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MSAP2000 AAMS User’s Guide

7.12.1.3 Unspecified Bit Rate (UBR)

The Unspecified Bit Rate (UBR) ATM traffic class is similar to the ABR traffic class

for bursty data transfers. However, while ABR gives subscribers a set amount of

bandwidth, UBR doesn't guarantee any bandwidth and only delivers traffic when

the network has spare bandwidth.

7.12.2 Traffic Parameters

These are the parameters that control the flow of ATM traffic.

7.12.2.1 Peak Cell Rate (PCR)

Peak Cell Rate (PCR) is the maximum rate at which the sender can send cells.

This parameter may be lower (but not higher) than the maximum line speed. 1

ATM cell is 53 bytes (424 bits), so a maximum speed of 832Kbps gives a

maximum PCR of 1962 cells/sec. This rate is not guaranteed because it is

dependent on the line speed.

7.12.2.2 Sustained Cell Rate (SCR)

Sustained Cell Rate (SCR) is the mean cell rate of each bursty traffic source. It

specifies the maximum average rate at which cells can be sent over the virtual

connection. SCR may not be greater than the PCR.

7.12.2.3 Maximum Burst Size (MBS)

Maximum Burst Size (MBS) is the maximum number of cells that can be sent at

the PCR. After MBS is reached, cell rates fall below SCR until cell rate averages

to the SCR again. At this time, more cells (up to the MBS) can be sent at the

PCR again.

The following figure illustrates the relationship between PCR, SCR and MBS.

Figure 32 PCR, SCR and MBS in Traffic Shaping

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ZyXEL MSAP2000 Manuel d'utilisateur Page 71