ZyXEL Communications ALC1224, User Manual

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ALC1224 User’s Guide
4-4 Virtual Channel Management
Variable Bit Rate (VBR)
The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections. Video conferencing is an example
of a VBR connection. It requires real-time data transfers and the bandwidth requirement varies in proportion to the
video image’s changing dynamics.
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.
4.4.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.
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.
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.
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.
Minimum Cell Rate (MCR)
Minimum Cell Rate (MCR) is the minimum rate at which the sender can send cells
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, MCR and MBS.