Logical channels were developed in the IS–136 DCCH technology to organize the PCS and other digital information flowing across the air interface.

Logical-Channels Configuration

The logical channels are depicted graphically in Figure 5. The figure shows how the forward DCCH (FDCCH) consists of many logical channels carrying information from the system to the phone. The reverse DCCH (RDCCH), carrying information from the phone to the system, consists of one logical channel.

Figure 5. Logical-Channels Configuration

Operating Principle

Logical channels sort and prioritize signaling information by functional use. The data is then mapped onto a DCCH, which is a physical channel. Physical channels are the actual portions of electromagnetic bandwidth consisting of frequencies and time divisions. Logical-channel data flows on the DCCH in both directions: from the system to the phone (downlink), and from the phone to the system (uplink).

Logical Channels Functions

The multiplexed broadcast channel (BCCH) shown in Figure 5 is designed to carry information about the system configuration and the rules that phones must follow at system access. Its primary logical channels are the following:

• fast broadcast channel (F–BCCH)—carries information that phones need immediately, such as the system ID and registration information
• extended broadcast channel (E–BCCH)—carries information that is not as time critical, such as neighbor cell lists

The system uses the multiplexed SMS point-to-point messaging, paging, and access-response channel (SPACH) shown in Figure 5 to communicate with a specific phone. Its logical channels are the following:

• short message service channel (SMSCH)—carries PCS messaging and over-the-air activation and programming (OAA/P)—PCS information is carried on the logical channels at both 800 MHz and 1900 MHz
• paging channel (PCH)—carries system pages to the phone
• access response channel (ARCH)—provides system response to phone queries and administration information

Table 2 outlines the logical channels.

Logical Channel	Description
BCCH	This is a downlink multiplexed channel comprised of F–BCCH and E–BCCH.
SPACH	This is a downlink multiplexed channel comprised of SMSCH, PCH, and ARCH.
RACH	This is a single uplink channel with all time slots used for system access.
SCF	The SCF fields in the downlink are used to provide a collision-prevention mechanism for the uplink.

Table 2. Description of Logical Channels