Cellular Network Architecture Ashish Sharma...Ashish Sharma Cellular Network Architecture. ETEC 405: Wireless Communications Faculty: ASHISH SHARMA,CSE,MAIT 1. Location Register (Database)

Ashish SharmaCellular Network Architecture

ETEC 405: Wireless Communications Faculty: ASHISH SHARMA,CSE,MAIT 1

LocationRegister(Database)

Mobile Switching Center MSC

Backbone Wireline Network

Base Station Controller

Base Station

MobileTerminal

RadioNetwork

Cell

Ashish SharmaMobility Management

• Enables telecomm networks to • Locate roaming MSs for call delivery• Maintain connections as the MSs move

between different cells• Involves two operations

• Location Management• Handoff Management

• Involves two types of mobility• Terminal mobility• Personal mobility

2ETEC 405: Wireless Communications Faculty: ASHISH SHARMA,CSE,MAIT

Ashish SharmaTypes of Mobility

• TERMINAL MOBILITY(Network should route calls to the MTregardless of its point of attachment)

• PERSONAL MOBILITY(Users should access the network wherever

they are; UPT (Universal Pers. Tel #))• SERVICE PROVIDER MOBILITY

(Allow user to roam beyond regional networks).


Ashish SharmaMobility Management

• Location Management

4

Handoff Management

Base Station

MT A is receiving a call !How will the networkdeliver the call to A ?

A

ETEC 405: Wireless Communications Faculty: ASHISH SHARMA,CSE,MAIT

Ashish SharmaLocation Management

5

Call Delivery(Paging)

Location Update(Registration)


Ashish SharmaCost Tradeoff

6

Too Many Location Updates

Too Few Location Updates

Low Paging CostsHigh Update Costs

High Paging CostsLow Update Costs


Ashish SharmaSolution

• Local Areas (GSM) = Registration Areas (IS-41)

7

Registration Area Boundary

Center Cell


Ashish SharmaHandoff Types

8

Intra-Cell Inter-Cell

Soft Handoff Hard Handoff


Ashish SharmaMobility Management:Location Management• Location management enables the system to track the locations

of MTs between consecutive communications• Trade-off between the costs of location update and paging

design optimal location management schemes to reduce the overall cost

9

LOCATIONMANAGEMENT

LOCATIONREGISTRATION

(UPDATE)

CALL DELIVERY

AUTHENTICATION

DATABASE UPDATES

DATABASE QUERIES

TERMINAL PAGING


Ashish SharmaMobility Management:Handoff Management

• Handoff management: an MT keeps its connection active when it moves from one access point to another one

• Four types of handoffs: Network Controlled Handoff (NCHO), Mobile Controlled Handoff (MCHO), Network Assisted Handoff (NAHO), and Mobile Assisted Handoff (MAHO)

10

HANDOFFMANAGEMENT

NEW CONNECTION GENERATION

USER MOVEMENT

NETWORK CONDITIONS

RESOURCE ALLOCATION

CONNECTION ROUTING

DATA FLOW CONTROL

INITIATION

BUFFERING/SEQUENCING

MULTICAST


Ashish SharmaLocation Management

11

BACKBONE TELEPHONE NETWORK

(HLR)

Mobile Switching Center

Visitor Location Register

Mobile Terminal(MT)

MSCVLR

Local Signaling Long Distance Signaling

(MSC)(VLR)

Home Location Register


Ashish SharmaLocation Registration

12

MT enters a new LA, and transmits location update to new BS

BS forwards update to MSC, which queries VLR

Does the MT have an existing record?

New LA is under same VLR. VLR updates the LA ID # for the MT.

Yes No

VLR determines address of HLR, and sends location registration message

HLR authenticates and registers MT by updating the VLR ID # for the MT. Then, HLR cancels former VLR.



• BS keeps broadcasting Location Area (LA) ID#. • MT listens to broadcast and will perform a location

update when:• Powering up• Crossing LA boundaries• After a defined period of time




Ashish SharmaCall Delivery

15

Incoming call for roaming MT reaches an MSC

The calling MSC determines the address of the MT’s HLR, and sends a location request message to the HLR.

The HLR sends a route request message to the VLR, which forwards the message to its MSC

The MSC gives the MT a Temporary Local Directory Number, and forwards the TLDN back to the HLR

The HLR forwards this message to the calling MSC, which sets up a route to the MT at its current MSC.

Finally, the current MSC tells all of the BSs in the MT’s LA to send a polling signal to page the MT. When the MT responds, the call is connected.


Ashish SharmaCall Delivery


Ashish SharmaHow is a Cell Phone Call made?

• When a mobile originates a call, a call initiation request is sent on the control channel to BS.

• With this request the mobile transmits its tel number (MIN;• Mobile ID Number), electronic serial number (ESN) and tel number of

the called party.• Base station receives this data and sends it to the MSC.• MSC validates the request make connection to the called party through

the PSTN and instructs base station and mobile to use an idle forward and reverse voice channel to allow conversation to begin.

• (in AMPS -> 10-60 voice channels; one control channel in each cell base station).


Ashish SharmaResearch Areas in Location Management• Database Architectures• Paging Techniques• Multi-network location management• Location Area Design


Ashish SharmaDatabase Architectures

• Centralized• Increase database hierarchy• Cache user locations at switching points• Replicate user profiles at more than one

database• Use pointers to follow a path of VLRs to the

MT’s current location• Distributed

• Database Trees• Partitioning


Ashish SharmaPaging Techniques

• Blanket paging• Paging the MS in all cells belonging to an LA

simultaneously. • Advantage: The delay of the response to paging is kept at a minimum.• Disadvantage: Paging has to be done in several cells.

• Closest-cells first• The cell where the MS was last seen is paged first

followed by subsequent equidistant ring of cells.• Several rings may be polled simultaneously in a paging cycle

to keep delay low.• Sequential paging

• Subsequent pages are performed in most likely locations based on past history and distance.


Ashish SharmaPaging

21

(MSC)

VLR


Ashish SharmaLocation Area Design

• Tradeoff• Location Updates versus Terminal Paging

• Goal: Improvements to tradeoff• Geographical• Fixed versus Dynamic• User-based versus Global definition• Network-specific


Ashish SharmaDynamic Location Update Schemes

• Movement-based• The MT performs an update each time it crosses a

certain movement threshold, where one movement is made by crossing a cell boundary.

• Distance-based• The MT performs an update when its distance from

the cell where it performed its last update surpasses a certain distance threshold.

• Time-based• The MT performs an update at a constant time

threshold, deltaT.


Ashish SharmaExample

• A MT is moving through the cellular network (R= km) as shown in the figure at a rate of 30km/hour.

• Label the cell ID’s where the MT will perform its updates for:

• Movement-based (T=3)• Distance-based (T=6km)• Time-based (30 minutes)

24

3


Ashish SharmaExample Figure

3

• A MT is moving through the cellular network (R= km) at a rate of 30km/hour.

• Where will updates be performed for:

• Movement-based (T=3)

• Distance-based (T=6km)

• Time-based (30 minutes)

25

A

D

C

B

H

G

FL

KP

NS

R

QM

J

IE O


Ashish SharmaAnswer-Movement-based

26

A

D

C

B

H

G

FL

KP

NS

R

QM

J

IE O

T = 3, 2 h = 3Update at S and G


Ashish SharmaAnswer-Distance-based

27

A

D

C

B

H

G

FL

KP

NS

R

QM

J

IE O

Update at O, and near the M/G border

T=6km

T = 6, 2 h = 3


Ashish SharmaAnswer-Time-based

28

A

D

C

B

H

G

FL

KP

NS

R

QM

J

IE O

Update only at M

3h*215km.30km/hour*0.5hoursevery update 1

-- minutes 30

==

>


Ashish SharmaGroup Problem

• Design a location update and paging scheme. • Provide a diagram with numbered steps.• Explain how your scheme reduces the signaling

overhead.


Ashish SharmaHandoff

• The transfer of a mobile terminal’s active connection(s) from one channel to another.

• Hard handoffs vs. soft handoffs• Hard handoff: break old connection, then form new

connection. • Soft handoff: Connect to several BSs simultaneously.

• In CDMA, handoff does not change the physical channel, it just changes the BS that handles the channel.


Ashish SharmaSIGNAL STRENGTH

• Cellular systems depend on the radio signals received by an MS throughout the cell and on the contours of signal strength emanating from the BSs of two adjacent cells i and j.


Ashish SharmaSignal Strength

32

Select cell i on left of boundary Select cell j on right of boundary

Ideal Boundary

Cell i Cell j

-60

-70-80

-90-

100

-60-70

-80-90

-100

Signal strength (in dB)


Ashish SharmaSignal Strength (2)

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Signal strength contours indicating actual cell tiling. This happens because of terrain, presence of obstacles and signal attenuation in the atmosphere.

-100

-90-80

-70

-60

-60-70

-80

-90

-100

Signal Strength

(in dB)

Cell i

Cell j


Ashish SharmaSIGNAL STRENGTH

• Signal strength goes down as a mobile terminal moves away from the BS.

• As the mobile terminal moves away from the BS of the cell, the signal strength weakens and the so-called HANDOFF occurs.

• This implies a radio connection to another adjacent cell.


Ashish SharmaHandoff Region

35

BSi

Signal strength due

to BSj

E

X1

Signal strength due

to BSi

BSjX3 X4 X2X5 Xth

MS

Pmin

Pi(x) Pj(x)

Pz(x) (for z=1,2) denote the power received at MS from BS z.

By looking at the variation of signal strength from either base station it is possible todecide on the optimum area where handoff can take place.


Ashish SharmaHANDOFF REGION

• At X1 the received signal from BSj is close to 0 and the signal strength at the mobile terminal could be primarily attributed to BSi.

• Similarly, at distance X2 the signal from BSi is negligible.

• To receive and interpret signals correctly at mobile, the received signal must be at a minimum power level Pmin(X3 and X4), i.e., between X3 and X4 the mobile terminal can be served either by BSi or BSj.



• The area between X3 and X4 is called HANDOFF AREA or HANDOFF REGION.

• Where to perform HANDOFF depends on many factors.

* Do handoff at X5 where two BSs have equal signal strength.HARD HANDOFF!!!!!



• * Avoid “Ping-Pong Effect”, if the mobile moves back and forth between BSi and BSj

* SOLUTION SOFT HANDOFF!!!!: Continue to maintain both links with BSi and BSj until the signal strength from BSjexceeds that of Bi by some pre-specified threshold value E as shown by point X in Figure.


Ashish SharmaHandoff Control

• NCHO (Network-Controlled Handoff) • All close-by BSs monitor signal strength from the mobile terminal. • MSC collects data from BSs, decides best candidate BS for the

mobile terminal, and initiates the MS’s handoff (CT-2, AMPS).• Results in heavy signaling load, handoff delay of many seconds.

• MAHO (Mobile-Assisted Handoff) • MT monitors signal strength from nearby BSs and reports the

measurements back to the BS/MSC (twice per second).• MSC decides best candidate BS and initiates the handoff (GSM)

• MCHO (Mobile-Controlled Handoff) • MT monitors signal strength from nearby BSs, decides best

candidate BS, and initiates handoff (DECT)


Ashish SharmaHandoff Management

40

(Detection &Decision)

Channel Assignment

Radio LinkTransfer


Ashish SharmaHandoff Management

• Initiation (Detection/Decision)• The user, the network, or changing channel conditions

detect the need for handoff.• New connection generation (Channel Assignment)

• The network must find new resources for the handoff call

• The network must also perform any needed routing operations.

• Data flow control (Radio Link Transfer)• Delivery of the data from the old path to the new

path is maintained according to agreed-upon service guarantees.


Ashish SharmaHandoff Initiation

• A balance of user movement versus network conditions

• Goals:• Keep user connected• Minimize network signaling• Minimize “ping-pong” handoffs


Ashish SharmaHandoff Initiation—What criteria should cause handoff?

43

1rP2rP


Ashish SharmaHandoff Management• Handoff in cellular telephony:

• Transfer of a voice call from one BS to another

• Handoff in WLANs:• Transfer of a connection from one AP

(Access Point) to another• Handoff in hybrid networks:

• From a BS to another, from an AP to another, from a BS to an AP, or vice versa


Ashish SharmaHandoff Decision Time Algorithms• Traditional algorithms employ thresholds• Channel measurements:

• Received Signal Strength (RSS)• Measures the co-channel interference power and noise

• Alternatively to RSS or in conjunction:• Path loss• Carrier-to-interference ratio (CIR)• Signal-to-interference ratio (SIR)• BER• Block error rate (BLER)• Symbol error rate (SER)• Etc.


Ashish SharmaHandoff Decision Time Algorithms• Goals:

• Keep user connected• Minimize network signaling• Minimize “ping-pong” handoffs

• In order to avoid the ping-pong effect, additional parameters are used such as hysteresis margin, dwell timers, and averaging windows.


Ashish SharmaHandoff Decision Time Algorithms• Received Signal Strength (RSS): The BS whose signal is received with

the largest strength is selected.

• RSS + Threshold: If the RSS of a new BS exceeds that of the current one and the signal strength of the current BS is below a threshold.

• RSS + Hysteresis: If the RSS of a new BS is greater than that of the old BS by a hysteresis margin.

• RSS + Hysteresis + Threshold: If the received signal strength of a new BS exceeds that of the current one by a hysteresis margin and the signal strength of the current BS is below a threshold.

• Algorithms + Dwell Timer: A timer is started at the instant when the condition in the algorithm is true. The handoff is performed is the condition continues to be true until the timer expires.


Ashish SharmaTraditional Handoff Algorithms

48

What kind of handoff isHappening in A, B, C, D?


Ashish SharmaSample RSS seen by MS traveling in a straight line between them


Ashish SharmaPerformance of Handoff Algorithms

• Performance measures (related to voice connections):

• Call blocking probability• Handoff blocking probability• Delay between handoff request and execution• Call dropping probability

• Objective: Minimize unnecessary handoffs• Overlooked issues:

• Throughput maximization• Maintaining QoS guarantees during and after

handoff


Ashish SharmaGeneric Handoff Management Process

51

(1) Decision to handoff is made (network-controlled, mobile-assisted or controlled)(2) MT registers with visiting database via a handoff announcement(3) New visiting database communicates with home database for authentication

and subscriber profile(4) Home database responds with authentication. Both databases are updated.(5) Home database communicates with old visiting database to clear registration information

for the MT(6) The old visiting database flushes or redirect packets to the new visiting database and removes

the MT form its list.ETEC 405: Wireless Communications

Faculty: ASHISH SHARMA,CSE,MAIT

Documents

Cellular Network Architecture Ashish Sharma...Ashish Sharma Cellular Network Architecture. ETEC 405: Wireless Communications Faculty: ASHISH SHARMA,CSE,MAIT 1. Location Register (Database)