5.3 IS-136 Protocol Layers

5.3 IS-136 Protocol Layers

The IS-136 standards defines three protocol layers.

• Layer 1: physical layer

• Layer 2: data link layer

• Layer 3: network layer

The functionality at higher layers is provided by the teleservices and data services and is described in later sections.

5.3.1 Layer 1: The IS-136 Physical Layer

The physical layer specifies the physical characteristics of the digital PCS air interface such as RF- related parameters, modulation format, power output requirements, and frame and time slot structure. The physical layer also defines hardware requirements for mobile stations and base stations and influences operational parameters of digital PCS networks, such as the number of RF channels possible for each cell or sector, the distance between cell sites, and the frequency reuse patterns.

IS-136 specifies operations of digital PCS in the 800-MHz band and 1900-MHz band, with each band divided in 30-kHz RF channels. IS-136 is a time division multiple access (TDMA) cellular technology that divides RF channels into frames . A single TDMA frame is composed of six time slots and is 40 ms in length. To understand how frames and time slots are used, the examples in Figure 5-2 are provided. A half-rate channel uses only one time slot, whereas a full-rate channel uses two time slots separated by two time slots (e.g., time slots 2 and 5 are paired). A double-rate channel uses four time slots.

Figure 5-2. IS-136 frame structure and time slot utilization.

The DCCH time-slot structure in IS-136 is described in Figure 5-3. The DCCH is structured in hyperframes of 1.28 seconds of length, composed of two superframes each divided in 16 frames of 40 ms.

Figure 5-3. IS-136 DCCH structure on forward direction.

The DCCH is divided in logical channels for the forward and reverse directions. The forward DCCH is divided into the following channels:

• SMS point-to-point, paging, and access response channel (SPACH). The SPACH channel is divided into SMS channel (SMSCH), used to deliver teleservice messages to mobile stations; the paging channel (PCH), used to deliver pages and other commands to mobiles; and the access response channel (ARCH), used to deliver responses to messages from the mobile station.

• Broadcast control channel (BCCH). The BCCH is divided in the fast broadcast control channel (F-BCCH), used to carry time-critical information needed by mobile stations to access the network; the extended broadcast control channel (E-BCCH), used to carry less time-critical information; and the SMS broadcast control channel (S-BCCH), used to carry broadcast teleservices.

• Shared channel feedback (SCF). The SCF is used to control access of mobile stations to the reverse DCCH to avoid collision. SCF provides information to mobile stations on when to initiate an access attempt, whether the access was successful, and whether the information has been received.

A fixed number of time slots is allocated for each superframe for F-BCCH, E-BCCH, S-BCCH, and SPACH, with the F-BCCH repeated at each superframe.

One logical channel is defined for the reverse DCCH, the random access control channel (RACH). The RACH is a point-to-point channel shared by all mobile stations used to access the DCCH. Mobile stations use the RACH in a contention mode, and collision avoidance is achieved through appropriate information provided on the forward DCCH.

5.3.2 Layer 2: The IS-136 Data Link Layer

The IS-136 data link layer provides the following set of functions:

• Addressing with mobile station ID (MSID).

• Error detection, recovery, and sequencing through monitoring of radio link quality, retransmission control, and CRC generation and verification.

• Media access control: Mobile stations access the network over the RACH in either a contention-based (random access) or reservation-based fashion. The base station controls the access through feedback on shared channels on the forward DCCH and random access on the reverse DCCH.

• Frame delimiting through header formatting, layer 3 message concatenation, layer 2 frame segmentation, and reassembly.

• Flow control on the forward and reverse DCCH.

The data link layer defines service access points and service primitives (i.e., the way layer 3 communicates with layer 2) and protocols for the logical channels on forward and reverse DCCH. The data link layer defines also the DTC in terms of the method of supervising the connection of the DTC to avoid collision between mobile stations on the same channel.

5.3.3 Layer 3: The IS-136 Network Layer

The IS-136 layer 3 or network layer provides a means to establish, maintain, and terminate connections between the mobile station and the network. The layer 3 message set includes messages transferred on both the DCCH and the DTC.

Layer 3 messages include information for registration, paging, DCCH structure, call setup/release, R-data transport for teleservices, and relay of higher-layer information for additional applications. Layer 3 messages are transported in layer 2 packets indicating the type of layer 3 information that is carried.

DCCH

DCCH is used to convey call setup information and to provide the platform for enhanced services through signaling and messaging. Figure 5-4 describes the layering of messages on the control plane.

Figure 5-4. IS-136 control plane layering.

The IS-136 digital control channel supports the following features:

• The sleep mode to preserve mobile station battery power

• Support of multiple vocoders to take advantage of improvements in voice technology

• The ability to seamlessly acquire the same services both in the cellular band (800 MHz) and the PCS band (1900 MHz)

• The support of teleservices to transfer application data to and from cellular phones

• A hierarchical macrocell-microcell environment providing support for microcellular operation

• Private and residential system identities providing the tools of wireless office service (WOS) operations

IS-136 DCCH occupies a full-rate digital channel (i.e., two time slots out of six). The DCCH is composed of a set of logical channels in the direction from the network to the mobile nodes (forward direction) and a single logical channel in the direction from the mobile node to the network (reverse direction). The logical channels in the forward direction are classified into broadcast channels and point-to-point channels, and the logical channel in the reverse direction is a point-to-point channel. Logical channels are introduced in the DCCH to organize the information flowing across the air interface (Figure 5-5):

Figure 5-5. Logical channels in DCCH.

1. Broadcast channel (BCCH): The BCCH is a downlink channel that provides continuous system information and the rules an MS needs to follow to access the system. The BCCH is divided in fast BCCH and extended BCCH. Fast BCCH is used for time-critical system information that needs to be sent at fixed repetition cycles, such as the system ID (SID) and information needed by the MS to access the system (e.g., network type, protocol version, mobile country code). E-BCCH is used to carry information that is less time critical and does not need a guaranteed rate (e.g., the neighbor cell list for MS mobility).

2. SMS paging and access channel (SPACH): SPACH provides MS with paging and system access parameter information. SPACH is divided in paging channel, used to transfer call setup pages to the MS; the access response channel, used to send system responses from the system to the MS (e.g., DTC assignment commands); and the SMS channel, used to transfer point-to-point teleservice data to and from MS.

3. Random access channel (RACH): RACH is a shared channel resource used by all MSs when attempting to access the system. RACH is an uplink channel used, for example, for MS response to an authentication request or to acknowledge delivery of a short message. RACH is used by MS to access the system either with a contention-based mechanism (i.e., several MSs try to get access to the system at the same time) or with a reservation-based mechanism (where the system indicates to MS which one is allowed to access and when). The system informs the MS of status of the access through the shared channel feedback.

4. Shared channel feedback (SCF): The SCF informs the MS of the status of uplink channels and controls and acknowledges the information sent by the MS uplink.

DTC OPERATIONS

The DTC allows two basic types of time division duplex (TDD) usage of a radio channel: half-rate, which allows six users to share the radio channel; and full-rate, which allows three users to share the same radio channel enabling a user data rate of 13 Kbps. Other channel usage consist of double and triple rate channels (i.e., where one user is assigned four or six time slots, therefore doubling or tripling the data rate available).

The DTC offers two methods of transferring control information: inband signaling, which replaces user data being transmitted, and out-of-band signaling, also called slow associated control channel (SACCH). SACCH is a continuous stream of signaling information sent besides the user data information. This means that transfer of SACCH information does not affect the transfer of user data since dedicated bits are used. However, transfer of information using SACCH is slow, and for rapid message delivery, the FACCH is used. FACCH implements in-band signaling by transporting FACCH information as a replacement to user data.

DTC performs other operations not described in this chapter, since they are not relevant to wireless data.