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Chapter 20
Inverse Multiplexing ATM, Bit by Bit

Robin D. Langdon

In the time division multiplexing (TDM) environment, there are many applications that require greater than T1/E1 bandwidth, but where the jump to T3/E3 is not possible due to cost or availability of service. The technology to bridge the bandwidth gap between TDM and ATM WANs is known as inverse multiplexing, which allows multiple T1 or E1 lines to be aggregated to form a single multimegabit virtual or “clear” channel.

INTRODUCTION

Not long ago, the deployment of ATM was considered a wildfire market — ATM would be available on the desktop, would be used in LAN backbones, and would provide transparent LAN-to-WAN interconnection at almost infinitely scalable bandwidths with all the benefits that classes of service could provide. However, the enthusiasm for ATM might be judged to be on the wane.

Completely dismissing ATM would be premature. ATM might be having trouble competing with switched and fast Ethernet for the desktop, but it has firmly established a home in corporate and carrier backbones, where it is experiencing strong growth. On the access side of the public arena, however, ATM’s migration into the WAN has been and is projected to be slow. Based on industry forecasts, ATM service revenues in the public network will remain relatively small when compared to alternative switched data network services such as frame relay. A new access option, inverse multiplexing over ATM, is becoming a reality for corporate users who want the benefits of ATM bandwidth but want to avoid the high costs of ATM service and implementation.

ATM’s POSITION IN THE MARKETPLACE

For end users and carriers, the real issue is what to do when the local network needs to expand to the enterprise network and runs into the bottleneck of the public WAN’s access bandwidth. ATM outside the backbone and in the wide area is a possible solution, if ATM had a larger presence in the public network. ATM service is by no means ubiquitous, nor is it expected to become so in the next several years. ATM in the WAN may not even be available for those customers who need it or are willing to pay for it.

Some network managers, even those running their corporate backbones at ATM’s OC-3c rates, may not be able to justify the steep price tags (relative to access alternatives) or backhauling expenses associated with OC-3c or DS3WAN connections. Although T1 ATM prices are coming down to the point where they are equal to or less than the prices of traditional T1 lines (especially in areas where the carriers are trying to encourage users to experiment with ATM services), the bandwidth lost to ATM cell overhead and partially filled cells reduces the available bandwidth to the point where the inefficiencies of T1 ATM may be too high a burden for an application to bear.

To complicate ATM’s position in the marketplace, it is also faced with formidable competition from frame relay — a service that has no cell overhead, is ubiquitous, is priced attractively, and has tremendous market momentum. Like ATM in the LAN, ATM in the WAN must compete against alternative technologies that are less expensive, readily available, or easier to use.

INVERSE MULTIPLEXING FOR ATM

T1 inverse multiplexing, or imuxing, is a process in which a single data stream is split across multiple T1 lines in a round-robin fashion; the T1s are logically combined to form a single virtual data channel that is the aggregate of the T1 bandwidths (minus a small amount for overhead). From the point of view of the device providing the data stream, it is communicating via a single, high-speed WAN channel at some multiple of the T1 rate — that is, at the bandwidth of a fractional T3 service, but using readily available, less expensive T1 services.

The similarities between inverse multiplexing and ATM are significant when planning for current and future network implementations. ATM and inverse multiplexing topologies can both provide the ability to link individual sites by clear channel broadband data pipes. Both imuxing and ATM provide scalability, and both seamlessly link LANs and WANs in enterprise networks. Inverse multiplexing and ATM complement each other and can work together hand-in-hand.


Exhibit 1.  Cell-based Inverse Multiplexing for ATM (IMA).

Where the rate of a traditional T1 is insufficient and T3 is too expensive or is unavailable, T1 inverse multiplexing is an efficient and immediate cost-effective solution to provide increased bandwidth. The concept of inverse multiplexing can be applied to ATM cells, where an ATM cell stream is transmitted across multiple T1/E1 links; alternatively, it can also allow ATM cell traffic to be transported across the existing T1/E1 network infrastructure as a bitstream. In either case, users avoid having to pay for the excessive bandwidth of a T3 or OC-3c line that they may not need. The customer also avoids the price, in dollars or bandwidth, of T1 ATM service.

There are two variations of inverse multiplexing for ATM: cell-based and bit-based. Each has its own strengths, discussed at greater length in the following sections.

Cell-Based ATM Inverse Multiplexing

The very nature of public carrier networks demands a stable, standardized mechanism for transport. The ATM Forum’s evolving standard for inverse multiplexing for ATM (IMA) will play a crucial role in the acceptance and implementation of ATM inverse multiplexing.

IMA is a new user-to-network interface (UNI) being specified by the ATM Forum. The physical interface (PHY) committee of the ATM Forum defines standard mappings of ATM cells onto existing physical layer media; UNIs and PHYs are usually inseparable. In this case, the IMA UNI rides on top of existing T1 or E1 ATM PHY, performing inverse multiplexing via a cell-based control protocol, which is a major departure from the normal PHY definition. (See Exhibit 1.)

IMA is expected to be widely accepted in both the user and equipment vendor communities. Within the carriers’ networks, IMA can be used instead of T1ATM for point-to-point trunking between frame relay/ATM switches, greatly improving bandwidth without upgrading to DS3 or OC-3c. On the customer premises, the IMA specification promises vendor interoperability, giving users maximum flexibility in the equipment selection process.

However, the definition of a specification is a long and painstaking process. The IMA specification is still being formulated. Once the standard is defined, the normal maturation process for a new technology will set in. Early adopters of IMA will have to accept a number of adjustments as new hardware, software, and protocols are rolled out. Those customers for whom the risk in cost and reliability of deploying a new technology is too high will have to wait until the dust settles. Multivendor, interoperable NxT1 ATM may take a while to become a reality.


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