18547980 demonstration of call flow in a cdma networkcpt3

8/8/2019 18547980 Demonstration of Call Flow in a CDMA NetworkCPT3

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CDMA MOBILE VOICE DATA S ERVICE SCALL PR OCES SINGCall processing is the complete process of routing, originating, terminating cellular

telephone calls, along with the necessary billing processes. Here we shall introduce

call processing from the MS perspective, understanding the states of the MS and the

functions it can carry out being in each of these states.

The CDMAOne mobile goes through four states from power-on to getting in a call.

The diagram below figure 3.1 captures this in great detail also giving details about

conditions, which cause state transition. Each state shall now be handled in more

detail


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Figure 3.1: mobile station states

1. MS-INI TIALIZA TION S TATEWhen the MS is powered-on, it enters the MS-Initialization state with a power-up

indication. In this state it performs cell search and carrier detection, finally camping

on to a cell.. Once t has camped on to a cell it listens to the Primary paging channel

in the Ms-Idle state. The state has MS-Initialization has 4 sub-states where the

mobile performs in which it performs all the vital functions before entering the MS-

Idle State. The detailed diagram below illustrates all the sub-state changes.

The Mobile Station Initialization State consists of the following sub states:

i. System Determination Sub state -The mobile station selects which system

to use and enters the next state if the selected system is a CDMA system.


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Figure 3.2: MS-Initialization State

ii. Pilot Channel Acquisition Sub state : The mobile station acquires the Pilot

Channel of a CDMA system. The MS shall tune to the CDMA Channel number in

CDMACH and search for Pilots. Goes to the next state if it acquires the Pilot in

T20ms.

iii. Sync Channel Acquisition Sub state :The mobile station obtains system

configuration and timing information for a CDMA system. On entering this state

the mobile shall set the code channel to W32. The mobile shall wait for a valid

Sync Channel for T21ms. It then goes to the next state if MOB_P_REV is greater


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than the MIN_P_REV in mobile. It updates the following parameters from the

Synch Channel Message:

Timing Change Sub state -The mobile station synchronizes its timing to that of a

CDMA system.

All mobiles maintain a PRL (Preferred Roaming List given by service provider), whichalong with the History list is used in selecting a carrier. The PRL is a guiding list of

carriers, which are permitted and forbidden to the subscriber.

2. MS-IDLE S TATEThe MS enters this state and listens to the Primary Paging Channel with the PRAT

read from the Sync channel message. Any kind of interrupt say from the User, such

as sending a SMS, making a call or network paging causes the Mobile to leave the

idle to enter the System Access State. Calling this state, the Idle state is a misnomer

as the mobile is busy but appears Idle to the user. We shall discuss this state in

detail now.

What does the mobile do being in this Idle State?

The name given to this state is a misnomer. Actually the mobile is very busy in this

state. In short the mobile constantly turns parts of itself on and off; on to perform

vital functions and off again to save power so that the battery lasts longer. This

periodic on and off of the mobile transceiver is called slotted mode of operation.

Note:

The mobile can operate in the slotted mode only in the idle state. While a mobile is

latched to a particular network, for most of the time is in the idle state. Being in this

state the mobile monitors the Paging Channel. If there is a procedure that requires

to be carried out for a message received on Paging Channel, the mobile enters the


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System Access State. Paging Channel messages or user actions can cause the

mobile to go from idle state to System Access State. So in this state the mobile

station can receive messages, receive an incoming call (mobile station terminated

call), initiate a call (mobile station originated call), initiate a registration, or initiate a

message transmission.

Idle Procedures

The mobile station performs various procedures for messages received on Paging

Channel. They are as follows.

a) Paging Channel monitoring procedures; the mobile monitors the Paging

Channel in the slotted or non-slotted mode of operation. In slotted mode of

operation the mobile monitors all the slots in the paging channel.

b) The mobile station performs acknowledgement procedures ;

acknowledgements of messages received on the Paging Channel shall be sent

on the Access Channel. When sending a message that includes an

acknowledgement, the mobile station shall set the VALID_ACK field to '1' and

shall set the ACK_TYPE and ACK_SEQ fields equal to the ADDR_TYPE and

MSG_SEQ fields, respectively, of the message being acknowledged. When

sending a message that does not include an acknowledgement, the mobile

station shall set the VALID_ACK field to '0' and shall set the ACK_TYPE and

ACK_SEQ fields equal to the ADDR_TYPE and MSG_SEQ fields, respectively, of

the last message received that required acknowledgement. If no such

message has been received, the mobile station shall set the ACK_TYPE field

to '000' and shall set the ACK_SEQ field to '111'.

c) The mobile performs Registration procedures; please see the section on

Registration in Chapter two for more information.

d) In idle state the mobile can perform an idle handoff (An idle handoff occurs

when a mobile station has moved from the coverage area of one base station

into the coverage area of another base station during the Mobile Station Idle

State).

e) The mobile shall perform the Response to Overhead Information Messages;

the overhead messages are System Parameters Message, CDMA Channel List

Message, Extended System Parameters message, Neighbor List Message,

Global Service Redirection Message, and Access Parameters Message. Other

than the last message all are called configuration messages. Associated with


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the set of configuration messages sent on each Paging Channel is a

configuration message sequence number (CONFIG_MSG_SEQ).

f) The mobile station can do Origination operation , if the user initiates a call.

g) The mobile station can do a Message Transmission, if the users direct the

mobile to transmit a message.

h) The mobile station has to responds to Page Messages ; the Page Messages are

General Page Message, Page Message and Slotted Page Message.

i) The mobile station had to respond to message or order received other than

Page Messages.

j) Power down operation whenever the user directs the mobile to do so.

"Idle" phone is plenty busy!

I said earlier that in the idle state the phone is busy. Let us go about seeing how the

mobile station is busy. See what the mobile does when is busy in a real

environment? The most important thing the mobile does is to wake periodically and

turn on its receiver briefly to see if it has been paged, which means to find out if

there is an incoming call (mobile termination call) or a message. This happens on

what is known as a slot cycle, and the base station controls the period of the slot

cycle. Recall this is Slotted mode of operation Slot cycle indices are numbers from 0

to 7, and for any index the period is 1.28 seconds multiplied by 2^index.

Note:

The receiver consumes quite a lot of power. Relatively speaking the purpose of the

slot cycle is to permit the phone to keep the receiver turned off most of the time.

This is vital to extend battery life.

When the mobile first registers with a base station, the base station and mobile

determine which paging channel the mobile will use (if there is more than one) and

what phase of the slot cycle that mobile will use. Thereafter, the phone wakes

periodically, turns its receiver on briefly to see if it has an incoming call or if there is

other traffic from the cell it must respond to, and if there is nothing then it shuts the

receiver down again and waits until the next slot time. This is slotted mode of

operation.

When an incoming call arrives at a base station for a given mobile, the phone

system of the caller generates the sound of a phone ringing as a comfort tone back

to the caller (this tone comes in Alert With Information Message), and the base

station waits until the slot time for the called mobile. When it comes around, the cell


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sends a message to the phone telling it that there is an incoming call. This causes

the phone to waken and set up the call, and to begin to ring. If the phone doesn't

respond to the page, the cell may try again on the next slot.

Note:

The advantage of a longer slot cycle is that the phone spends a lower percentage of

the time with its receiver on and thus the battery will last longer. It also means

there is more capacity on the paging channel. The advantage of a shorter slot cycle

is that the phone gets more chances to receive the page, and will receive the page

sooner.

The mobile also has to perform Registration, to keep informing the base station

what is its current location. More details on this in the Registration section.

3. MS-SY STEM A CCESS ST ATEThe MS enters this state on indications as received from the idle state to request

resources from the network. This shall get clearer by studying the diagram on sub-

states below. The MS transmissions are in the slotted ALOHA mode, which is

based on fair contention on the air interface. This behaviorof the mobile on the airinterface is controlled by the parameters in Access Parameter Message received as

a part of Configuration Messages. All transmissions are under the Open Loop Power

control.

The MS moves to a traffic channel on receiving the Channel Assignment Message. In

short this state is a transition from idle to connected mode, wherein the network


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has not committed resources to the MS. Once the network realizes that the requests

can be accommodated the mobile is allocated a traffic channel.

The diagram below explains the sub states in the state.

figure 3.3: MS-System Access State diagram

Power control in System Access State

We shall now discuss the phone operation in on access channel little more in detail.

As earlier said that Power Control is necessary for efficient operation in a CDMA

system. Information transmitted in the System Access state is of bursty nature and

there is no scope of power control since the MS is not set-up on a dedicated link

with the BS. Hence in this case the power control in use is the Open Loop power

control.

The basis of this power control is setting up the mobile with parameters that make it

transmit at intervals, which prevent collisions from other mobiles by using a hash


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equation (which has minimum collisions). The hash equation uses some general

parameters received in the Access Parameters message (APM) and parameters,

which are unique to the mobile.

Phone operation on Access Channel

The MS transmits access probes to the BS with increasing power levels in a probe

sequence. A bunch of 15 such probe sequences is called an access attempt. The

power increment between probes, time interval between probes and sequences are

all set from values obtained in the APM.

Figure 3.4: Schematic of a typical Access Parameter Message


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Figure 3.5: phone Operation on access channel

figure 3.5 is recommended for study to understand phone operation on the access

channel.

4. MS CONTR OL ON TRAFFIC CHANNEL ST ATEThe mobile enters this state from the System Access State either to originate or

answer a call. Apart from this, the mobile may come to the traffic state either to

send/receive a long SMS or OTASP procedure depending on the Service Option in

the Origination Message/Page Response Message.

The state machine in the traffic state is shown below.


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Figure 3.6: MS-control on traffic channel state diagram

The MS enters the traffic state either when it has placed a call or to enter a call. The

flow sequence is different for these two cases. The blue arrows and orange arrows

give transitions in the MT /MO call scenario respectively. The black arrows give

common flow after the mobile has entered the Conversation sub state.

After call release the Ms enter the System Determination sub state with the

appropriate indication.

Call Flo w CDMA2000 1x ( voice)In this section we shall discuss some common scenarios. Important fields of the

messages have been added for detail.


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Figure 3.7: CDMA2000 voice/data call flow

The main difference between the CDMAOne and CDMA2000 call flow is that, in

CDMA2000 the mobile initiates the decision as to whether the session will be a

packet data session, voice session, or concurrent (meaning voice and data). After

the decision has been made, the mobile sends an origination message on the

access channel that includes an indication that this is a voice or packet data

session.

Considering the circuit switch core network domain


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Figure 3.8: CDMA2000 voice call flow diagram

Standardizing the open interfaces between the radio/access network and core

network allows opportunity and flexibility for operators to purchase equipment from

different vendors. In 1998, the CDMA Development Group began work on an

interoperability specification (IOS), which defines a standardized protocol interface

between the CDMA basestation, MSC and packet-switching equipment (PDSN, PCF).

This open interface is termed CDG-IOS.


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Figure 3.9: CDMA2000 Originating voice call flow diagram

The basestation and MSC use CDG-IOS protocol messages to exchange call setup

information, and coordinate the setup and connection of the call. The call flow is

described as follows:

Origination Message: The mobile user dials the numbers and presses the

SEND button causing the mobile to transmit an Origination Message on the

access channel.

CM Service Request message: The basestation sends the Origination

information, such as service option and called party digits, to the MSC in a CDG-

IOS CM Service Request message.

BS Acknowledgement: The BS also acknowledges receiving the origination by

sending a BS Acknowledgementon the paging channel.


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Channel Assignment Message:The MSC allocates a circuit resource based on

a specific resource-selection scheme, and informs the base station through a

CDG-IOS Assignment Request message.

Channel Assignment Message: At this time, the basestation allocates radio

resources (Walsh code, power, channel resources and so on) for the call and

starts transmitting (NULL frames) on the traffic channel. The BS notifies the

mobile in a Channel Assignment Message on the paging channel and the MS

goes to a traffic channel.

Traffic channel acknowledgment messages: The mobile on receiving at

least 2 NULL frames concludes that it is on the right TCH. The MS and BS

acknowledge each others with traffic channel acknowledgment messages. Now

the mobile is on Traffic Channel.

If there are any changes in the types of service during the call setup, a Service

Negotiation use case is executed before the basestation sends the Assignment

Complete to the MSC.

Service Negotiation: The base station and the mobile negotiate the type of

call.

Service connect Order:Once the MS and BS negotiate the type of call, the BS

sends a service connect message to the MS

Service Connect Completion message:The MS also acknowledges receiving

the service connect message by sending a Service Connect Completion

message.

Assignment Complete message: Once the radio link has been fullyestablished at both the mobile station and basestation, the basestation indicates

to the MSC that the setup procedure has been completed, through a CDG-IOS

Assignment Complete message.


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Outgoing ring alert: The audio circuit is completed and the mobile is made to

ring to present the user an outgoing call display using theAlert with Information

Message.

Paging a Mobile for an Incoming Call (Mobile Terminated call)

A mobile-terminated call-setup scenario is very similar to the mobile-originated call

setup, except that it is initiated by the MSC to bring the mobile onto the access

channel. Also, from the basestation perspective, a hard-handoff addition scenario is

very similar to a call attempt in the system. For every call attempt into the

basestation, the basestation goes through the same steps with some minor

differences for signaling. This common behavior among various use cases can be

easily captured via object-oriented modeling.

Figure 3.10: mobile terminated call flow diagram


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General Page Message: In the idle state the mobile monitors the pagingchannel to receive incoming calls. A General Page Message on the PagingChannel notifies an incoming call.

Page Response Message: The paged mobile sends a Page Response Messageon the access channel.

Channel Assignment Message: The system sets up a traffic channel for thecall, and sends a Channel Assignment Message.

Traffic channel acknowledgment messages: The mobile and the basestation notice each others traffic channel signals and confirm their presence byexchanging Acknowledgment messages.

Service Negotiation: The base station and the mobile negotiate the type ofcall eg. 13k voice, etc. Service Negotiation takes place.

Incoming ring alert: The mobile is made to ring to present the user anIncoming call display using theAlert with Information Message.

Acknowledging the previous message, either of the following two steps is possible:

I. Answerin g a CallConnect Order: The Connect Order is sent when the user presses the ANSWERkey. Taking to account the previous flow, this is a continuation.

II. Rel easi ng CallRelease order: This scenario can happen either from the MO (calling) or MT(called) end.Release orderis sent to the BS when the User presses the DISCONNECT button.See the release call flow diagram in figure 3.11.


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Figure 3.11: Release call-MS Initiated call flow diagram


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Figure 3.12: Release call-MSC Initiated call flow diagram

HANDOFFIn this section we have to remember the Pilot Channel, it is like a lighthouse to a

ship. It acts as a beacon for the mobile and identifies the BS. When the MS powers

on it gets latched to a BS by searching for the Pilots. Now which pilot does it latch

on to or say which BS does it latch to? Obviously it has to latch to the BS, which is

nearest to it. So how does the MS know which BS is the nearest. Well, the MS willscan for the strongest (in terms of power) Pilot Channel and latch to it.

Now why handoff? When the MS goes from one cell to another cell Handoff occurs.

As the MS goes away from the BS the power level of the pilot channel may decrease

and hence it looks for a pilot of stronger strength to latch on.


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This is to ensure that when a mobile station is using traffic channel resources, the

information flow does not stop when control goes from one base station to another

base station. Thus we prevent Call Dropping.

At this point there may be another important question? Does the mobile station

scan all the short PN offsets to search for a Pilot during handoff? Well, Pilot channels

having the largest power; the mobile station from its position will receive varying

power levels of different offsets. Here let me introduce the term Pilot Databases.

Pilot Databases

The Pilots are divided into sets, which are used to search for pilots during Handoff.

The mobile maintains four sets:

a) Active Set: Pilots associated with forward traffic channels assigned by the base

station

b) Candidate Set: Pilots not currently in the Active Set, but whose level is high

enough to be there (but others are stronger)

c) Neighbor Set: Pilots that are not currently in the Active Set or Candidate Set

and are likely candidates for handoff. The initial neighbor list is sent to the

mobile in the System Parameters Message on the Paging Channel.

d) Remaining Set: Includes all pilots in the system which are not in another set

Handoff ProceduresWe have different types of Handoff Procedures depending upon the situation

1. Soft Handoff: Soft Handoff is when the mobile goes from one cell to another

cell but uses the same frequency. We also can have softer handoffwhen the

mobile goes to a different sector within a cell.


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figure 3.13: soft handoff process flow diagram


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figure 3.14:: softer handoff process flow diagram

2. Hard Handoff: Hard Handoff is when

I. MS is transferred between disjoint active sets

II. CDMA frequency assignment change

III. The frame offset changes

IV. When the mobile is sent from CDMA channels to analog voice channels.


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Figure 3.15: hard handoff -Intra BSC process flow diagram


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Figure 3.16: hard handoff -Inter BSC process flow diagram

3. Idle Handoff: When the Paging Channel is transferred from BS to another BS.

An idle handoff occurs when a mobile station has moved from the coverage areaof one base station into the coverage area of another base station during the

Mobile Station Idle State).

4. Access Handoff: When the mobile sends the access attempts to another BS.

CDMA sm s ca ll f lowIn case of CDMA networks, each MS belongs to one and only one Message Center

(MC or the SMSC). SMS being a store-and-forward type of application, it is the

responsibility of the Home MC of any given CDMA subscriber, to store &

subsequently deliver the terminating message to the subscriber.


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When a CDMA user originates a Short Message, there are two methods of routing

the message to its destination. One method is called as the Direct method& the

other is called as Indirect methodof routing. In case of the Direct method, the

Originating MSC delivers the message to the Destination subscribers MC. In case of

the Indirect method, the message is first delivered to Originators MC & then

forwarded to the Destination subscribers MC. Ultimately, the message gets

deposited in the destination subscribers MC for delivery.

Figure 3.17: block diagram of sms routing methods

SMS Call Flows (A) explains the s uccessful SMS delivery scenario

Figure 3.18: successful sms flow diagram

a. MS B registers at MSC/VLR-2. MSC sends REGNOT message to the HLR with

its own address (PC/SSN) as SMSAddress to the HLR.

b. HLR Stores the SMSAddress and sends regnot response to the MSC/VLR-2.

c. MS A sends an SMS for MS B. The MSC-1 delivers the message to the MC of

MS B (i.e. MC-2) through the IS-41 message called as SMDPP (Short Message

Delivery Point-to-Point).

d. The Message Center sends smdpp response to acknowledge the receipt of

the message.

e. The MC-2 sends SMSREQ (SMS Request) message to the HLR of MS B, to

find out the subscribers current location & status.

f. If HLR finds that the MS B is registered, it sends smsreq response with the

MIN & the SMSAddress (the address of the serving MSC) to the MC-2.


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g. On receipt of response to SMSREQ the MC-2 sends SMDPP message to the

MSC identified by the SMSAddress.

h. The MSC-2 pages the MS B & delivers the message & sends successful

response to the MC-2 in smdpp message.

SMS Call Flows (B) explains the first unsuccessful SMS delivery attempt

Figure 3.19:un successful sms flow diagram

a. MS B de-registers from MSC/VLR-2. MSC sends MSINACT message to the

HLR, which indicates to HLR that the MS B is not available any more.

b. HLR marks the MS B as Inactive and sends msinact response to theMSC/VLR-2.

c. MS A sends an SMS for MS B. The MSC-1 delivers the message to the MC of

MS B (i.e. MC-2) through the IS-41 message called as SMDPP (Short Message

Delivery Point-to-Point).


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d. The Message Center sends smdpp response to acknowledge the receipt of

the message.

e. The MC-2 sends SMSREQ (SMS Request) message to the HLR of MS B, to

find out the subscribers current location & status.

f. HLR finds that the MS B is marked as Inactive and sends smsreq response

with the SMSAccessDeniedReason parameter set to a value indicating that

the SMS to this MS (B) cannot be delivered at this moment. The HLR also

sets a flag called SMS Delivery Pending Flag (SMDPF) indicating that there

was an attempt to deliver an SMS to this MS, but, it was not delivered.

g. After some time when the MS B turns ON, it performs Power-Up

Registration. The MSC/VLR-2 sends REGNOT to the HLR.

h. HLR responds to REGNOT with the subscribers profile. The HLR also findsthat, against this MIN, the SMDPF flag is set.

i. Because the SMDPF flag for MS B was set, the HLR sends a message called

SMSNOT (SMS Notification) to the Home MC of MS B. The SMSNOT

message includes the MIN, MDN & the SMSAddress.

j. The MC-2 responds with smsnot to the HLR. On receipt of the smsnot

message, the HLR resets the SMDPF flag.

k. On receipt of SMSNOT, the MC-2 comes to know that the MS B is now

available & is located at the MSC identified by the SMSAddress received in

SMSNOT. The MC-2 retries the delivery of message by sending SMDPP

message to the MSC-2.

l. On successful delivery of message to MS B, the MSC sends the smdpp

response message to the MC-2. If the originating MS had requested for

delivery acknowledgement, then the MC-2 forms the delivery confirmation

message to be sent to MS A & goes ahead with SMS delivery call flow for

message delivery confirmation to MS A.

18547980 demonstration of call flow in a cdma networkcpt3

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