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Figure 7-5. ATM "Cloud"
It takes about 30 seconds to reroute if a link goes bad. Time to reroute is switch dependent and also dependent upon the number of switches in network. It is very important to determine time to reroute if traffic is heavy. Ask your service provider for this information.
Figure 7-6. Typical national ATM commercial backbone network
Typically, the ATM backbone network is referred to as the "ATM Cloud" and is depicted as a cloud in the literature. The cloud may consist of many ATM switches interconnected by some particular topology. Various network topologies including point-to-point, star, partial mesh, and full mesh are utilized by the service providers. The interconnections between switches are referred to as trunks. Also, the connection between any two switches is called a Network Node Interface (NNI). Typical trunk speeds are OC-12 and OC-48. Contrast the trunk speeds to the customer access speeds of T1/T3/NxDS-1/OC-3. Of course, the trunk speed must be greater as the trunk is carrying the traffic of many customers simultaneously.
Figure 7-6 depicts a typical ATM backbone network. The network shown in some detail in the figure might also be shown as a network "cloud." Notice the network topology is a partial mesh topology. The choice of a particular network topology is dependent upon the origin and destination of customer traffic. Like most businesses, ATM service providers seek to minimize costs and maximize revenues and profits. Switches are geographically located with the profit motive clearly in view.
Each service provider has a Network Management Center (NMC) to monitor the health of their respective network. When there is a network trunk failure, the NMC will detect the failure very quickly and dispatch a maintenance crew to fix the problem. Such a situation might arise when someone cuts the fiber interconnecting two switch sites (nodes). The typical NMC mean-time-to-repair (MTTR) goal is four hours. Network coverage is maintained 24 hours per day, seven days per week (7x24).
Figure 7-7. ATM corporate subscriber unit
The User Network Interface (UNI) is the physical point where the user equipment is connected to the service provider’s backbone. The typical location of the physical connection is the customer’s premises. Currently, corporate customers use equipment such as ATM access switches or concentrators to connect to the ATM network. In the future, consumers may use set-top boxes as the UNI for ATM access.
The typical UNI access speeds now are NxDS-1, DS-3, and OC-3.The number of ports per PVC is most often between 64 and 256. A typical corporate customer has three to five ATM UNI sites. As more corporate players enter the market, there could be some additional port and PVC choices available. For consumer access, 7xDS-1 access speed is the most interesting speed due to the bandwidth requirements for full motion video.
Figure 7-8. ATM consumer subscriber unit
Service providers offer UNI managed services including CPE installation and maintenance, and training. Network monitoring of CPE is provided by the service provider’s NMC. Just as in NNI trunk monitoring and maintenance, the NMC will dispatch a maintenance crew if the equipment has failed. The user can choose from several maintenance options including 5x8 coverage (five days per week/eight hours per day), 7x24 coverage (seven days per week, 24 hours per day), or some combination of coverage such as 5x12 (five days per week/12 hours per day). Or, the user can elect to maintain the CPE themselves after the warranty period has expired. If the user elects to maintain the CPE themselves, there are usually several choices available. One maintenance approach is to purchase spare cards and replace failed cards as required, returning the failed card to the manufacturer for repair at the user’s leisure. Another approach does not require the purchase of spare cards. The user requests a replacement card from the manufacturer who ships immediately and bills the user after the failed card is returned or 30 days later, whichever occurs first.
The typical 7x24 CPE maintenance contract specifies a four- or six-hour MTTR. The "M" in MTTR means the average. While any particular failure might take many hours to repair, the goal is an average time of four hours. Since these things are specified in the corporate user’s contract, a site MTTR log that records CPE downtime is imperative if the user wishes to enforce the service provider’s legal obligations. If the actual MTTR is significantly above the stated contract MTTR, a user can negotiate consideration from a service provider for the perceived, and actual, loss of user network resources. Also, if any legal dispute arises concerning the service provider’s legal obligations, the MTTR log becomes an indispensable document. The importance of keeping records cannot be overstated.
The typical service provider is not really providing the UNI network monitoring and CPE maintenance. The service providers do not have internal business systems capable of dealing with ATM. To provide the UNI monitoring and CPE maintenance, they are subcontracting the work out to small, third-party vendors. And the ability of the service provider to monitor the performance of the third-party vendor may not be adequate. If the user does not keep records, then disputes concerning monitoring and maintenance become the user’s word against the third-party vendors. Such situations could possible give rise to some ugly disputes over the next couple of years, as several thousands of dollars for each incident are at risk.
Corporate ATM access is characterized by the volume of diverse, non-native traffic, and the need to maintain site integrity 7x24. Figure 7-7 depicts a typical corporate UNI. All data traffic from the user is piped to concentrators, also known as aggregators or access builders. Concentrators take the non-native upstream bits and convert/combine them into an ATM cell stream. The downstream ATM cells are desegmented into their native bit streams and routed to the proper port by the concentrators. The CSU/DSU function may or may not be integrated into the concentrator functionality.
Figure 7-9. Frame Relay to ATM Interworking
Consumer access to ATM is enabled by the ADSL middleman. ADSL is necessary to provide the 1.5 Mbps to 10 Mbps bandwidth necessary to provide full motion video over the copper wire pair into the home (or business, cheap corporate access!). Figure 7-8 depicts a typical consumer ATM UNI interface, also called a set-top box.
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