The Virtues of Narrowband ATM Daniel A. Kosek James P. Cavanagh

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1 Previous screen The Virtues of Narrowband ATM Daniel A. Kosek James P. Cavanagh Payoff When deciding on a transport technology, network administrators should consider the enhanced network performance and cost benefits offered by asynchronous transfer mode (ATM). This article discusses the benefits of ATM, especially the advantages of cell-based architectures over time-division multiplexing architectures. Introduction Asynchronous transfer mode (ATM) is a cell-switching architecture that allows the efficient sharing of a network's transport backbone among sources. Asynchronous Transfer Mode is a streamlined transport technique that relies on devices outside of the cellswitching fabric to perform any error correction or retransmission that might need to be done on a high-speed digital communications system. Many people seem to agree that ATM is the way of the future for communications. ATM is being embraced by standards-setters, communications products manufacturers, network service providers, traditional telephone companies, and most important, business communications planners and implementers as a cure for almost all of the problems facing users of communications products and services. For the medical community, ATM represents a way of transporting medical images and other data at the speed needed to save lives. Retailers view ATM as the underpinning for a revolution in the way consumers select and purchase goods and services. Entertainment companies anticipate the use of ATM networks to distribute movies and music on a custom basis to the consumer. Researchers see ATM as a means of constructing a vast information superhighway that will support the sharing of information in ways never before thought possible. However, it takes time for carriers to deploy the ubiquitous, high-speed infrastructure required to support Broadband Asynchronous Transfer Modecommunications. It takes time to develop a whole host of new software applications to take advantage of WAN that exceed the speed of today's local area networks by several magnitudes. It also takes time for the cost of these new high-speed transport mechanisms to decrease enough to become affordable to those other than big-budget research labs and government agencies. Broadband and Narrowband ATM Business users can, however, take advantage of a smaller version ofasynchronous Transfer Mode that is available today: narrowband ATM. Narrowband ATM moves mixed loads of voice, traditional data, bursty LAN data, and video and image information in small, efficient, fixed-length cells. Comparison of Transmission Speeds and Standards The differences between broadband and narrowband ATM are in fundamental areas: transmission speeds and standardization. Broadband ATM operates at E3 (i.e., speeds of 34M b/s and faster), including the North American T3 rate of maps, and uses a larger cell format 53bytes, which is a combination of a five-byte overhead and 48-byte payload.

2 Previous screen Narrowband ATM operates at speeds of El (i.e., 2.048M b/s)and slower, including the North American T1 rate of 1.544M b/s, and uses a smaller cell format 24 bytes, which is a combination of a three-byte overhead and a 21-byte payload. This article focuses on benefits that highlight the technical and economic advantages of a cell-based architecture over Time-Division Multiplexing architectures. Although work is being done to scale down the low-end speed of Broadband ATM to T1 speeds, several technical and practical hurdles must be overcome before that can happen. Broadband ATM is based on international and domestic standards. Narrowband ATM, which was developed before standards for the movement of cells existed, is based on proprietary implementations by specific manufacturers. One example of a Broadband ATM switching product is StrataCom's (San Jose CA) BPX. The BPX offers multiplexing that transports mixed traffic loads in small, efficient fixed-length 24-byte cells. Comparison to Time-Division Multiplexing In order to understand the important business advantages of narrowband ATM, it can be compared to the predominant networking technique used in T1/E1 networking today: timedivision multiplexing (TDM), or circuit switching. ATM networks assign virtual circuit that only consume bandwidth when real traffic is offered to the network. time-division multiplexing (TDM) networks permanently assign bandwidth regardless of utilization. The permanent assignment used by time-division multiplexing (TDM) wastes bandwidth because there is no mechanism in time-division multiplexing (TDM) for sharing bandwidth between circuits when it is not utilized. The financial impact is substantial, considering that bandwidth is typically 75% to 85% of a network's five-year operating budget. For example, if time-division multiplexing (TDM) network hardware costs $5.8 million, and the recurring monthly trunking costs are $585,000, the five-year project cost would be $40.9 million. If bandwidth savings of Asynchronous Transfer Mode are applied, the new monthly trunking costs would be$269,100, bringing the five-year project costs down to about$21.9 million. That is a full 54% less bandwidth cost delivering a total five-year cost savings of about $18.9 million. Exhibit 1 compares time-division multiplexing (TDM) costs to ATM costs over five years. A Cost Comparison of ATM and TDM Network Resiliency Narrowband ATM also provides a substantial improvement over conventional Time- Division Multiplexing in the area of network resiliency. Routing/rerouting based on cell addressing greatly reduces the complexity of managing a network when compared with rerouting rigid, static circuits. In time-division multiplexing (TDM) architectures, mapping each connection from one point to the next presents several substantial technical barriers; it also takes up a lot of time. Many network management teams have a person or group that must constantly update the maps that represent the network in different operational modes. One map represents normal operations, and other maps are needed for rerouting during failures. Although automated tools have been created to help, the work must still be continuously monitored. In an ATM network, narrowband or Broadband, no maps are required because each cell contains the knowledge (address) for the network to deliver the traffic to the

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4 Previous screen appropriate location. Rerouting uses the same addressing capability. An Asynchronous Transfer Mode network can reroute all traffic on a failed T1 in less than two seconds compared with the 10 to 40 seconds, or longer, required to reroute the same traffic in a Time-Division Multiplexing network. The fast rerouting of ATM networks ensures that data operations like System Network Architecture sessions do not time-out and that voice users do not hang up on calls in progress. Bandwidth In an ATM network, bandwidth is viewed as a common pool, available to all virtual circuit. ATM guarantees fairness between ports on the network. It offers guaranteed bandwidth to all applications, and a bandwidth reservation with the right of first refusal. All unused bandwidth is placed back in the pool for any other connection to use. Cell-based switching enables the statistical pooling of traffic; time division architectures cannot. A TDM's control of bandwidth is archaic compared to ATM. time-division multiplexing (TDM) switching is much like an interstate with concrete barriers between lanes. Users are assigned a lane that no one else can travel on, even when it is not being used. This is due to the inability of time-division multiplexing (TDM) architectures to dynamically assign bandwidth. This problem is exacerbated by today's LAN traffic characteristics. A LAN requires large amounts of bandwidth for a few moments, followed by long periods of silence. time-division multiplexing (TDM) networks must dedicate large amounts of bandwidth to service LAN traffic effectively. A Time-Division Multiplexing approach forces users to spend a great deal on excess bandwidth to support LAN traffic, or suffer degraded performance. An ATM approach is superior to time-division multiplexing (TDM) for the support of bursty LAN traffic because of its ability to pool and assign bandwidth on demand. Why Move to Atm? Once the basic benefits of Asynchronous Transfer Mode are understood in terms of networking needs and the business function that the network is intended to perform, the question of moving to ATM becomes threefold. How Can ATM Benefits Be Applied in the T1 Environment? The simplest approach is to place the 53-byte ATM cell on a T1trunk. The problem with this approach is latency on a T1. To send a standard ATM cell on a T1 takes two DS1 frames plus five bytes of a third frame. If a narrowband implementation of ATM is to be successful, the cell size should be scaled to match the available facilities: T1, Fractional T1, or E1. An ATM 53-byte cell has five bytes of address and 48 bytes of user payload. To fit this information in a 24-byte T1 frame, the same payload-to-address ratio as the 53-byte cell's could be used (three bytes of address space and 21 bytes of user payload). If the narrowband cell was designed properly, the cell could be converted to and from the 53-byte size. This would allow one large ATM network with both broadband and narrowband segments. Are the Economics of ATM Valid at T1 Speeds? Without a doubt the answer is yes. In current T1 networks supporting a variety of voice and data applications, the results are impressive. The following sections describe the four types of traffic 9.6K-b/s data, 19.2K-b/s data, voice, and frame relay at 256K-b/s and

5 Previous screen the economic benefits available. 9.6K-b/s Data. Data speeds of 9.6K-b/s still represent a major portion of corporate terminal traffic, like System Network Architecture. Exhibit 2 compares the number of 9.6K-b/s circuits a single T1 can support when operating as a Time-Division Multiplexing or narrowband ATM trunk. Since the typical utilization of an individual 9.6K-b/s circuit is between 25% and 50%, the benefit available is from 73 to 274additional circuits by using narrowband ATM services. This 46% to 175% increase in ports can be used to: Reduce bandwidth in the network. Provide more connections in the current bandwidth available. Allow reroutability during span failures. Free space for new services. T1 Capacity Comparison at 9.6K b/s A TDM's T1 capacity for 9.6K-b/s traffic is 156 connections. Once the 8K-b/s overhead bit are removed from a full T1, the remaining capacity is 1,536M b/s. From this capacity, a control channel must be established for intermultiplexer communication. This control channel is typically a 32K-b/s channel, which leaves 1,502M b/s for user data. When this rate is divided by a consumption of 9.6K b/s of data per connection, the resulting capacity is , which is rounded down to 156 connections. This same math is done for all the examples in this article using the 32K-b/s control channel requirement. Although this article has only used examples of T1 capacity, the benefits apply to E1 facilities as well. The math for E1 capacity is slightly different. The typical E1 (2,048M b/s) circuit has only 30DSOs (64K b/s) available to the user, for a total available capacity of 1,920M b/s. Removing the control channel (32K b/s) leaves 1,888M b/s for user capacity for a time-division multiplexing (TDM) connection capacity of This number rounds down to 196. A cell-based network on the same facility could support from 168 connections at 100% utilization to 655connections at 25% utilization. 19.2K-b/s Data. The same comparisons of 19.2K b/s yield even better results. With similar utilizations, the number of circuits supported represent an increase of 51% to 193% over Time-Division Multiplexing. The narrowband ATM benefits discussed previously allow the same gains and choices in 19.2K-b/s traffic. Overall bandwidth in the network is reduced, which can save monthly recurring costs over the life of the network and expand the services or capacities of the network on the existing bandwidth. Exhibit 3 compares time-division multiplexing (TDM) and narrowband ATM T1 capacity at 19.2K b/s.

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8 Previous screen T1 Capacity Comparison at 19.2K b/s Voice Services The benefits for voice services include many new features that time-division multiplexing (TDM) technology cannot match. For example, the ability to detect fax or high-speed modem traffic and change compression quickly is conveniently done on a cell-switched narrowband or Broadband ATM network. Narrowband ATM delivers benefits for traffic with typical voice utilizations between 25% and 40% by an increase in ports on a T1 from 63% to 390%. While many companies withtime-division multiplexing (TDM) solutions have moved to SDN voice networking, the increased capacity advantage of voice on ATM makes voice services attractive for both narrowband and Broadband ATM networks. Exhibit 4 illustrates voice utilizations for time-division multiplexing (TDM) and narrowband ATM. Voice Utilizations for TDM and Narrowband ATM Frame Relay Frame relay is the industry's most widely used means of providing LAN connectivity. The interface specification International Telecommunications Union-TS 122 was written to take advantage of the switching architecture described in specification ITU-TS 121, ATM. While time-division multiplexing (TDM) manufacturers have created interface modules that support the Frame relay interface standard, their internal switching architecture does not support Frame relay connections. The dedication of bandwidth in a time-division multiplexing (TDM) for Frame relay connections is the worst offender of all connection types. The inability to share unused bandwidth is evident. The typical utilization of Frame relay connections is between 8% and 12%. Utilization of 3% is not uncommon. ATM's ability to share bandwidth between these connections again shows the cost-effective nature of narrowband Asynchronous Transfer Mode over time-division multiplexing (TDM). Exhibit 5 shows the difference between time-division multiplexing (TDM) and ATM support of Frame relay traffic. Differences in TDM and ATM Support of Frame Relay Traffic In this example, a monthly cost of $53,500 per T1 is divided by the number of circuits supported (of each type), resulting in a cost per circuit. A direct comparison of typical circuits and their associated cost per month is shown in Exhibit 6. The savings gained by using a narrowband ATM solution are outstanding. Narrowband ATM outperforms timedivision multiplexing (TDM) in each circuit type, especially inframe relay. frame relay is the first of the packet mode services. Soon, new ATM services will be added to networking products.

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12 Previous screen Combined Comparison of TDM and Narrowband ATM Can a Seamless Network Be Created with Narrowband (T1, E1, FT1)Trunks and Broadband (45MB, OC-1, OC-3) Trunks? The answer to this question is yes, provided the narrowband ATM system uses a cell that can be transformed to a Broadband cell and back again. Only a cell-based solution offers the benefits outlined in this article. Atime-division multiplexing (TDM) switching product cannot be easily merged with a Broadband ATM network. ATM interfaces are available for routers, hubs, and desktop systems. Users must ask if their network should be Asynchronous Transfer Mode at the desktop,time-division multiplexing (TDM) locally, and ATM in the wide area again. The far better solution may be ATM at the desktop through the wide area and beyond. Conclusion While the standards-setters discuss practical answers to the questions of implementing high-speed Broadband Asynchronous Transfer Mode networks of the future, business users need a practical alternative today. NarrowbandATM allows business users to gain the financial benefits and performance improvements in today's networks, while positioning users in the middle of the on-ramp to the multigigabit information superhighways of tomorrow. Author Biographies Daniel A. Kosek Daniel A. Kosek is a freelance WAN specialist with more than seventeen years of experience in telecommunications. For the past six years he has held an applications engineering role at San Jose-based StrataCom, Inc., assisting customers with the implementation of frame relay and ATM networking technologies. He has spoken internationally and nationally on ATM and frame relay technologies. James P. Cavanagh James P. Cavanagh has been an applications engineer with the Carrier Group at StrataCom, Inc. for four years, and has worked in the telecommunications industry for more than 12 years. His articles have appeared in a variety of trade publications, and he participates in many industry conferences and symposia as both a panelist and moderator. He is also involved with the International Communications Association's Summer Program at the University of Colorado at Boulder.