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New Member

Wireless IP Phone and Quantity of Call per Access point

Hi,

I put this on the wrong forum here we go again:

Does anybody knows the number for the quanty of call using the new 7920 phone at the same time when connected to the same Access Point? G711 & G729? What about Latency?

Also any overhead information about voice on wireless MAC will be apreciated?

Regards

6 REPLIES
Cisco Employee

Re: Wireless IP Phone and Quantity of Call per Access point

The design guide recommends a max of 7 concurent calls for G.711 and 8 for G.729

Overhead

For the VoIP calculations below, a VoIP call has the following characteristics:

(a) The packets are made up of a 20byte IP header, an 8byte UDP header, a 12byte RTP header, and the RTP data.

(b) The RTP data is a 20ms voice sample. For G.729, this is 20bytes. For G.711, this is 160bytes.

(c) The total VoIP packet is 200bytes of (IP+UDP+RTP) headers + RTP Data. The 802.11 header (L2 MAC) is 24bytes long, so the total packet is 224bytes

(d) RTP data is transmitted at 50 packets/sec in each direction, or 100pps for a full-duplex conversation.

Hope this helps

David

New Member

Re: Wireless IP Phone and Quantity of Call per Access point

Thanks David,

The 24 bytes wireless MAC header size long does already include the encryption right (this is in the payload)??

Where can I found the design guide?

Thanks a bunch

Gab

Cisco Employee

Re: Wireless IP Phone and Quantity of Call per Access point

Hi Gab,

Yes that MAC header is regardless of WEP

At this stage the design guide is not published but a colleague of mine is working on getting this published on CCO but if you contact your account team you should be able to get more details from the design guide. If they have trouble finding it give them my user id of derwin and I will point them to it for you.

David

New Member

Re: Wireless IP Phone and Quantity of Call per Access point

Thanks darwin,

I have one more question with Voice application and wireless.

In the case that you have romaing and IP mobility enable in your network and you are crossing one cell to another cell belonging to a second AP. What happesn with the call? Since the voice RTP goes in UDP I'm worry with consecutive packet loss, How the roaming avoid this? Do you need two antennas in the Mobile pHone?

Thanks again

gab

Cisco Employee

Re: Wireless IP Phone and Quantity of Call per Access point

You need to seperate out layer 2 roaming and layer 3 roaming (IP mobility)

Layer 2 roaming.

When you move from one AP to another AP in the same LAN then from IP perspective there has been no change and packet flow will continue as per normal. The changes in MAC forwarding tables are taken care of by IAPP between the 2 AP's. The time taken for this will vary between static WEP and EAP, but there are enhancemnets in the next version of firmware to speed up roaming with EAP and this will be highly recommended.

Layer 3 roaming (mobile IP)

In this case the FA will redirect the packets to the new FA until the HA has swtiched over to the new FA

The IP address has not changed only the care of adress so packet flow as per normal

Gab, I strongly recommend that you talk to your account team as this is a rapidly changely area of this techonology and they will be able to give you more help with the exact details as they pertain to your network

Silver

Re: Wireless IP Phone and Quantity of Call per Access point

David:

Is the wireless design guide now available? And if so, can you send me a link to it?

johnruffing@suntel.com

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