Chapter 6

Mobility Management
Outline
 Overview of the PCS system architecture
 Mobility management
PCS system architecture
 The mobile service area is covered by a set of base stations (BSs),
which are responsible for relaying the calls to and from the mobile
stations (MSs) located in their coverage areas (or cells).
 The BSs are connected to mobile switching centers (MSCs) by lan
d links.
 MSC
 a telephone exchange configured specifically for mobile applica
tions.
 interfaces the MSs (via BSs) with the PSTN.
 Databases are used for roaming management:
 Home location register (HLR)
 Visitor location register (VLR)
 There are two aspects of mobility in a PCS network:
 Handoff
 Roaming
Handoff
 When a mobile user is engaged in conversation, the
MS is connected to a BS via a radio link.

 If the mobile user moves to the coverage area of

another BS, the radio link to the old BS is eventually
disconnected, and a radio link to the new BS should
be established to continue the conversation.

 This process is variously referred to as automatic

link transfer, handover, or handoff.
Roaming
 When a mobile user moves from one PCS system
(e.g., the system in New York City) to another (e.g.,
the system in Los Angeles), the system should be
informed of the current location of the user.
Otherwise, it would be impossible to deliver the
services to the mobile user.
 To support mobility management, protocols such as
EIA/TIA Interim Standard 41 (IS‑41 or ANSI‑41)
or Global System for Mobile Communications
(GSM) Mobile Application Part (MAP) have been
defined for PCS networks.
Handoff
 Three strategies have been proposed to detect
the need for handoff:
 mobile‑controlled handoff (MCHO)

 network‑controlled handoff (NCHO)

 mobile‑assisted handoff (MAHO)

Mobile‑Controlled Handoff (MCHO)
 The MS continuously monitors the signals of t
he surrounding BSs and initiates the handoff p
rocess when some handoff criteria are met.
 MCHO is used in DECT and PACS.
Network‑Controlled Handoff (NCHO)
The surrounding BSs measure the signal from
the MS, and the network initiates the handoff p
rocess when some handoff criteria are met.
 NCHO is used in CT‑2 Plus and AMPS.
Mobile‑Assisted Handoff (MAHO)
 The network asks the MS to measure the signa
l from the surrounding BSs. The network mak
es the handoff decision based on reports from t
he MS.
 MAHO is used in GSM and IS‑95 CDMA.
Two types of handoff
 The BSs involved in the handoff may be conne
cted to the same MSC (inter‑cell handoff or i
nter‑BS handoff)

 The BSs involved in the handoff may be conne

cted to two different MSCs (inter­system han
doff or inter‑MSC handoff ).
Inter‑BS Handoff
 The new and the old BSs are connected to the same
MSC.
 Assume that the need for handoff is detected by the MS
; the following actions are taken:
 1. The MS momentarily suspends conversation and initiates t
he handoff procedure by signaling on an idle (currently free)
channel in the new BS. Then it resumes the conversation on t
he old BS.
 2. Upon receipt of the signal, the MSC transfers the encryptio
n information to the selected idle channel of the new BS and
sets up the new conversation path to the MS through that cha
nnel. The switch bridges the new path with the old path and i
nforms the MS to transfer from the old channel to the new ch
annel.
Inter‑BS Handoff
 3. After the MS has been transferred to the new
BS, it signals the network, and resumes
conversation using the new channel.
 4. Upon receipt of the handoff completion signal,
the network removes the bridge from the path and
releases resources associated with the old channel.

 This handoff procedure is used with the

mobile‑controlled handoff strategy
Inter‑BS Handoff
Inter‑BS Handoff
 For the network‑controlled handoff strategy,
all handoff signaling messages are exchanged
between the MS and the old BS though the
failing link.

 The whole process must be completed as

quickly as possible, to ensure that the new link
is established before the old link fails.
Inter‑BS Handoff
 If the new BS does not have an idle channel, the handoff
call may be dropped (or forced to terminate).
 The forced termination probability is an important
criterion in the performance evaluation of a PCS network.
 Forced termination of an ongoing call is considered less
desirable than blocking a new call attempt.
 Most PCS networks handle a handoff in the same manner as
a new call attempt. That is, if no channel is available, the
handoff is blocked and the call is held on the current
channel in the old cell until the call is completed or when
the failing link is no longer available.
 This is referred to as the non-prioritized scheme.
Channel Assignment Schemes
 To reduce forced termination and to promote c
all completion, three channel assignment sche
mes have been proposed:
 Reserved channel scheme
 Queuing priority scheme

 Subrating scheme
Reserved channel scheme
 Similar to the non-prioritized scheme, except
that some channels in each BS are reserved for
handoff calls.
Queuing priority scheme
 Adjacent coverage areas of BSs may overlapped.
 Thus, there is a considerable area where a call can be
handled by either BS. This area is called the handoff
area.
 If no channel is available in the new BS during hando
ff, the new BS buffers the handoff request in a waitin
g queue.
 The MS continues to use the channel with the old BS
until either a channel in the new BS becomes availabl
e (and the handoff call is connected) or the MS moves
out of the handoff area (and the call is forced to termi
nate).
Subrating scheme
 Creates a new channel for a handoff call by sharing r
esources with an existing call if no channel is avail
able in the new BS.
 Subrating means an occupied full‑rate channel is tem
porarily divided into two channels at half the original
rate:
 one to serve the existing call
 and the other to serve the handoff request.
 When occupied channels are released, the subrated ch
annels are immediately switched back to full‑rate cha
nnels.
Inter‑BS Handoff
 These handoff schemes can significantly
reduce the probability of forced termination as
well as the probability of call incompletion
(new call blocking plus handoff call forced
termination).
Intersystem Handoff
 In intersystem handoff, the new and old BSs ar
e connected to two different MSCs.
 We trace the intersystem handoff procedure of
IS‑41, where network‑controlled handoff (NC
HO) is assumed.
 In this figure, a communicating mobile user m
oves out of the BS served by MSC A and enter
s the area covered by MSC B.
MSC MSC MSC MSC
Intersystem Handoff
 Intersystem handoff requires the following
steps:
 Step 1. MSC A requests MSC B to perform
handoff measurements on the call in progress.
MSC B then selects a candidate BS2, BS2, and
interrogates it for signal quality parameters on
the call in progress. MSC B returns the signal
quality parameter values, along with other
relevant information, to MSC A.
Intersystem Handoff
 Step 2. MSC A checks if the MS has made too
many handoffs recently (this is to avoid, for
example, numerous handoffs between BS1 and
BS2 a where the MS is moving within the
overlapped area) or if intersystem trunks are
not available. If so, MSC A exits the
procedure. Otherwise, MSC A asks MSC B to
set up a voice channel. Assuming that a voice
channel is available in BS2, MSC B instructs
MSC A to start the radio link transfer.
Intersystem Handoff
 Step 3. MSC A sends the MS a handoff order.
The MS synchronizes to BS2. After the MS is
connected to BS2, MSC B informs MSC A
that the handoff is successful. MSC A then
connects the call path (trunk) to MSC B and
completes the handoff procedure.
Intersystem Handoff
 In this intersystem handoff process, MSC A is
referred to as the anchor MSC, and is always in the
call path before and after the handoff, as illustrated in
the four cases in Figure 2.4.
 This anchor approach is used in all existing mobile
phone networks because the re‑establishment of a
new call path (without involving MSC A) between
MS and the new MSC would require extra trunk
release/setup operations in PSTN, which is not
available or is not cost‑effective.
Intersystem Handoff
 If the MS moves back to MSC A again, the
connection between MSC A and MSC B is removed
(handoff backward).
 If the MS moves to the third MSC C, then MSC B
will be in the call path (handoff to third).
 That is, the link between MSC B and MSC A is
disconnected, and MSC C connects to MSC A
directly.
 This process is called path minimization.
Roaming Management
 Two basic operations in roaming management
are
 registration (or location update), the process
whereby an MS informs the system of its current
location, and
 location tracking, the process during which the
system locates the MS. Location tracking is required
when the network attempts to deliver a call to the
mobile user.
Roaming Management
 The roaming management strategies proposed in the
IS‑41 and GSM MAP standards are two‑level
strategies in that they use a two‑tier system of home
and visited databases.
Home Location Register (HLR)
 When a user subscribes to the services of a PCS
network, a record is created in the system's database,
called the home location register (HLR).

 The HLR is a network database that stores and

manages all mobile subscriptions of a specific
operator.

 Specifically, the HLR is the location register to which

an MS identity is assigned for record purposes, such
as directory number, profile information, current
location, and validation period.
Visitor Location Register (VLR)
 When the mobile user visits a PCS network other than the
home system, a temporary record for the mobile user is created
in the visitor location register (VLR) of the visited system.

 The VLR temporarily stores subscription information for the

visiting subscribers so that the corresponding MSC can provide
service.

 In other words, the VLR is the "other" location register used to

retrieve information for handling calls to or from a visiting
mobile user.
Registration Procedure
Registration Procedure
 Step 1. Suppose that the home system of a mobile user is in
Morristown. When the mobile user moves from one visited
system (e.g., New York City) to another (e.g., Los Angeles), it
must register in the VLR of the new visited system.
 Step 2. The new VLR informs the mobile user's HLR of the
person's current location‑the address of the new VLR. The
HLR sends an acknowledgment, which includes the MS's
profile, to the new VLR.
 Step 3. The new VLR informs the MS of the successful
registration.
 Step 4. After step 2, the HLR also sends a deregistration
message to cancel the obsolete location record of the MS in
the old VLR. The old VLR acknowledges the deregistration.
Call delivery procedure
 To originate a call, the MS first contacts the
MSC in the visited PCS network.

 The call request is forwarded to the VLR for

approval.

 If the call is accepted, the MSC sets up the call

to the called party following the standard
PSTN call setup procedure.
Call delivery procedure
 Step 1. If a wireline phone attempts to call a mobile subscriber
, the call is forwarded to a switch, called the originating switc
h in the PSTN, which queries the HLR to find the current VLR
of the MS. (1) The HLR queries the VLR in which the MS resi
des to get a routable address. (2) If the originating switch is no
t capable of querying the HLR (i.e., it is not equipped to suppo
rt mobility), the call is routed through the PSTN to the subscri
ber's gateway MSC, which queries the HLR to determine the c
urrent VLR serving the MS.
 Step 2. The VLR returns the routable address to the originatin
g switch through the HLR.
 Step 3. Based on the routable address, a trunk (voice circuit) is
set up from the originating switch to the MS through the visite
d MSC.
Call delivery procedure

MSC
Roaming Management under SS7
 The missing parts in the picture are the
interactions between the PCS network and the
PSTN.
 This section briefly describes how mobile
roaming is managed by the PSTN signaling.
Common channel signaling (CCS)
 Common channel signaling (CCS) is a
signaling method that provides control and
management functions in the telephone
network.
 CCS consists of
 supervisory functions
 addressing

 call information provisioning

CCS
 A CCS channel conveys messages to
 initiate
and terminate calls
 determines the status of some part of the network

 controls the amount of traffic allowed.

 CCS uses a separate out‑of‑band signaling

network to carry signaling messages.
SS7
 Signalling System No. 7 (SS7) is a CCS system devel
oped to satisfy the telephone operating companies' re
quirements for an improvement to the earlier signalin
g systems, which lacked the sophistication required to
deliver much more than plain old telephone service (P
OTS).
 Signaling between a PCS network and the PSTN are t
ypically achieved by the SS7 network.
SS7
SS7
 Figure shows the network elements that are
involved in the interconnection between a PCS
network and the PSTN. In the figure, the
dashed lines represent the signaling links; the
solid line represents a trunk.
SS7
 The SS7 network consists of three distinct
components:
 Service Switching Point (SSP)
 Signal Transfer Point (STP)

 Service Control Point (SCP)

Service Switching Point (SSP)
 A telephone switch interconnected by SS7 link
s. The SSPs perform call processing on calls th
at originate, tandem, or terminate at that node.
 A local SSP in the PSTN can be a central offic
e (CO) or end office (EO).
 An SSP in a PCS network is called a mobile s
witching center (MSC).
Signal Transfer Point (STP)
 A switch that relays SS7 messages between ne
twork switches and databases. Based on the ad
dress fields of the SS7 messages, the STPs rou
te the messages to the correct out­going signali
ng links. To meet the stringent reliability requi
rements, STPs are provisioned in mated pairs,
as shown in Figure.
Service Control Point (SCP)
 Contains databases for providing enhanced
services. An SCP accepts queries from an SSP
and returns the requested information to the
SSP .
 In mobile applications, an SCP may contain an
HLR or a VLR.
SS7
 In this network, the trunks (voice circuits) connect SS
Ps to carry user data/voice information.

 The signaling links connect SCPs to STPs, and STPs

to SSPs.

 The SSPs and SCPs are connected indirectly through

STPs.
Registration
 In this example, the MS moves from VLR1 to
VLR2.
 Step 1.
 The MS enters the area controlled by MSC2.
 MSC2 launches a registration query to its VLR
through STP2, assuming that VLR2 and MSC2 are
not co-located.
Registration
 Step 2.
 VLR2 sends a registration message to the MS's HLR
(HLR4 in Figure 2.8).
 VLR2 may not know the actual address of HLR. Instead,
VLR2 sends the message containing the MS identity,
called the Mobile Identification Number (MIN), to an
STP (STP3 in our example) that can translate the MIN
into the HLR address.
 Step 3.
 The MIN‑to‑HLR address translation is performed at
STP3 by a table‑lookup technique called global title
translation (GTT). STP3 then forwards the registration
message to HLR.
Registration
 Step 4.
 After the registration, HLR sends an acknowledgment back
to VLR2.
 Since the address of VLR2 is known, the acknowledgment
may be sent to VLR2 using a shortcut, without passing
through STP3.
 Step 5.
 After step 3, HLR sends a deregistration message to VLR1
to cancel the obsolete record.
 VLR1 then acknowledges the cancellation (not shown in
Figure 2.8).
Registration
 In steps 2, 3, 4, and 5, the messages may visit several
STPs before arriving at their destinations, and the regi
stration process may generate considerable traffic in t
he SS7 network.
 Thus, it is desirable to reduce the registration traffic.
 Two approaches have been proposed to reduce the "c
ost" of deregistration at step 5 in Figure 2.8:
 implicit deregistration

 periodic re-registration
MSC1 MSC2
Implicit deregistration
 Obsolete VLR records are not deleted until the
database is full.
 If the database is full when an MS arrives, a record is
deleted, freeing storage space to accommodate the
newly arrived MS.
 A replacement policy is required to select a record for
replacement (it is possible that a valid record is
replaced, and the information is lost).
 Advantage: no deregistration messages are sent
among the SS7 network elements.
Periodic re-registration
 The MS periodically reregisters to the VLR.
 If the VLR does not receive the re-registration
message within a timeout period, the record is
deleted.
 This approach only creates local message traffi
c between the MSC and the VLR. Furthermore
, no SS7 signaling messages are generated if th
e VLR is co-located with the MSC.
Pointer Forwarding Scheme
 To reduce the registration traffic at steps 2 and
3 in Figure 2.8, a pointer forwarding scheme
was proposed, which consists of two
operations:
 Move operation (registration).
 Find operation (call delivery).
Move operation (registration)
 When an MS moves from one VLR to another,
a pointer is created from the old VLR to the
new VLR. No registration to the HLR is
required (see Figure 2.9(a)).
Find operation (call delivery)
 When the HLR attempts to locate the MS for
call delivery, the pointer chain is traced. After
the find operation, the HLR points directly to
the destination VLR (see Figure 2.9(b)).
Call Delivery
 Depending on the memory capacities of the VLRs, the pointer
s in the obsolete chain may or may not be deleted.
 To limit the pointer traversal time in the find operation, the reg
istration procedure in Figure 2.8 may be performed for every k
move operations.
 In other words, the number of pointers visited in the find opera
tion will be limited by k. The pointer forwarding scheme shoul
d not be considered when the new cost of pointer creation and
pointer traversal is higher than the cost of accessing the HLR.
 As performance studies indicate, the pointer forwarding schem
e significantly reduces the network traffic in many cases.
Call Delivery
 Similar to the registration process, visits to several STPs and a
GTT may be required to access the HLR in call delivery.
 Several STPs may be visited to obtain the routable address fro
m the VLR.
 To reduce the call delivery traffic, a cache scheme was propos
ed to maintain a cache in the originating SSPs.
 Another possibility is to maintain the cache in the STP that pe
rforms GTTs, that is, STP3 in Figure 2.11.
 A cache entry consists of two fields: the MIN of an MS and t
he address of the current visited VLR of the MS. The cache
contains entries for MSs recently accessed from the SSP
Cache Scheme
 When the calling party originates a call to an MS, the SSP first
checks if the cache entry for the MS exists. There are three pos
sibilities:
 Case 1: The cache entry does not exist. The call delivery proce
dure illustrated in Figure 2.10 is performed.
 Case 2: The cache entry exists and is current. The VLR is dire
ctly accessed as shown in Figure 2.11.
 Case 3: The cache entry exists but is obsolete. The procedure d
etects that the cache entry is obsolete if the queried VLR's resp
onse is negative. The call delivery procedure illustrated in Fig
ure 2.10 is performed.