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Community Member

Codec Bandwidth Calculaion


In Cvoice Studentguide Datalink overhead

for Ethernet is 18 bytes

for Frame-relay is 6 bytes

However in Cisco IP telephony QOS design guide Datalink overhead

for Ethernet is 14 bytes

for Frame-relay is 4 bytes

During Codec Bandwidth Calculaion which one should be considered?

Community Member

Re: Codec Bandwidth Calculaion

In real world you probably need to consider 18 and 6.

It is higher because of CRC.

It also depends where are you going to use the calculations:

* Calculations are statis for CCM (G729-24 and G711-80) and Gatekeeper (G729-16 and G711-128)

* Calculations are dynamic for real-time RTP streams (this is where you probably need to consider 18 and 6)



Community Member

Re: Codec Bandwidth Calculaion

It depends on what you are using this for - link capacity or QoS settings ("priority" and "bandwidth" statements in the router).

For capacity calculations on WAN links, you need to take into account the full overhead. This includes the flags that separate the frame relay frames and the FCS. So a full frame bandwidth overhead is 7 bytes (you only count 1 flag) For LAN line capacity, you also need to take into account the FCS, the preamble and start of frame delimiter, which is another 8 bytes on top of your 18 (or 22 in the case of 802.1Q)

For QoS settings, you disregard the FCS/CRC bytes. This is because these are normally stripped off in the interface hardware and are therefore not seen by IOS. This means that the QOS design guide is right for frame relay (4 bytes). However, most ethernet access is 802.1Q nowadays which requires a layer 2 overhead of an extra 4 bytes on top of the standard 14 bytes ethernet header, giving a total of 18 bytes.

ATM is an interesting case, where the QoS mechanisms look at the AAL5 level, with just 8 bytes overhead per packet. The actual line overhead with ATM is much greater.

So they are all sort of right in their own way! It depends what question you are answering.


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