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Fast Secure Roaming



The last blog, Fast BSS Transition, was based on the 802.11r amendment. There are two other amendments that make roaming more efficient. Those two will be discussed in this blog. We are talking about 802.11k (RRM) and 802.11v (WNM).

802.11k
Radio resource measurement (RRM) is a mechanism that helps to understand RF better. RRM can adjust power levels and change the channel of the access points. In this case, when an access point fails, another access point can adjust its power level to (temporarily) fill up the gap that is created by the failed access point. Those measurements can be done by clients and access points and the request can come from the local access point or requested from another station. The information can come from channel load reports and from neighbour reports and the purpose is to find the most effective way to use a shared spectrum.

The neighbour report is an important key in the 802.11k process. As you know, clients choose to roam to another access point. Based on the information they receive from the neighbour reports the client makes those decisions. Those reports are from neighbour access points within the same mobility domain. The information reaches the clients by 802.11 action frames. The following information is shared about the neighbour access points:
- BSSID
- Mobility Domain
- Security capabilities
- Quality of Services
- Automatic Power Save Delivery (APSD)
- Radio measurement
- BlockAck method
- Spectrum management
- Regulatory class
- Channel number
- PHY type
As you can see, this is more information than just the RSSI value. Clients will use those neighbour reports for making better decisions. Access points get information from the clients as well to update their neighbour reports and share that with other clients again.
- Transmit Power Control (TPC) for calculating the link budget before association
- Power capabilities to compute the range data
- QoS data and metrics provide troubleshooting data
- Client device statics includes voice and management diagnostics for QoS
- Information to reduce co-channel interferences (CCI)
- Quiet-time announcements for troubleshooting and diagnostics

802.11v
Wireless network management (WNM) is information about the network resources that are exchanged between the access point and the client. This is based on performance. Where the access point uses the information in 802.11k to change itself, 802.11v is for telling the client that there might be a better access point than the one the client is connected to, based on the exchanged information. The information that is exchanged are:
- BSS Max Idle Period Management
- BSS Transition Management
- Channel Usage
- Collocated Interference Reporting
- Diagnostic Reporting
- Direct Multicast Services (DMS)
- Flexible Multicast Services (FMS)
- Multicast Diagnostic Reporting
- Event Reporting
- Locations Services
- Multiple BSSID Capability
- Proxy ARP
- QoS Traffic Capability
- SSID List
- Triggered STA Statistcs
- TIM Broadcast
- Timing Measurement
- Traffic Filtering Service
- U-APSD Coexistence
- WNM-Notification
- WNM-Sleep Mode

Voice Enterprise certification is created by the Wi-Fi Alliance and is for testing voice applications in the real-world environments. 802.11r, 802.11k, and 802.11v are for roaming in general, but voice applications are applications which are very roaming sensitive. Latency is a problem for a phone call (Voice over Wi-Fi). Latency should be, one way, under 50ms as well as the jitter, and no more than 3 consecutive packets lost. The voice quality needs to be good, but it shares the medium with data traffic as well.

That is the reason that VoWiFI needs prioritization, and this can be done with Wi-Fi Multimedia (WMM) QoS that recognize voice traffic over data traffic. Support for WMM-Admission Control will optimize traffic management and load balancing for traffic. To save the battery life of clients, WMM-Power Save can be enabled, but this is not mandatory.

Mobile IP is a layer 3 roaming solution for roaming sensitive applications like VoWiFI. Roaming is mostly Layer 2, but there are environments that use layer 3 roaming. With layer 3 roaming the connection needs to restart, since the client gets a new IP address. Mobile IP is an IETF standard protocol that gives the client the possibility to keep its IP address from another VLAN. To established this, tunnelling is used with IP header encapsulation. The client has a home address that is registered at a device named home agent (HA). This HA is the current access point. When the client roams to a new access points in another subnet, the HA is the single point of contact. The IP and MAC information is shared in a database called the home agent table (HAT) that another device, the foreign agent (FA), can access. The new access point handles all the communication with the original (HA) access point. The FA IP address is a care-of address. The HAT table is used for the FA to locate the HA from the client, and a tunnel will be established between the FA and the HA.