Basic MAC Message Formats
MAC deals with all the information in three different formats. First, there are ten logical channels that are defined for different kinds of data transfer services offered by the MAC entity. Each logical channel type is defined by the type of information it carries and the interpretation of the values in the corresponding messages. Logical channels can be considered to operate between logical connection end points and between logical entities. They are referred to with four-letter abbreviations, e.g. BCCH. The names of the logical channels will mostly be used when message contents and their meaning are addressed and the names of the transport channels should reflect message lengths, rules to assemble a MAC frame, and access methods. In some other systems, logical channels are called simply messages with various message types and subtypes.
The logical channels are mapped onto transport channels that are referred to with three-letter abbreviations, e.g. BCH. They describe the basic message format, while the message contents and their interpretation are subject to the logical channels. Thus the transport channels are just a kind of link in information delivery from higher layers to the physical layer. They are useful in describing the mapping from logical channels onto radio bursts the physical layer deals with. “Logical Channels,” Later in this chapter, describes the most important logical channels.
Transport channels are concatenated to construct PDU trains. PDU trains are used in data exchanges with the physical layer. The PHY layer maps the PDU trains provided by the MAC onto the PHY bursts. Six different kinds of PDU trains are defined to be used in different parts of the MAC frame. “Transparent Channels” provides a more detailed description of the transport channel.
All ten logical channels are listed in Table 7.1 and all six transport channels are listed in Table 7.2. The logical and transport channels are listed in Tables 7.1 and 7.2, respectively, as they appear within the frame. Figure 7.4 illustrates all the possible mappings between logical and transport channels for the downlink (DL), the uplink (UL), and the direct link (DiL) cases separately. The six PDU train types are listed in Table 7.3.
Figure 7.4. Mapping between logical channels and transport channels for (a) the downlink, (b) the uplink, and (c) the direct link.
| Transport Channel | Abbreviation | DirectionDL/UL/DiL |
|---|---|---|
| Broadcast Channel | BCH | DL |
| Frame Channel | FCH | DL |
| Access Feedback Channel | ACH | DL |
| Long Transport Channel | LCH | DL/UL/DiL |
| Short Transport Channel | SCH | DL/UL/DiL |
| Random Channel | RCH | UL |
| PDU train | Transport Channels | Direction DL/UL/DiL |
|---|---|---|
| Broadcast PDU train | BCH/FCH/ACH | DL |
| FCH-and-ACH PDU train | FCH/ACH | DL |
| Downlink PDU train | SCH/LCH | DL |
| Uplink PDU train with short preamble | SCH/LCH/RCH | UL |
| Uplink PDU train with long preamble | SCH/LCH/RCH | UL |
| Direct link PDU train | SCH/LCH | DiL |
Transport Channels
The transport channels are the basic elements to construct PDU trains and they describe the basic message format. In the case of the RCH, the random access method and the collision resolution scheme are also properties of the transport channel. Transport channels carry a fixed amount of data, except the FCH that can carry a variable amount. A short description of each transport channel and their characteristics are given in the following. Their characteristics are given in Table 7.4.
| BCH: | The BCH carries only the BCCH and is broadcasted in downlink direction. Its length is fixed and equal to 120 bits. One BCH is sent per MAC frame per sector antenna. |
| FCH: | The FCH carries only the FCCH and is broadcasted in downlink direction. The FCH comprises of fixed size information element (IE) blocks. Every block contains three IEs, each with a length of eight octets, and a 24-bit CRC calculated over the three IEs of the block. The FCH structure is shown in Figure 7.5. |
| IEs carry resource grants in downlink direction. Resource grants are used to communicate resource allocation information to mobile terminals and are responses to resource requests from the terminals. All the IEs are discussed in more detail later in the chapter describing the logical channels. The access point or centralized controller determines the number of the blocks on the basis of the resource allocation status in the frame. | |
| ACH: | The ACH is used for sending RFCH in downlink direction. It has a total size of nine octets. The format is identical to the SCH format described below. ACHs are identified by a type field with binary coding 0110 at its beginning. |
| LCH: | The LCH transports user data for the connections related to the granted user channels (UDCHs, UBCHs, and UMCHs). Further, it conveys control information for the connections related to the DCCH and RBCH. The LCH consists of 54 bytes. The two first bits indicate the LCH type. Only two types are defined: one for the logical channels mapped onto the LCH and another for a dummy LCH. The actual payload is always fixed in length and equals to 406 bits. The last three bytes of the LCH are for 24-bit CRC. |
| SCH: | The SCH carries short control information for three control channels: DCCH, LCCH, and RBCH. It consists of nine bytes of which the first four bits are to determine the type of information in the channel. The following 52 bits are for information delivery and the last two bytes are for 16-bit CRC. |
| The SCH is primarily used to convey resource requests for both uplink and direct link, and to deliver RLC messages both in downlink and uplink. It is used also as a feedback channel in automatic repeat request (ARQ), and to deliver encryption information in downlink direction. | |
| RCH: | The RCH is used by a terminal to send control information to the access point or centralized controller when it has no granted SCH available. It consists of nine bytes and its format is identical to the one of the SCH. Only a subset of the SCH message types are defined for RCH transport. It is used to convey resource requests for both uplink and direct link. Further, it can be used to convey RLC messages to the access point or centralized controller. |
Figure 7.5. Data field structure of FCH frame.
| Transport Channel | PHY Mode | Length [bytes] |
|---|---|---|
| BCH | BPSK, r=1/2 | 15 |
| FCH | BPSK, r=1/2 | Na*27 |
| ACH | BPSK, r=1/2 | 9 |
| LCH | Set in FCCH | 54 |
| SCH | Set in FCCH | 9 |
| RCH | BPSK, r=1/2 | 9 |
| a.N is an integer. | ||