Multiplex communications – Pathfinding or routing – Store and forward
Reexamination Certificate
1999-02-08
2002-11-19
Kizou, Hassan (Department: 2738)
Multiplex communications
Pathfinding or routing
Store and forward
C370S389000, C370S412000, C714S748000
Reexamination Certificate
active
06483845
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a packet transmitting/receiving apparatus for transmitting and receiving packets using a transmission medium, and also relates to a packet receiving apparatus.
BACKGROUND ART
For digital data transmission and reception, communication is usually performed using packets as units. When a transmitting device transmits a packet, a receiving device receives the packet and takes out data carried in the packet. In such data communications using packets as units, when a packet is received the receiving device may return an acknowledge (hereinafter designated as ACK) indicating the status of the received packet to the transmitting device, to enhance communication reliability. This ACK includes an error occurrence state during transmission, the condition of the receiving device, etc. as well as information indicating whether or not a retransmission is necessary.
One of the interfaces that perform such packet communication is the IEEE 1394 interface. IEEE 1394 is the next-generation high-speed serial interface for multimedia, standardized by IEEE (refer to High Performance Serial Bus P1394/Draft 8.0v2). Currently, development is under way to use this IEEE 1394 for digital A/V equipment.
IEEE 1394 provides two types of packet transport modes: isochronous packet transport mode for the transport of data that needs real-time transmission, and asynchronous packet transport mode for the transport of data, such as a control command, etc., that does not need real-time transmission. Of these mode, in the transport of normal asynchronous packets, the receiving device returns an ACK to the transmitting device for each received packet. using this ACK, the receiving device can request a retransmission. On the other hand, for special asynchronous packets used for transmission to all the devices connected to the same interface, an ACK cannot be sent. In this case, none of the receiving devices can request a retransmission.
In IEEE 1394, there are two kinds of asynchronous packets: a request packet transmitted to request an action and a response packet transmitted to return the result of the action requested by the request packet. In either case, when a packet is received, an ACK is returned to the transmitting device. These two kinds of packets are usually used as a pair, except the case where processing is completed by returning an ACK to a request packet.
FIG. 1
shows a sequence diagram illustrating the case where a first device
1
transmits a request packet to a second device
2
. When the first device
1
sends a request packet to the second device
2
, the second device
2
, upon receiving the packet, returns an ACK to the first device
1
. The ACK in this case can indicate a retransmission request, completion of reception (request being processed), completion of processing, etc. When the ACK indicates a retransmission request, the first device
1
retransmits the request packet, as shown in FIG.
1
. When ACK indicates the completion of the reception, this means that the second device
2
is processing the request, so that the first device
1
waits until the corresponding response packet is returned. When the ACK indicates the completion of processing, transmission of a response packet is not performed.
On the other hand, after transmitting the ACK indicating the completion of the reception, the second device
2
transmits to the first device
1
a response packet carrying the result of the processing requested by the request packet. Upon receiving the response packet, the first device
1
sends an ACK indicating a retransmission request or completion of processing. In this case also, if the ACK indicates a retransmission request, the second device
2
retransmits the response packet. Note here that since the response packet is not a packet requesting processing, an ACK indicating the completion of reception (request being processed) cannot be used.
In IEEE 1394, transmission and reception of a request packet and a response packet are performed as described above. However, in the case of the previously mentioned special request packets transmitted to all the devices connected to the same interface, ACKs are not transmitted.
IEEE 1394 specifies a timeout interval for detecting an interruption of processing during communications using such request and response packets. More specifically, when a request packet is transmitted, if a response packet is not received within a predefined time after receiving an ACK indicating the completion of the reception, then it is determined that the processing requested by the request packet has been interrupted because of some failure. The device that transmitted the request packet can then proceed to the next processing. On the other hand, the device that received the request packet must send a response packet within the predefined time. The default value of this predefined time is 100 ms, and the time setting can be changed arbitrarily.
On the other hand, IEEE 1394 provides a counter of about 25 MHZ, called a cycle time register, synchronized in all devices. As a function of the interface, it is guaranteed that the counter value is the same for all the devices. Therefore, the cycle time register can be used as a clock in the IEEE 1394 interface.
In IEEE 1394, where processing is performed using the request/response packet pair, there is a possibility that while waiting for a response packet after sending a request packet, a request packet or the like may arrive from another device. This necessitates increasing the capacity of a packet receiving buffer so that a plurality of packets can be received. Further, if the packet receiving buffer cannot be used because a new request packet has been received, an ACK indicating a retransmission request for a response packet will have to be transmitted. Because of such a retransmission request, a timeout may occur, interrupting the processing. Such interruption of the processing can significantly reduce communication reliability.
Furthermore, since the processing of a response packet cannot be carried out while processing the new request packet, an ACK requesting a retransmission will be returned in reply to the received response packet. In this case also, an interruption of processing may occur, as in the above case. Therefore, there has been a need for a packet transmitting/receiving apparatus that can receive a response packet reliably without causing an interruption in processing.
Further, while waiting for the expected response packet, if a new request packet is received from another device, multiple processes that require servicing will occur simultaneously. Accordingly, all devices are required to have the capability to carry out multiple processes, the resulting problem being that the processing and configuration of each device increase in complexity.
On the other hand, in IEEE 1394, a special type of packet may be used that is sent to all the devices connected to the same interface. This special type of packet is primarily used when transmitting important information, for example, to manage the entire interface. For this special type of packet, however, a retransmission request cannot be made using an ACK. As a result, when the packet receiving buffer cannot be used because it is used for other packets, for example, the special packet cannot be received. This degrades the reliability of the entire interface, and can render correct packet reception and transmission impossible.
As earlier stated, in IEEE 1394, the maximum time interval between the transmission of a request packet and the transmission of a response packet is predefined. If the corresponding response packet is not received within the predefined time, the device that sent the request packet determines that the processing has been interrupted. On the other hand, the device that received the request packet must send a response packet within the predefined time interval, but there can be cases where the device is unable to send a response packet within the predefined time, depending on the condition of the
Asano Makoto
Takeda Hidetoshi
Kizou Hassan
RatnerPrestia
Tsegaye Saba
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