ZyXEL Communications IES-1248-51 Guia do Utilizador descarregar pdf (Página 143)

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Chapter 17 xDSL Profiles Setup

IES-1248-51V User’s Guide

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17.3.1.2 Variable Bit Rate (VBR)

The Variable Bit Rate (VBR) ATM traffic class is used with bursty connections.

Connections that use the Variable Bit Rate (VBR) traffic class can be grouped into

real time (rt-VBR) or non-real time (nrt-VBR) connections.

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.

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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