Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-12-03
2004-04-06
Ton, Dang (Department: 2666)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
Reexamination Certificate
active
06717947
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates the transport of data packets through a network such as a serial bus or a packet switched network. More particularly, the invention relates to mechanisms which allow isochronous packets to be delivered with improved accuracy.
2. Description of the Related Art
Communication systems are often classified as employing time division multiplexing (TDM) or statistical multiplexing (SM). TDM systems assign to a transmitter one or more periodically available timeslots. The transmitter is then allowed to transmit information bits into a communication medium during its assigned timeslots. TDM systems allocate bandwidth to a given transmission channel and thereby guarantee a time of delivery for information transmitted thereon. SM systems involve transmitters which transmit data packets into a network. The data packets are passed along a set of communication links on a best-efforts basis. Depending on the protocol used, packets often may not arrive within a prespecified time. However, SM systems allow bandwidth to be more efficiently used because transmitters which are not transmitting useful data do not consume bandwidth.
TDM systems have some distinct advantages and disadvantages when compared with SM systems. TDM systems are inherently useful when a quality of service (QOS) is required. QOS refers to a communication system's ability to guarantee a specified transmission bandwidth, and time-of-arrival of data packets. In some cases QOS also involves using channels which provide a specified bit-error rate. For example, in a video conferencing system using leased lines, enough bandwidth is allocated to guarantee video and audio data will pass through the connection at all times with a specified level of quality. However, TDM systems are inherently inefficient when data transfer rates are considered from a network perspective. Instead of using network bandwidth only when useful data needs to be sent, the TDM transmitter occupies network bandwidth continuously.
Communication systems often involve data channels which transfer data packets using an “acknowledged protocol.” An example of an acknowledged protocol is the transport control protocol (TCP) as used in the Internet. An acknowledged protocol is designed to provide reliable point-to-point data transfer and thereby provides methods to retransmit data packets found to be in error. Error control codes such as CRC codes are often sent to allow a receiver to determine whether a received data packet contains errors. If the data packet does contain errors, a retry-request packet is forwarded back to the transmitter so the errant packet may be resent. An example of a non-acknowledged protocol is the user datagram protocol (UDP) as also used in the Internet. When a UDP packet is sent, no acknowledge is required. Rather UDP packets can be flooded across a network to one or more destinations using broadcast or multicast methods. A broadcast involves sending one or more packets to all nodes on the network. A multicast involves sending one or more packets to more than one node on the network, but not necessarily all of the nodes. When compared to TCP, UDP packet streams also consume less bandwidth because no acknowledge packets and no resent packets are associated with the UDP packet stream. Also, for certain data types such as real-time multimedia viewers and players, a late packet needs to be discarded just like an erroneous packet. Hence network bandwidth is conserved by not re-transmitting data which will only be later discarded at the receiving end. Also, UDP packets enable broadcast and multicast data transfer methods. The cost of using a non-acknowledged protocol such as UDP is the loss in reliability of data.
UDP datagrams are often used within protocols designed to provide data transmissions having guaranteed bandwidth and on-time delivery. The resource reservation protocol (RSVP) is one example. RSVP is a protocol employed by network routers to establish a path with guaranteed bandwidth and packet delivery times. RSVP and similar protocols are needed to insure multimedia data streams involving real-time voice and video data can be played by a media player at a receiving end of a connection with a specified QOS.
Variants of aforementioned modes of data transfer are also used in communications involving locally connected computers and peripherals. For example, the Institute of Electrical and Electronic Engineers (IEEE) 1394 specification entitled “High Performance Serial Bus” defines a communication system used to transfer data between locally connected equipment. For example, the equipment may include computers, digital video players, digital audio players, camcorders, remote control devices, and related equipment including computer mouse and keyboard interfaces. A system compliant with the IEEE 1394 specification will be referred to hereinafter as a “1394 system.” A given 1394 system involves a collection of nodes connected in a tree topology without any loops. Both computer backplane and cable based topologies are supported and can be mixed in a given system configuration. A full set of details regarding supported network topologies and configurations are documented in the IEEE 1394 specification.
The 1394 system includes two main types of bus accesses, namely “isochronous” and “asynchronous.” In the context of the 1394 specification, a “bus access” involves a data transfer. These two main types of bus accesses address different sets of needs experienced in locally connected computer system arrangements involving computer devices as well as multimedia devices. Isochronous transfers use a non-acknowledged protocol similar to UDP, and provide a guaranteed bandwidth and latency much like RSVP. Isochronous transfers are broadcast to all nodes in the 1394 system. These nodes pass the information stream on to one or more neighboring nodes. One or more selected nodes may be programmed to accept and process the isochronous transmission.
By definition, on an “isochronous” channel, a determined maximum bandwidth may be transmitted periodically within a repeating time interval. Isochronous channels guarantee a packet stream will be provided with enough bandwidth to meet that packet stream's needs, and that all packets will arrive at all receiving nodes within a specified jitter tolerance. By definition, an “asynchronous” bus access is one where packets or groups of packets are transmitted to a receiver, and an acknowledge packet is then sent back from the receiver to the transmitter. In an asynchronous protocol, when a receiver detects an errant packet, it transmits an error acknowledge back to the transmitter. The transmitter then resends the packet. In the 1394 system, asynchronous packets are sent using a best efforts access strategy whereby competing nodes arbitrate to gain access to the 1394 bus. Hence while asynchronous bus access have a guaranteed reliability, they do not provide a guaranteed bandwidth or delivery time. Also, unlike isochronous data streams which are broadcast to all nodes, asynchronous transfers involve point-to-point communications.
Isochronous bus accesses support data transfers which require guaranteed bandwidth and latency. This is equivalent to assuring a QOS in a communication network. Like TDM channels, isochronous transfers are assigned a channel which corresponds to a periodically available timeslot of specified duration. Unlike TDM systems, the beginning point of this timeslot within an isochronous cycle is not prespecified, only the fact that the timeslot will be available within the cycle is assured. In the 1394 specification, an isochronous cycle time is set at 125 us. As much as 80% of this isochronous cycle time may be assigned to isochronous bus accesses. The remaining 20% is dedicated to support asynchronous transfers as are described herein below. Timeslots are assigned in “bandwidth allocation units.” A bandwidth allocation unit corresponds to a 20 ns timeslot of which there are 6144 in a 125 us interval. Timeslots are alloca
Ghodrat Fataneh F.
Stein Michael R.
Thomas David A.
Carstens Yee & Cahoon LLP
LSI Logic Corporation
Ton Dang
LandOfFree
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