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DCN Next Generation Open Interface Release 2.4 General

en | 24

Bosch Communications Systems | 2007 February | SRS_INF | Du010933

• Simultaneously transmission on both sides

This kind of problem arises when

both sides of the communication

line sent a message and one of

the messages fails due to

transmission errors (see Figure

4). In this case the badly

received request-message is

assumed to be an ACK-packet.

This packet will be ignored.

Then the actual ACK-packet is

received and accepted.

The remote controller still has a

message waiting for an

acknowledge. Therefore after

the time-out a retry of the

request-message transmission

will take place.

If, by some circumstances (as shown in Figure 4), a message is received correctly twice, the

sequence number must be checked to see if the difference between the previously

received message and this sequence is positive. If not, then the message may be

assumed to be received a second time.

• Receiving checksum error in expected ACK-packet

The receiver side of the communication line ignores the packet. Using this kind of error

occurs during a burst of errors on the communication line. By ignoring the packet, the

communication line gets time to recover from the errors burst. After a timeout on the

sending-side the packet will be retransmitted and shall be received correctly.

4.1.2.4.4 Heartbeat for connection checking

During the connection, we like to know if the connection is still valid. To control the validity of

the connection a heartbeat check is included in the protocol.

The heartbeat mechanism includes the transmission of sync-packets to the other side every 5

seconds. The receiver must check if he has received a message within 10 seconds. If not,

then the receiver may assume that the connection is lost. The heartbeat check time should be

reset whenever a message (of any type) is received.

Note that during many message transmissions the transmission of the heartbeat message

may be omitted, because any message received from the CCU is valid to be used as

heartbeat.

4.1.3 ‘Simple’ feature protocol

The ‘Simple’ feature protocol is meant to be used with small DCN NG systems. The protocol

is designed to use a minimum amount of memory on the CCU.

The ‘simple’ feature protocol handling incorporated the following features:

⇒ Header and Message data is checked against a checksum, which is sent directly after the

message (according the layout as described in §4.1.1).

⇒ On reception of a bad checksum the CCU will send a NAK-message.

⇒ A message is expected to be sent as one block. The CCU checks if each byte is received

within 50 ms of the previous byte. If not, a NAK will be sent.

⇒ Messages sent by the remote controller are not acknowledged, because each Remote

Function Call should result in a response.

CCU

Upd. MessageReq. Message

Controlling PC

ACK

Ignored

Retry of

Req. Message

Figure 4 Simultaneous transmission with error

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