Using All Your Pipes

A Basic Rate Interface (BRI) ISDN connection provides two 64-Kbps B channels for data communications. Inverse multiplexing can aggregate both into a single 128-Kbps virtual channel--if your terminal adapter supports it. Two of the most popular ISDN inverse multiplexing schemes are Bandwidth on Demand Interoperability (Bonding) and Multilink PPP (MP).

Bonding is typically used for aggregating multiple B channels during videoconferencing sessions. MP, as defined in the specification called RFC 1717, is more commonly used for network connectiv ity. The main difference between the two protocols is in how the additional bandwidth is handled.

Bonding aggregates channels in hardware, and the ba ndwidth negotiation occurs during call setup. Once the call is in place, there is no mechanism to add or remove channels. MP channel aggregation, on the other hand, is generally done less expensively in software. MP can also allocate and deallocate channels on the fly, and negotiation is rapid, making it ideal for the bursty nature of data applications.

A proposed MP extension is called Bandwidth Allocation Control Protocol (BACP). BACP allows both ends of an ISDN connection to inform each other when they are about to bring up an additional channel or when they are preparing to tear down a channel. This will allow for cleaner administration of bandwidth changes than occurs now with MP.

For remote LAN connections and Internet hookups, the ability to add or shed B channels is desirable, making MP preferred. For a WAN, it keeps remote-connection costs down by trimming channels to fit the current load. For a single user, you can have a fast two-channel Internet connection and yet drop a channel to t ake an incoming voice call. V.120 doesn't support channel aggregation.