Method of re-synchronizing a group of B channels in an ISDN...

Multiplex communications – Diagnostic testing – Fault detection

Reexamination Certificate

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C370S394000, C370S524000, C370S904000

Reexamination Certificate

active

06560201

ABSTRACT:

TECHNICAL FIELD
This invention relates to an ISDN network interface on networking equipment. More particularly, the invention applies to the re-synchronization of a group of B Channels in an ISDN network. Still more specifically, this invention relates to such a method and apparatus that reissues the sequence of initial synchronization as described in the BONDING™ (Trademark of the Bonding Consortium) Mode 1 specification.
BACKGROUND ART
Backbones for local area network (LAN) connections within large companies, as well as the connections between those companies (World Wide Web—www), are typically operated using proprietary or leased glass fiber networks employing an ATM protocol. Often the need for additional bandwidth arises in these networks, either because of increased network traffic demands or to provide backup bandwidth for dropped connections. To supply this additional bandwidth, several digital telephone channels (Bearer channels) of the public switched network are bundled together to provide an aggregate channel of increased bandwidth.
For example, a bandwidth of 64 Kbit/s can be achieved using one Bearer channel (B channel). Typically, 24 (T1 standard) or 30 (E1 standard) of these channels are contained in a primary multiplexed line, thus providing a raw data rate of up to 2 Mbit/s when fully utilized.
It is, however, problematic that network suppliers can give no warranty that each of these B channels are operated synchronously to one another when telephone channels of the public switch network are used to form the primary multiplexed line. Certainly the entire telephone network is operated by a synchronous clock, however, differences in transmit time between the respective bundled channels can occur. Because the different data packets making up a data package (ATM frame) will pass through different paths in the network on their way from sender to receiver, the contents of the data packages (ATM frames) which are multiplexed and transmitted in these channels may not reach the addressee in their original form. In addition, there will be no warranty that the individual channels will follow a certain sequence (channel shuffling) when arriving at the addressee. This problem is also known from the field of picture telephony.
The inverse multiplexing function converts a wide-band communication on a high speed link into a group of combined switched channels. For example, this function can be used to convey 2 Mbits bandwidth over ISDN B channels of 56 or 64 Kbit/s. Other examples are the European primary ISDN (Integrated Services Digital Network) which provides 30 B channels for data transfer and one channel for call control (D channel) and basic rate ISDN which utilizes two B channels for data transfer and one D channel for control.
A challenge associated with inverse multiplexing is to be able to re-synchronize the bundled channels such that the high speed link can be rebuilt on the remote (receiving) side of an established link between two peer stations connected to one another through the ISDN network. This challenge exists for data transfer in both directions, as the established link can be used to support full duplex communication. Because the path between peers in one direction may have different characteristics as compared to the path in the opposite direction, the inverse multiplexing function must be performed symmetrically at each receiving station.
The BONDING™ (Bandwidth On Demand Interoperability Group) specifications written by the BONDING consortium on Sep. 1, 1992 (and later releases) offer some guidance to solving this problem. These specifications define a frame structure and procedures for establishing a wide-band communication link by combining multiple switched 56 or 64 Kbit/s channels that connect network nodes over a switched digital network (e.g. public switched telephone network, private network, etc.). At the transmitting end of the link, user data is placed in repeated frames that are carried over multiple, independent B channels. At the receiving end of the link, all channels are phase aligned and synchronized by an apparatus known as an Inverse Multiplex Unit (IMU), such that the original wide-band communication can be reassembled. The skilled reader will appreciate that this invention is not restricted to the use of the IMU described herein, any type of apparatus can be used which is able to cancel the varying channel delays present at the receiver side of the network.
Because the channels used for the wide-band connection are routed through the network independently of each other, the data in each channel may be individually delayed relative to the data in other channels. Overall transit delay for the end-to-end connection is equal to the longest transit delay between each of the individual channels plus a constant delay due to the data reordering function performed by the receiving IMU. To allow for the correct realignment of the data at the receive end, the BONDING specification calls for the transmission of a training sequence which is to be sent in advance of a group of several channels being bonded to form a high speed link. This training sequence is made up of a special framing pattern that is exchanged between each of the locations participating in the connection. The analysis of this pattern, when received, allows for the exact calculation of delays introduced by the switched network. Once aligned, data transfer is continually monitored throughout a transmission for failures such as channel disconnection, phase slip or high error rate. Various fault isolation and recovery procedures are defined in the specification to address these failure modes.
The BONDING specification describes four modes of operation:
BONDING Mode 0 calls for initial parameter negotiation and directory number exchange to take place over a master channel before data transmission begins. No delay equalization is specified under this mode. Thus, the mode is useful when the calling endpoint requires directory numbers and when delay equalization is performed by some other means (e.g., an attached video codec).
BONDING Mode 1 supports user data rates that are multiples of a bearer rate. It provides a user data rate equal to the full aggregate bandwidth of the bearer channels, but does not provide for in-band monitoring of transmission quality. This mode also supports delay equalization by providing for the sending of an initial set of frames, or so called training sequence, to synchronize (phase align) the sending of multiplexed data over the various bearer channels. Error conditions on one or more the channels that disturb overall system synchronization are not automatically recognized.
BONDING Modes 2 and 3 are each based upon Mode 1, however, these modes support transmission quality monitoring by using data transfer frames that include bits dedicated to storing transmission control information. This information can be analyzed at the receive side of the network to determine if transmission failures have occurred and to drive appropriate fault recovery routines. The ability to monitor the transmission quality does not come without a price, however, as that portion of the bandwidth being used for the transport of control information reduces the overall bandwidth available for data transmission. For Mode 2 the effective B channel rate is reduced by 1/64
th
. Mode 3 requires one additional B channel to provide for the control information overhead.
Because of its simplicity and full bandwidth support, BONDING Mode 1 has become the most popular mode employed to bond channels for high speed data transmission. Typically, the quality monitoring and failure recovery routines, unsupported by Mode 1, are performed at some upper protocol layer of the application using the BONDING function. However, challenges remain in performing the above mentioned training sequence and subsequent delay compensation at high data rates (sustained 2 Mbit/s) at the receiver. This challenge is augmented by the need to synchronize the data path in both directions in order to support full duplex mod

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