ZyXEL Communications IES-1248-71, User Manual

Page: 141 / 464

Chapter 17 xDSL Profiles Setup
IES-1248-71/73 User’s Guide
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17.3.1.3 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.
17.3.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.
17.3.2.1 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.
17.3.2.2 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.
17.3.2.3 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.
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If the PCR, SCR or MBS is set to the default of “0”, the system will assign a
maximum value that correlates to your upstream line rate.
The following figure illustrates the relationship between PCR, SCR and MBS.