1_2mobile comm

Mobility ManagementMobility Management

A Common PCS NetworkArchitecture

March 21, 2009 Girish Kumar Patnaik 2

Two Aspects of Mobility in a p yPCS Network

• Handoff– Link transfer, or Handover

bil f f ld– A mobile user moves from one coverage area of an old BS to the coverage area of a new BS during the conversation.

– The radio link to the old BS is disconnected and a radio link to the new BS should be established to continue the conversation.

• Roaming– When a mobile user moves from one system to another,

h l i f h h ld ll h PCSMarch 21, 2009 Girish Kumar Patnaik 3

the location of the user should tell the PCS system.

Strategies to detect the need gfor Handoff

• Mobile-controlled handoff (MCHO)– MS continuously monitors the signals of the surrounding BSs and

initiates handoff process when some criteria are metp– Used in DECT and PACS

• Network-controlled handoff (NCHO)– The surrounding BSs measure the signal from the MS and the– The surrounding BSs measure the signal from the MS, and the

network initiates the handoff process when some criteria are met – Used in CT-2 Plus and AMPS

• Mobile-assisted handoff (MAHO)• Mobile-assisted handoff (MAHO)– The network asks the MS to measure the signal from the

surrounding BSs. The network makes the handoff decision based on reports from the MS


epo ts o t e S– Used in GSM and IS-95 CDMA

Inter-BS Handoff N d Old BS d h MSC• New and Old BSs are connected to the same MSC

• Need for Handoff is detected by the MS• Steps of Actions:• Steps of Actions:

– MS momentarily suspends conversation and initiates the Handoff procedure by signaling on an idle channel in the new BS. Then it resumes conversation on the old BSresumes conversation on the old BS

– MSC transfers the encrypted information to the selected channel of the new BS and setup the new conversation path. The switch bridges the new path with the old path and informs the MS to transfer from the oldnew path with the old path and informs the MS to transfer from the old channel to the new channel

– After the MS has been transferred to the new BS, it signals the network and resumes conversation using the new channelnetwork, and resumes conversation using the new channel

– Upon receipt of the Handoff completion signal, the network removes the bridge from the path and releases resources associated with the old channel


channel

Inter-BS Handoff


What happens if no new channel is available at new BS?available at new BS?

• Nonprioritized Scheme– Handoff is blocked - keep using the existing channel until either:Handoff is blocked keep using the existing channel until either:

• call is over or• link fails (or forced termination)

• Reserved channel schemeReserved channel scheme– Keep some channels reserved for handoff

• Queuing priority schemeE l it ll l (h d ff ) t b ff iti f bil– Exploit cell over lap (handoff area) to buffer a waiting queue of mobiles waiting for handover

• Subrating scheme – Subtrating means dividing full-rate channel temporarily into two half-rate

channels by sharing resources, One for existing call and the other for handoffWhen occupied channels are released the subtrate channels are switched


– When occupied channels are released, the subtrate channels are switched back to full-rate channels

Intersystem Handoff• New and Old BSs are connected to two different MSCs• Network-controlled Handoff• Steps of Actions:

– MSC A requests MSC B to perform handoff measurements on the call in progress. MSC B selects a candidate BS and interrogates it for signal p g g gquality parameters on the call in progress. MSC B returns the signal quality parameter values to MSC A.

– MSC A checks if the MS has made too many handoffs recently or y yIntersystem trunks are not available. If so MSC A exits the procedure. Otherwise MSC A asks MSC B to setup a voice channel, and then MSC B instructs MSC A to start the radio link transfer.

– MSC A sends the MS a handoff order. The MS synchronizes to new BS. After the MS is connected to new BS, MSC B informs MSC A that handoff is successful. MSC A then connects the call path to MSC B and completes


the handoff procedure.

Intersystem Handoff


Handoff between two MSCs


What happens if the mobile i ?moves again?


Roaming ManagementRoaming Management

Basic OperationsBasic Operations

• Two basic operations in roaming management.g– Registration (location update): an MS

informs the system of its current locationy– Location Tracking: the system locates the

MS


Location Tracking and Updateg p• When a mobile user

moves from one PCSmoves from one PCS system to another, the current location of the user should be updated.H t t th t• How to get the current location of the user ?

• Where to store theWhere to store the current location of the user ?


Two-level Hierarchical Strategygy• In IS-41 and GSM MAP, the two-level strategies

dare proposed.• A two-tier system of home and visited databases

– Home Location Register (HLR)– Visited Location Register (VLR)g ( )


Home Location Register (HLR)• HLR is a network database that stores and

manages all subscriptions of a specificmanages all subscriptions of a specific operator.

• The information in HLR:• The information in HLR:– MS Identity, directory number, profile

information current location validation periodinformation, current location, validation period.


Visitor Location Register (VLR)• The VLR has temporary information for the

visiting mobile users.visiting mobile users.• The information in VLR:

MS Id tit di t b t l ti– MS Identity, directory number, current location.


MS Registration Process


MS Registration Process1. When the mobile user moves from one visited system to

another, it must register in the VLR of the new visited systemsystem

2. The new VLR informs the mobile user’s HLR of the person’s current location – address of the new VLR. The HLR d k l d t hi h i l d MS’HLR sends an acknowledgement, which includes MS’s profile, to the new VLR.

3. The new VLR informs the MS of the successful registration.

4. After step 2, the HLR also sends a deregistration message to cancel the obsolete location record of the MS gin the old VLR. The old VLR acknowledges the deregistration.


Call Origination ProcedureCall Origination Procedure


Call Origination Procedure

• MS contacts the MSC in the visited PCS network

• The call request is forwarded to the VLR for approvalfor approval

• If the call is accepted, the MSC sets up the call to the called party following thecall to the called party following the standard PSTN call setup procedure


Call DeliveryCall Delivery


Call Delivery1. Call attempted by a wireline phone is

for arded to a s itch in PSTN hichforwarded to a switch in PSTN, which queries the HLR to find the current VLR of the MSof the MS.

2. The VLR returns the routable address to h i i i i h h h hthe originating switch through the HLR

3. Based on the routable address, the trunk is setup from the originating switch to the MS through the visited MSC


Roaming Management under SS7• How mobile roaming is managed by the PSTN signaling?• Common Channel Signaling (CCS) is a signaling method that

id t l d t f ti i th t l hprovides control and management functions in the telephone network.

• CCS channel conveys messagesCCS channel conveys messages – to initiate and terminate calls– determines the status of some part of the network

t l th t f t ffi ll d– controls the amount of traffic allowed

• CCS uses a separate out-of-band signaling network to carry signaling messagesg g g

• Signaling System No. 7 (SS7) is a CCS system• Signaling between a PCS network and the PSTN network are

hi d b h kMarch 21, 2009 Girish Kumar Patnaik 24

achieved by the SS7 network

Interconnection between a PCS network and the PSTNPCS network and the PSTN

• Trunks (Voice circuits) connect SSPs to carry user data/voice


( ) y

Components of SS7• Service Switching Point (SSP)• Service Switching Point (SSP)

– Telephone switch interconnected by SS7 link– SSPs perform call processing on calls that originate or p p g g

terminate at that node– A local SSP in the PSTN can be central office or end office

An SSP in a PCS network is MSC– An SSP in a PCS network is MSC• Signal Transfer Point (STP)

– Switch that relays SS7 messages between network switchesSwitch that relays SS7 messages between network switches and databases

– Based on the address fields of the SS7 messages, the STPs route the messages to the correct outgoing signaling linksroute the messages to the correct outgoing signaling links

• Service Control Point (SCP)– Contains databases (HLR or VLR) for providing enhanced


( ) p gservices, and accepts queries from SSP

Registration through SS7


Registration through SS71. MSC2 launches a registration query to its VLR through STP2,

assuming that VLR2 and MSC2 are not colocated2 VLR2 d i t ti t th MS’ HLR (HLR4)2. VLR2 sends a registration message to the MS’s HLR (HLR4).

VLR2 may not know the actual address of HLR. Instead, VLR2 sends the message containing Mobile Identification Number (MIN) to an STP (STP3) that can translate the MIN into the HLR address

3 MIN to HLR address translation is performed at STP3 by a3. MIN-to-HLR address translation is performed at STP3 by a GTT. STP3 then forwards the registration message to HLR.

4. HLR sends an acknowledgement back to VLR2g5. HLR sends a deregistration message to VLR1, and then VLR1

acknowledges the cancellation


Traffic Cost

• The traffic in the SS7 network is heavy.• Approach to reduce the deregistrationApproach to reduce the deregistration

traffic:Implicit deregistration + Periodic re registration– Implicit deregistration + Periodic re-registration

• Approach to reduce the registration traffic:P i f di h– Point forwarding scheme


Implicit deregistration


Implicit deregistration• Obsolete VLR records are not deleted until the

database is full.f h d b i f ll h i• If the database is full when an MS arrives, a

record is deleted, freeing storage space to accommodate the newly arrived MS.accommodate the newly arrived MS.

• A replacement policy is required to select a record for replacement.

• Advantage: No deregistration messages are sent among the SS7 network elements.


Periodic re-registration• The MS periodically re registers to the VLR• The MS periodically re-registers to the VLR.• If the VLR does not receive the reregistration message within a timeout period, the

record is deletedC t l l t ffi b t MSC d VLR• Creates local message traffic between MSC and VLR

• No SS7 signaling messages are generated if VLR is collocated with the MSC


Pointer Forwarding Scheme


Pointer Forwarding SchemeM O ti (R i t ti )• Move Operation (Registration):– When MS moves from one VLR to another, a pointer is

created from the old VLR to the new VLR.– No registration to HLR is required

• Find operation (Call delivery):– When the HLR attempts to locate the MS for call delivery, the

pointer chain is traced.– After find operation, the HLR points directly to the destinationAfter find operation, the HLR points directly to the destination

VLR.• Number of pointers visited in the find operation is

li it d b klimited by k• Pointer forwarding scheme should not be considered

when the net cost of pointer creation and pointerMarch 21, 2009 Girish Kumar Patnaik 34

when the net cost of pointer creation and pointer traversal is higher than the cost of accessing the HLR

Call Delivery through SS7


Traffic Cost for Call DeliverySSthrough SS7

• Cache scheme was proposed to reduce theCache scheme was proposed to reduce the call delivery traffic.

• Two possible positions for the cache:p p– Method 1. In the originating SSPs– Method 2. In the STP that performs GTTs

T fi ld i h• Two fields in cache:– MIN of an MS– Address of the current visited VLR of the MS– Address of the current visited VLR of the MS

• The cache contains entries for MSs recently accessed from the SSP (MSC).


( )

Cache Scheme


Three Possibilities• When the calling party originates a call to

an MS the SSP first checks if the cachean MS, the SSP first checks if the cache entry for the MS exists.C 1 C h t d t i t C ll• Case 1: Cache entry does not exists. Call delivery through SS7

• Case 2: Cache entry exists and is current.VLR is directly accessed.

• Case 3: Cache entry exists but is obsolete. Call delivery through SS7


AcknowledgementAcknowledgement

• Slides obtained from home page of Prof.Phone Lin

• Slides obtained from home page of Prof Gerald Q Maguire JrProf.Gerald Q. Maguire Jr.


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