Sending Data

Once in a connection, devices can send data to one another. This is done by sending data packets at connection events. Data packets are distinct from advertising packets because they are private communications between two devices rather than broadcast communications to any device that is listening. The biggest differences between advertising packets and data packets are the length of the payload that is possible and the packet header.

The length of the payload in a data packet can be anywhere from 0 octets in length to 31 octets. A zero-length payload in a data packet is an empty packet; it has no application data but can still include some information from the packet header. The maximum length payload

(at 31 octets) is smaller than the advertising packet’s maximum length. It is also only possible to have this large a packet if it is encrypted.

An unencrypted data packet has a maximum of just 27 octets of data in it to allow for the retransmission of a data packet, even after encryption has been established between the data packet being sent into the controller and encryption being enabled in the controller.

7.8.1. Data Header

The data packet header, as shown in Figure 7–20, contains just the following four fields:

• Logical link identifier (LLID)

• Sequence number (SN)

• Next expected sequence number (NESN)

• More data (MD)

Figure 7–20. The data packet header

There is no “packet is encrypted” bit because this is a modal property of the connection, just like for the adaptive frequency hopping or the connection event intervals.

7.8.2. Logical Link Identifier

The logical link identifier (LLID) is used to determine if the packet contains one of the following types of data:

• Link Layer control packet (11)—This is used for managing the connection

• Start of a higher-layer packet (10)—Or for a complete packet

• Continuation of a higher-layer packet (01)

If the packet is a Link Layer control packet, this is indicated by the logical link identifier, and this data is passed directly to the Link Layer control entity. The meaning of the data within this packet is therefore determined by the Link Layer control entity, as described in Section 7.10.

All other packets are to or from the host. The host is able to send packets larger than the maximum 27 octets of data that can be included in a single Link Layer data packet;

therefore, it is able to segment these. To do this, the packet is labeled as either a start of a higher layer data packet or a continuation of a higher layer data packet. This is illustrated in Figure 7–21, in which a very long higher layer data packet is split over three Link Layer data packets. The first data packet is labeled with an LLID of “start”, whereas the other two data packets are labeled with an LLID of “continuation”.

Figure 7–21. Packet fragmentation

This is interesting from two standpoints. First, the Link Layer doesn’t require knowledge of the ultimate size of the packet at the start of the packet. It is possible to send start,

continuation, continuation, ... continuation, continuation, before sending another start message. The number of continuation messages is not fixed at the start of the message.

This allows the second interesting side effect: it is possible to always send zero-length continuation messages without any impact on the higher layer data. This allows empty packets to always be sent, meaning that simple acknowledgement messages can always be sent as zero-length continuation messages. These zero-length continuation packets are known as empty packets.

7.8.3. Sequence Numbers

To enable the reliable transfer of data, all data packets have sequence numbers. The sequence number for each new data packet sent is different from the last data packet’s sequence number, with the first packet in a connection having a sequence number of zero.

This allows a receiving device to determine if the next packet that is received is a retransmission of the previous packet because the sequence number is the same or a transmission of a new packet because the sequence number is different.

In data packets, there is a single bit for the sequence number, starting at zero for the first data packet that is sent. The sequence number then alternates between one and zero for each new data packet that is sent by a device.

7.8.4. Acknowledgement

To perform acknowledgement of a data packet, a single bit is used. This is called the next expected sequence number bit. This informs the receiving device of the next sequence number that the transmitting device is expecting it to send.

If the packet received by a device has the sequence number zero, the next expected sequence number that it receives must be one; otherwise, the packet would have been

retransmitted. Therefore, it’s possible to signal if the packet received was received correctly or if the packet needs to be retransmitted. This is illustrated in Figure 7–22.

Figure 7–22. Sequence numbers

7.8.5. More Data

The final bit in the data channel packet header is the more data bit. This signals to the peer device that the transmitting device has more data ready to be sent. The peer receiving device upon seeing the more data bit set in a received packet should continue to communicate in this connection event. This automatically extends connection events while there is still data to be sent. It also quickly closes connection events for which there is no more data to be sent. The more data bit also allows a device that needs to save power to close the connection event gracefully and quickly by setting its more data bit to zero. The more data bit can therefore be used to enable lots of data to be reliably delivered in a very efficient manner by using as few transmitted packets as possible.

7.8.6. Examples of the Use of Sequence Numbers and More Data

The processing of sequence numbers, next expected sequence numbers, and more data bits is shown in Figure 7–22 and described in the following:

1. The master transmits its first packet by using the default sequence number of zero