CDMA MOBILE VOICE DATA SERVICES

CALL PROCESSING

Call 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

Figure 3.1: mobile station states

1. MS-INITIALIZATION STATE

When 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.

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 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), which along 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 STATE

The 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 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 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 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-SYSTEM ACCESS STATE

The 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

behavior of the mobile on the air interface 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 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 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

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 CONTROL ON TRAFFIC CHANNEL STATE

The 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.

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 Flow CDMA2000 – 1x (voice)

In this section we shall discuss some common scenarios. Important fields of the messages have

been added for detail.

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

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.

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

Acknowledgement on the paging channel.

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 other’s 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 fully established 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.

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

the user an outgoing call display using the Alert 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

General Page Message: In the idle state the mobile monitors the paging channel to receive incoming calls. A General Page Message on the Paging Channel notifies an incoming call.

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

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

Traffic channel acknowledgment messages: The mobile and the base station notice each other’s traffic channel signals and confirm their presence by exchanging Acknowledgment messages.

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

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

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

I. Answering a Call

Connect Order: The Connect Order is sent when the user presses the ANSWER key. Taking to account the previous flow, this is a continuation.

II. Releasing Call

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

Figure 3.11: Release call-MS Initiated call flow diagram

Figure 3.12: Release call-MSC Initiated call flow diagram

HANDOFF

In 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 will scan 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.

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 Procedures

We 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 handoff when the mobile goes to a different sector

within a cell.

figure 3.13: soft handoff process flow diagram

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.

Figure 3.15: hard handoff -Intra BSC process flow diagram

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 area of 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 sms call flow

In 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.

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 method” of

routing. In case of the “Direct method”, the Originating MSC delivers the message to the Destination

subscriber’s MC. In case of the “Indirect method”, the message is first delivered to Originator’s MC &

then forwarded to the Destination subscriber’s MC. Ultimately, the message gets deposited in the

destination subscriber’s MC for delivery.

Figure 3.17: block diagram of sms routing methods

Network NetworkMCMCIndirect

Direct SMDPP

SMDPP

SMDPP / SMPP

SMDPP

Origination Home

Destination Home

SMS Call Flows (A) explains the successful 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

subscriber’s 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.

g. On receipt of response to ‘SMSREQ’ the MC-2 sends ‘SMDPP’ message to the MSC identified

by the ‘SMSAddress’.

MSC-1 MC-2 MSC-2HLR-2REGNOT

(SMSAddress)regnot (profile)

A

B

SMDPP (SMS Bearer, MDN)smdpp (ACK)

SMSREQ (MDN)

smsreq (MIN, SMSAddress)

SMDPP (MIN, SMS Bearer)

smdpp (ACK)

a

b

c

d

e

f

g

h

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 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

subscriber’s 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 subscriber’s profile. The HLR also finds that, 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’.