ZyXEL Communications SAM1316-22, User Manual

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Chapter 14 xDSL Profiles Setup
SAM1316-22 User’s Guide
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The rt-VBR (real-time Variable Bit Rate) type is used with bursty connections that
require closely controlled delay and delay variation. An example of an rt-VBR
connection would be video conferencing. Video conferencing requires real-time
data transfers and the bandwidth requirement varies in proportion to the video
image’s changing dynamics.
The nrt-VBR (non real-time Variable Bit Rate) type is used with bursty connections
that do not require closely controlled delay and delay variation. An example of an
nrt-VBR connection would be non-time sensitive data file transfers.
14.5.1.3 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.
14.5.2 Traffic Parameters
These are the parameters that control the flow of ATM traffic.
14.5.2.1 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.
14.5.2.2 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.
14.5.2.3 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.
Note: 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.