Multiplex communications – Communication over free space – Using atm as a wireles protocol
Reexamination Certificate
1999-06-21
2003-05-06
Patel, Ajit (Department: 2664)
Multiplex communications
Communication over free space
Using atm as a wireles protocol
C370S333000, C370S395100, C714S746000
Reexamination Certificate
active
06560206
ABSTRACT:
TECHNICAL FIELD
This invention relates to methods of transmitting data over a communications link and to cell-based methods of transmitting data over a communications link and to corresponding systems, apparatus and software for transmitting or receiving data and in particular to transmitting such data over a communications link which is subject to a high error rate, such as a wireless link.
BACKGROUND OF THE INVENTION
Cell-based protocols for transmission systems are well known. Cells (also called packets) are sent individually, and routed over a network using addressing information in the cell. Usually the data needs to be fitted into a number of such cells. Some protocols specify variable length cells, for example IP, and others specify cells of fixed length, such as ATM. Some are connection oriented, such as SNA, ATM, X.25 and frame relay. Others are connectionless, for example IP.
One known cell-based protocol, the Asynchronous Transfer Mode (ATM) protocol, will now be discussed in more detail. The ATM protocol is designed for data transfer over high speed, low error rate digital networks for multiple service types. ATM has generally been considered unsuitable for wireless transmission due to its low tolerance to errors.
ATM Adaptation Layers (AALs) shape the service data for the ATM protocol and provide error protection characteristics dictated by the properties of the transmission medium and the service. There are currently a number of different AALs specified, each with different error handling features. For example AAL 5 includes error detection information, though not correction information, at the frame level, but none is provided at the cell level, at least for the cell payload. This error protection is achieved by concatenating each data unit with a header and trailer, resulting in a variable length frame structure. The frame is then segmented into cells for transmission by the ATM protocol.
ATM is a transmission protocol based upon asynchronous time division multiplexing using fixed length data packets or cells. These cells typically have a length of 53 bytes: each cell containing 48 octets of user data (payload) and 5 octets of network information (header). Cells of a length of 55 bytes are also known, but in less frequent use. The ATM protocol segments data into cells. The header of a cell contains limited header error correction information. The header of a cell also contains address information which allows the network to route the cell. The address information is made from a concatenation of the Virtual Path Identifier and the Virtual Channel Identifier. These two fields require 28 bits (3.5 octets) of information.
FIG. 1
shows an example of an ATM cell header.
ATM communications technology will have an important part to play in the evolution of global communications networks, especially the internet. Considering trunk communication, each trunk link will be used to carry several different types of traffic, the two most common traffic types being generally known as voice and data. Voice traffic can contain errors and still be understood to a reasonable quality due to the brain's ability to cope with noise. The key criterion of voice is that it cannot withstand large variations in delay. Data traffic, such as a file transfer, can tolerate extreme delays but cannot tolerate lost or corrupt information. In general these two different types of traffic have been carried by different networks operating significanty different protocols. More recently Asynchronous Transfer Mode (ATM) has provided a common network protocol for these two traffic types.
One characteristic of ATM is the provision of an idle cell. Some data sources exhibit a variable bit rate and in order to support the transmission data from such sources it is sometimes necessary, when the bit rate decreases, to transmit empty cells, referred to as idle cells. This enables ATM to transport traffic with a wide range of characteristics.
ATM has been designed to operate over low error-rate trunk networks, which generally use reliable optical communication techniques, and assumes that the data traffic suffers a low error rate. The international Telecommunications Union (ITU) has recommended that erroneous frames be recovered by retransmissions of the entire frame in preference to individual cell retransmissions. Assured services carry significant disadvantages because of the retransmission system, including prolonged buffering at the transmitter, prolonged buffering at the receiver, complex protocol acknowledgement structures, and increased latency from acknowledgement messages and data retransmission.
Wireless communication is becoming one of the most popular commercial methods for providing access and trunk communications. With the recent launch of Low Earth Orbit (LEO) satellites, there are now many applications within commercial and military environments that may use terrestrial or satellite-based wireless communication links. The problem with wireless communication is that the fundamental error rates are significantly greater than those experienced in wireline systems. Wireless systems must cope with a harsher signal propagation environment that is subject to noise, interference, fading and delay. This is further compounded by the restrictions on the power levels at which wireless systems can operate. Mobile handsets and orbiting satellites are restricted in their transmit power by battery life, and wireless systems are generally constrained by regulatory limits on transmit power.
Wireless communication suffers errors within the traffic and the distribution of errors is uneven. The error rate has an underlying random independent bit error rate, overlaid by a burst error rate. Whilst ATM has therefore generally been considered unsuitable for wireless transmission, some attempts have been made to adapt ATM for transmission over wireless systems.
U.S. Pat. No. 5,568,482 (Li et al., assigned to Yurie Systems Inc.) describes a low speed radio link system for ATM transport. An incoming stream of ATM cells intended for transmission over the radio link is segmented into a plurality of subframes, each subframe carrying a plurality of ATM cells and having additional framing bytes. One example uses nine subframes, each carrying five ATM cells. The structure of this protocol allows synchronisation to be more easily maintained under burst error conditions on the link.
U.S. Pat. Ser. No. 09/222,557 (Bentall, assigned to Nortel Networks Corporation) describes a technique for improving ATM operation over a communications link whereby bandwidth is improved on a communications link by sending a header that supports fewer addresses. An 8 bit address field permits 256 different addresses. This header is associated at both ends of the link such that the original header can be removed, the packet is associated with one of the 256 available channels, and the original header is reconstructed at the far end. By doing this some bandwidth is gained which can be used to improve the quality of the link.
Radio channels such as those found in access networks and radio links typically exhibit bit-error rates (BER) of 10
−3
to 10
−5
. These error rates are too high for ATM transmission.
The solutions available in the open literature typically rely on a combination of forward error correction (FEC) and backward error correction (BEC or ARQ). This results in excessive cell delay variation. Furthermore, they often allow an increase in transmitted bit-rate, which restricts the choice of switch/radio combination, as the link-enhancing device is no longer transparent in this sense. It is believed that this occurs in several link accelerators for satellite radio links. However, as ATM has been proposed for the next generation of military communications, some form of link hardening is seen as essential in military radio links for both terrestrial and satellite applications.
OBJECT OF THE INVENTION
The present invention seeks to provide a more reliable method for transmitting data packets over a communications link such as
Naden James M
Tait Mark W
Barnes & Thornburg
Jain Raj
Patel Ajit
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