different SSIDs and wireless roaming

I'm new to wireless technology,I'm confused with some aspects about wireless roaming?

1. If I want to use some APs to build a roaming environment,do I "have to" use same SSIDs?I know many many materials and web pages suggest all the APs should use same SSIDs to build a roaming environment,but I don't know if this is a Mandatory requirement.
2. What is "Roaming" definition?some web pages say "roaming is moving between cells".Does it mean the different cells could have different SSIDs? If a wireless client moves between AP1 with SSID1 and AP2 with SSID2,may I call this "roaming"?
3. A lot of web pages say that during the roaming process,the wireless client will try to find a better signal offering the same SSID,is this the default behavior of client?what if it can't find a wifi signal with the same SSID?
4. If a client finds a better AP signal offering the same SSID,and it tends to roam,after the reassociation to the new AP, will the client send a disassociation frame to the old AP ? or the new AP will send the disassociation frame?

1 Answer

If I want to use some APs to build a roaming environment,do I "have to" use same SSIDs?I know many many materials and web pages suggest all the APs should use same SSIDs to build a roaming environment,but I don't know if this is a Mandatory requirement.

That's kind of what a SSID is for. It tells devices that these APs belong to the same network and that it is possible to seamlessly roam between them.

If the APs use different SSIDs, the device will automatically assume they belong to different networks and therefore roaming is impossible. (There's no other way for it to know.) Of course the client will automatically reconnect to a new network if it completely loses the old connection, but that's not really called "roaming".

What is "Roaming" definition?some web pages say "roaming is moving between cells".Does it mean the different cells could have different SSIDs? If a wireless client moves between AP1 with SSID1 and AP2 with SSID2,may I call this "roaming"?

In Wi-Fi, it can mean both things, but the most common definition is moving between BSS (APs) while remaining in the same ESSID and without disrupting data transfer.

For example, to quote a blog post found at wi-fi.org: "When roaming is effective, client devices can move from one area to another and roam from AP to AP without disrupting applications that require a persistent network connection."

The part I emphasised implies that roaming devices need a way to know that the connection will be preserved – and that usually means looking for an identical SSID.

A lot of web pages say that during the roaming process,the wireless client will try to find a better signal offering the same SSID,is this the default behavior of client?what if it can't find a wifi signal with the same SSID?

Different clients have different behavior.

• Android:
• iOS:
• Intel:
• wpa_supplicant:

If a client finds a better AP signal offering the same SSID,and it tends to roam,after the reassociation to the new AP, will the client send a disassociation frame to the old AP ? or the new AP will send the disassociation frame?

AFAIK, the client will.

Note that "basic" roaming requires the client to disassociate from the old AP before it can associate to the new one. For "seamless" roaming all devices need to support 802.11r.