Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Utility Patent
1997-12-18
2001-01-02
De Cady, Albert (Department: 2784)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
Utility Patent
active
06170075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to telecommunications systems and more particularly to a method and apparatus for improving the speed and quality of data communications through a packet switched network. The invention is particularly useful for enhancing the communication of real-time media signals, such as audio and video, through a congested and lossy network such as the Internet.
2. Description of the Related Art
The Internet is a world-wide network of computers and computer networks of a configuration well known to those in the art. The Internet operates according to a set of standard protocols known as Transmission Control Protocol/Internet Protocol (TCP/IP). Each protocol in the TCP/IP suite is designed to establish communication between common layers on two machines, or hosts, in the network. The lowest layer in the Internet is the “physical” layer, which is concerned with ensuring that actual bits and bytes of information pass along physical links between nodes of the network. The next layer is the “network” or “IP” layer, which is concerned with permitting hosts to inject packets of data into the network to be routed independently to a specified destination. The next layer in turn is the “transport” layer, which is concerned with allowing peer entities on source and destination hosts to carry on a conversation. Generally speaking, the IP and transport layers of the Internet are not concerned with the physical arrangement of the network, such as whether source and destination machines are on the same sub-network or whether there are other sub-networks between them.
The transport layer of TCP/IP includes two end-to-end protocols, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is a reliable connection-oriented protocol, which includes intelligence necessary to confirm successful transmission between the sending and receiving ends in the network. UDP, in contrast, is an unreliable connectionless protocol, which facilitates sending and receiving of packets but does not include any intelligence to establish that a packet successfully reached its destination. In general, UDP is used by applications that do not want TCP's sequencing or flow control and wish to provide their own.
According to the TCP/IP model, the TCP transport layer takes a data stream to be transmitted and breaks it up into independent connectionless segments or “datagrams.” TCP adds to each of these packages a 20 byte header, which includes overhead information such as a source port number, a destination port number and a sequence number designed to allow the receiving end to properly reassemble the datagrams into the original message. The transport layer then “passes” each of these packages to the IP layer.
The IP layer in turn adds another header to each package, providing additional overhead information, such as a source IP address and a destination IP address. The IP layer then transmits the resulting packages through the Internet, possibly fragmenting each package into pieces or as it goes. As the pieces of the package finally reach the destination machine, they are reassembled by the IP layer and passed to the transport layer. The transport layer then inserts the original datagrams in proper sequence in an effort to reconstruct the original data stream for use by the receiving process and ultimately by an end user.
As a computer network, the Internet thus serves to provide communication between two nodes, such as a local subscriber computer/modem (which may be referred to as the “source” equipment) and a remote computer/modem (which may be referred to as the “destination” equipment), for example. In practice, the source equipment packetizes a stream of useful data and adds to each packet the header information required by TCP/IP for transmission over the Internet. The source then forwards a sequence of these packets via a communications link to a network access server (often in the form of a “hub” or “router”) at the edge of the Internet.
The communications link from the source equipment to the network access server may take any of a variety of forms. As an example, if the source equipment is a connected to the public switched telephone network (PSTN), the communications link may consist of an unshielded twisted pair (UTP) of copper wires extending from the subscriber's modem to a telephone company central office, and then a T1 line extending from the central office to the network access server. As another example, if the source equipment is connected to a local area network (LAN), the communications link may consist of the LAN and then a transmission line extending from the LAN to the network access server. In that case, the source equipment may even have its own Internet address (IP address). Nevertheless, the source equipment should still be viewed as being connected “to” the Internet via a communications link, as it is connected via that link to a network access server at the edge of the Internet. In any event, this communications link is generally reliable, in the sense that little if any perceptible loss will result to data being carried by the link.
At the edge of the Internet, the network access server receives the incoming stream of data packets provided by the source and routes the packets onto the Internet for transmission to a remote location, or other edge, of the Internet. This network access server is commonly owned by an Internet Service Provider (ISP) organization. Due to the growing demand for Internet access, a network access server usually contains a plurality of modems or other circuitry arranged to simultaneously receive and process multiple incoming calls. In addition, the network access server often includes or is connected to a gateway, sometimes in the form of a discrete processor, for forwarding the packets onto the Internet. Of course, the network access server may take other forms as well, generally serving the function of passing data between the Internet and some external communications link (even if that external communications link is an offshoot of the Internet).
Ideally, the packets transmitted into the Internet by the network access server should arrive successfully at a remote edge of the Internet and pass to the specified destination equipment. Generally, similar to the source equipment as described above, the destination equipment is connected via a communications link to a network access server at the remote edge of the Internet. This communication link may be of the same or different type than that used to connect the source equipment. In any event, the destination equipment should ideally receive the transmitted IP packets, extract the payload from the packets and reconstruct an ordered data stream for use by an remote subscriber.
Unfortunately, deficiencies in the existing communication infrastructure have precluded the successful widespread transmission of real time media signals, such as digitized voice, audio and video, from end-to-end over the Internet. The principle reasons for this lack of success have been a limited bit rate in the communications link and, to a greater degree, a high rate of packet loss and delay in the Internet.
First, the conventional telephone circuit often used to carry packets between the source equipment and the network access server is limited in bit rate. In particular, according to Shannon's Law, which is well known in the art, the bit rate or capacity C in a transmission line having a bandwidth B and a signal-to-noise ratio SNR is defined as the product of B and log
2
(1+SNR). While the quality of transmission along a conventional telephone line may vary, the line typically has a bandwidth of about 3 kHz and a signal-to-noise ratio of about 30 dB. With these values, the line would be limited to a bit rate of about 30 kpbs.
In an effort to work with this bit rate limitation, the telecommunications industry has recognized that Internet users (and other modem users) are more likely to download complex media signals (such as audio
Borella Michael
Mahler Jerry
Schuster Guido M.
Sidhu Ikhlaq
3Com Corporation
Cady Albert De
Chase Shelly A
McDonnell & Boehnen Hulbert & Berghoff
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