WLAN Roaming

Roaming in WLAN (also in GSM networks) is associated with physical movement and connectivity while moving.

You could apply following terms to roaming in WLAN networks:

1. Pre-call roaming.
It occurs when user moves before making a call. This movement could occur between Layer 2 and Layer 3 boundaries. In Layer 3 roaming phones will change its IP address.

2. Mid-call roaming.
It occurs when the user moves with an active call.

So as I said before, we could divide device roaming on:

1. Layer 2 roaming
Layer 2 roaming occurs when a station (PC or wireless IP phone) moves from one access point to another. Both access points are on the same IP subnet, so station doesn’t have to change its IP address.

2. Layer 3 roaming
Layer 3 roaming occurs when a client moves from one access point to another. These two access points work on different IP subnets. So phone will have to change its IP address. The problem with changing IP address is that its voice calls or data sessions will drop.

If you are client in wireless networks with multiple access points, you could have lots of problems if the wireless roaming is no configured properly.

There should be the same SSID, and the same security settings (type of security and keys), but the frequency channels should not overlap. Three channels do not overlap 1, 6 and 11 do not overlap each other, the same as 2, 7 and 12. You can check this on the picture below. So this is not a problem if you have less than three access points or wireless routers in the wireless network. However, if you have more than three access points, make sure that access points which are close to each other are not running on the overlapping channels. You can see on the picture which channels do not overlap.

Wireless client will automatically switch from one access point or wireless router to another and select the one with the best signal in this particular place and time.

If you are expert in wireless and want find out more about WLAN Roaming, check the book - Roaming in Wireless Networks. It covers not only WLAN, but also CCS7, GSM, GPRS, 3G.

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