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Clear channel ATM lets users keep their options open with regard to WAN access and ATM transport. They can use the DS3 and/or OC-3c UNI interfaces they have available on their backbone ATM switches without having to deploy a new UNI. Users do not even need to use an ATM switch. With a clear channel ATM inverse multiplexer, a single device such as a high-speed workstation or server equipped with an OC-3c or DS3 ATM NIC can connect directly into the CCA imux.

Support for Non-ATM and Multimedia Traffic

Network managers can build private networks and intranets between two or more sites with ATM backbones by using multiple, private line T1 or E1 WAN circuits in a virtual fractional T3 or E3 pipe. It is not necessary to make any changes to the backbone ATM switches that are being interconnected (see Exhibit 3). When they wish to upgrade to a different type of transport (i.e., IMA or higher-speed ATM), they can upgrade without changing their user UNI interface, thereby protecting their original ATM investment.


Exhibit 4.  Simultaneously Supporting TDM and ATM Traffic over M13 Service.

For those network environments where ATM is required for some, but not necessarily all, applications, clear channel ATM imuxing can be used to combine ATM traffic over the same fractional T3/E3 links used for non-ATM traffic. In Exhibit 4, a channelized DS3 link in an M13 (multiplexer DS1 to DS3) network can support both traditional non-ATM traffic (including imuxed router traffic and T1 tail circuits from a PBX), as well as specific multimedia applications being run over ATM. Users need not convert all of their WAN access connections to ATM to support what may be a relatively small set of applications; the ATM traffic can be transported transparently over NxT1 links within the channelized DS3 that is used for other types of traffic. It is not an either/or scenario. Users do not have to cut over from TDM to ATM WAN access; they can have both at the same time, over facilities with which they are already familiar. NxT1/E1ATM access can be added incrementally, making the migration to ATM less expensive and less risky.

Compatibility and Cost Savings

If a carrier is using inverse multiplexing to provide multimegabit-per-second frame services, clear channel ATM could be provided with the same equipment in the infrastructure. An ATM service can be offered using the existing transport technology. Other than adding inverse multiplexers that support clear channel ATM, the carrier’s customers need not change their ATM backbone equipment. Neither does the carrier need to change its service facilities, nor retrain and retool to support a new inverse multiplexing technology. The carrier is using the same bit-based inverse multiplexing technology that it does for frame relay, the only difference being that the imux now has an ATM interface facing the customer premises, instead of the imux’s customary HSSI or V.35 interface. Circuit provisioning is identical, requiring no retraining other than how to address the expected ATM protocol and interfacing issues.

A carrier’s ability to offer virtual ATM transport in the form of NxT1/E1 clear channel ATM imuxing allows it to serve more customers, particularly those who are not quite ready for high-speed DS3 or OC-3c ATM service. A clear channel ATM NxT1 link can provide significantly higher-speed access to a switched ATM service than single T1/E1 links, using readily available lines and without higher bandwidth’s price tag. The UNI interfaces already deployed at the customer premises and within the service provider’s cloud remain unchanged, and the carrier can now expand its ATM offerings to more than just those few customers in selected geographical areas that can access or afford DS3 and OC-3c bandwidth.

CONCLUSION

Using clear channel ATM inverse multiplexing does not preclude a user from upgrading to IMA when it is available or affordable, or to DS3 or OC-3c when the user’s applications environment demands even greater bandwidth. A benefit of clear channel ATM inverse multiplexing is that both of the technologies that are combined into a solution — a DS3 or OC-3c UNI, with bit-based NxT1/E1 multiplexing — are readily available and well understood. Whether support for IMA is added to a clear channel ATM inverse multiplexer, or the inverse multiplexer is replaced by a higher-speed ATM access device, there is no need for the user or the carrier to restructure the network.

Although it is still undetermined as to the ultimate role that ATM will play, it is clearly an important technology with significant benefits. It is also a technology that will exist in concert with the installed base of TDM WAN access devices for a long time. Clear channel ATM inverse multiplexing leverages both bit-based inverse multiplexing and ATM technologies, and is perhaps the best example of how simply and seamlessly ATM and TDM can coexist — to the benefit of end users and carriers alike.


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