Seamless Roaming 

You can continue talking on your cellphone even as you move accross different cellphone coverage areas. In fact, you can see the names of the nearest towers change on your cellphone display as you cross over from one zone to another. How does this happen? The process is called seamless roaming.

For seamless roaming to work, you should have a set of similarly configured access-point equipment at the right distances. The access-point equipment can be anything, your cellular towers or the wireless access points you have on your WLAN. Why should they be similarly configured? Well, consider that your cellular networks are all intermixed in reality, sharing the same airspace. You would see towers of all cellular operators in your area, but your phone roams only between those of your operator. Why? That is because it is on a particular frequency range and encryption from the others. Your operator's SIM, which is in your handset, tells the instrument to look only for that set when negotiating a transfer between two phones. If you change one tower (or access point in your case) to a different frequency range or encryption, you will see it instantly vanishes from your phone or notebook's list of possible suspects it can connect to. Finally, it is pointless to have two or more access points well within the same coverage area, since they would overlap quite a bit, wasting wireless energy.

For seamless roaming to work, the adjacent access points should use non-overlapping channels, which are channels 1,6 and 11

One thing you can readily measure is distance. Your regular off-the-shelf access points can do about 100 mtrs on 802.11b. The newer 802.11g ones can do 500 mtrs or more. Place your first access point where it needs to be and imagine a sphere around it that has a 500 mtrs radius. If you have wireless-opaque objects such as walls along the way, then you need to reduce the distance. How much distance needs to be reduced depends upon a number of factors, which are calculated using a scientific formula (called the simplified pathloss equation)-Pr = Pt * K * ( ^ r ) * g. Here Pr and Pt are the receiver and transmitter powers, r is gamma (2 if open-space, 4 if there are obstacles), K is a constant, d0 is the standard distance and d is the distance between the access point and wherever you want to connect from. The final g is dependent on things like weather, humidity and so on, and is called the 'log normal shadowing constant'. 



g comes into play when you have walls, trees and things in between. r also increases as the density of medium (such as under water) increases. Discussion of further mathematics on this is beyond the scope of this article, but you can refer to
http://www.ecsl.cs.sunysb.edu/tr/wlandeployment.pdf.    

The easiest way to solve this of course is to treat the walls and obstacles as black-spot agents and place your equipment in a way that it provides maximum coverage. Remember even furniture will cause attenuation of the wireless signal and needs to be compensated against.

You can select the channel to be used by the access point using the access point's management interface

We also said that they must have identical configurations. This means that they are all configured: (1) with the same frequency spectrum (2) they have the same encryption settings. So, if one of them is on the 2.8 to 3.2 GHz band, all of them need to be on the same band; otherwise, they will still roam, though the roaming won't be seamless. The encryption setting is a must. If you configured one with an RSA WEP key that was 128-bits long, all of them need to be on the same key type and length. And yes, the keys must be the same too.

Finally, your devices must talk IRAP (International Roaming Access Protocol). This is a non-proprietary standard developed under the initiative of Intel and already supported by the GSM Association, IEEE and the WiFi Alliance. You can find out more on this at

http://www.intel.com/technology/comms/roaming/irap

With the help of IRAP, your WLAN no longer needs to depend on having equipment all from the same vendor. IRAP enables wireless equipment from different vendors to seamlessly communicate and hand off of the wireless client.

All the three access points have the same SSID but use different channels to eliminate collision

Problems while implementing seamless roaming



Today, even with the coming of the 802.11g, signal quality and strength, reliability, signal reliability and consistency (constancy) remain questionable. This makes a complete mainstream switchover from wired to wireless impossible. Handoffs between wireless zones are still not good enough to be deemed 'mission critical friendly'. At best, you can use it to provide seamless wireless access to your knowledgebase than move your mission-critical applications and systems to this.

Setting up a seamless roaming wireless network



The 2.4 GHz IEEE 802.11b and g, frequency band is divided into 14 partially overlapping channels. For seamless roaming to work, the adjacent access points should use non-overlapping channels, which are channels 1, 6 and 11. We used three access points (CISCO Aironet 1200, D-Link DWL-2100AP and Netgear WGR-614) to create a wireless network, providing seamless roaming to users over an entire floor. The access points were placed such that when the range of one access point ended, another access point was there to provide WiFi coverage. The access points used the 1, 6 and 11 channels, which can be set for each access point using the management interface of each access point, (as shown in the screenshot on the left). Also, you do not need access points from the same vendor for this to work. All access points had the same SSID, but worked on different frequency channels, which can be seen from the 'Network Stumbler' software. Now, whenever the user moves from the range of one access point, there is another access point with the same SSID to provide coverage. Users do not have to do anything on their notebooks or PDAs. As soon as the device finds a high power signal for the network they are connected to, it connects to that access point leaving the access point whose signal power has decreased since the user has moved out of its range. You can use various combinations of these channels to provide seamless roaming and eliminate dead spots places-where there are no wireless signal-for your entire building or floor. In such a case you need to place the access points in such a manner, that there are no access points using overlapping channels. This requires careful planning.