Layer 3 Messages

Layer 3 manages registration, call setup, service negotiation, and control operations. Defined in C.S0005-E, carried by the LAC sublayer.

Message Categories

Layer 3 messages fall into three categories based on which channel carries them:

Overhead Messages (Forward Common Channel)

These are broadcast continuously on the paging channel to all mobiles in the cell. They define the operating parameters of the base station:

These messages are not acknowledged — they repeat on a cycle, and a mobile that misses one will receive it on the next pass. The base station configures the broadcast interval for each message type.

Access Messages (Reverse Common Channel)

Sent by the mobile on the access channel to initiate contact with the network:

Each access message includes the mobile’s identity (MSID_TYPE + identity payload) so the network knows who is transmitting. See Identity & Addressing for details on identity encoding.

Directed Messages (Forward Common Channel)

These target a specific mobile on the paging channel using the addressing mechanism:

Traffic Channel Messages (Dedicated Channel)

Once a mobile is assigned a traffic channel, all signaling moves to the dedicated channel (f-dsch / r-dsch):

Forward (BS → MS)

Reverse (MS → BS)

Order Messages

The Order message is a versatile container used for many control functions. The ORDER field (6 bits) identifies the specific order:

The BS Ack Order is the most common — it’s the mechanism for acknowledging traffic channel messages when the base station has no other pending signaling to send.

Registration & Call Setup

Registration is how a mobile announces its presence to the network. CDMA2000 supports several registration types, controlled by parameters in the System Parameters Message:

The Registration Message on the access channel includes:

• Mobile identity (ESN, IMSI, or both)
• Registration type
• Protocol revision (MOB_P_REV)
• Slot cycle index (power saving preference)
• Station class mark (device capabilities)

The base station acknowledges successful registration with an Order message or implicitly by including the mobile in a Page or Channel Assignment.

Service Negotiation

When a call or data session is established, the service option must be negotiated between the mobile and the base station:

MS                                    BS
 │                                     │
 │  Origination (requested SO)         │
 │ ──────────────────────────────────▶ │
 │                                     │
 │        Channel Assignment (ECAM)    │
 │ ◀────────────────────────────────── │
 │                                     │
 │  ── mobile tunes to traffic ch ──   │
 │                                     │
 │        Service Connect              │
 │        (accepted SO, RC, RLP)       │
 │ ◀────────────────────────────────── │
 │                                     │
 │  Service Connect Complete           │
 │ ──────────────────────────────────▶ │
 │                                     │
 │     ═══ service active ═══         │

The Service Connect Message specifies:

• Service option — SO3/SO17/SO68/SO73 for voice, SO7/SO33 for data
• Radio configuration — RC1 or RC3 for forward and reverse
• RLP parameters — For data sessions: RLP type, sequence number width, NAK parameters
• Forward/reverse traffic config — Rate set, supplemental channel allocation

If the mobile doesn’t support the proposed service option, it can reject or counter-propose during traffic-channel negotiation. 1xBTS decodes traffic-channel Mobile Station Reject Orders with the rejected message type, order, parameter ID, record, connection reference, and any trailing bytes when those fields are present.

General Page Message

The General Page Message (GPM) is the primary mechanism for reaching idle mobiles. It can carry multiple page records in a single message, each targeting a different mobile:

Page Record Types

Each page record includes:

• Address — The target mobile’s identity
• Special Service flag — Whether a specific service option is being requested
• Service Option — If special service is set, the requested SO

Slotted Paging

Mobiles don’t monitor the paging channel continuously — they wake up only during their assigned paging slot (computed from IMSI_S, see Identity & Addressing). The base station must transmit the page during the correct slot for the target mobile to receive it.

Data Burst Message

The Data Burst Message is a general-purpose container for non-voice, non-packet data payloads. It’s used for:

• SMS delivery — Both MO (mobile → network) and MT (network → mobile) SMS messages are carried as data bursts
• Position location — GPS or network-based location data
• Short data burst — Application-specific small data payloads

Data bursts can be sent on either the paging channel (for idle mobiles) or the traffic channel (for active sessions). The burst type field identifies the payload format.

Message Encoding

Channel-Specific Encoding

Different channels use different message identifier formats:

PDU Structure (Traffic Channel Example)

A typical forward dedicated channel PDU:

MSG_TYPE (8)  — Message identifier
ACK_SEQ  (3)  — Acknowledging reverse message
MSG_SEQ  (3)  — Sequence number of this message
ACK_REQ  (1)  — Requesting acknowledgment
ENCRYPT  (2)  — Encryption mode (00 = none)
SDU    (var)  — The Layer 3 message body
PADDING (var) — Zeros to byte boundary

This PDU is then wrapped in the SAR envelope (MSG_LENGTH + CRC-16) and fragmented into MuxPDU frames for physical layer transmission. See Protocol Layers for the full encapsulation chain.