Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1999-02-26
2003-03-11
Nguyen, Steven (Department: 2665)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S474000, C710S024000, C710S308000
Reexamination Certificate
active
06532232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to computer systems, and more particularly to methods for efficiently transmitting isochronous data between a computer system and a peripheral.
2. Description of the Related Art
Modern computer systems require a communication link to connect various subsystem components to other subsystem components. The various subsystem components such as a processor, memory, and input/output (I/O) devices are coupled to the bus for communication. Typically, the communication link is a bus, which includes a set of wires coupling the subsystem components and the host computer.
Buses broadly fall into two categories: parallel bus and serial bus. The parallel bus transmits a set of data bits in parallel over parallel lines. Thus, the parallel bus generally provides a fast data transfer rate. However, due to physical factors such as cross-talk, clock skew, and synchronization problems, the use of parallel bus has been limited to short distances.
On the other hand, the serial bus transmits data serially over a longer distance than the parallel bus by using a cable to communicate serial data. The serial bus is especially useful for connecting a computer system to a variety of peripheral devices that have a wide range of data bandwidth requirements. Currently several standards for serial data transmission exist, such as Ethernet, Localtalk, RS-422, Universal Serial Bus (USB), etc. Typically, these types of serial transmission systems can transmit information up to about 10 megabits per second. More recently, the computer industry has been driven toward a faster serial data transmission standard, especially in communicating with peripheral devices.
In 1995, the Institute of Electrical and Electronic Engineers (IEEE) approved a serial bus standard for a high-speed serial data transmission architecture. The standard is known as 1394-1995 IEEE Standard for a High Performance Serial Bus, which is incorporated herein by reference. The IEEE 1394 bus is often referred to as FireWire™. The purpose of the 1394 standard is to provide a high-speed low cost serial bus for use as a peripheral bus or a parallel back-plane bus.
The IEEE 1394 serial bus standard provides significant advantages over conventional bus standards. One advantage of the 1394 standard is the ability to transmit data over a cable medium at variable speeds, including very high speeds. For example, transceiver chip sets for the 1394 standard are now running at speeds of up to 400 Mbps, and many companies anticipate reaching speeds of up to one Gbps (gigabits per second). The 1394 cable comes in two versions, a and b, and starts at 400 Mbps (megabits per second) to 1, and up to 1.6 Gbps. Due to its high transmission rate, the IEEE 1394 bus is projected to be the standard cable to connect high speed drives that are connected by parallel cables such as IDEs and SCSIs. The basic clock frequency of the 1394 standard is 24.576 MHz, and data is transmitted in multiples of 24.576 MHz.
Another advantage of the 1394 cable is that it allows both data and power to be transmitted such that simple, low power devices can be powered directly from the cable. To implement the dual functions of carrying data and power, the 1394 cable contains two twisted pairs of wires for carrying data signals and two additional wires for power. Furthermore, the 1394 cable can connect two components up to about 15 meters apart without a repeat assignment. In addition, it allows daisy chain connection of subsystem components.
The IEEE 1394 serial bus standard also supports a variety of protocols such as IP, ethernet, SCSI, digital audio, digital video, etc. The support for digital audio and digital video protocols, in particular, allows the IEEE 1394 serial bus to be used in communicating digital audio and/or video (A/V) data between a computer system and consumer electronic products. For example, the IEEE 1394 bus can be used to communicate digital A/V data between a host computer system and conventional consumer electronic devices such as camcorders, DVD players, CD players, digital cameras, HD TVs, etc.
FIG. 1
shows a computer system
100
including a host computer
102
coupled to a camcorder
104
. Thee host computer
102
is coupled to the camcorder
104
via an IEEE 1394 serial bus
106
. In this arrangement, the host computer
102
transmits digital A/V data to the camcorder
104
over the IEEE 1394 bus
106
. The camcorder
104
receives the A/V data for display. Alternatively or simultaneous with displaying the A/V data, the camcorder
104
may record the received A/V data on a recording medium for storage.
The transmission of digital A/V data over the IEEE 1394 serial bus between a host computer and a peripheral electronics device is generally carried out in isochronous transactions. In isochronous transactions, a peripheral device is provided a guaranteed access to the bus at specific time intervals. Specifically, IEEE 1394 serial bus A/V devices transmit data in the form of data packets commonly known as Common Isochronous Packets (CIPs). Using these data packets, the IEEE 1394 serial bus allows a peripheral device to transmit/receive a CIP via the bus at 125 &mgr;s intervals or 8,000 times per second.
FIG. 2
illustrates a structure of a CIP
200
for transmitting A/V data over an IEEE 1394 serial bus. The CIP
200
includes a CIP payload
202
and a CIP header
204
. The CIP payload
202
corresponds to the payload portion of the packet that contains data defined by an application layer. On the other hand, the CIP header
204
includes various header fields including a format dependent field (FDF)
206
.
The FDF
206
includes a plurality of fields. One of the fields in the FDF
206
is a synchronization time (SYT) field
106
. The SYT field
106
in the CIP header
104
is used to store a presentation time stamp for the associated CIP
200
. The presentation time specifies the time at which to present the packet data for display in the case of a video data or for play in the case of an audio data.
The IEEE 1394 serial bus is often used in computer systems that operate under an operating system, (e.g., Microsoft® Windows 95®, Windows 98®, and the like), which may provide preemptive multitasking environment. In such an environment, transmission of CIP based data from a host computer to a peripheral A/V device may present several problems due to the real time requirements of A/V protocols. More specifically, the A/V protocols were originally specified to facilitate transfer of A/V data between consumer electronic devices such as VCRs, TVs, camcorders, and the like. These consumer devices typically include a micro-controller that provides the customized hardware support for the A/V protocols. For instance, the micro-controllers provide time-stamp, synchronization, and other support for the real time transmission and presentation of A/V data.
Within a computer system, however, the real time A/V data processing requires substantial processor time to set up and transmit CIPs. For example, in a preemptive multitasking environment, the host computer typically computes and creates a presentation time in advance of the transmission of a data packet. On the receiving end, the A/V devices generally include a high-resolution timer, which allows creation and attachment of SYT field data on-the-fly. For proper synchronization, the host computer adjusts the presentation time during the transmission of a series of CIP data packets to ensure that it matches the data rate expected by an A/V device receiving the data packets. In general, the transmission rate of the full CIP data packets exceeds the expected data rate. Hence, if the presentation time of the SYT field
106
is not properly matched with the expected data rate, the A/V device may exhibit jitter during the display of video data or during the play of audio data.
To reduce such jitter effect, conventional techniques have sought to match the transmission data rate with the expected data rate. To that end, these approaches have typicall
Adaptec, Inc.
Nguyen Steven
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