Extending ATM to Midsize Companies - Technology Information

Telecommunications, Nov, 2000 by Rob Carlson, Nathan Muller

Inverse multiplexing over ATM allows customers to scale their bandwidth requirements as their businesses grow.

ATM service has been available since the early-1990s and has emerged as the best networking technology for integrating all types of traffic, including voice and video. ATM services have benefited large companies, but today even midsize companies with multiple traffic types and three or more distributed locations can benefit from ATM's sustained throughput, low latency, adept traffic handling and appropriate QoS mechanisms. The recent availability of new access equipment and aggressive pricing for network service is making ATM suitable for mainstream use, particularly for midsize companies that have been locked out of the service due to its high cost of implementation.

For smaller companies, the key to unlocking this powerful service's potential comes from IMA (inverse multiplexing over ATM), which solves the bandwidth gap problem. Without IMA, midsize companies have few choices for local access to a service provider's network: the DS1 rate of 1.544 Mbps, which offers less bandwidth than they need, or the DS3 rate of 45 Mbps, which offers more bandwidth than they need and may be cost-prohibitive. With IMA, companies can aggregate multiple DS1 circuits to achieve the right amount of bandwidth for their applications and pay for only that amount on an xTl basis.

For example, when the bandwidth of four T1s is bonded by the IMA device, the virtual connection through the service provider's network is provisioned at 6 Mbps. When the bandwidth of eight T1s is bonded, the virtual connection is provisioned at 12 Mbps. Regardless of the number of T1 access links in place, the IMA device bonds them, combining the bandwidth into a fatter logical pipe that can support mixed-media applications running over interconnected LANs (see Figure 1).

IMA works by distributing ATM cells over multiple T1 physical links to the carrier's switch. In round-robin fashion, the cells are placed on the links so that the first cell is sent out over the first T1 circuit, the second over the second circuit, and so on. Control information is also sent over the access links, enabling the status of each link and the quality of the connection to be supervised and automatically corrected on a continual basis. At the remote end, the carrier's switch feeds the cells through a local T1 trunk group to another IMA device, where they are consolidated into a single data stream.

Since the IMA access device at the receiving end requires a constant stream of cells to recreate the original stream, the sending device adds filler cells whenever there is a drop in the traffic rate to keep the round-robin process at both ends in synchronization. To reduce bandwidth consumption, IMA removes idle and unassigned cells from the original stream and reinserts them at the receiving end. Since cell order and format are retained and the delay variations within each trunk group are smoothed out through the use of buffers in the IMA equipment at each end, inverse multiplexing is transparent to the customer's applications and the service provider's network.

Equipment Requirements

While IMA has been around for several years, equipment has been relatively scarce. Until recently, IMA devices have been pricey and lacking in features, discouraging many midsize companies from further considering ATM services, even with xT1 access. As equipment prices have fallen and more functionality has been added to IADs (integrated access devices), ATM is now worthy of serious consideration. Since midsize companies greatly outnumber large-scale enterprises, IMA opens a vast, largely untapped market for ATM.

IADs deliver voice and data to the carrier's ATM network through a variety of network interfaces. In addition to T1/El or N x T1/El IMA, there are T3/E3 ATM interfaces. Consequently, if a company eventually needs more bandwidth, the same system can support the increase without requiring the company to buy an entirely new system or pay for an upgrade to an existing system. IMA vendors expect to accommodate customer bandwidth requirements at the OC-3 level (155 Mbps) in the near future.

IMA systems also feature built-in failure protection. If one of the T1 links in a trunk group fails, traffic continues to be distributed over the remaining links. When the failed link is restored, traffic automatically distributes across the entire trunk group. This level of failure protection is not available with multiport ATM routers that support IMA. In fact, if one of the T1 links from the router goes down, they all go down.

The ability to maintain traffic flow in the event of a T1 circuit failure has proven attractive to ATM customers. IMA is built on redundant T1 circuits, and the probability of multiple T1 access circuits failing at the same time is low compared to the possibility of a single DS3 access circuit failure. Local circuit redundancy is a significant and sought-after benefit of IMA among smaller companies considering ATM services.