Light weight access point, vlans, multiple ssids

[sarahr202]

Hi everybody

Let say we have an light weight access point ap1.  Ap1 is broadcasting two ssids:

cisco1  which is mapped to vlan 1

cisco 2  which is mapped to vlan 2

If ap1 is using channel 6 for cisco 1, does it mean ap1 will also use same channel i.e channel 6 for cisco2?

thanks and have a great weekend.

[Leo Laohoo · Accepted solution]

sarahr202 wrote:
Hi everybody
Let say we have an light weight access point ap1.  Ap1 is broadcasting two ssids:
cisco1  which is mapped to vlan 1
cisco 2  which is mapped to vlan 2
If ap1 is using channel 6 for cisco 1, does it mean ap1 will also use same channel i.e channel 6 for cisco2?
thanks and have a great weekend.

Lightweight WAP right?  As in controller-based WAP?

If this is the case, then the answer is both a yes and a no.

Let me explain:

Throw away the notion that you can set the channel down.  I mean, if you have a controller-based WAP, the last thing you want to do is "micro-manage" which channels your WAPs operate on.   I mean, you can but as a rule-of-thumb, you don't and let the controller sort things out.

So, going back to your question:  You whave multiple WAPs and two SSID:  1 and 2.  Let's presume that you've configured that all your WAPs will be broadcasting SSID 1 and SSID 2.

The decision about what channels each WAP will be operating on falls squarely on the Wireless LAN Controller (WLC).  The WLC makes this decision based on a blah-blah-blah algorythm.  If, for example, WAP A and, say, WAP R can "hear" each other on the same channel, the WLC will make the decision and say, "Hey WAP R, since you and WAP A are operating in the same channel and both of you can hear each other, why don't you, WAP R, operate in channel 11.".

However, if WAP A and WAP R can't see each other then both of them can operate in the same channel.

NOW, here's comes the tricky question ... Here's the scenario:  You have SSID 1 and SSID 2.  You want all your WAPs to broadcast both SSID.  HOWEVER, you want SSID 1 to operate at, say, 1 Mbps rate only while SSID 2 can operate at all other data rates.

Yes, this can be done using RF Profile and AP Groups.

Is this what you are asking?

[sarahr202]

NOW, here's comes the tricky question ... Here's the scenario:  You have  SSID 1 and SSID 2.  You want all your WAPs to broadcast both SSID.   HOWEVER, you want SSID 1 to operate at, say, 1 Mbps rate only while SSID  2 can operate at all other data rates.

So we want the AP to allow wireless clients in SSID 1 to connect at 1 Mbps only regardless of signal quality i.e a wireless client  very close to ap ,receives a better signal( more power) and can use higher data rate but we want AP to allow it to connect at 1 Mbps only.

Did i understand the design requirement correctly?

thanks Loelahoo and have a great weekend.

[George Stefanick · Accepted solution]

Its Airtime. Let me explain...

Wireless is a half duplex medium, which means ONLY 1 device can talk on channel at a time. It uses CSMA-CA which uses a phyiscal and virtual carrier sense.

Access points send out 1 beacon per SSID 104ms. if you have the max SSID set, which on controllers is 16. You can see you need to send 16 beacons every 104ms.

I did test, 1 access point, 16 ssid. NO TRAFFIC. I used 22% wifi ultization on channel JUST for beacons, NO TRAFFIC.

Yes, as you add more clients, its just like a wired hub. Everyone has to use CSMA and play nice.

What applications are you running ?

[sarahr202]

What applications are you running ?

None. LoL  i am just learning.the basics.

thanks

[George Stefanick · Accepted solution]

Oh, i c... Well welcome to the wolrd of WiFi.

If I can offer some good reading for you..

CWDP is a great book for surveys.

CWAP is great for adavnce 802.11.

CWSP is great for wifi security.

CWNA is great for enry level

Tom Lammles CCNA Study guide is good

Ive had a part or mention in most of the books above. The matrial is very very good.

and there is a awsome blog www.my80211.com  (mine)

LOL

Enjoy!

[sarahr202]

Thanks George.

Currently I am studying for CCNP switch exam. It does overlaps with wireless lan a little bit .   The exam does not cover wireless topics in detail but I just got very interested in wireless. I am planning to get few certs in wireless once I am done with CCNP. Thanks for the list of books, I definately keep them in mind when i start my wireless study.

thanks and have a great weekend.

[George Stefanick]

Good deal. Good luck on the switch exam. I heard horror stories about that exams accuracy (check the cisco cert forums). Good thing you are going deep on the related items.

Good luck

[sarahr202]

Great blog with tons of information 802.11.

I will defeinately use the atricles,videos when i start my wireless study.

keep up the good work!

[George Stefanick · Accepted solution]

You can not set PHY rates by SSID, ONLY by radio. So your exmaple, Client one is limited to 1 and client 2 on ssid 2 gets 54 ... no can do ..

Remember, this is a radio setting on Cisco. So any chnages made on the radio impacts all SSIDs on that radio.

[Amjad Abdullah · Accepted solution]

Hi,

On one AP, the channel is decided per-radio basis. If an AP 802.11b/g band is using channel 6 for example, then all SSIDs on that AP will be broadcasted on channel 6 on 802.11b/g band. Multiple channels on same band can not be used on same SSID. only one channel per band per AP.

So as an answer to your concern the answer is "Yes. Cisco1 and Cisco2 will be broadcasted on same channel".

HTH

Amjad

[George Stefanick · Accepted solution]

Sarah,

1 radio = 1 channe; regardless of SSIDs

This is also true of 5 GHz. If you have a dual band ap, say 2.4 GHz is channel 1 and 5 GHz is channel 161 ..

Cisco 1 = would be broadcasted out of 1 and 161

Cisco 2 = would be broadcasted out of 1 and 161

This is another reason why you dont want to have a lot of SSIDs, say more than 5 or 6. Becuase all these SSIDs will fight for air space.

[sarahr202]

Thanks George and Amjad.

So far I learned   be it  an autonomous access point or light weight ,it will use one channel per radio for different ssids configured on it.

What are the some  ramifications of using multiple ssids on single ap?

The following is my observation :

Let say we have an ap1 which is using channel 6; ap1 is also broadcasting ssids cisco1 and cisco2

If we have client1 in cisco1, and client2 in cisco2, then client 1 has to make sure medium first (channel 6) is  available and not used by any wireless clients in CIsco1 or Cisco 2 before client1 can transmit its frame.

The question as the number of clients increases in both SSIDS i.e cisco 1 and cisco 2 ,the probability of channel usage also increases which could make a wireless client to wait longer before it could transmit.

So having a number of SSIDS with each having a number of clients on single AP, does affect the performance.

I appreciate your input.

thanks and have a great weekend.