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How to give DSL concentrators a boost

By Richard Sekar
Network World, 11/29/99

Carriers can meet growing bandwidth demand by using digital subscriber line concentrators that employ an Inverse Multiplexing (IMUX) capability, also known as DSL bonding, to turbocharge business DSL services. IMUX bonds up to eight copper pairs to deliver high-bandwidth services - up to 12M bit/sec - for little more than the cost of a traditional dedicated T-1 connection.

IMUX becomes an important option as competitive local exchange carriers (CLEC) launch symmetric DSL (SDSL) and ISDN DSL (IDSL) services nationwide to deliver business-class communications services that fill the bandwidth gap between 56K bit/sec and 1.5M bit/sec.

There are three main applications driving vendors to incorporate IMUX functionality into their DSL concentrators. First, IMUX eliminates the problem of declining DSL speeds at longer distances. For example, serving a midsize business located more than 24,000 feet from a central office may cost a carrier $500 to $900 per month for a T-1 circuit. But the 320K bit/sec available at that distance from SDSL may not be sufficient given the company's growing bandwidth demands. By bonding up to five DSL lines, it costs the carrier only $25 to $100 per month to offer the customer 1.54M bit/sec service.

Second, IMUX enables CLECs to offer speeds of up to 1.15M bit/sec to IDSL customers served through a Digital Loop Carrier. Until now, these customers have been limited to a top rate of 144K bit/sec.

Third, the IMUX capability lets carriers sell "up market" to larger organizations that require fractional T-3 speeds. By bonding together as many as eight DSL lines, a CLEC can offer speeds as fast as 12M bit/sec symmetric for only $40 to $160 per month.

While the multilink PPP protocol (MLPPP) or the Multi-Link Frame Relay (MFR) User-to-Network Interface draft standards under review by the Frame Relay Forum could become the basis for IMUX, MFR is fast becoming the solution of choice. MLPPP enables only Internet access; it does not support the end-to-end private virtual circuits (PVC) that frame relay service requires.

MFR enables Internet access and frame relay services by provisioning end-to-end PVCs.

MFR performs load balancing across multiple bonded DSL lines. It also actively monitors the performance of the bonded connection. If one line goes down, MFR automatically distributes the load across the remaining lines, then transparently restores the bond and balances traffic when the line comes back up.

Finally, a DSL concentrator that implements MFR can automatically notify the network operations center of any changes, along with customer names and addresses, for proactive customer management.

While the DSL concentrators can now bond up to eight copper pairs, customer premises equipment (CPE) currently supports two bonded pairs for access rates of up to 3M bit/sec with SDSL. IDSL CPE enables access rates of up to 288K bit/sec. CPE for four bonded pairs will be available in 2000. Because products are based on the MFR standard, many different types of CPE from several third-party vendors will be available and will interoperate seamlessly with DSL access concentrators that employ MFR.

The availability of turbocharged SDSL and IDSL will enable a variety of high-speed Internet access applications, such as streaming video and distance learning. Financial services companies, for example, can use streaming broadcasting to provide up-to-the-second financial information. Telecommuters will finally get the access speeds they want at a price they can afford.

Turbocharged SDSL provides CLECs with a major opportunity to switch frame relay users to a less-expensive, high-speed IP service without throwing away their frame relay routers. These customers can connect their existing routers to an MFR-based DSU with a V.35 interface that connects multiple lines to the DSL concentrator. IMUX even enables voice services to run over the same circuit as frame relay.

By multiplying bandwidth, IMUX greatly expands the potential market for DSL services, scales to meet subscribers' needs for higher bandwidth and makes possible a host of new applications.

How to give DSL concentrators a boost

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