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VoIP packet length

I'm testing VoIP over 64kbps Leased Line.


I'm using two cisco 1750 routers with FXS-FXO vics and ios 12.2(2)T4.

I configured LLQ, I reserved 12kbps (codec g729 8kbps 50pps) for each voip conversation in strict PQ (24kbps for 2 voip stream, max).

I used "ip rtp header-compression", interleaving, and "ppp multilink fragment-delay 30" (fragment lenght 240byte)on serial interface S0.


A)If I compute the dimension of a voip packet it would be: (20byte payload )+ (4 byte IP/UDP/RTP compressed headers) + (6byte MLP header) = 30byte !

B)If during a voip conversation (only one of two, max) I do "sh queue virtual-access 1":

cisco1750_FXS_04#sh queue virtual-access 1

Input queue: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: weighted fair

Output queue: 4/1000/64/0/1175 (size/max total/threshold/drops/interleaves)

Conversations 2/3/16 (active/max active/max total)

Reserved Conversations 0/0 (allocated/max allocated)

Available Bandwidth 24 kilobits/sec

(depth/weight/total drops/no-buffer drops/interleaves) 3/0/0/0/0

Conversation 24, linktype: ip, length: 62

source:, destination:, id: 0x0236, ttl: 254,

TOS: 160 prot: 17, source port 17296, destination port 19540

(depth/weight/total drops/no-buffer drops/interleaves) 1/32384/0/0/1175

Conversation 13, linktype: ip, length: 1502

source:, destination:, id: 0x2849, ttl: 127,

TOS: 0 prot: 6, source port 1041, destination port 524


I can see that queued voip packet have length 62byte and data packet 1502 byte !!!


Why is there that difference in packets length (voip and data fragments between calculated and showed by router ?

Is "Conversation 24, linktype: ip, length: 62" a Layer3 length ?


Re: VoIP packet length

The answer to your question here is "compression and

fragmentation is done after queueing. What you seeing is the packet

size of the original voice or data packet before

compression or fragmentation is done to it.

voice would be 12 byte rtp header + 8 byte udp + 20 byte ip header and 20byye payload

which should come to about 60 byte, the 2 byte is for additional overheads.

When the voice packet is about to be clocked out onto the wire from the

queues, the header would then be compresed to about 4 bytes.

I would suggest using a serial sniffer to see the packet sizes after transmission.

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