Error detection/correction and fault detection/recovery – Pulse or data error handling – Error count or rate
Reexamination Certificate
2000-01-18
2002-11-05
Baker, Stephen M. (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Error count or rate
C714S774000
Reexamination Certificate
active
06477669
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the efficient and reliable transmission of packet or cell-based information, such as frame relay, SS#7, ISDN, Internet or asynchronous transfer mode (ATM) based information, via wireless links. More specifically, the present invention relates to a method and apparatus for the adaptive control of forward error correction codes associated with cell-based ATM formatted data and packet-based frame relay, SS#7, ISDN and Internet-formatted data for transmission over communication channels. While the present invention is applicable all of the foregoing types of transmission formats, the ATM format will be the exemplary focus of one preferred embodiment for purposes of providing an enabling disclosure, written description and best mode for the present invention.
BACKGROUND OF THE INVENTION
There are a variety of methods for transmitting information via a broadband Integrated Services Digital Network (B-ISDN), using a variety of protocols related to Asynchronous Transport Mode (ATM), frame relay mode, ISDN, Internet and SS#7 modes of transmission. The ATM mode, as the exemplary preferred embodiment, was originally investigated by a group called the International Telephone and Telegraph Consultative Committee (CCITT). The group, currently called the International Telecommunication Union-Telecommunications Standards Sector (ITU-TSS), investigated a new form of ISDN that would have the flexibility to accommodate a large number of channels and the ability to transfer large amounts of data at a very fast rate. At the end of the study, the committee decided to adopt ATM as the target transfer mode for the B-ISDN. The ITU-TSS is currently defining the wide area network (WAN) standards for ATM.
ATM is a transfer mode that sends 53 octet packets (also known as cells) of information across a network from one communication device to another. The 53 octets are comprised of 48 octets of data information, referred to as the payload, and 5 octets of header information (including the routing information). The header and data information must be organized into cells so that when the cells are filled, they can be sent when an open slot of 53 octets becomes available.
Although ATM based transmission, switching, and network technology has been employed in broadband integrated services digital networks (B-ISDN) which rely on fiber optics, there are numerous difficulties associated with implementing ATM based technology in a wireless communication network. These difficulties include the fact that ATM based networks and switches rely on a number of high speed interfaces. These high-speed standard interfaces include OC-3 (155 Mbit/s), OC-12 (622 Mbit/s) and DS3 (45 Mbit/s). However, a few ATM based networks and switches support lower speed interfaces, such as T
1
(1.544 Mbit/s) and the programmable rate RS-449 interface.
As a consequence, there are only a few interfaces which can support the comparatively low transmission rates (less than 1 Mbit/s to 8 Mbit/s) used in wireless communication. Although commercial satellite and wireless modems support these low transmission rates using an RS-449 programmable rate interface, it is difficult to implement ATM based technology in a wireless environment using conventional interfaces because most ATM traffic is transmitted over high rate data interfaces.
Another difficulty associated with implementing ATM based technology in a wireless communication network has to do with the fact that ATM based protocols rely on extremely low bit error ratios which are typical of fiber optics based networks. By way of example, ATM protocols assume that the transmission medium has very low Byte Error Ratios (BER) (10
−12
) and that bit errors occur randomly.
In contrast, the bit error ratios associated with wireless communication are much higher (on the order of 10
−3
to 10
−8
) and tend to fluctuate in accordance with atmospheric conditions. In addition, the errors associated with wireless communication tend to occur in longer bursts. Thus, a robust error correction scheme must be employed in a wireless network in which ATM based technology is to be implemented.
In addition to the difficulties discussed above, there is another significant constraint placed on wireless communication networks which is not imposed on terrestrial based fiber optics networks. This constraint has to do with the fact that the cost of bandwidth in a wireless network is much higher than for fiber optics networks. As a consequence of having been traditionally implemented in fiber optics networks, ATM based technology is not particularly efficient in its use of transmission bandwidth. Therefore, if ATM based technology is to be implemented in wireless networks, it must achieve a more efficient use of bandwidth.
Traditionally, wireless and satellite communication systems used Forward Error Correction (FEC) Codes, such as Viterbi codes and Reed-Solomon codes, to improve the bit error characteristics of wireless and satellite links because such links are inherently noisy. The quality of wireless or satellite links vary with atmospheric conditions such as rain, fog and snow.
One problem with the use of FEC codes is that they reduce the available bandwidth by a certain percentage. The magnitude of the reduction depends on the particular code selected and the rate of the code. By way of example, a rate ½ Viterbi code consumes 50% of the available bandwidth. Often, the codes are selected for a given link based on a worst case scenario and are fixed for the link. Although fixed coding simplifies the design and implementation of a particular error correction scheme, it results in a severe loss of available bandwidth.
Other primary access interfaces that confront the same problem include the “frame relay” system (for LAN interconnection and Internet access) using TCP/IP or other LAN protocols (ISDN and SS#7) and Internet protocols. Considerations similar to those for ATM are relevant to the transmission of traffic using these other interfaces, as exemplified by the transmission of frame relay, ISDN/SS#7 and Internet traffic over satellite/wireless networks, although some differences are known in the art.
For example, unlike ATM cells, the frame relay, ISDN/SS#7 and Internet use packets that are variable lengths. Thus, the frame formats used to communicate between the satellite/wireless terminals are arranged to transport variable length packets efficiently.
As explained in the Provisional Application Ser. No. 60-052,359, which is incorporated herein by reference, the frame relay system uses a robust, flexible frame format between two communicating terminals which allows the transport of several variable sized Spackets (segmented packets) in a frame and also allows a single Spacket to be carried over several frames, whichever the case might be. Also, the frame format allows fast synchronization and the exchange of coding information. Each frame contains Reed-Solomon (RS) check bytes that are used for error correction and to enhance the quality of the satellite/wireless link. The number of RS check bytes in a frame can be changed on the fly, without any loss of data, to compensate for varying link conditions. The decision to change the RS check bytes in a frame is based on the constant monitoring of the link quality. Several frames are also interleaved before transmission over the satellite/wireless link, to help spread the effect of burst errors over several frames, all of which can then be corrected by the FEC in the frames.
Also, Virtual Channels (VCs) can be configured to be enabled for data compression, which means that the Spackets belonging to the VC are passed through a data compressor/decompressor combination to save bandwidth. VCs can also be configured to be either high or low priority VCs and the scheduler then, uses this information to fairly transmit the various Spackets over the satellite/wireless link. To minimize the large delays introduced by the transmission of low priority packets on a low bit rat
Agarwal Anil K.
Chitre Dattakumar M.
Baker Stephen M.
Comsat Corporation
Sughrue & Mion, PLLC
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