Architecture and Schemes for Intelligent Mobility Management in

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Architecture and Schemes for Intelligent Mobility Management in
Future Mobile Telecommunication Systems
Kuo-Hsing Chiang, Nirmala Shenoy
Department of Computer Systems Engineering, RMIT University
GPO Box 2476v, Melbourne, Victoria, 3000, Australia
Email: kuo_hsing@hotmail.com, nirmala.shenoy@rmit.edu.au
Tel: +61 3 9925 5360, Fax: +61 3 9925 5340
Abstract - In this paper, we present a novel terrestrial mobile
network architecture that aims at the simplification of
mobility management
for the terrestrial
mobile
telecommunication networks. The work is in line with the
UMTS approach. The proposed architecture uses the
hierarchical cell concept and facilitates strategic placement of
database control. A macro-cell based approach for location
updating and a combined macro/micro cell based paging
control plays a key role in our proposal. Another significant
feature of the proposed scheme is that, each micro-cell has the
same signalling load, regardless of the location updating
traffic. A Location Agent (LAg) that is co-located with
Signalling Transfer Point (STP) and hierarchically structured
is also proposed to reduce mobility management signalling
traffic to and from the HLR. A swap technic is applied at the
location databases, which further reduces the signalling
traffic. The simplifications achieved through the use of this
architecture benefit both location updating and paging
procedures by optimising the traffic in both cases.
I. INTRODUCTION
The future mobile systems aim at enabling subscribers to
communicate with anyone, anytime and anywhere. Mobility
management is always a key issue for the mobile networks to
achieve more efficient location management and handover
control. The existing techniques for location management are
location updating (registration) and paging. The first
generation mobile networks, such as Advanced Mobile Phone
Service (AMPS), page the Mobile Station (MS) in every cell
within the network. The second generation mobile networks,
such as Global System for Mobile communication (GSM) [1]
use Home Location Register (HLR), Visitor Location
Register (VLR) and Location Area (LA) concepts to achieve
more bandwidth efficiency in air interface. In this paper, we
propose an architecture for an integrated wireless/fixed
network in line with the UMTS approach [2,3] but is very
much superior from the point of view of optimal signalling
traffic and shared signalling loads with faster access during
paging. The architecture has been modeled to achieve
efficient mobility management by using hierarchical
distributed database and intelligent paging techniques as
suggested in the review paper [9]. The basic architecture
described in section II, is the significant contribution based on
which optimal mobility management schemes are proposed.
A hierarchical Location Agent concept used for location
management is explained in section III A. In section III B, the
swap technic is applied to location database management to
further reduce the signalling traffic. In section III C, a
location updating using macro-cell and the information flows
are discussed. A macro/micro cell based intelligent paging
scheme as applied in a location area is proposed in section III
D. Section III E describes the location management in picocell environment and the handover control is briefly discussed
in section III F. Section IV provides discussion and
conclusions.
II. BASIC ARCHITECTURE
Figure 1 illustrates the proposed network architecture. The
proposed network is built on concepts from UMTS and GSM.
The changes proposed by us have been highlighted using
Italics. The basic network consists of three sub-networks:
radio access network, core network, and a signalling and
intelligent network.
• Radio access network provides mainly radio related
functions, the basic local cell site switching (CSS) and
transmission functions, and interfaces with core and
signalling networks. A group of pico-cells are
connected to one Pico-cell CSS (PiCSS) and a group of
micro-cells and one macro-cell are connected to a
BSC/CSS. The BSC/CSS coverage area equals to one
macro-cell coverage area and also equals to a Location
Area. The PiCSS and BSC/CSS are at the same level of
hierarchy and connected to LE/MSC. This type of
umbrella architecture has been proposed earlier, but
here we have slightly rearranged it and utilised this
architecture to help strategic placement of databases,
thereby achieving optimised mobility management.
• The core network consists of enhanced local and transit
switches of B-ISDN and also includes a transmission
system to provide switching, call control and
connection control.
•
Intelligent and signalling network will provide specific
services and location management. Distributed
databases will be required in Intelligent Network (IN)
to store user related data (user/service profiles) to
support access to the network and mobility services. A
LAg co-located with STP is proposed. The function of
the LAg briefly, is to provide hierarchical pointers to
the VLRs a mobile station is traversing, where the
hierarchy is controlled by the mobility of the user. A
detailed explanation of this concept follows.
STP
Intelligent and
Signaling Network
HLR
MS initial entry into remote or visited network:
Signaling Network
MS
STP
STP/LAg
LE/MSC/VLR
Core Network
LE/MSC/VLR
( Remote or Visited Network )
VLRnew
STP/LAg
HLR
Location updating (or registration)
Location updating (or registration)
Location updating (or registration)
LE/MSC/VLR
Location updating (or registration) ACK
Location updating (or registration) ACK
PiCSS
BSC/CSS
PiBCSS
MS moving from VLRold to VLRnew :
Radio Access Network
MS
Pico-cells
Pico-cells
Micro-cell
Micro-cell
Figure 1. Proposed Network Architecture
III. MOBILITY MANAGEMENT
A. Location Management Using Hierarchical LAgs at STPs
In GSM to locate a mobile user, the HLR is always accessed
first and the locality of connection is not exploited. In the
future network, the Local Exchange (LE) may be enhanced to
include Mobile Basic Call State Model. The LE shall be able
to handle both fixed and mobile calls [4]. Based on this
approach, a local database at the LE can control both fixed
and mobile calls that are in the same LE coverage area. To
reduce the signalling traffic when an MS roams to other
networks or away from its home LE, a Location Agent (LAg)
concept is used. In our configuration the LAg is distributed at
the SS7 STPs. This means that the SS7 STP will be enhanced
to include extra information. If the MS is roaming to other
VLRs under the same LAg coverage area, the LAg will
record the VLR identifiers. On the other hand, if the MS is in
the remote part of the network or in the other networks, the
LAg may acts like HLR Agent and replicates all the necessary
information for the call set-up and mobility management.
This will be a distributed HLR in STP level in the network.
Figure 2 illustrates the information flows of the location
updating by LAg in remote or visited network. A STP/LAg
connects from one to several LE/MSCs.
We propose locality of connection to be exploited as
follows: in Figure 3 the LE is an integrated MSC and SSP
therefore the call control can be localised at LE/MSC level.
For example, when a fixed user initiates a call to a mobile
user and both users are under the same LE control, the
network will first check the local database which is co-located
with LE therefore the call can be set up faster and the
signalling traffic will be reduced. The other approach shown
in Figure 4 is to connect the LEs and MSCs to a common STP
that will be the signalling anchor point and therefore saves the
signalling traffic to HLR. This approach can be used in the
current mobile network when the LE and MSC are not
integrated.
STP/LAg
HLR
Location updating
Macro-cell
BSC: Base Station Controller HLR: Home Location Register LAg: Location Agent
LE: Local Exchange
MSC: Mobile Switching Centre PiCSS: Pico-cell CSS
STP: Signaling Transfer Point VLR: Visitor Location Register CSS: Cell Site Switch
( Remote or Visited Network )
VLRold
VLRnew
Location updating
Location updating ACK
Location updating ACK
Figure 2. Information Flows of the Simplified Location Updating by Lag
in Remote or Visited network
In our configuration, the LAgs are distributed at SS7 STPs.
Based on the users’ mobility patterns any one of the LAgs
will be the location agent for that mobile user. For example in
figure 5, LAg111 is at the lowest level for low mobile users,
LAg11 at the next level for medium mobile users and LAg 1
at the top level for highly mobile users. MS1 is a subscriber
who spends most of his time under the local STP/LAg111 and
his VLR pointer is recorded only at Lag111 while MS3 has
higher mobility and often goes across different STP coverage
areas and hence his VLR pointer will be at LAg1. An inherent
advantage of this scheme is that the information to be stored
in the LAgs is distributed fairly across the different
hierarchical layers of STP/LAgs. If the LAg acts as a HLR
Agent then for highly mobile users like MS3, having his
profiles recorded at LAg1 will save some traffic when he
moves from coverage under STP/LAg112 to STP/LAg121. If
LAg1 didn’t have his profiles then the user’s information has
to be transmitted via LAg112, LAg11, LAg1, LAg12, and
LAg121 to the new VLR. Our approach has a shorter
signalling path to transmit user’s information comparing with
location forwarding strategy proposed in [10].
The proposed scheme will avoid heavy database processing
at a particular STP and also reduce the signaling traffic to a
remote HLR. The signaling load will be localized and
distributed more evenly in all STP regions. Analytical studies
of comparing different location management schemes are
being conducted.
STP/LAg
LE/MSC
LE/MSC
STP/LAg
LE/MSC
Figure 3. LE and MSC are Integrated
LE
MSC
LE
MSC
LE
Figure 4. LE and MSC are Separated
STP/LAg1
STP/LAg12
STP/LAg11
STP/LAg111
STP/LAg112
VLRold
STP/LAg121
STP/LAg122
VLRnew
MS1(LAg111)
MS2(LAg11)
MS3(LAg1)
Figure 5. Distributed LAg’s for Different Class of User
B. Swap technic for Location Management
We have further divided the conventional location database
(e.g. VLR) into two parts: current and history databases. The
current database records the user/service profiles of the MSs
that are currently residing in the VLR coverage area and is in
the primary storage. The history database is stored in the
secondary storage. It records the profiles of the MSs that
visited the VLR previously in a certain period. The swap
technic is used to exchange information between the primary
and secondary databases.
In the conventional mobile network (e.g. GSM, IS-41), the
location updating algorithm will update the profiles at
VLRnew and the profiles at VLRold will be deleted. In our
approach, the profiles at VLRold will be swapped from the
primary database to secondary database. The profiles are
deleted only based on the mobility patterns and the timers. By
using the swap technic at VLR, the signalling traffic for
profiles download will be reduced significantly. The
comparison between conventional and our approach is
illustrated in Figure 6. There is a profiles download procedure
every time when the MS roaming from a VLR to another
VLR in the conventional approach. In our approach, the
profile can be swapped and it is not necessary to be
downloaded to the VLRs. In conjunction with the hierarchical
algorithm proposed in the last section, the swap algorithm
applied at location databases can further reduce the signalling
traffic for location management.
C. Location Updating Using Macro-cell
In the conventional location updating approach (e.g. GSM),
the location updating occurs only in perimeter cells,
regardless of periodical updating. The radio resources
consumed in the perimeter cells are much more than the inner
cells. This causes load unbalance for each cell and the
different design for perimeter and inner cells. Other
researchers [11,12] use different dynamic LA management
and updating schemes which need to record the different
users’ mobility parameters. However, it is generally not easy
to have dynamic LA for different MS’s as the MS’s must be
able to identify the boundaries of LA’s which are
continuously changing [9]. In our proposal, the existing
macro-cells are used as LAs and a BSC/CSS control area
equals to a macro-cell coverage area and also equals to a LA.
This is the novelty of our approach as this leads to simplified
approaches to location updating and paging. Figure 7 shows
the network planning for the BSC or CSS. In this approach,
the macro-cell will broadcast the LA identifier which, could
be the same as macro-cell identifier to save signalling
overhead. The main advantage of this architecture is to
simplify the location updating control through macro-cell.
Another important effect is that each micro-cell will have the
same signalling load, regardless of the location updating
traffic, as it does not have to broadcast the LA identifier. The
macro-cell can also act as backup resources during inter
micro-cell handover failures. Because the macro-cell and
micro-cells are under the same BSC/CSS control it makes
BSC/CSS control of radio resources easier during handover
and location management. This concept can be extended to
the pico-cell and micro-cell environments. The architectural
and functional models proposed in this work can be easily
scaled to fit the environment. Figure 8 illustrates the
simplified location updating procedures and information
flows.
Conventional Location Updating Procedures:
VLRnew
STP
VLRold
HLR
Location update req.
Location update resp. (with Auth. parameters or profiles)
Location update req.
Location update resp. (with or without profiles)
Location cancellation req. via STP11
Location cancellation resp. via STP11
Location Updating Procedures Using Swap technic:
A. MS moves from VLR1 to VLR2:
STP11
VLR2
VLR1
HLR
Location update req.
Location update resp. (with profiles)
SWAP at VLR1
Update location pointer at STP11
Location update resp. (with profiles)
B. MS returns to VLR1 from
VLR2:
VLR1
STP11
VLR2
HLR
Location update req.
Location update resp. (without profiles)
SWAP at VLR2
Update location pointer at STP11
Location update resp. (without profiles)
SWAP VLR1
Figure 6. Information Flows of Conventional and Swap Location database
Management
BSC/CSS
xi  2 R yi  R
  P( x, y )  {( xi  2 R, yi  R), ( xi  2 R, yi  R), ( xi  2 R, yi  R), ( xi  2 R, yi  R)},
xi  2 R yi  R
0  x  16,0  y  8
•
For example at P2 (16,4), R=2, the first paging area
composes cells {(14,16), (13,15), (15,15), (12,4),
(14,4), (16,14), (13,3), (15,3), (14,2)}.
If there is no response from PA1, the rest of cells in the
LA will be paged. Due to the service reliability reasons,
if the MS is still not responding to the first 2 pages, we
need to page the whole LA either using macro-cell (if
micro-cell paging failed) or micro-cells again.
LA
Macro-cell
•
Micro-cells
Figure 7. Overlaid Cell Planning Model
MS
BSC/CSSold
BSC/CSSnew
VLR
Y
Monitoring the LA
or macro-cell identifiers
Location updating request
Location updating request
LA update
Location updating ACK
Location updating ACK
Figure 8. Information Flows for Location updating Control Using
Macro-cell
D. Macro/micro Cell Paging
In the conventional network, the whole service area is
divided into LAs which are a logical group of cells. In our
approach, the LAs are geographically distributed according to
BSC/CSS (macro-cell). We are proposing a combined macrocell and micro-cell paging which is depending on the user’s
class (slow/fast). From figure 7 and 9, each macro-cell
coverage area is one LA. Each user has his own Paging Area
(PA), as we propose a per-user position based PA algorithm
with more information and intelligent control [6,7] based on
user’s speed, last contact cell, etc. [8]. The multiple-step
paging scheme has already been proposed in [13], we plan to
use it in our architecture and have proposed a technique
through which we can determine the cells to be paged easily.
Hence the BSC/CSS covering one LA will have a number of
records of paging details for the different users. If a user
belongs to the fast moving class then the macro-cell based
paging scheme may be applied. In general the paging scheme
is explained below and shown in figure 9.
• Each micro-cell’s position is identified with references
to X-Y axis.
• R = paging radius and all cells within this radius will be
paged. R is dependent on the user’s class (e.g.
pedestrian, car, public transport, etc.). For example in
Figure 8, P1 is a user in office, P2 is a pedestrian, and
P3 is a user driving a car in the city.
• When a user is called, the first paging area = PA1, and
is given by
8
7
6
5
4
3
2
1
..
..
...
.
0
Y
First PA
8
7
(4,6)
6
(8,4)
5
4
3
(16,4)
(5,3)
. .......
2
4
6
8
10 12 14 16
R
P3
R
R
P1
P2
2
1
X
0
2
4
6
8
10 12 14 16
X
* The shadow area is covered a macro-cell and is equal to one BSC/CSS coverage area
** P1, P2, P3 are the last contact cells of the users
Figure 9. Planning for Location and Paging Area
A description of the macro/micro-cell based paging
algorithm is given in the flow chart of figure 10. The network
first checks the user’s class whether the user is fast or slow
moving. In the case of fast moving user, the macro-cell
paging will be used. If the network realises that user belongs
to slow moving class then the intelligent paging algorithm is
launched. The information flows of an incoming call for fast
moving user are shown in figure 11.
GSM uses a time-triggered periodic location updating to
trace the MS’s position and status (ON/OFF). In our
approach, when the MS is turning off, the MS will send a
signalling message to inform the network (BSC, VLR, and
HLR) its status so there is no necessity to have periodic
location updating to make sure of the MS’s status. This
reduces the signalling traffic considerably and can be easily
adapted to the proposed architecture. This signalling message
can be sent by the macro-cell base station. In the case where
MS moves outside the network coverage area or moves into a
blind spot, there is no signal between MS and network. This
is a critical situation in our approach. During the noncontactable period, if there is an incoming call to the MS,
paging will fail and the network will treat the MS as switched
off and the following incoming calls will not be paged. Once
the MS moves into the coverage area and regains the signals,
the MS will send a location updating message to inform
network its status/position immediately.
between PiBSC and BSC and the PiBSC communicates
directly to the LE.
Paging Request
User's Class
Selection
Fast Moving
User?
Yes
Page the LA
using Macro-cell
No
Paging the surrounding cells of the
last contact cell within the LA
according to
•R "
MS Response?
Yes
Stop
No
Paging the remain
cells in the LA
MS Response?
REFERENCES
Yes
Stop
No
Paging the whole LA
using macro-cell
Figure 10. Flow Chart of Paging Control
MS
Micro-cell
BS
Macro-cell
BS
BSC/CSS
LE/MSC
Incoming call to MS
Paging MS using macro-cell (for fast moving class user)
Paging MS
{
Paging response
Connection setup
OR
{
IV. CONCLUSION
In future mobile network systems, signaling traffic for
mobility is predicted to be much higher and complicated. In
this paper we have approached this problem by suggesting
some enhancements to the proposed UMTS architecture,
resulting in reasonable signaling traffic for mobility
management. The model has been studied with respect to
location updating and paging signaling traffic and the
corresponding information flows and control points proposed.
However we further need to study and analyse this model to
provide estimation on the actual signaling traffic and perform
some comparative studies with existing architecture.
Paging response
Connection setup
Figure 11. Information Flows of an Incoming Call for Fast Moving User
E. Location Management in Pico-cell Environment
In figure 1, a functional entity called Pico-cell BSC (PiBSC)
which connects to LE is proposed. The functions of the
PiBSC are similar to BSC. The mobility management in the
pico-cell environment is similar to the micro-cell
environment.
F. Handover
When the MS moves from a pico-cell to a micro-cell, the
forward handover control should be used instead of backward
handover control which been used in most of micro-cell and
macro-cell cases. This is because there is no connection
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