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    Overhead Calculation Background

    Overhead impacts how the available bandwidth per interface is calculated. Therefore, it plays a key part in the capacity planning process. This topic an relate topics provide background on how IP/MPLSView computes overhead.

    Note that overhead calculation applies to IP Layer 3 only.

    The following are categories of overhead in IP/MPLSView:

    • Overhead triggered by the mapping of Layer 3 user frame into a lower level frame (e.g. IP over AAL5). This is also called padding.
    • Overhead triggered by the encapsulation method used by the interface (e.g. Frame Relay or ATM).
    • Overhead triggered by the Layer 2 VPN encapsulation (e.g. Martini L2VPN).
    • Overhead triggered by the transport protocol (e.g. POS).

    Note: Unless Frame Size is specified for a demand (i.e. through the demand file or demand window), IP/MPLSView will not consider encapsulation overhead for that demand.

    As a general matter the overhead of a demand is the sum of the VPN overhead and the link overhead. A generic value is used for all types of VPN; for the link overhead, a specific value is used.

    The following table provides the list of interfaces and protocols supported by IP/MPLSView along with the associated overhead. It has to be stressed that these values are used by default and can be modified by the user in the last section of the IP/MPLSView dparam file.

    Interface/Encapsulation Type

    Encapsulation overhead (bytes)

    AAL5 overhead

    16

    AAL0 overhead

    16

    PPP overhead

    4

    HDLC overhead

    4

    ETH overhead

    18

    VLAN overhead

    18

    FR overhead

    8

    DOT1Q overhead

    18

    SONET overhead

    9

     

    Labelling overhead (bytes)

    VPN overhead

    12

    MPLS overhead

    4

    GRE overhead

    24

    Here are some examples of the overhead calculation:

    • The overhead for a demand whose average frame size is 100 bytes using a VPN routed over an Ethernet is 12 + 18 = 30 bytes
    • The overhead for a demand whose average frame size is 100 bytes using a VPN routed over a POS is 12 + 9 = 21 bytes
    • The overhead for a demand whose average frame size is 100 bytes using a VPN routed over a GRE tunnel is 12 + 24 = 36 bytes

    For IP traffic over ATM, the following specific cascading procedure is applied to determine how much bandwidth is required to transport customer traffic :

    1. VPN overhead is added to the user frame if the demand is mapped with a particular VPN.
    2. MPLS overhead is added to the previous frame if a tunnel is used to transport the VPN traffic.
    3. AAL5 overhead is added to the previous frame.
    4. Then, the PDU is split into a number of ATM cells.

    Modified: 2016-11-08