Common packet types

There are five common packets. In addition to the types listed in segment 1 of the previous table, there is the ID packet not listed. Each packet will now be examined in more detail.

4.4.1.1 ID packet

The identity or ID packet consists of the device access code (DAC) or inquiry access code (IAC). It has a fixed length of 68 bits. It is a very robust packet since the receiver uses a bit correlator to match the received packet to the known bit sequence of the ID packet. The packet is used, for example, in pag-ing, inquiry, and response routines.

4.4.1.2 NULL packet

The NULL packet has no payload and therefore consists of the channel access code and packet header only. Its total (fixed) length is 126 bits. The NULL packet is used to return link information to the source regarding the success of the previous transmission (ARQN), or the status of the RX buffer (FLOW). The NULL packet itself does not have to be acknowledged.

4.4.1.3 POLL packet

The POLL packet is very similar to the NULL packet. It does not have a pay-load either. In contrast to the NULL packet, it requires a confirmation from the recipient. It is not a part of the ARQ scheme. The POLL packet does not affect the ARQN and SEQN fields. Upon reception of a POLL packet the slave must respond with a packet. This return packet is an implicit acknowledgement of the POLL packet. This packet can be used by the master in a piconet to poll the slaves, which must then respond even if they do not have information to send.

4.4.1.4 FHS packet

The FHS packet is a special control packet revealing, among other things, the Bluetooth device address and the clock of the sender. The payload contains 144 information bits plus a 16-bit CRC code. The payload is coded with a rate 2/3 FEC which brings the gross payload length to 240 bits. The FHS packet covers a single time slot.

Figure 4.6 on page 56 illustrates the format and contents of the FHS payload.

The payload consists of eleven fields. The FHS packet is used in page master response, inquiry response and in master slave switch. In page master

response or master slave switch, it is retransmitted until its reception is

acknowledged or a timeout has exceeded. In inquiry response, the FHS packet is not acknowledged. The FHS packet contains real-time clock information.

This clock information is updated before each retransmission. The retransmis-sion of the FHS payload is thus somewhat different from the retransmisretransmis-sion of ordinary data payloads where the same payload is used for each retransmis-sion. The FHS packet is used for frequency hop synchronization before the piconet channel has been established, or when an existing piconet changes to a new piconet. In the former case, the recipient has not been assigned an active member address yet, in which case the AM_ADDR field in the FHS packet header is set to all-zeroes; however, the FHS packet should not be considered as a broadcast packet. In the latter case the slave already has an AM_ADDR in the existing piconet, which is then used in the FHS packet header.

Figure 4.6: Format of the FHS payload

Each field is described in more detail below:

Parity bits

This 34-bit field contains the parity bits that form the first part of the sync word of the access code of the unit that sends the FHS packet. These bits are derived from the LAP as described in section 13.2 on page 143.

LAP This 24-bit field contains the lower address part of the unit that sends the FHS packet.

Undefined This 2-bit field is reserved for future use and shall be set to zero.

SR This 2-bit field is the scan repetition field and indicates the interval between two consecutive page scan windows, see also Table 4.4 and Table 10.1 on page 101 Table 4.3: Description of the FHS payload

Parity bits LAP

Un-defined SR SP UAP NAP

LSB MSB

SP

This 2-bit field is the scan period field and indicates the period in which the mandatory page scan mode is applied after transmission of an inquiry response message, see also Table 4.5 and Table 10.6 on page 112.

UAP This 8-bit field contains the upper address part of the unit that sends the FHS packet.

NAP

This 16-bit field contains the non-significant address part of the unit that sends the FHS packet (see also section 13.1 on page 143 for LAP, UAP, and NAP).

Class of device

This 24-bit field contains the class of device of the unit that sends the FHS packet. The class of device has not been defined yet.

AM_ADDR

This 3-bit field contains the member address the recipient shall use if the FHS packet is used at call setup or master-slave switch. A slave responding to a master or a unit responding to an inquiry request message shall include an all-zero AM_ADDR field if it sends the FHS packet.

CLK_27-2

This 26-bit field contains the value of the native system clock of the unit that sends the FHS packet, sampled at the beginning of the transmission of the access code of this FHS packet. This clock value has a resolution of 1.25ms (two-slot interval). For each new transmission, this field is updated so that it accurately reflects the real-time clock value.

Page scan mode

This 3-bit field indicates which scan mode is used by default by the sender of the FHS packet. The interpretation of the page scan mode is illustrated in Table 4.6. Currently, the standard supports one mandatory scan mode and up to three optional scan modes (see also “Appendix VII” on page 999).

SR bit format b₁b₀ SR mode

00 R0

01 R1

10 R2

11 reserved

Table 4.4: Contents of SR field

SP bit format b₁b₀ SP mode

00 P0

01 P1

10 P2

11 reserved

Table 4.5: Contents of SP field

Table 4.3: Description of the FHS payload

The LAP, UAP, and NAP together form the 48-bit IEEE address of the unit that sends the FHS packet. Using the parity bits and the LAP, the recipient can directly construct the channel access code of the sender of the FHS packet.

4.4.1.5 DM1 packet

DM1 serves as part of segment 1 in order to support control messages in any link type. However, it can also carry regular user data. Since the DM1 packet is recognized on the SCO link, it can interrupt the synchronous information to send control information. Since the DM1 packet can be regarded as an ACL packet, it will be discussed in Section 4.4.3 on page 60.