ZyXEL Communications P-660RU-T1 v3, User Manual, Page: 71 / 230

Chapter 7 Internet Setup
P-660RU-Tx v3 User’s Guide
71
7.3.5 Always-On Connection (PPP)
An always-on connection is a dial-up line where the connection is always up regardless of
traffic demand. The ZyXEL Device does two things when you specify an always-on
connection. The first is that idle timeout is disabled. The second is that the ZyXEL Device will
try to bring up the connection when turned on and whenever the connection is down. An
always-on connection can be very expensive for obvious reasons.
Do not specify an always-on connection unless your telephone company offers flat-rate
service or you need a constant connection and the cost is of no concern.
7.3.6 ATM QoS
ATM QoS is an agreement between the carrier and the subscriber to regulate the average rate
and fluctuations of data transmission over an ATM network. This agreement helps eliminate
congestion, which is important for transmission of real time data such as audio and video
connections.
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) 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) 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.
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.
Figure 27 Example of ATM OoS