What do I need to know about deploying APs over low-speed links?
Product and Software: This article applies to all Aruba controllers and ArubaOS versions.
Aruba's Mobile Edge architecture consists of thin access points (APs) wherever wireless coverage is needed and centralized Aruba mobility controllers for security, management, and WLAN functions. The architecture has been designed with the utmost flexibility for different deployment models where Aruba mobility controllers and access points can be interconnected over any IP network (LAN or WAN) using Layer 3 communications.
Today, a number of customers have deployed APs that connect to controllers located across low-speed (defined as less than 1 Mb/s capacity) or high-latency (defined as greater than 100 ms) links. This document provides recommendations for deploying the Aruba Mobile Edge architecture across low-speed and high-latency links, discusses the potential problems involved in this type of deployment, and also discusses upcoming product enhancements to alleviate some potential problems.
Link-Speed Constraints for Remote Networks
Aruba recommends that controllers and APs be connected over a link with a capacity of 1 Mb/s or greater, and requires a minimum link speed of 64 Kb/s per tunnel-mode SSID. LANs, many metro area networks, and most broadband DSL or cable connections today provide link speeds greater than 1 Mb/s. For low-speed links such as ISDN, T1/E1, and many Frame Relay WAN connections, specific design guidelines and Aruba configuration are required to avoid connectivity problems.
To achieve high reliability and fast failover in the event of a network or controller outage, APs and controllers maintain "heartbeat" or "keepalive" packets between themselves. Failure to receive these heartbeat packets - described in detail below - can cause APs to "rebootstrap", going through a process of tunnel re-establishment with the controller. During the bootstrap process, the AP shuts off all radios for approximately 20 ms, and all clients are required to re-associate. Two types of heartbeat packets are sent: GRE tunnel keepalives and PAPI keepalives.
GRE Tunnel Keepalives
• Sent once every second.
• Bi-directional: Sent from AP to controller and from controller to AP.
• By default, AP rebootstraps after missing eight consecutive keepalive packets. This number can be adjusted using the "bootstrap-threshold" parameter.
• By default, the controller will remove an AP's tunnel after 12 seconds of inactivity. In ArubaOS 2.x, this number can be adjusted using "stm ap-inactivity-timeout". In ArubaOS 3.0, this setting is equal to 1.5x the bootstrap-threshold.
• Does not apply to bridge mode SSIDs (Remote AP).
• Sent once every minute and retransmitted every 5 seconds when not acknowledged.
• By default, APs rebootstrap after communication with the controller is interrupted. In ArubaOS 2.5, the timeout occurs after 10 consecutive missed keepalives, which is approximately 50 seconds. In ArubaOS 3.1, this takes approximately 4 minutes. Can be adjusted using "max-imaliveretries" parameter (in ArubaOS 2.x) or "maxrequest-retries" (in ArubaOS 3.x).
• Effectively applies only to bridge-mode SSIDs. For tunnel-mode SSIDs, GRE tunnel keepalive intervals will time out long before PAPI keepalive intervals.
Failure to Receive Heartbeats
If a low-speed link is saturated, the AP heartbeat packets might be dropped, which causes the AP to rebootstrap. Rebootstrapping is the primary cause of connectivity problems for APs connected across low-speed links. An additional consideration for low-speed links is the throughput possible over the wireless LAN for tunnel-mode SSIDs. Tunnel-mode SSIDs transport all wireless traffic over GRE or IPsec tunnels back to the mobility controller for processing. When used with low-speed links, tunnel-mode SSIDs are best suited for low-bandwidth applications such as barcode scanning, small database lookups, and Telnet to avoid saturating the WAN link. As an alternative, APs can be deployed as remote APs using the ArubaOS Remote AP module, and SSIDs can be configured as bridge-mode SSIDs.
In a bridge-mode SSID, all wireless traffic is terminated locally at the AP and bridged onto the local Ethernet segment. If much of this traffic will remain local rather than crossing the WAN, most saturation issues can be avoided. In addition, heartbeat timers for bridge-mode SSIDs can be set to be much more tolerant of packet loss. There are no hard rules to categorize what will work and what will not. A number of Aruba customers have deployed APs across 64 Kb/s WAN links without difficulty, because packet loss is low and the throughput requirements are not high. Other customers with unpredictable traffic loads have experienced some problems due to AP heartbeat timeouts. Each customer must analyze realistic traffic patterns before deployment to minimize risk of link saturation.
Latency Constraints for Remote Networks
When deploying APs across high-latency links of 100 ms or greater, special considerations are required due to the timing constraints of some client devices. Aruba APs locally process 802.11 probe requests and probe responses, but the 802.11 association process requires interaction with the mobility controller. Certain models of handheld computers and barcode scanners based on Windows Pocket PC 4.2 are known to have very tight timing requirements that cause the association process to time out if an association response is not received within 102 ms.
Other devices and operating systems may be affected as well. Check with your device vendor for the latest versions of firmware and drivers that are designed to be less sensitive to WAN latency. Aruba recommends that customers test high-latency links to ensure that timing issues will not become a problem. For deployments where latency problems cannot be avoided, Aruba recommends that a mobility controller be installed closer to the APs.
When deploying APs across low-speed links, the following recommendations apply:
• Adjust the AP bootstrap-threshold if the network experiences packet loss. This will make the AP recover more slowly in the event of an actual failure, but it will be more tolerant to loss of heartbeat packets. The recommended setting is 30.
ap location 0.0.0 bootstrap-threshold 30 (ArubaOS 2.x)
ap system-profile default bootstrap-threshold 30 (ArubaOS 3.x)
• Reduce the number of tunnel-mode SSIDs if possible - each SSID creates a tunnel to the mobility controller with its own tunnel keepalive traffic.
• If much of the data traffic will remain local to the site, deploy Remote APs in bridging mode.
• If high-latency links (transoceanic or satellite) are used in the network, deploy a mobility controller geographically close to APs. For example, consider deploying a mobility controller per continent.
• If high latency is causing association issues with certain handheld devices or barcode scanners, inquire about recent firmware and driver updates that have been made available by device manufacturers to reduce the sensitivity to link latency.