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Questions Answered in This Chapter
What are SCR and PCR?
How do audio, video, and multimedia signals fit in with ATM transmission?
What is an ATM switch, NNI, and UNI?
What does a service provider ATM backbone look like?
What are corporate and consumer ATM access costs?
What is interworking and what technologies are ATM compatible?
What is shared access?
What transmission technologies does ATM compete with?
This chapter introduces some new ATM concepts and terminology and explains their function and importance. Also, since ATM is the transmission technology that can seamlessly transmit audio, video, and multimedia, the issues regarding ATM transmission of these applications are given treatment in some detail.
Peak cell rate (PCR) is the maximum allowable cell rate for any given PVC or SVC. PCR is the upper boundary on the rate that data can be loaded onto the ATM connection without potential loss of cells. Cells exceeding PCR are tagged Discard Enable (DE) and are eligible to be dropped. Whether or not the cells are actually discarded depends upon the volume of traffic transiting the switch at the same time, the switch capacity, and the service provider’s attitude toward PCR and the user. Currently, ATM backbones are lightly loaded and service providers are not discarding DEs unless switch capacity is maxed out. Since switch capacity is seldom maxed, some service providers are giving users a free ride, for now. PCR is expressed in megabits per second. Some service providers are setting the PCR to the port connection speed, allowing users to burst to the maximum possible. Setting PCR to port speed is another freebie from the service providers to encourage migration to the lightly loaded ATM backbones. Do not expect such generous handouts as the backbones begin to exceed 25 percent of capacity.
Peak cell rate—sounds simple enough. The maximum number of cells transmitted. Yes, but when? Over the highest one second of traffic of the last one minute? The highest one minute of the last hour? Or the highest one hour of the past one day? Confusing? Well, ATM switch manufacturers have pretty much removed the confusion. Most ATM switches have their databases polled every 15 minutes with polling times set to coincide with the quarter intervals of the clock. The switch polls on the hour and every quarter hour thereafter. The data reporting systems take either 15 or 30 minutes, depending upon switch manufacturer, to present the data to the user. So every 15 or 30 minutes you can view the switch statistics, at least those statistics the service provider deems appropriate for user consumption, of the previous 15-minute time slot. This means the peak cell rate that is visible to the user and to the service provider is going to be an average cell rate taken over at least a 15-minute interval. So, a clever user can burst above the PCR any portion of a 15-minute time period, as long as the average cell transfer rate remains at or below the PCR without incurring any monetary penalties from the service provider.
PCR is a traffic parameter that is most often used in the VBR class of service. PCR is intended to allow the user to burst above some sustainable cell rate (SCR) for short durations. Applications requiring PCR are real-time based, such as audio and video.
Sustainable cell rate (SCR) is the cell rate the user is allowed, by contract, to transmit over the ATM network without incurring any monetary penalties or resulting in dropped cells. SCR is measured in megabits per second (Mbps). SCR is an average, like PCR, as service providers poll their switches’ Management Information Base (MIB) periodically. The typical polling time is 15 minutes, although some service providers poll at 30-minute intervals. So, if the number of bits transmitted over the polled period are equal to or less than the contracted SCR, there is no monetary penalty. However, during some portion of the period, the actual transmit rate might be much greater than the SCR.
Low-speed data requires a bit rate of 64 Kbps or less. Examples of low-speed data are Morse code communications (still used in military and maritime applications), telemetry, remote monitoring, and data gathering applications. Low-speed data does not present any technical challenges to ATM and/or ADSL technologies.
Bandwidth is the commodity ATM (and Frame Relay) service providers sell. Like most commodities, the more bandwidth you want, the more you have to pay. High-speed data is the same thing as large bandwidth. The higher the speed, the larger the bandwidth required to transmit the data and the higher the price to place the data on the backbone of some service provider.
Some applications, such as real-time video- and audio-based applications, require, for acceptable performance, the transmission of data at very high speeds. Some applications do not necessarily need to transmit data at high speeds for acceptable performance, but the sheer volume of data may necessitate a large bandwidth. Such applications include banks and financial institutions that need to ship huge volumes of data concerning financial transactions in short time periods. Another example is a government bureaucracy such as the Social Security Administration. Unless these large entities use a large bandwidth transmission medium, they cannot get all their data transported in the time required. Unlike audio and video applications, if they do not succeed in getting the data from Point A to Point B, the quality of the end-user application does not suffer. While our bank account might not be debited or credited in a timely fashion if the financial institution does not transmit data at a sufficiently high speed, the end result is still the same. Eventually the account is updated, although the account holder might suffer some financial loss due to the delay. These large, commercial enterprises and government agencies are the ground-breaking leaders in adopting ATM service. The amount of data they need to ship is sufficient that the economies of scale justify the current expense of ATM.
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