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Minimum Cell Rate

The minimum cell rate (MCR) applies only to the available bit rate (ABR) class of service. The MCR is the transmit rate in cells/second that the source is always allowed to send. It is the minimum bandwidth required by the end application.

Maximum Burst Size

The maximum burst size (MBS) is defined as the maximum number of cells that can be transmitted per burst at PCR. A burst is defined as the consecutive transmission of two or more cells. A typical maximum burst size is 256 cells. However, certain access technologies have a specific MBS. See Table 6-2.

Access Network AAL5 (cells)
Ethernet 32
FDDI 91
IP over ATM 192

Table 6-2. MBS of several access technologies

Cell Loss Priority

Cell loss priority (CLP) is a bit field of the ATM header. The CLP field is used by the ATM network to determine the eligibility of a particular cell for discard. Normally, the CLP bit is set to binary 0. When the user exceeds, or bursts above, the contracted SCR, the CLP bit of each cell exceeding the SCR is tagged Discard Enable (DE) by setting the CLP bit to 1. If the carrier backbone becomes congested, then the carrier can discard the DE tagged cells. When the network becomes congested, the DE tagged cells can be discarded, even if the transmission rate is still less than the PCR.

Maximum Cell Delay

Maximum cell delay is the time required for a cell to travel from the first ATM ingress port to the last ATM egress port. Put another way, maximum cell delay is the time required for a cell to travel from the source ATM port to the destination ATM port. Maximum cell delay is measured in milliseconds and is measured in one direction only. The transmit path from point A to point B may be different from the transmit path from point B to point A, according to the carrier’s network topology and cell routing strategy. So, the maximum cell delay in a duplex communication mode can vary considerably in the transmit direction between two locations.

Cell Delay Variation

Cell delay variation is a measurement of the maximum variance from the average time interval between successive cells transmitted from source to destination. Like the maximum cell delay descriptor, it is measured from ingress ATM port to egress ATM port, and it is measured in milliseconds. Cell delay variance describes the maximum delay any cell has experienced as the cell transisted the carrier’s ATM network. This descriptor is extremely important to real-time audio and video applications where a delay of microseconds can distort the signal, resulting in poor quality of the received signal.

Cell Loss Probability

Cell loss probability is a measurement of the possibility that a particular cell will be discarded by the carrier’s network. Cell loss probability is different for each Quality of Service which defines specific user service categories such as constant bit rate (CBR).

Traffic Shaping

Traffic shaping is the ability to change the rate of cell transmission on the fly at the customer’s premises in order to comply with the carrier’s traffic contract requirements such as SCR, PCR, and MBS. Traffic shaping in this context is performed by CPE. However, not all CPE can perform traffic shaping. Let’s say the user network is pumping data into the ATM CPE at a rate far above the SCR of a CBR service class. If there is no traffic shaping capability in the ATM CPE, all the cells exceeding the SCR are tagged Discard Enable and may or may not be transmitted to the destination, depending upon how the carrier’s network is loaded at the moment of attempted transmission and the carrier’s attitude toward allowing customers to burst above the service contract limits. However, if ATM CPE with traffic shaping capability is used, the excess cells are held in abeyance and allowed to enter the ATM port at the SCR rate. Think of traffic shaping as an ATM cell bucket with a funnel of varying diameter, as set by the user’s service contract, attached to the bottom of the bucket that all the cells must pass through upon egress. Do big buffers come to mind?

Carriers may be lax right now, due to very lightly loaded networks, in policing their ATM networks for contract compliance by users. As the lightly loaded networks begin to experience capacity ratios of 25 percent or better, users can expect carriers to rigorously enforce contractual obligations. If the CPE equipment in the user location is not equipped to perform traffic shaping, plan on either upgrading or changing CPE or experiencing some degree of network performance less than optimum.

Here is a traffic cost analysis that may be fruitful for users to perform: ATM traffic volume is not constant all through the day and night. Traffic waxes and wanes just like the moon. The period of least traffic is midnight to 6 a.m. CST. Sending traffic that exceeds the contractual rates during lightly loaded periods instead of the busy part of the day may not get the user’s cells discarded. Remember, ATM is the statistical multiplexing of transmission and signaling theory. Some combination of traffic shaping and transmitting during off peak hours may yield higher network throughput while allowing the user to purchase lower rate, and less expensive, PVCs. The difference can be several hundreds of thousands of dollars per year for the heavier hitters. Statistical multiplexing. Probability. If the user understands the concepts as used in ATM networks, significant cost savings can be realized.

Service Classes

Service classes are really nothing more than a hierarchical set of user categories that determine the access priority to the switch and transmission facilities. Service classes are tied directly to carrier revenue potential and are therefore of great significance to carriers. From the user’s perspective, choosing the proper service class for the network application is important for two reasons. If the user is paying for a service class that results in circuit underutilization, the unused bandwidth is money wasted. However, if the service class chosen is not adequate for the volume and nature of data transmitted, dropped cells mean data must be retransmitted, perhaps innumerable times. Of course, for real-time audio and video applications, dropped cells are not good. The user should give careful consideration to the application and the various service classes offered by the carriers before choosing the service class for the application.


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