Filling the T1-T3 gap.
Today, businesses are experiencing explosive growth in the use of all types of data networks. Internet access, for example, is becoming an increasingly important strategic element for all businesses. As their Internet usage increases--and as high-bandwidth applications, such as streaming media, videoconferencing, and e-commerce, become more common on the Internet--businesses are running out of bandwidth far earlier than they ever imagined.
At 1.5 Mbps, T1 lines simply do not have sufficient bandwidth to deal with the new demands being made on networks. Yet fiber-based T3 circuits are overkill for many small and midsized businesses. T3 provides 45-Mbps bandwidth, but it comes at a steep premium: T3 circuits lease for upwards of $4,000 a month, compared to as low as $400 for T1 lines. Moreover, T3 circuits are not easily available to many businesses, while T1 lines are ubiquitous.
SHOPPING AT THE GAP
The price/bandwidth/availability gap between TI and T3 is sending businesses and service providers alike scrambling for cost-effective ways to fill needs. They are finding that, because until recently there was little demand for bandwidth between T1 and T3, no one has developed a truly effective means to fill the T1-T3 gap.
Many business users expected that digital subscriber line (DSL) technology, cable, wireless, or ATM would solve the gap problem. For a variety of reasons, however, ranging from insufficient bandwidth to asymmetric bandwidth (bandwidth upstream is lower than bandwidth downstream), efficiency, or cost, no one technology offers all the characteristics required by business applications.
Symmetric DSL (SDSL), a scalable solution that does a terrific job of filling in the bandwidth gap between 56 kbps and 1.5 Mbps, offers an upgrade path from dial-up modems. It is an economical solution for businesses that need less-than-T1 bandwidth and are close enough to the central office to get high enough speed. While it suffers from availability and standards problems, SDSL is gaining significant momentum in the market.
Asymmetrical DSL (ADSL), which is primarily a residential service and is focused on universal ADSL (UADSL), promises to offer speeds up to 7 Mbps downstream. However, it suffers from a low upstream speed of only 768 kbps today. In addition, ADSL has very limited deployment. DSL in general also suffers from a lack of standards and from bandwidth variation depending on the location, length, and quality of the copper lines.
Broadband cable offers high speeds but is also highly asymmetrical, and it was developed for residential use. Cable transmissions are shared between multiple users, creating security and availability concerns that make cable a questionable choice for business use.
Wireless technology is used in multitenant facilities, but undertaking an antenna-construction and line-of-site engineering project is far too expensive for the typical individual business subscriber. Wireless can be expensive at the service provider's point of presence (POP) as well, due to the expense associated with separate radio and aggregation equipment. In addition, mere may be security and availability concerns with wireless technology, depending on equipment and installation variations.
ATM, which is preferred by many carriers, is suitable where end-to-end quality of service (QoS) is an important consideration and where the service provider's infrastructure is based on ATM. Use of ATM carries a bandwidth penalty, however, since it assesses a stiff "cell tax" that can reduce the payload of a circuit by 20% to 30%. As a result, ATM is not an efficient access technology for TCP/IP-based networks, such as the Internet. Moreover, packet-based QoS schemes are now evolving to compete with ATM.
BUNDLING CAN FILL THE GAP
A practical way to bridge the T1-T3 gap is to bundle multiple T1 lines into one larger circuit. However, existing NxT1 solutions may be proprietary, and they were originally developed for niche markets. They do not provide the cost-effective, high-performance, bandwidth-efficient, scalable solution needed for large-scale deployment of a new multimegabit solution.
Legacy-, bit-based interleaved multiplexing, for example, is a proprietary, expensive solution that creates point-to-point connections between two routers. It does not scale for use in the Internet or other high-speed data networks that connect many users to a central facility. Another NxT1 technique, load balancing, is used in most routers to intelligently direct packets down parallel paths. Load balancing can provide higher-capacity WAN access for up to two T1 lines, but it is expensive, cumbersome, and does not scale well to more lines. Inverse multiplexing over ATM (IMA) also enables the transport of packets over multiple T1 lines simultaneously; but, since it is an ATM solution, it suffers from high overhead.
A NEW SOLUTION
Now, there is an exciting, new approach to bundling T1 circuits that solves the issues that limited the value of earlier NxT1 solutions, promising to finally fill the T1T3 gap. This solution employs the industry standard multilink PPP (MLPPP) and multilink Frame Relay (MFR) protocols to bundle multiple T1 and Fractional T1 lines, creating a single, symmetrical, virtual multimegabit access path with bandwidth equivalent to that of the aggregated copper lines. For instance, a multimegabit circuit created by bundling together six T1 lines will behave as a single circuit with approximately six times the throughput of a single T1 line but only one-sixth of the latency. The solution is scalable, enabling businesses to start with network access at 1.5 Mbps and move upward as their bandwidth requirements grow.
The solution segments packets into fragments and transports them over separate lines in the T1 bundle; varying the size of the fragments achieves optimal bandwidth efficiency and latency for business applications. Use of the MLPPP or MFR sequence numbers ensures that the fragments are reassembled in the correct order into packets.
If a T1 line within a circuit fails, the bandwidth is downshifted; service is not interrupted. When the T1 line comes back up, it is automatically added back to the bundle. To ensure continuous operation, service providers can transport fragments over T1 circuits from different carriers. There may be a differential delay of many milliseconds between the different T1 links, but MLPPP and MFR can handle these delays and still deliver reassembled packets, in sequence.
The solution creates a virtual multimegabit access path between the end user and the service provider's POP. This requires both multilink customer access and POP aggregation equipment. Products implementing the solution can connect seamlessly into the current network infrastructure, thus reducing the overall cost of deployment. Business users can take advantage of customer premises equipment (CPE) to create an affordable multimegabit Internet access path without having to lease an expensive T3 line.
This virtual multimegabit access path solution provides businesses with a number of benefits, foremost among them being supercharged business Internet access with scalable bandwidth, high throughput, and low latency. Businesses can also use the solution to upramp their Frame Relay networks, which suffer from a similar bandwidth gap between T1 and T3. Services based on the solution are reliable, since the access path is built on the proven T1 infrastructure. Because the solution is standards-based, products that employ it are transparent to existing network equipment and will integrate seamlessly with an existing network infrastructure.
Service providers are beginning to take advantage of this new exciting NxT1 service to offer their business customers affordable and available high-speed access with a wide range of value-added services, such as virtual private networks (VPNs) and QoS guarantees. Since these services are layered on bundled T1 lines, which are fast becoming a commodity as prices decline, they can be extremely cost-effective, opening high-speed access to more and more business users.
Mitra is vice president of marketing, Tiara Networks, Inc., San Jose, Calif.
Circle 261 for more information from Tiara Networks, Inc.
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|Title Annotation:||Technology Information|
|Comment:||Explosive growth in data networking and growing bandwidth needs leave T1 lines inadequate for many businesses, but a large gap remains between the 1.5Mbps single T1 connection and the 45Mbps T3 line, which costs over $4,000 per month and is often overkill.|
|Date:||Oct 1, 1999|
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