- Layer-2 Roaming occurs when a host moves between two APs that are connected to the same Layer-2 domain and served by the same Router interface or default gateway. In other words this type of roaming is within the same Subnet or broadcast domain.
- Layer-3 Roaming is described as roaming between roaming domains that are in separate VLANs or broadcast domains.
Applications that use a connection-orientated transport layer protocol such as TCP should recover from the disruption caused by roaming but sessions using connectionless protocols like UDP can be difficult to maintain. The IEEE 802.11 framework was designed to support Layer-2 roaming with mechanisms such as scanning of radio channels playing a part in this.
The time it takes to make the move from one domain to another is dependant upon a number of factors such as the duration of the following processes:
- The probing process
- The 802.11 authentication process
- The 802.11 association process
- The 802.1x authentication process
Layer-2 roaming occurs when a client physically moves from the domain of one Access Point to the domain of another. When the switchover actually takes place is determined by how the specific vendor has set various parameters and such factors as signal strength, missed beacons and frame acknowledgements are usually taken into account.
One method is known as preemptive roaming, where the host scans a range of wireless channels and listens for other APs (Access Points) or actively probes for other APs. Using the probing method, the host device cannot transmit and receive data frames at the time of probing so this can have some impact on the data throughput.
The second method is to use what is known as Roam Time Access Point Discovery, where the host sends a re-association message to the new access point.
The following is a breakdown of a typical Layer 2 roaming process.
- The host uses rules set by the equipment vendor such as relative signal strength to decide to roam.
- The host decides where to roam to and usually passively or actively scans all channels for a new AP.
- The host initiates a re-association with the new AP./
- Connection-orientated processes can be maintained provided the IP address does not change.
Layer 3 roaming occurs when a host device roams between two distinct broadcast domains, either VLANs or Subnets. Obviously, moving from one Layer-3 domain to the other normally requires a change of IP address, but in WLANs we allow the host to maintain the same IP address through the process highlighted below:
1. The client device is known as the MN (Mobile Node), and the MN detects the presence of another Access Point that we refer to as the FA (Foreign Agent).
2. The Foreign Agent will communication with the client's original AP known as the HA (Home Agent), and will establish a tunnel.
3. Packets that are received are sent by the FA to the HA through the tunnel.
4. The HA uses the tunnel in the reverse direction by forwarding any packets for the MN via the FA.
5. When the Mobile Node sends packets destined for the HA itself or its associated devices, the FA forwards those packets via the tunnel.
WLAN Roaming is discussed on some of our instructor-led training courses, which we are constantly updating.