X25AM Programming Manual

PTP Applications

X25AM Programming Manual—527201-001

3-6

D, Q, and M Bits

The Q and M bits are used for data-exchange modes 1, 2, and 3. The D bit is used for

data-exchange mode 2. The bits are located in the MCW and need to be set properly

when data is written. The bits are translated by PTP to packet D, Q, and M bits in the

packet or packets that contain the data.

Delivery (D) Bit

If the D bit is set to 0 in an outgoing data packet, then the local DCE can return P(R),

the next expected packet sequence number. Some networks provide the facility to

specify that P(R) comes from the remote DTE instead. If the D bit is set to 1, then this

facility is used. The D-bit facility does not function as an end-to-end protocol.

Qualifier (Q) Bit

Use of this bit is left to the user. It is typically used to determine whether the data is

actually data or some other kind of information, such as PAD control data. The

accepted convention is that setting it to 0 indicates data and setting it to 1 indicates

some other kind of information.

More (M) Bit

The M bit indicates whether the current data is the end of the logical record or more

data will follow. If it is set to 1, then there is more data for the logical record. If it is set

to 0, then the current data is the end of the logical record.

Table 3-2. The Basic Differences Among Data-Exchange Modes

Mode Data Exchange and Your Application

0 Data exchange is done at the logical record level. Your application has no control

over anything except data. Used to send and receive data in simple READ/WRITE

transfers.

1 Data exchange is done at the packet level for both data and interrupt packets. Your

application has control over data and the Q and M bits.

Used to send and receive data and interrupt packets. Only 1 packet of data will be

sent. Data that does not fit in 1 packet is discarded.

2 Data exchange is done at the packet level for almost all kinds of packets. Your

application has almost complete control over packet content on the virtual circuit in

use.

Used to send and receive most types of packets.

3 Data exchange is done at the packet level for both data and interrupt packets. This

is similar to mode 1, except that the file-system buffer can hold the contents of

multiple packets. Your application has control over data and the Q and M bits.