Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Community Member

7920 and 1200 AP's

Greetings,

I had a 7920 about 2 months ago until it crapped out. I have been setting up AP's but have been unable to test them with the phone.

Please correct me if I'm wrong.... One 1200 AP will support 5 phones. What if you are in a remote location away from the LAN and have to set up an AP as a repeater? Does the 1200 support a 7920 in repeater mode? And if so, what are the limitations as far as number of phones?

Thanks

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: 7920 and 1200 AP's

Although, the design guide really does not say dont do it I would not design any WLAN network running VoIP with repeaters in fact for data only networks a repeater is my last choice only.

You can have 7 or 8 phones per AP depending on codec but if you use a repeater you will need to half this as the throughput will be halved. The 7 or 8 phones is derrived from calculations based on bandwidth, I can post them if you need. You will also find a increase in latency and may find your channel utilization figures much higher than if you dont use a repeater. This is due to the fact that every packet recieved by the repeater is transmitted again by the repeater.

4 REPLIES
Cisco Employee

Re: 7920 and 1200 AP's

Although, the design guide really does not say dont do it I would not design any WLAN network running VoIP with repeaters in fact for data only networks a repeater is my last choice only.

You can have 7 or 8 phones per AP depending on codec but if you use a repeater you will need to half this as the throughput will be halved. The 7 or 8 phones is derrived from calculations based on bandwidth, I can post them if you need. You will also find a increase in latency and may find your channel utilization figures much higher than if you dont use a repeater. This is due to the fact that every packet recieved by the repeater is transmitted again by the repeater.

Community Member

Re: 7920 and 1200 AP's

Thank you for the response. Can you please post the calculations based on bandwidth.

This was not a design by choice, but the customer did not want to trench a fiber run. I assured him the quality would not be as good.

Also, can you please clarify the difference between non-overlapping channels and different channels. I thought they were the same.

Thanks again.

Cisco Employee

Re: 7920 and 1200 AP's

One of the key aspects to remember when calculating network capacity for 802.11b networks is that it is a shared medium. So consideration must be given for “radio contention” amongst the various devices. This means that the back-off algorithms in 802.11b that allow multiple devices to access the medium will affect the overall throughput.

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.

Looking at the 11Mb column of the chart, we can make the following calculations for G.711:

256 byte packet size = 2,596,588 bit/second = 324,573 bytes/second (Theoretical Packet Rate)

100 packets/second * 224 bytes/packet = 22,400 bytes/second (Bandwidth of G.711 VoIP call)

324,573 / 22,400 = 14.489 calls (Theoretical Maximum VoIP capacity per 802.11b channel)

14.489 * .6 = 8.69 calls (Theoretical Maximum number of VoIP calls * 60% of bandwidth)

NOTE: 60% of bandwidth is used to calculate the number of VoIP calls

(a) it allows bandwidth to be available for data traffic

(b) it provides bandwidth consideration for 802.11b management traffic and acknowledgements

The design consideration for G.711 calls is not to exceed 7 concurrent VoIP calls per AP.

This number has been proven in lab testing to provide acceptable voice quality.

The design consideration for G.729 calls is not to exceed 8 concurrent VoIP calls per AP.

This number has been proven in lab testing to provide acceptable voice quality.

If you talk to your account team and ask them for the 7920 design guide you can see the full details

"Also, can you please clarify the difference between non-overlapping channels and different channels. I thought they were the same. "

In 802.11b there is 11 channels (for the US some countries have different laws) each with a channel width of 22Mhz, this is a large channel width and means it overlaps some other channels This means that there is only 1 combination of 3 channels that do not overlap with each other 1,6 and 11 and several combinations of 2 channels that do not overlap eg 2 and 7 etc

This link has a good diagram showing the channel overlap

http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.html

David

Community Member

Re: 7920 and 1200 AP's

Remember it is never really 11mbs becausse of the csma/ca protocol overhead for dcf the real rate is 5.5mbs closest to the ap. So when you look at the drs/ars points of 11, 5.5, 2 and 1 mb it is really 5.5, 2.5, 1 and 500kbs.

211
Views
0
Helpful
4
Replies
CreatePlease to create content