Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010, Configuration Manual

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To adjust the shaping rates to account for different layer 2 encapsulations as well as
the ATM cell pad, header, and trailer on interfaces, apply a parameter with the cell
byte-adjustment application (
qos-byte-adjustment ).
When you apply a parameter with the qos-byte-adjustment application to an interface
with frame shaping mode, you adjust shaping rates to account for different layer 2
encapsulations only.
Calculation and Example of Byte Adjustment for Cell Shaping
The system counts the bytes transmitted to track the shaping rate. Instead of counting
the actual packet size, the system uses the CPE packet size. You can configure the
byte adjustment so that the shaping rate matches the CPE bandwidth.
By default, the byte adjustment is set to 0. If the overhead between the access node
and CPE is 0, you do not need to configure the byte adjustment value.
Figure 65 on page 294 displays an example of an Ethernet encapsulation and an ATM
encapsulation.
Figure 65: Byte Adjustment Calculation for Ethernet and ATM Encapsulations
Table 29 on page 294 lists the header lengths for the Ethernet encapsulation, which
represents the CPE protocol overhead. The hierarchy is PPPoE over S-VLAN over
Ethernet.
Table 29: Header Lengths for Ethernet Encapsulation
Number of Bytes Header
14 bytes (6-SA, 6-DA, 2-ethertype) EnetHeader
8 bytes (2-vmanTci, 2-ethertype,
2-vlanTci, 2-ethertype) Vstack
6 bytes (1-version/type, 1-code,
2-session id, 2-length) PppoeHeader
2 bytes (2-protocol id) Ppp
4 bytes FCS
294 ■
Byte Adjustment for ADSL and VDSL Traffic Overview
JUNOSe 11.1.x Quality of Service Configuration Guide