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2. INTERFACES
8 2- DPT100-B/I/BT
2.3.1.2 IrLAP
The IrLAP protocol provides:
•
Management of communication processes on the link between devices.
• A device-to-device connection for the reliable, ordered transfer of data.
• Device discover procedures.
• Hidden node handling.
Figure 2-6 identifies the key parts and hierarchy of the IrDA protocols. The bottom layer is the Physical layer,
IrPHY. This is the part that converts the serial data to and from pulses of IR light. IR transceivers can’t transmit
and receive at the same time. The receiver has to wait for the transmitter to finish sending. This is sometimes
referred to as a “Half-Duplex” connection. The IR Link Access Protocol (IrLAP) provides the structure for packets
or “frames” of data to emulate data that would normally be free to stream back and forth.
X BOFs BOF A C I FCS
EOF
(1+N) of C0h
Payload 2 bytes
C1h
Figure 2-7 shows how the IrLAP frame is organized. The frame is proceeded by some number of Beginning of
frame characters,(BOFs). The value of the BOF is generally $C0, but 0xFF may be used if the last BOF
character is a $C0. The purpose of multiple BOFs is to give the other station some warning that a frame is
coming.
The IrLAP frame begins with an address byte (“A” field), then a control byte(“C” field). The control byte is used
to differentiate between different
types of frames and is also used to count frames. Frames can carry status, data, or commands. The IrLAP
protocol has a command syntax of it’s own, and these commands are part of the control byte. Lastly, IrLAP
frames carry data. This data is the information or “I” field. The integrity of the frame is ensured with a 16-bit
CRC, referred to as the Frame Check Sequence (FCS). The end of the frame is marked with an EOF
character which is always a $C1. The frame structure described here is used for all versions of IrDA protocols
used for serial wire replacement for speeds up to 38400 baud.
In addition to defining the frame structure, IrLAP provides the “housekeeping” function of opening and closing
connections, and maintaining connections once they’ re open. The critical parameters that determine the
performance of the link are part of this function.
These parameters control how many BOFs are used, identify the speed of the link, how fast either party may
change from receiving to transmitting, etc. IrLAP has the responsbility of negotiating these parameters to the
highest common set so that both sides can communicate as fast and as reliably as possible.
2.3.1.3 IrLMP
The IrLMP protocol provides:
•
Multiplexing of the IrLAP layer. This allows multiple channels above an IrLAP connection.
• Protocol and service discovery. This is via the Information Access Service (IAS).
When two devices that contain the IrDA standard feature are connected, there is generally one device that has
something to do, and the oder device has the resource to do it. For example, a laptop may have a job to print
and an IrDA standard compatible printer has the resources to print it. In IrDA standard terminology, the laptop is
a Primary device and the printer is the Secondary device. When these two devices connect, the Primary device
must determine the capabilities of the Secondary device to determine if the Secondary device is capable of
doing the job. This determination is made by
the Primary device asking the Secondary device a series of questions. Depending on the answers to these
questions the Primary device may or may not elect to connect to the Secondary device.
The queries from the Primary device are carried to the Secondary device using IrLMP. The responses to
these queries can be found in the Information Access Service (IAS) of the Secondary device.
The IAS is a list of the resources of the Secondary device. The Primary device compares the IAS responses
with its requirements and then makes the decision if a connection should be made.
(Fig.2.7)