Multimegabit Service Closes The Broadband Bandwidth Gap.

As the demand for business data access continues to outpace expectations, a lot of companies are running out of bandwidth. T1 lines delivering 1.5Mbps were once considered more than adequate for many companies, but no more. Today, even small companies are using high-bandwidth applications such as e-commerce, video-conferencing, and streaming video, and their T1 circuits are becoming bottlenecks that limit productivity and reduce customer satisfaction.

In the past, the common remedy for the T1 bottleneck was to make the jump to a 45Mbps T3 circuit, but, at six to eight times the price of a T1 line, going to T3 is like trying to take a drink from a fire hose and at a price that companies can't afford. In fact, new High-bit-rate Digital Subscriber Line (HDSL II) technology is driving down the cost of T1 service by delivering it over one copper pair instead of two. HDSL II deployment this year will widen the price gap between T1 and T3 service so that T3 local access costs will range from eight to twelve times T1 pricing. Additionally, T3 is provisioned through fiber, which is not available to most of the businesses in the United States. Where it is available, T3 may take months to provision.

Now, new Multilink Frame Relay (MFR) and Multilink PPP (MLPPP) standards allow companies to scale up bandwidth to match their data needs and budgets. The MFR standards, FRF.15 and FRE.16, and the IETF standard, Layer 2 MLPPP protocol, provide scalable access at speeds between T1 and T3 for cost-effective data service. FRF.15 describes the procedures and frame format used by Customer Premises Equipment (CPE) for Aggregated Virtual Circuit (AVC) service that allows for the transport of multiple virtual circuits to transport a single stream of sequenced data frames. FRF.16 specifies the standards for physical emulation of frame relay devices for the user-to-network interface and the network-to-network interface, making it possible for the interconnection of networks using MFR protocols. In a similar way, MLPPP aggregates the bandwidth of two or more T1 or ISDN telephone lines for use as a single data circuit.

Multimegabit Alternatives Today

Broadband access is spreading across the business landscape in various forms. T1 is the oldest form of broadband, but newer technologies, like Symmetric DSL (SDSL), Asymmetric DSL (ADSL), Asynchronous Transfer Mode (ATM), wireless, and cable, are also available. Despite the emergence of broadband alternatives, however, T1 has remained the most popular method of business Internet access because it retains several advantages over the other approaches.

For example, SDSL service typically peaks at 1.5Mbps, so it's more suitable for bridging the gap between dial-up modems and T1 speed. ADSL service offers faster download speeds than upload speeds and is targeted at the consumer market. Broadband wireless has limited range and requires antennae to implement. ATM service uses a transmission scheme that imposes a "cell tax," which decreases the actual throughput of a circuit by 15 to 40 percent, based on the application. T1 service remains the choice of business data access because it is inexpensive, it's available everywhere, it offers guaranteed symmetrical transport with low latency, and it provides secure connectivity with a dedicated circuit.

Because of T1's inherent advantages over other broadband access solutions, telecommunications engineers have looked for ways to bundle two or more T1 lines together into one virtual circuit as a way of increasing throughput. This multimegabit T1 service, known as NxT1, allows companies to scale up bandwidth as their needs and budgets require without switching to a different access solution.

NxT1 service is not a new idea, but previous solutions have been expensive, proprietary, and difficult to scale. One approach called bit-based interleaved multiplexing creates a proprietary multimegabit point-to-point connection between two routers, which doesn't scale for use in the Internet or other high-speed data networks that connect many users to a central facility. Load balancing is another method by which routers exchange traffic along two parallel paths, but while this works for two T1 lines, it doesn't easily scale to higher bandwidth levels.

With the adoption of the FRF.15, FRF.16, and MLPPP standards, there is now a better way to bundle T1 circuits for multimegabit service that is scalable for any data transmission need. These standards allow for aggregation of the bandwidth from multiple T1 or fractional T1 lines.

Today, MFR and MLPPP have been implemented in multimegabit access concentrators that enable NxT1 service for service providers and their business customers. When installed at the customer premises and at the service Provider Point Of Presence (POP), multimegabit access concentrators deliver reliable and scalable NxT1 service to meet the bandwidth needs of businesses, as well as multi-tenant buildings.

Beyond Extra Bandwidth

By aggregating bandwidth from a bundle of T1 or fractional T1 circuits, multimegabit access concentrators enable NxT1 service that offers secure and symmetrical transport with low latency. In addition, they also offer higher reliability than a single T1 circuit and they give service providers the flexibility to add customers and retain existing ones.

With a single T1 circuit, any failure in the line or its connection interfaces eliminates the service. However, the multimegabit access concentrator protects users against such failures. The MFR and MLPPP protocols divide data packets into fragments and then transport them over each of the separate T1 or fractional T1 lines in the bundle. Each fragment is assigned a sequence number that ensures that the fragments are reassembled properly into packets at the receiving end. A failure in any one T1 line doesn't bring the service down because the transmission occurs over several physical T1 circuits. If one of the T1s in the bundle fails, the other circuits are still working.

When implemented at a service provider POP, multimegabit access concentrators allow providers to gain leverage from their existing POP equipment, offer bandwidth options to their customers, and reduce customer provisioning time and costs. The access concentrator allows the service provider to divide the bandwidth of a T3 line among several customers, which reduces wasted bandwidth and improves overall network efficiency. Multimegabit access concentrators also make it easier for service providers to deploy a distributed, hierarchical POP architecture where traffic is aggregated in small remote POPs and large regional POPs while reducing switching and backhaul needs and, when installed in a multi-tenant building, access concentrators make it possible to add new customers or change bandwidth allocation remotely, eliminating on-site service calls. Multimegabit access concentrators that implement the MFR and MLPPP standards have made NxT1 service scalable and cost effective for business service providers and, by providing NxT1 service that is available virtually everywhere, multimegabit access concentrators extend the inherent advantages of T1 service to meet the bandwidth demands of today's businesses.

Kash Mitra is the vice president of marketing at Tiara Networks, Inc. (San Jose, CA).