BSS handoff. However if the serving and
target base stations are not in the same BSS, and external handoff is performed in which a
MSC coordinates the handoff performs the switching task between the serving and target base
stations. External handoffs can be either in the same MSC (intra-MSC) or between different
MSC's (inter-MSC).

22

Handoff Techniques - 12.5.1

Handoff techniques can be classified as mobile-controlled handoff (MCHO), network
-controlled handoff (NCHO), and mobile-assisted handoff (MAHO). In MCHO the
mobile continuously monitors the signal strength and quality from the serving base station
and several handoff candidate base stations. When the handoff criteria are met, the
mobile checks the candidate base station for an available traffic channel and launches a
handoff request. NCHO is employed by a low-tier CT-2+ and a high-tier AMPS system.
In this case the base station monitors the strength and signal quality from the mobile.
When these fall below a certain threshold, the network arranges for a handoff to another
station. The network asks all surrounding base stations to monitor the signal strength
from the mobile and report the measurements back to the network. The network
selects a new base station for handoff and informs both the mobile and new base station.
MAHO strategy has been employed by high-tier 2G (GSM, IS-95 CDMA, IS-136 TDMA)
and 3G (WCDMA, cdma200) systems. In this approach the network provides the mobile
with a list of base station frequencies (those of nearby base stations). The network asks
the mobile to measure the signal strengths and signal quality from the surrounding base
stations and report the measurements back to the serving base stations so the network
can decide whether a handoff is required and to which base station.

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Handoff Types - 12.5.2

Handoff can be categorized as hard handoff, soft handoff, and softer handoff. Hard
handoff can be further divided into intrafrequency and interfrequency hard handoffs. A
hard handoff occurs when the old connection terminated before making a new
connection. In the case of a interfrequency hard handoff, the carrier frequency is different
from the old carrier frequency which the MS was connected to. If the new frequency of
the carrier is the same as the old frequency in which the MS was connected to, this is
called a intrafrequency handoff. In 2G TDMA systems the majority of the handoffs are
intrafrequency handoffs. Interfrequency handoffs may occur between two different radio
access networks. In this case it can also be called a intersystem handoff. A intersystem
handoff is always a type of interfrequency since different frequencies are used in different
systems. A soft handoff occurs when a new connection is made before the old
connections is released. In 3G systems the majority of handoffs are intrafrequency
handoffs. A 2-way soft handoff happens when two sectors with different BS's that do
not belong to the same BSC. A softer handoff occurs when a BS transmit through one
sector but receives responses from more than one sector. When a soft and softer
handoff occurs at the same time, this is called a soft-softer handoff.

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Handoff Process and Algorithms - 12.5.3

A basic handoff process consists of three main phases which include measurements,
decision and execution phase.

• Measurement phase - the MS continuously measures the signal strength of of the
serving and neighboring cells, and reports the results to the network.

• Decision phase - consists of an assessment of the overall quality of service (QoS)
of the connection and comparing it with the requested QoS attributes and estimates
from neighboring cells. A handoff my occur depending on the outcome of the
comparisons.

• Execution phase - involves handoff signaling and radio resource allocation. Radio
signal strength (RSS) measurements from the serving cell and neighboring cells
are primarily used in most networks.

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Handoff Process and Algorithms – 12.5.3
(Continued)

The following parameters are generally used in the handoff algorithm:

• Upper threshold: the level at which the signal strength of the connection is at the maximum
acceptable level with respect to the required QoS.

• Lower threshold: the minimum level of the signal strength to satisfy the required QoS, thus
the signal strength of the connection can not fall below this level.

• Handoff margin: a predefined parameter set in which the signal strength of neighboring cell
exceeds that of the serving cell by a certain amount or a certain time.

Some of the traditional handoff algorithms are as follows:

i. RSS type: the BS with the largest signal strength is selected
ii. RSS plus threshold type: a handoff is performed if the RSS of the new BS exceeds that of
the serving BS and the signal strength of the serving BS is below the lower threshold value.
iii. RSS plus handoff margin type: a handoff is performed if the RSS of the new BS is more
than that of the serving BS by a handoff margin/

* RSS (Radio Signal Strength)

26

Figure 12.9 Handoff process

27

Handoff Call Flows (Intra-MSC Handoff) - 12.5.4

In Intra-MSC Handoff the MS constantly monitors the signal
quality of the BSS link, or optionally the BSS will forward its
measurements to the MS. When the signal quality os poor, the MS
will attempt to maintain the desired signal quality of the radio link
By requesting a handoff. The following steps on the left occur in a
Intra-MSC Handoff.

28

Figure 12.10
Call flow for intra-MSC handoff

29

Handoff Call Flows (Inter-MSC Handoff) - 12.5.4

In this scenario we assume that a call has already been
established. The serving BSS is connected to the
servingMSC, and the target BSS to the target MSC (see
figure 12.11).

30

Figure 12.11
Call flow for inter-MSC handoff

31

Figure 12.11
Call flow for inter-MSC handoff
(Continued)

32

Thank You

References: Wireless Communications and Networking, Vijay K. Garg.

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Mobility Management in Wireless Communication

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