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What is BONDING?

To understand what BONDING is, first it is important to understand that H.320 is the ITU recommendation for video conferencing using ISDN networks. ISDN or "Integrated Services Digital Networking" is a form of digital communications that can provide up to 64 kbits/second of bandwidth per ISDN channel.

In order for video conferencing systems to use ISDN to deliver dial-up digital bandwidth, some form of reverse multiplexing or bandwidth aggregation technique is required.

BONDING algorithms do this. These algorithms allow Inverse Multiplexing of channels to create call sessions in bandwidth increments of 64kbits/second. BONDING stands for "Bandwidth On Demand" and several Bonding algorithms were developed to create aggregated high bandwidth ISDN calls. BONDING is essential to H.320 video conferencing above 64Kbps.

Various forms of BONDING

Three variants of BONDING were developed to enable inverse multiplexing of ISDN Bearer channels into higher bandwidth call sessions. They are:

BONDING Mode 1: Most commonly accepted and widely used. Delay equalization between Bearer channels is negotiated at call startup. This BONDING algorithm does not generally revise sync in response to dynamic changes in Bearer channel delay, and so call sessions based on this algorithm will fail if Bearer channels deliver data in varying timeframes.

AIM: This is a dynamically changing BONDING method that can accommodate some changes in transport delay among Bearer channels. Originally named "AIM" after the source of its creation at Ascend Communications, later acquired by Lucent and even later Alcatel, AIM stands for Ascend Inverse Multiplexing. This BONDING mode is resilient and will maintain call sessions even with some changes in the delay synchronization between Bearer channels.

STATIC: This BONDING algorithm negotiates delay sync just after the call setup acknowledge and thereafter performs minimal revision to the delay equalization timing. It is prone to sync loss and therefore, call drop, if delay between channels varies. This algorithm most closely matches the BONDING Mode 1 algorithm that has been widely adopted as an inverse multiplexing standard.

What do I need to know about BONDING?

ISDN calls and digital networks are engineered to be reliable. Further, all digital networks follow clocking which in theory means they all derive clocking from a similar stable source. In reality, however, calls traverse many networks and many pieces of equipment even when made simultaneously. This can cause variations in signal transmission flow, or delay. This is important because delay equalization is critical to H.320 video conferencing.

Two or more video conference calls made on an ISDN network may cause data to arrive at different times. BONDING handles this through a part of its design called "delay equalization". However, if the delay between one call and another change for some reason (such as a bad network path for one call) BONDING can lose the delay sync and H.320 call sessions can fail.

Generally, if an H.320 call drops due to delay sync problems, ISDN networking diagnostics often report this as "Far End User Disconnect" when, this may not be the case at all.

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