FR, FR + FRF.12, and MLPoFR L2 overhead

I've always followed the SRND in terms of calculating the L2 overhead for the LLQ policy-map values.

Copied from the QoS SRND (pg 33, Chapter 1-15)

--
A more accurate method for provisioning VoIP is to include the Layer 2 overhead, which includes
preambles, headers, flags, cyclic redundancy checks (CRCs), and ATM cell-padding. The amount of
overhead per VoIP call depends on the Layer 2 technology used:
• 802.1Q Ethernet adds (up to) 32 bytes of Layer 2 overhead.
• Point-to-point protocol (PPP) adds 12 bytes of Layer 2 overhead.
• Multilink PPP (MLP) adds 13 bytes of Layer 2 overhead.
• Frame Relay adds 4 bytes of Layer 2 overhead; Frame Relay with FRF.12 adds 8 bytes.
• ATM adds varying amounts of overhead, depending on the cell padding requirements.
--

For a g711 call, I understood the frame size would be-

FR = 160B + 40B + 4B
FR + FRF.12 = 160B + 40B + 8B
MLPoFR = 160B + 40B + 13B (MLPoFR)

This thread from an online forum seems to also follow this calculation;

http://www.mail-archive.com/[email protected]/msg06146.html

But then I came upon Petr's post on "Computing VoIP traffic bandwidth consumption"

http://blog.internetworkexpert.com/2008/10/17/computing-voip-traffic-bandwidth-consumption/

FR = 160B + 40B + 7B
FRF.12 = 160B + 40B + 7B
MLPoFR = 160B + 40B + 9B

This blog makes complete sense on how voice packets are not fragmented so there should be no additional FRF.12/MLP overhead for voice packets.

But for the lab should we be following the SRND L2 overhead values, Petr's post, others? What if the question states to refer to the SRND?

Thanks,

Comments

  • Hi,

    when i was working on the blog post, i was comparing my computations with the table from the following document:

    http://www.ciscosystems.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

    So far it looks like the approach suggested in the blog post matches numbers in the table. This is why, instead of memorizing the numbers, I simply memorized the basic overhead and was always using the bandwidth consumed with L2 headers in my configurations. The same as well applies to the real exam.

    Petr




    ----- Original Message -----
    From: "antyeung" <[email protected]>
    Sent: Wed, January 28, 2009 22:59
    Subject: [CCIE Voice] FR, FR + FRF.12, and MLPoFR L2 overhead


    I've always followed the SRND in terms of calculating the L2 overhead for the LLQ policy-map values.

    Copied from the QoS SRND (pg 33, Chapter 1-15)

    --
    A more accurate method for provisioning VoIP is to include the Layer 2 overhead, which includes
    preambles, headers, flags, cyclic redundancy checks (CRCs), and ATM cell-padding. The amount of
    overhead per VoIP call depends on the Layer 2 technology used:
    • 802.1Q Ethernet adds (up to) 32 bytes of Layer 2 overhead.
    • Point-to-point protocol (PPP) adds 12 bytes of Layer 2 overhead.
    • Multilink PPP (MLP) adds 13 bytes of Layer 2 overhead.
    • Frame Relay adds 4 bytes of Layer 2 overhead; Frame Relay with FRF.12 adds 8 bytes.
    • ATM adds varying amounts of overhead, depending on the cell padding requirements.
    --

    For a g711 call, I understood the frame size would be-

    FR = 160B + 40B + 4B
    FR + FRF.12 = 160B + 40B + 8B
    MLPoFR = 160B + 40B + 13B (MLPoFR)

    This thread from an online forum seems to also follow this calculation;

    http://www.mail-archive.com/[email protected]/msg06146.html

    But then I came upon Petr's post on "Computing VoIP traffic bandwidth consumption"

    http://blog.internetworkexpert.com/2008/10/17/computing-voip-traffic-bandwidth-consumption/

    FR = 160B + 40B + 7B
    FRF.12 = 160B + 40B + 7B
    MLPoFR = 160B + 40B + 9B

    This blog makes complete sense on how voice packets are not fragmented so there should be no additional FRF.12/MLP overhead for voice packets.

    But for the lab should we be following the SRND L2 overhead values, Petr's post, others? What if the question states to refer to the SRND?

    Thanks,



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