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freinds i went through the study guide completely
i have following doubts pls help me clear them

1) difference between BSSID,SSID,ESSID?
is ESSID and SSID same? ie when SSID is used by more than 1 BSS SSID is called ESSID. is that the meaning?

2) can upfade be used as advantage for WLAN?

3) root mode and non root mode though the text says its covered in more detail in CWAP, i dint get the basic idea related to it.

4) SMA connector dint find much on it in book

5) correct me if i am wrong, at higher frequencies the throughput decreases so why is the 915MHz band not used. is it due to overcrowded frequency band users?

1. BSSID identifies a single BSS--that is, an AP and all of the clients that are associated with the AP. The BSSID is usually (always?) the MAC address of the wireless interface on the AP.

ESSID is a text string that can be configured on the AP. ESSID has several purposes. First, stations use the ESSID to identify which network they want to join. Few client utilities give the user the ability to specify by MAC address the AP with which the client should associate. Nearly all client utilities require the user to enter the ESSID. Then the client station will associate with any AP that has that ESSID. A second purpose for the ESSID is that, if two APs have the same ESSID, clients should be able to roam between them.

SSID is a short-hand for ESSID. Because the BSSID is hardly ever encountered by casual 802.11 users, there is seldom a need for casual 802.11 users to disambiguate between ESSID and BSSID, so they just shorten it to SSID.

2. The real question would be "how would you know whether signal strength was being increased by upfade?" Multipath is SUCH an unpredictable phenomenon, especially in indoor environments where there are essentially an infinite number of paths between the transmitter and receiver, that attributing a particular increase or decrease in signal strength to multipath is very difficult. Maybe somebody who is experienced with a spectrum analyzer could do it.

Even if I did design a network around upfade, I would be worried that the environment would change and my extra signal strength would go away. That's why I always design wireless networks with a fade margin--a little bit of extra signal strength that can compensate for fluctuations in the environment, etc...

3. I think you're talking about bridges. Bridges can be set to either root mode or non-root mode. In a given bridge system, exactly one bridge must be set to root mode. All other bridges must be set to non-root mode. Other than that, there's not much difference between them.

4. It's another connector. What do you want to know about it?

Thanks Joshua.

I thought I'd clarify for you about the "always" - The BSSID is, per the standard, the mac address of the radio card....however, in the real world that's not always true.

Microsoft released a whitepaper some time ago explaining "virtual access points" and then vendors began implementing things this way. Take Colubris as an example. Their APs can send out several beacon streams - each with a unique BSSID/ESSID pair - such that each AP looks like multiple APs to the clients.

Hope this clarifies. thanks,

Devinator

I think you got it wonr by saying that the higher the frequency, the lower the throughput. What is correct to say is that the higher the frequency, the less range it has. This is probably what confuses you.

For example, if a DSSS system operates in 2.4 GHz and 5.8 GHZ then at the same distance, the 2.4 DSSS might have a higher data rate than the 5.8 DSSS (assuming all other things equal). Therefore you might conclude that the lower frequency one has higher throughout but i think it is not right to say it that way....it is more of...

Higher frequency -> less distance

Anyone wants to correct me?

All other things being equal, higher frequency signals are capable of carrying more data than lower frequency signals. In a digital modulation system, the carrier frequency must be higher than the frequency of the modulated data. Therefore, higher frequency signals can have data modulated onto them at higher bit-rates than lower frequency signals.

But all other things are never equal. Higher-frequency signals are also more prone to attenuation and suffer from more free space path loss. Therefore, the data rate of a high frequency signal may be reduced below its theoretical maximum in order to introduce error correction that compensates for the decreased SNR caused by the increased attenuation of the higher-frequency signal. As a result, the practical data-carrying capacity of the higher-frequency signal at a given range may not be significantly higher than that of the lower-frequency signal.

Another factor is interference. The 2.4 GHz range is so full of interference that it usually (in my experience) achieves less throughput than the 5 GHz range, even though 5 GHz is theoretically more subject to attenuation.

I couldn't say for sure why the 900 MHz band isn't more widely used. I do know of a few companies that make 900 MHz digital wireless systems. All of them top out at around 1 Mbps to 1.5 Mbps. I wonder whether the lower frequency signal of the 900 MHz system places a practical limit on the maximum bit-rate that can be pushed through the system. Or maybe it's just not economically feasible to do it. There's no doubt that there are political and market considerations in the decision of which frequency band to use, as well as purely technical/engineering ones.

thank u joshua, cgo, devinnator
thank u very much for ur explainations,really got my doubts cleared
as i am jus a student and dont have much hands of experience on wi-fi devices these answers were really helpful.
regarding the bridges root mode i really dint get the exact point.
the text says it is covered in detail in CWAP but the term 'root mode' has left me in bit confusion and hence finding it difficult to understand different modes of bridges
once again thank u

I couldn't say for sure why the 900 MHz band isn't more widely used. I do know of a few companies that make 900 MHz digital wireless systems. All of them top out at around 1 Mbps to 1.5 Mbps. I wonder whether the lower frequency signal of the 900 MHz system places a practical limit on the maximum bit-rate that can be pushed through the system. Or maybe it's just not economically feasible to do it. There's no doubt that there are political and market considerations in the decision of which frequency band to use, as well as purely technical/engineering ones.

Yes Joshua,

That 900MHz range is utilized by few... I didn't know this till recently;

DirectTalk, a feature on some of the Nextel/Sprint cell phones is using the 900Mhz ISM band as the communications medium for private communications off the air. It is a walkie talkie feature.

BTW, it is a 700mW radio they are using. Your range should be pretty impressive.



http://www.nextel.com/en/support/faq/direct_talk.shtml

[quote="Joshua Bardwell"]
3. I think you're talking about bridges. Bridges can be set to either root mode or non-root mode. In a given bridge system, exactly one bridge must be set to root mode. All other bridges must be set to non-root mode. Other than that, there's not much difference between them.
/quote]

Not quite true. Bridges run a thing called Spanning Tree Protocol (STP) based on IEE 802.1D, which describes the process of automatically selecting the root, and calculating distances to reach the root for a multi bridge system. If 2 bridges are set to the same low priority, the lower MAC addres wins.

http://en.wikipedia.org/wiki/Spanning_tree_protocol

Bridges can be either set to participate in STP or not. You cannot force a bridge to be the root if there is already a bridge with the lowest priority and a lower MAC. You cannot set a bridge to be root without knowing the priority and bridge MACs of the other bridges. I suppose a vendor might automate forcing a root by dynamically picking its MAC address.

If there is a chance that there could be 2 connections to the backbone from a given bridge, you really should enable STP. Otherwise, disable it. For single ethernet port APs it's safe to leave it off. SOHO APs with built in switches may not have a setting for STP - be careful!

Many managed switches also have a mode to allow you to set an ethernet port to come online quicker if it's connected to a PC or server instead of to another switch. Cisco calls it PortFast.

=seymour=

Seymour,

I'm familiar with STP. It's my experience that very few, if any, wireless "bridges" implement STP. I think there is at least one out there that can do it, but I can't think of the vendor off the top of my head. I don't work with wireless bridges every day, and I tend to work with a limited pool of vendors, so I'm open to having my information updated.

The "root bridge" in an 802.11 bridging scenario is unrelated to the root bridge in a STP scenario. It's the same term, but they're being used in two completely different ways. For example, "root bridges" in an 802.11 bridging scenario are usually manually configured, wereas STP automatically and dynamically selects a root based on various priority factors.

All:

BSS -- one set of synchronized stations, infrastructure or independent
ESS -- one infrastructure BSS, or several of them interconnected
IBSS -- one independent BSS
SSID -- a text string identity of a BSS, not necessarily unique
BSSID -- a unique MAC address identity of a BSS

IEEE does not use the term ESSID. It should be avoided. Ok, the term "ESS ID" (sic) was used twice in IEEE 802.11-1999 section 5.7.2 and 5.7.3 where the term "SSID" would have been more in keeping with the text. This has been offered for correction in 802.11n draft 5.2 with the deletion of these sections.

Please notice that one infrastructure BSS is also an ESS. There is no special name for a single infrastructure BSS. When the number of BSSs in an ESS doesn't matter, the IEEE uses the term "ESS". When the number of BSSs in an ESS must be one, the IEEE uses the term "infrastructure BSS".

For example: Section 7.3.2.1 "SSID element. The SSID element indicates the identity of an ESS or IBSS."

It is common practice to use one SSID for all BSSs in an ESS but this is not required. Each of multiple SSIDs used by an ESS identifies the ESS that uses them.

I hope this helps. Thanks. /criss