Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
2000-03-03
2004-11-16
Vu, Huy D. (Department: 2665)
Multiplex communications
Communication over free space
Repeater
C370S347000
Reexamination Certificate
active
06819658
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, SS7, ISDN or asynchronous transfer mode (ATM) based information, via wireless links. More specifically, the present invention relates to a method and apparatus for segmentation, reassembly and inverse multiplexing of packets and ATM cells over satellite and wireless links in a way that minimizes the requisite overhead contained in packet, cell and frame headers.
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 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-sized packets of information across a network from one communication device to another. The 53 octets are assembled as a “cell”, which comprises 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.
There are two forms of headers that are specified in the CCITT Recommendation I.361. Each form is 5 octets long. There also are two different ATM network connections, each one having a different type of header. One connection form is the user-network interface (UNI), which is used between the user installation and the first ATM exchange and also within the user's own network. The other form of connection is the network-node interface (NNI) which is used between the ATM exchanges in the trunk network. The header format for the UNI consists of the following fields:
Generic flow control (GFC) field of 4 bits. It can provide flow control information towards the network from an ATM endpoint.
Routing field of 24 bits. Eight of the bits are VPI (virtual path identifiers) and 16 bits are VCI (virtual channel identifier). Empty cells with both the VCI and VPI set to zero indicates that the cell is unassigned.
Payload type (PT) field of 3 bits. This field is used to provide information on whether the cell payload contains user information or network information. This field is used by the network to intercept any important network information.
Cell loss priority (CLP) field containing 1 bit. This field may be set by the user or service provider to indicate lower priority cells. If the bit is set to 1 the cell is at a risk of being discarded by the network during busy times.
Header error control (HEC) field of 8 bits. This field is processed by the physical layer to detect errors in the header. The code used for this field is capable of either single-bit error-correction or multiple-bit error-detection.
As seen in
FIGS. 1A and 1B
, the header format for the NNI is the same as the header information of the UNI except that there is no GFC, and the VPI of the routing field is 12 bits instead of 8 bits.
Error detection occurs only within the header message. No error detection of the data occurs within the information portion of the cell. The receiving endpoint determines whether the data can be corrected or whether it must be discarded. When a link or node becomes busy, an ATM network must discard cells until the problem is resolved. The first cells to be discarded are the cells that have a CLP bit in the header set to a “1”. Since connection endpoints are not notified when a cell is discarded, higher layers of protocols are needed to detect and recover from errors.
A cell is sent along a channel called a Virtual Channel Connection (VCC). A VCC consists of a series of links that establish a unidirectional connection through the network that allows the flow of information from one endpoint to another endpoint. Cells on a VCC always follow the same path through the network. Therefore, each cell arrives at its destination in the same order in which it was transmitted. VCCs can be unidirectional or may occur in pairs, thus making the connection bi-directional. VCCs can be within a Virtual Path Connection (VPC), meaning a group of virtual channels that are associated together, so as to be sent as a large trunk for a part of network. The VCCs are multiplexed and demultiplexed at appropriate network nodes in the network. Each VCC and VPC have specially assigned numbers called Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI), respectively. These numbers help the system determine the direction in which the cells belonging to the connection should be sent and which applications should be associated with the connection.
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 a 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, such as the satellite environment seen in
FIG. 2
, because most ATM traffic is transmitted over high rate data interfaces.
FIG. 2
illustrates the interconnection by a satellite relay between multiple terminals (Terminal 1 and Terminal 2, merely by way of example), using a time division multiple access (TDMA) network wherein bursts of information are sent to the satellite in a time divided manner for assembly and distribution to the terminals in a frame format.
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 Bit 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
Agarwal Anil K.
Bokar Udayan N.
Hariharan Moorthy N.
Patankar Shekhar V.
Comsat Corporation
Ryman Daniel
Vu Huy D.
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