Table Of Contents
Cisco Aironet 1500 Series Wireless Mesh AP Version 5.0 Design Guide
Solution Features and Benefits
Cisco Wireless LAN Controllers
Point-to-Multipoint Wireless Bridging
Point-to-Point Wireless Bridging
Mesh Neighbors, Parents, and Children
Bridging Packets From and To a LAN
Multiple Wireless Mesh Mobility Groups
Layer 2 or Layer 3 Encapsulation
Indoor WLAN Network to Outdoor Mesh
Connecting the Cisco 1500 Series Mesh AP to Your Network
Fresnel Zone Size in Wireless Mesh Deployments
Mesh AP and Controller Configuration
Layer 2 and Layer 3 Deployments
AP RADIUS Authentication with Cisco Secure ACS Server
Controller Configuration for RADIUS Authentication
Switch or Router Configuration
Troubleshooting Considerations
Misconfiguration of the MESH AP IP Address
Identifying the Node Exclusion Algorithm
Managing the Cisco 1500 Series Mesh AP with WCS
Hierarchical Mesh AP Management
Cisco Aironet 1500 Series Wireless Mesh AP Version 5.0 Design Guide
Last revised: July 2008This document provides design guidance for the deployment of the Cisco Aironet 1500 Series Lightweight Outdoor Wireless Mesh Access Point (referred to subsequently as the Cisco 1500 Series Mesh AP or simply 1500 Series Mesh AP), which operates with Cisco Wireless LAN Controllers (WLCs) and Cisco Wireless Control System (WCS) software to provide centralized and scalable management, high security, and mobility that is seamless between indoor and outdoor deployments. Designed to support zero-configuration deployments, the Cisco 1500 Series Mesh AP easily and securely joins the mesh network, and is available to manage and monitor the network through the controller and WCS graphical or command-line interface (CLI). Compliant with Wi-Fi Protected Access 2 (WPA2) and employing hardware-based Advanced Encryption Standard (AES) encryption between wireless nodes, the Cisco 1500 Series Mesh AP provides end-to-end security.
Contents
Solution Overview
This section provides an overview of the Cisco Aironet 1500 Series Wireless Mesh AP Version 4.1 solution.
Outdoor Wireless Benefits
The Cisco wireless mesh networking solution enables cost-effective and secure deployment of enterprise, campus, and metropolitan outdoor Wi-Fi networks. Standards-based wireless access takes advantage of the growing popularity of inexpensive Wi-Fi clients, enabling new service opportunities and applications that improve user productivity and responsiveness.
Outdoor Wireless Challenges
As the demand for outdoor wireless access increases, customers faced with tight budgets and reduced resources must respond with wireless LAN (WLAN) solutions that take full advantage of existing tools, knowledge, and network resources to address ease of deployment and WLAN security issues in a cost-effective way. An outdoor WLAN solution that excels in the unique attributes of wireless mesh technology, effectively supports current networking requirements, and lays the foundation for the integration of business applications is needed.
Outdoor wireless solutions offer a number of challenges, compared with a standard indoor WLAN, particularly in the following areas:
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The outdoor environment is harsher than the indoor environment and therefore requires specialized equipment or enclosures to contain and protect indoor equipment that is deployed outdoors.
Outdoor deployments attempt to cover wider areas than indoor deployments. The main challenges for the outdoors are interference and finding a wired connection, although power is often available.
Outdoor deployments might require specialized radio frequency (RF) skills, might have a lower user density than indoor deployments, and might exist in an environment that is less regulated than inside a building. These features put pressure on the TCO of the outdoor solutions, and require a solution that is easy to deploy and maintain.
Solution Features and Benefits
The Cisco 1500 Series Mesh AP provides the following features and benefits:
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The Cisco 1500 Series Mesh AP can be installed anywhere power is available, without the need for a network connection. Intelligent wireless routing is based on the Adaptive Wireless Path Protocol (AWPP), which is designed specifically for wireless environments. AWPP enables a remote access point to dynamically optimize the best route to the connected network within the mesh, providing resiliency to interference and helping ensure high network capacity.
Deployment and management costs for the 1500 Series Mesh AP are reduced through support of zero-configuration deployments and through the ability of the APs to self-heal in response to interference or outages. The 1500 Series Mesh AP can act as a relay node and can associate clients at the same time. The 1500 Series Mesh AP has a dedicated radio for the backhaul and another radio for the local access, allowing the mesh network to maximize use of the total available channels and minimize the occurrence of interference. This results in more capacity than is available with solutions that use only a single radio. When more capacity is needed, additional sectors can be enabled, such as provisioning a network connection to a remote access point. The mesh dynamically re-optimizes itself when this is done.
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Using the Cisco Lightweight Access Point Protocol (LWAPP) features, the 1500 Series Mesh AP can discover its LWAPP controller and automatically download the correct configuration and software for its role in the wireless mesh network.
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Wireless mesh networks have unique features and requirements, and to address these features and requirements, Cisco Systems has developed a new protocol, AWPP, which allows each node to determine its neighbor or parent intelligently, choosing the optimal path toward the controller. Unlike traditional routing protocols, AWPP takes RF details into account.
The AWPP automatically determines the best path back to the LWAPP controller by calculating the cost of each path in terms of signal strength and number of hops. After the path is established, AWPP continuously monitors conditions and changes routes to reflect changes in conditions. AWPP also performs a smoothing function to signal condition information to ensure that the ephemeral nature of RF environments does not impact network stability.
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The Cisco wireless mesh solution brings all the ease of deployment and management of the Cisco Unified Wireless Solution to the wireless mesh solution.
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A core component of the Cisco Unified Wireless solution is the use of X.509 certificates and AES encryption for LWAPP transactions. This X.509 and AES encryption is embedded into the wireless mesh solution with LWAPP transactions and all traffic between 1500 Series Mesh AP nodes being AES encrypted. The complete packet path is from the Cisco controller to APs, and eventually to the users. The controller encapsulates user packets and forwards them to the correct RAP over Ethernet. RAP then encrypts the user data packets and transfers them over the backhaul. Data packets might travel through multiple MAPs before reaching the destination MAP. After receiving the encrypted user data, the destination MAP decrypts them and sends them over the air to the client using the encryption method specified by the client.
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The Cisco Mesh solution provides robust, optimal parent selection and fast convergence. Cisco Mesh software contains mechanisms to guard against stranded AP conditions. The Cisco Mesh solution has a software-based recovery mechanism so the customer does not have to dispatch a technician to fix an AP problem. The network automatically recovers from misconfigurations, such as a wrong IP address, DHCP server errors, bridge group name typo's or misprovisioning of the network. In addition, Cisco Systems has an exclusion list algorithm that allows the child node to intelligently put the parent node in the exclusion list and its future reuse would depend on the loyalty and history of the parent.
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The same seamless mobility features delivered through the Cisco Unified Wireless solution are delivered in the wireless mesh solution.
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Just as the Cisco Unified Wireless solution allows the LWAPP APs to communicate with the controller via a Layer 2 or Layer 3 network, this flexibility is extended to the wireless mesh solution.
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The Cisco 1500 Series Mesh AP solution can scale to 24 controllers each with up to 16 MBSSIDs and 256 VLANs. Each 4400 controller can support more than 100 1500 Series Mesh APs. Capacity in a mesh network can be increased conveniently by adding MAPs at the edge of the network or configuring more RAPs in the network. This is covered in more detail in Controller Planning.
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The Cisco 1500 Series Mesh AP uses the same tools and features as other Cisco Unified Wireless solutions. The management platform, Wireless Control System (WCS), is a much stronger product that not only offers more advanced features, but is also more scalable. Up to 150 controllers and 2500 access points can be managed by a single WCS.
Solution Components
The Cisco Wireless Mesh solution has three core components:
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Cisco 1500 Series Mesh AP
The Cisco 1500 Series Mesh AP is the core component of the wireless mesh solution, and leverages existing and new features and functionality in the Wireless LAN controllers and the WCS.
The Cisco 1500 Series Mesh AP, as shown in Figure 1, is the primary component for outdoor bridging and wireless mesh solutions.
Figure 1 Cisco 1500 Series Mesh AP
There are two types of Cisco 1500 Series Mesh APs:
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A wide variety of antennas that provide flexibility when deploying the 1500 Series Mesh AP over various terrains are available. The 5.8 GHz frequency radio uses 802.11a technology and is used in the system as the backhaul or relay radio. Wireless LAN client traffic passes either through the AP backhaul radio, or is relayed through other 1500 Series Mesh APs until it reaches the LWAPP controller Ethernet connection.
The 1500 Series Mesh AP also has a 10/100 Ethernet connection to provide bridging functionality. This Ethernet connection supports power over Ethernet (PoE) through a separate power injection system.
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The Cisco 1500 Series Mesh AP uses LWAPP to communicate to a wireless controller and other 1500 Series Mesh APs in the wireless mesh.
The 1500 Series Mesh AP is designed to be mounted upside-down with its antennas pointed toward the ground, as shown in Figure 2.
Figure 2 1500 Series Mesh AP Installation
Cisco Wireless LAN Controllers
The wireless mesh solution is supported by the Cisco 2000 Series and Cisco 4400 Series Wireless LAN Controllers (WLCs). The Cisco 4400 Series WLC (see Figure 3) is recommended for wireless mesh deployments because it can scale to large numbers of access points, and can support both Layer 2 and Layer 3 LWAPP.
Figure 3 Cisco 4400 Wireless LAN Controller
For more information on the Cisco 4400 Wireless LAN controller, see the following URL: http://www.cisco.com/en/US/products/ps6366/index.html
Wireless Control System (WCS)
The Cisco Wireless Control System (WCS) is the platform for wireless mesh planning, configuration, and management. It provides a foundation that allows network managers to design, control, and monitor wireless mesh networks from a central location.
With Cisco WCS, network administrators have a solution for RF prediction, policy provisioning, network optimization, troubleshooting, user tracking, security monitoring, and wireless LAN systems management. Graphical interfaces make wireless LAN deployment and operations simple and cost-effective. Detailed trending and analysis reports make Cisco WCS vital to ongoing network operations.
Cisco WCS runs on a server platform with an embedded database. This provides the scalability necessary to manage hundreds of WLCs, which in turn can manage thousands of Cisco lightweight access points. WLCs can be located on the same LAN as Cisco WCS, on separate routed subnets, or across a wide-area connection.
Figure 4 shows the interconnections between the controllers, WCS, and the 1500 Series Mesh APs.
Figure 4 Interconnections to the Solution
Frequency Bands
The 5GHz Band is actually a conglomerate of three bands in the USA: 5.150-5.250(UNII 1), 5.250-5.350(UNII 2), and 5.725-5.875(UNII 3) GHz. UNII-1 and the UNII-2 bands are contiguous and are indeed treated by 802.11a as being a continuous swath of spectrum 200MHz wide, more than twice the size of the 2.4GHz ISM band (see Figure 5).
Figure 5 Frequency Bands
In addition to FCC, other main regulatory domains for operation in 5GHz are the European Telecommunications Standards Institute (ETSI), Japan, China (Mainland China), Israel, Singapore and Taiwan (Republic of China).
Refer to the Cisco web site for compliance information, and also verify with your local regulatory authority what is permitted within your country:
http://www.cisco.com/warp/public/779/smbiz/wireless/approvals.html
The ETSI recommended frequency band for bridging is 5.470 to 5.725 GHz offering almost eleven channels with the same EIRP rules as the FCC. In exchange of this wide spectrum, the ETSI recommendation mandates the inclusion of two features not currently found in 802.11 products, Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC). DFS and TPC are the two functions handled quite well by the HyperLAN2 specification. The IEEE 802.11h standard covers the DFS and TPC that will apply to the 5 GHz band.
Figure 6 shows the RF power (conducted) allowed in the 2.4 GHz band.
Figure 6 RF Power (Conducted) Allowed in the 2.4 GHz Band—A=America, E=Europe, J=Japan
Figure 7 shows the antenna gains certified for use with the 1500 Series AP.
Figure 7 Antenna Gains Certified for Use with the 1500 Series AP.
Figure 8 shows the additional third-party antennas supported with the 1500 Series AP.
Figure 8 Additional Third-Party Antennas Supported with the 1500 Series AP
Deployment Modes
The Cisco 1500 Series Mesh AP solution supports multiple deployment modes, including the following:
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Wireless Mesh
In the wireless mesh deployment, there are multiple 1500 Series Mesh APs deployed as part of the same network, as shown in Figure 9.
Figure 9 Wireless Mesh Deployment
One or more of the 1500 Series Mesh APs have a wired connection to their WLC, and these are designated as rooftop mesh APs (RAPs). Other 1500 Series Mesh APs that relay their wireless connections to connect to the controller are called mesh access points (MAPs). The MAPs use the AWPP to determine the best path through other 1500 Series Mesh APs to their controller. The various possible paths between the MAPs and RAPs form the wireless mesh that is used to carry traffic from WLAN clients connected to MAPs in that mesh, and also to carry traffic from devices connected to the MAP Ethernet ports.
The WLAN mesh can simultaneously carry two different traffic types: WLAN client traffic and MAP Ethernet port traffic. WLAN client traffic terminates on the WLC, and the Ethernet traffic terminates on the Ethernet ports of the 1500 Series Mesh APs. Mesh membership in the WLAN mesh is controlled in a variety of ways. MAC authentication of the 1500 Series Mesh APs can be enabled to ensure that the APs are included in a database of APs authorized to use the WLAN controller. 1500 Series Mesh APs are configured with a shared secret for secure AP-to-AP intercommunication, and a bridge group name can be used to control mesh membership, or segmentation. The configuration of these features is described later in this document.
Wireless Backhaul
Cisco 1500 Series Mesh APs can provide a simple wireless backhaul solution, where the 1500 Series Mesh AP is used to provide 802.11b/g services to WLAN and wired clients. This configuration is basically a wireless mesh with one MAP. Figure 10 shows an example of this deployment type.
Figure 10 Wireless Backhaul Deployment
Point-to-Multipoint Wireless Bridging
In the point-to-multipoint bridging scenario, a RAP acting as a root bridge connects multiple MAPs as non-root bridges with their associated wired LANs. By default, this feature is disabled for all MAPs. If Ethernet bridging is used, you must enable it on the controller for the respective MAP and for the RAP. Figure 11 shows a simple deployment with one RAP and two MAPs, but this configuration is fundamentally a wireless mesh with no WLAN clients. Client access can still be provided with Ethernet bridging enabled, although if bridging between buildings, MAP coverage from a high rooftop might not be suitable for client access.
Figure 11 Wireless Point-to-Multipoint Bridge Deployment
Point-to-Point Wireless Bridging
In a point-to-point bridging scenario, a 1500 Series Mesh AP can be used to extend a Layer 2 network by using the backhaul radio to bridge two segments of a switched network, as shown in Figure 12. This is fundamentally a wireless mesh network with one MAP and no WLAN clients. Just as in point-to-multipoint networks, client access can still be provided with Ethernet bridging enabled, although if bridging between buildings, MAP coverage from a high rooftop might not be suitable for client access.
If you intend to use an Ethernet bridged application, we suggested that you enable the bridging feature on the RAP and on all MAPs in that segment. Also verify that any attached switches to the Ethernet ports of your MAPs are not using VLAN Trunking Protocol (VTP). VTP can reconfigure the trunked VLANs across your mesh and possibly cause a loss in connection for your RAP to its primary WLC. If improperly configured, it can take down your mesh deployment.
Figure 12 Wireless Point-to-Point Bridge Deployment
Solution Description
The wireless mesh solution has the following three main components:
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LWAPP WLAN
The wireless mesh solution provides the same feature set to mesh WLAN clients as are provided by the Cisco Unified Wireless solution set for the indoor WLAN. Because the design and configuration of this solution is adequately covered in other documents, it is not addressed in this document.
Wireless Mesh Connections
The Ethernet ports of the 1500 Series Mesh AP are bridged with the wireless mesh, acting as a transparent bridge between all Ethernet ports of nodes within that mesh. For example, the simple mesh shown in Figure 13 results in a logical multi-port bridge of all Ethernet ports, as illustrated in Figure 14.
Figure 13 Simple Mesh Example
Figure 14 Wireless Mesh Virtual Multi-Port Bridge
Note that the controller does not participate in this bridging, and that the traffic terminates at the 1500 Series AP Ethernet port. Take care in mesh deployments to block unnecessary multicast traffic to prevent wireless backhaul capacity from being consumed unnecessarily.
Also note that for bridged traffic, the controller does not act as a central coordination point. The data traffic for the multipoint bridge is simply bridging traffic through the shortest path calculated by the AWPP.
The bridge network is transparent to dot1q and Spanning Tree protocols.
Mesh Authentication
When a 1500 Series Mesh AP comes up in a mesh, it uses its Primary Master Key (PMK) to authenticate to a parent or a neighboring 1500 Series Mesh AP. There is a four-way handshake using this primary key to establish an AES session. Next, the new AP establishes an LWAPP tunnel to the controller and is then authenticated against the MAC filter list of the controller.
Next, the controller pushes the bridge shared secret key to the AP via LWAPP, after which it re-establishes the AES session with the parent AP.
Wireless Mesh Encryption
As previously described, the wireless mesh bridges traffic between the MAPs and the RAPs. This traffic can be from wired devices being bridged by the wireless mesh, or LWAPP traffic from the mesh APs. This means that the wireless mesh could be carrying traffic that is either clear text or encrypted, depending on the wireless LAN settings and other overlaying applications; this traffic is always AES encrypted when it crosses a wireless backhaul link. The AES encryption is established as part of the Mesh AP establishing neighbor relationships with other Mesh APs. The bridge shared secret is used to establish unique encryption keys between mesh neighbors. All APs establish an LWAPP connection to the controller through AES-encrypted tunnels between APs.
Simple Mesh Deployment
The key components of the simple mesh deployment design (see Figure 13) are the following:
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The router also provides Layer 2 isolation of the RAP. This is necessary because the RAP bridges traffic from its local Ethernet port to the mesh, so this traffic must be limited to that necessary to support the solution so that resources are not consumed by the unnecessary flooding of traffic.
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Note that the RAP wireless connection is to the center of the MAP mesh, which is an optimal configuration that minimizes the average number of hops in the mesh. A RAP connection to the edge of a mesh would result in an increase of hops.
Figure 15 shows one possible logical view of the physical configuration shown in Figure 13, with MAP5 as the path home for all other MAPs.
Figure 15 Logical View
Figure 16 shows an alternate logical view, in which the signal-to-noise ratio (SNR) on the diagonal paths to MAP5 is small enough for the MAPs to consider taking an extra hop to get to MAP5.
Figure 16 Unequal Paths
In both the cases above, MAP5 is the path home for all traffic. Ideally, the coverage from the RAP should be such that other MAPs, such as MAP2 for example, have a path back to the RAP, and traffic could be routed via MAP 2 in case of a loss of signal to MAP 5, as shown in Figure 17.
Figure 17 MAP2 Path Home
AWPP Wireless Mesh Routing
The introduction of wireless mesh brings with it a new routing protocol, the Cisco Adaptive Wireless Path Protocol (AWPP).
This protocol is designed specifically for wireless mesh networking in that its path decisions are based on link quality and the number of hops. AWPP is also designed to provide ease of deployment, fast convergence, and minimal resource consumption. AWPP takes advantage of the LWAPP WLAN, where client traffic is tunneled to the controller and is therefore hidden from the AWPP process. Also, the advance radio management features in the LWAPP WLAN solution are available to the wireless mesh network and do not have to be built into AWPP.
Cisco is a leading member of the Simple, Efficient, and Extensible Mesh (SEEMesh) consortium. The Cisco mesh model has become solidly embedded in one of the main contending proposals for the 802.11 task group, which is moving towards becoming a mesh standard for the industry. The combined design, known as Hybrid Wireless Mesh (routing) Protocol (HWMP), serves both the fixed type of deployments and the mobile deployments. HWMP is favored by other SEEMesh supporters because it combines low complexity with great flexibility. AWPP has been selected as the draft foundation for HWMP. Cisco Systems has taken a leading role in setting standards in the mesh field. The 802.11 standard is expected to be published by September of 2007.
Traffic Flow Within the Mesh
In Wireless Mesh Connections, a model of a virtual multi-port bridge is suggested for explaining how traffic is bridged across the wireless mesh. This model is equally applicable for traffic going to RAP and MAP MAC addresses as it is destined for MAC addresses connected to the RAPs or MAPs; that is, the 1500 Series Mesh AP builds a table of MAC addresses to associate with the peer in the mesh. It is by this table that the 1500 Series Mesh AP knows where to forward a frame, and AWP is used to build this table on each mesh AP.
An important point to remember is that the WLAN clients are not involved in AWP or the MAC address tables as they LWAPP tunnel to the controller, and the wireless mesh routing and addressing is transparent to the WLAN clients. The MAC address tables that are built by AWP contain only the MAC addresses of the 1500 Series Mesh APs, and wired clients connected to the 1500 Series Mesh APs.
Mesh Neighbors, Parents, and Children
A neighbor within a mesh is an AP that is within RF range that has not been selected as a parent or a child because its "ease" values are lower than another neighboring AP (refer to Ease Calculation).
A parent AP is one that is selected as the best route back to the RAP based on the best ease values. A parent can be either the RAP itself or another MAP. A child of an AP is an AP that has selected the parent AP as the best route back to the RAP. (See Figure 18.)
Figure 18 Parent, Child, and Neighbor
The goal of AWPP is to find the best path back to a RAP that is part of its bridge group name (BGN). To do this, the mesh AP actively solicits for neighbor APs. During the solicitation, the mesh AP learns all of the available neighbors back to a RAP, determines which neighbor offers the best path, and then synchronizes with that neighbor.
Figure 19 shows the state diagram for a mesh AP when it is trying to establish a connection.
Figure 19 Mesh AP State Diagram
Using release 4.0 software, the AWPP state machine has been optimized to offer better routing and convergence and reconvergence capabilities to AP1500 nodes. These optimizations enable faster channel scanning for neighbor nodes, discovering and constructing neighbor lists across all backhaul channels, selecting parent nodes from this list and quickly converging to a different parent on the same channel or a different channel in case of current parent failure.
The mesh AP must first decide whether it is a RAP. A mesh AP becomes a RAP if it can communicate with an LWAPP controller through its Ethernet interface. If the mesh AP is a RAP, it can go straight to the maintain state. In the maintain state, the mesh AP has established an LWAPP connection to the controller so it does not need to seek other mesh APs, but simply responds to solicitations. If the mesh AP is not a RAP, it starts a scan process where the mesh AP scans all available channels and solicits information from other mesh APs.
This behavior has two main implications:
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If the mesh AP is not a RAP, it follows the state diagram above in the following modes:
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The passive scanning mechanism enables a new mesh node to scan through all available channels and discover neighbors who might belong to different sectors. A typical mesh backhaul design should be around per-sector channel allocation. In such a design, if the new node does not belong to a particular sector, it can quickly move to other channels which are likely to have neighbors of its compatible sector. If a mesh backhaul is designed around different sectors with the same bridgegroupname and different channels, the passive scanning mechanism is also useful for nodes in the bordering areas of the sectors where there might be closely comparable neighbors on different channels.
The passive scanning mechanism using mesh beacons is efficient because it minimizes the amount of time spent on each channel, minimizes the number of channels sought by the Optimal Parent Selection (OPS) algorithm, and turns around scanning results quickly for later states of the AWPP state machine. Despite consuming periodic airtime, the mechanism brings significant benefits to the 802.11 backhaul radio with omni antennas.
Choosing the Best Parent
The OPS algorithm is implemented in the Seek state of the AWPP state machine. The basic idea of the parent selection in the new AWPP is as follows for both a RAP or MAP with radio backhaul:
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All AWPP parent selection metrics remain unchanged from the pre-release 4.0 implementation. This algorithm is run at startup and whenever a parent is lost and no other potential parent exists, usually followed by an LWAPP network and controller discovery. All neighbor protocol frames carry the channel information. Both parent maintenance and optimization techniques remain unchanged, as described in the following paragraphs, for completion.
Parent maintenance occurs by the child node sending a directed NEIGHBOR_REQUEST to the parent and the parent responding with a NEIGHBOR_RESPONSE.
Parent optimization and refresh occurs by the child node sending a NEIGHBOR_REQUEST broadcast on the same channel it has a parent on, and evaluating all responses from neighboring nodes on this channel. Until background scanning is implemented, off-channel optimization cannot occur. However, in most practical mesh networks, only a single channel backhaul is designed, especially with the current AP1500. Therefore, this should not be an issue except in a network where the same bridgegroupname is used across sectors and there are MAPs in bordering regions of the sectors.
A parent AP is the AP that has best path back to a RAP. AWPP uses ease to determine the best path. Ease can be considered the opposite of cost, and the preferred path is the path with the higher ease.
Ease Calculation
Ease is calculated using the SNR and hop value of each neighbor, and applying a multiplier based on various SNR thresholds. The purpose of this multiplier is to apply a spreading function to the SNRs that reflects various link qualities.
In Figure 20, MAP2 prefers the path through MAP1 because the adjusted ease (436906) though this path is greater then the ease value (262144) of the direct path from MAP2 to RAP.
Figure 20 Parent Path Selection
Parent Decision
A parent AP is chosen by using the adjusted ease, which is the ease of each neighbor divided by the number of hops to the RAP:
adjustedease = min (ease at each hop)
Hop countSNR Smoothing
One of the challenges in WLAN routing is the ephemeral nature of RF. This must be considered when analyzing an optimal path and deciding when a change in path is required. The SNR on a given RF link can change substantially from moment to moment, and changing route paths based on these fluctuations results in an unstable network, with severely degraded performance. To effectively capture the underlying SNR but remove moment-to-moment fluctuations, a smoothing function is applied that provides an adjusted SNR.
In evaluating potential neighbors against the current parent, the parent is given 20% of "bonus-ease" on top of the parent's calculated ease, in order to reduce the ping-pong effect between parents. This implies that a potential parent must be significantly better in order for a child to make a switch. Parent switching is transparent to LWAPP and other higher-layer functions.
Loop Prevention
To ensure that routing loops are not created, AWP discards any route that contains its own MAC address. That is, routing information apart from hop information contains the MAC address of each hop to the RAP; therefore, a 1500 Series Mesh AP can easily detect and discard routes that loop.
CLI Commands
The LWAPP controller on the WCS provides a number of views of the wireless mesh state. The following controller commands are useful for viewing the wireless mesh:
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These commands use the AP name as input. The AP names can be found using the show AP summary command:
(Cisco Controller) >show ap summaryAP Name Slots AP Model Ethernet MAC LocationPort------------------ ----- ----------------- ----------------- ----------------Rap:5f:fb:10 2 AP1500 00:0b:85:5f:fb:10 default_location 1Map1:5c:b9:20 2 AP1500 00:0b:85:5c:b9:20 default_location 1Map2:5f:fa:60 2 AP1500 00:0b:85:5f:fa:60 default_location 1Map3:5f:ff:60 2 AP1500 00:0b:85:5f:ff:60 default_location 1Note how the AP names in this example are a combination of a meaningful name and the AP MAC address. This is a recommended practice because it makes it easier to find a particular AP among a list of MAC addresses.
show mesh path
The following is an example of the show mesh path command, where the path through the wireless mesh from a mesh AP to the RAP is given:
(Cisco Controller) >show mesh path Rap:5f:fb:1000:0B:85:5F:FB:10 is RAP(Cisco Controller) >show mesh path Map1:5c:b9:2000:0B:85:5F:FB:10 state UPDATED NEIGH PARENT BEACON (86B), snrUp 65, snrDown 56,linkSnr 5100:0B:85:5F:FB:10 is RAP(Cisco Controller) >show mesh path Map2:5f:fa:6000:0B:85:5F:FB:10 state UPDATED NEIGH PARENT BEACON (86B), snrUp 72, snrDown 63,linkSnr 5600:0B:85:5F:FB:10 is RAP
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show mesh summary
The following are examples of the show mesh summary command for both a RAP and a MAP.
Notice that the RAP has only children, and a MAP has at least a parent but can also have children.
(Cisco Controller) >show mesh summary Rap:5f:fb:1000:0B:85:1B:78:90 state DEFAULT (1060), snrUp 0, snrDown 5, linkSnr 000:0B:85:5C:33:20 state (60), snrUp 0, snrDown 10, linkSnr 000:0B:85:5C:B9:20 state CHILD (160), snrUp 0, snrDown 55, linkSnr 000:0B:85:5F:FA:60 state CHILD (160), snrUp 0, snrDown 63, linkSnr 000:0B:85:5F:FF:60 state (60), snrUp 0, snrDown 6, linkSnr 0(Cisco Controller) >show mesh summary Map1:5c:b9:2000:0B:85:09:93:10 state UPDATED (61), snrUp 14, snrDown 15, linkSnr 1500:0B:85:1B:78:90 state DEFAULT (1061), snrUp 0, snrDown 7, linkSnr 000:0B:85:5C:1E:10 state NEEDUPDATE (260), snrUp 9, snrDown 13, linkSnr 900:0B:85:5C:33:20 state UPDATED CHILD (161), snrUp 20, snrDown 22, linkSnr 1200:0B:85:5F:FA:60 state UPDATED NEIGH BEACON (869), snrUp 45, snrDown 51, linkSnr 5000:0B:85:5F:FB:10 state UPDATED NEIGH PARENT BEACON (86B), snrUp 66, snrDown 55, linkSnr 5100:0B:85:5F:FF:60 state UPDATED BEACON (861), snrUp 7, snrDown 2, linkSnr 1In the output from this command, the snrUP is how this AP sees its received-signal-strength-indication (RSSI) from its neighbor. snrDown is what that neighbor is reporting back as its RSSI to the AP. The linkSnr is a weighed and filtered measurement based on the snrUp value. Note that snrUp and linkSnr is zero (0) for the RAPs.
show mesh neigh
The following are examples of the show mesh neigh command. The child AP sample shows the examples of the information used by AWPP, showing the ease values, and the vector information that gives the path back to the RAP.
(Cisco Controller) >show mesh neigh poletop:7a:70AP MAC : 00:0B:85:1B:78:90FLAGS : 161 UPDATED CHILDworstDv 255, Ant 0, channel 0, biters 0, ppiters 10Numroutes 1, snr 0, snrUp 37, snrDown 35, linkSnr 32adjustedEase 0, unadjustedEase 0txParent 0, rxParent 0poorSnr 0lastUpdate 1120196364 (Fri Jul 1 05:39:24 2005)parentChange 0Per antenna smoothed snr values: 32 0 0 0Vector through 00:0B:85:iB:78:90Vector ease 1 2648576, FWD: 00:0B:85:1B:D6:80 00:0B:85:1B:7a:70 00:0B:85:1B:78:90AP MAC : 00:0B:85:1B:D6:80FLAGS : 6B UPDATED NEIGH PARENTworstDv 0, Ant 0, channel 0, biters 0, ppiters 10Numroutes 0, snr 0, snrUp 0, snrDown 17, linkSnr 0adjustedEase 2207146, unadjustedEase 2648576txParent 2327, rxParent 2242poorSnr 0lastUpdate 1120196367 (Fri Jul 1 05:39:27 2005)parentChange 1009152070 (Mon Dec 24 00:01:10 2001)Per antenna smoothed snr values: 25 0 0 0Vector through 00:0B:85:1B:D6:80Vector ease 1 -1, FWD: 00:0B:85:1B:D6:80show mesh stats
The following is an example of the show mesh stats command where traffic statistics for a given mesh AP are given.
(Cisco Controller) >show mesh stats MAP:03:70AP MAC : 00:0B:85:53:03:70Poletop AP in state MaintrxNeighReq 840151, rxNeighRsp 938730txNeighReq 315153, txNeighRsp 840151tnextchan 0, nextant 0, downAnt 0, downChan 0, curAnts 0tnextNeigh 1, malformedNeighPackets 0, poorNeighSnr 52174blacklistPackets 0, insufficientMemory 0authenticationFailures 0Parent Changes 4, Neighbor Timeouts 21show mesh linkrate
The following is an example of the show mesh linkrate command where the SNR and bit rates between mesh APs are shown.
(Cisco Controller) >show mesh linkrate Rooftop:D6:80 poletop:7a:70MAC : 00:0B:85:18:7A:70State : assoc|joinedRxSignalStrength : 29 AckSignalStrength : 29Rx Data Rate : 18 Tx Data Rate : 18(Cisco Controller) >show mesh linkrate poletop:7a:70 poletop 78:90MAC : 00:0B:85:18:1B:78:90State : assoc|joinedRxSignalStrength : 35 AckSignalStrength : 35Rx Data Rate : 18 Tx Data Rate : 18
Note
This command is not used extensively. If you enter the show mesh linkrate command several times in succession, you will see the rates change from 6mbps to 18mbps and 12mbps (ACKs). 6 Mbps is the data rate for LWAPP, 18 Mbps is the data rate for backhaul data, and 12 Mbps is the rate for acknowledgements.
show mesh range
With the software releases prior to release 4.0 (4.0.155.0), there was a hard-coded bridging distance limitation of 12000 feet (2.25 miles) between the 1500 Series Mesh APs, even though the radio had a capability to go much further in distance. This distance limitation has been removed in the release 4.0 software release. The distance is configurable up to 132,000 feet (25 miles):
(Cisco Controller) >config mesh
range range from RAP to MAP Cisco Bridge (150..132000)
The default setting for range is 12000 feet. When you change the setting, the access points reboot (RAP and MAPs). Use the show mesh range command to view the settings:
(Cisco Controller) >show mesh range
MESH Range 14000
Traffic Flow
The traffic flow within the wireless mesh can be divided in to the following three components:
•
•
•
Because the LWAPP model is well known and the AWP protocol is proprietary, only the wireless mesh data flow is described. The key to the wireless mesh data flow is the address fields of the 802.11 frames being sent between mesh APs.
An 802.11 data frame can use up to four address fields: receiver, transmitter, destination, and source. The standard frame from a WLAN client to an AP uses only three of these address fields because the transmitter address and the source address are the same. However, in a WLAN bridging network, all four address fields are used because the source of the frame might not the transmitter of the frame, because the frame might have been generated by a device "behind" the transmitter.
Figure 21 shows an example of this type of framing. The source address of the frame is MAP:03:70, the destination address of this frame is the controller (the mesh is operating in Layer 2 mode), the transmitter address is MAP:D5:60, and the receiver address is RAP:03:40.
Figure 21 Wireless Mesh Frame
As this frame is sent, the transmitter and receiver addresses change on a hop-by-hop basis. AWP is used to determine the receiver address at each hop. The transmitter address is known because it is the current AP. The source and destination addresses are the same over the entire path.
Note that if the RAP controller connection is Layer 3, the destination address for the frame is the default gateway MAC address, because the MAP has already encapsulated the LWAPP in IP to be sent to the controller, and is using the standard IP behavior of using ARP to find the MAC address of the default gateway.
Each AP within the mesh forms an LWAPP session with a controller. WLAN traffic is encapsulated inside LWAPP and is mapped to a VLAN interface on the controller. Bridged Ethernet traffic can be passed from each Ethernet interface on the mesh and does not have to be mapped to an interface on the controller. (See Figure 22.)
Figure 22 Logical Bridge and WLAN Mapping
Design Details
Each outdoor wireless mesh deployment is unique, and each environment has its own challenges with available locations, obstructions, and network infrastructure availability, in addition to the design requirements based on users, traffic, and availability. This section describes important design considerations and provides an example of a wireless mesh design.
Wireless Mesh Constraints
When designing and building a wireless mesh network with the 1500 Series Mesh AP, there are a number of system characteristics to consider. Some of these apply to the backhaul network design and others to the LWAPP controller design:
•
18 Mbps is chosen as the optimal backhaul rate because it aligns with the maximum coverage of the WLAN portion of the client WLAN of the MAP; that is, the distance between MAPs using 18 Mbps backhaul should allow for seamless WLAN client coverage between the MAPs.
A lower bit rate might allow a greater distance between 1500 Series Mesh APs, but there are likely to be gaps in the WLAN client coverage, and the capacity of the backhaul network is reduced.
An increased bit rate for the backhaul network either requires more 1500 Series Mesh APs, or results in a reduced SNR between mesh APs, limiting mesh reliability and interconnection.
The wireless mesh backhaul bit rate, like the mesh channel, is set by the RAP.
The required minimum LinkSNR for backhaul links per data rate is shown in Table 1.
•
–
–
–
Table 2
6
5
9
14
9
6
9
15
12
7
9
16
18
9
9
18
24
13
9
22
36
17
9
26
Minimum Required LinkSNR Calculations by Data Rate
•
The number of hops is recommended to be limited to three-four primarily to maintain sufficient backhaul throughput, because each mesh AP uses the same radio for transmission and reception of backhaul traffic. This means that throughput is approximately halved over every hop. For example, the maximum throughput for an 18 Mbps is approximately 10 Mbps for the first hop, 5 Mbps for the second hop, and 2.5 Mbps for the third hop.
•
There is no current software limitation of how many MAPs per RAP you can configure. However, it is suggested that you limit this to 20 MAPs per RAP.
•
•
The number of controllers per mobility group is limited to 24.
Client WLAN
The mesh AP client WLAN delivers all the WLAN features derived by a standard LWAPP deployment for b/g clients with the full range of security and radio management features.
The goals of the client WLAN must be considered in the overall mesh deployment:
•
–
•
–
•
–
QoS Features
Cisco supports 802.11e on the local access and on the backhaul. The mesh APs prioritize user traffic based on classification, and therefore all user traffic is treated on a best-effort basis.
We do not generally recommend that QoS profiles be applied to users of the mesh network. Resources available to users of the mesh vary, according to the location within the mesh, and a configuration that provides bandwidth limitation in one point of the network can result in oversubscription in other parts of the network.
Similarly, limiting clients on their percentage of RF is not suitable for mesh clients. The limiting resource is not the client WLAN, but the resources available on the mesh backhaul.
Similar to wired Ethernet networks, 802.11 WLANs employ Carrier Sense Multiple Access (CSMA), but instead of using collision detection (CD), WLANs use collision avoidance (CA). This means that instead of each station trying to transmit as soon as the medium is free, WLAN devices will use a collision avoidance mechanism to prevent multiple stations from transmitting at the same time.
The collision avoidance mechanism uses two values, called aCWmin and aCWmax. CW stands for contention window. The CW determines what additional amount of time an endpoint should wait, after the interframe space (IFS), to attend to transmit a packet. Enhanced distributed coordination function (EDCF) is a model that allows end devices that have delay-sensitive multi-media traffic to modify their aCWmin and aCWmax values to allow for statically greater (and more frequent) access to the medium.
Cisco APs support EDCF-like QoS. This provides up to eight queues for QoS. These queues can be allocated in several different ways:
•
•
•
•
The Cisco Aironet 1500, in conjunction with Cisco controllers, provides a minimal integrated services capability at the controller, in which client streams have maximum bandwidth caps, and a more robust differentiated services (diffServ) capability based on the IP DSCP values and QOS WLAN overrides.
When the queue capacity has been reached, additional frames are dropped (tail drop).
Encapsulations
There are several encapsulations used by the mesh system. These include LWAPP control and data between the controller and RAP, over the mesh backhaul, and between the mesh AP to the client. The encapsulation of bridging traffic (non-controller traffic from a LAN) over the backhaul is the same as the encapsulation of LWAPP data.
There are two encapsulations between the controller and the RAP. The first is for LWAPP control, and the second for LWAPP data. In the control instance, LWAPP is used as a container for control information and directives. In the instance of LWAPP data, the entire packet, including the Ethernet and IP headers, is sent in the LWAPP container (see Figure 23).
Figure 23 Encapsulations
For the backhaul, there is only one type of encapsulation, encapsulating MESH traffic. However, two types of traffic are encapsulated: bridging traffic and LWAPP control and data traffic. Both types of traffic are encapsulated in a proprietary mesh header.
In the case of bridging traffic, the entire packet Ethernet frame is encapsulated in the mesh header (see Figure 24).
All backhaul frames are treated identically, regardless of whether they are MAP to MAP, RAP to MAP, or MAP to RAP.
Figure 24 Encapsulating Mesh Traffic
In case of bridging, the frames are transmitted as they are received at the ingress to the AP Ethernet port.
Queuing on the Access Point
The AP uses a high speed CPU to process ingress frames, Ethernet, and wireless on a first-come first-serve basis. These are queued for transmission to the appropriate output device, either Ethernet or wireless. Egress frames can be destined for either the 802.11 client network, the 802.11 backhaul network, or Ethernet.
The Cisco Aironet 1500 Series AP supports four FIFOs for wireless client transmissions. These FIFOs correspond to the 802.11e platinum, gold, silver, and bronze queues, and obey the 802.11e transmission rules for those queues. The FIFOs have a user configurable queue depth.
Likewise, the backhaul (frames destined for another outdoor Access Point) uses four FIFOs, though user traffic is limited to gold, silver, and bronze. The platinum queue is used exclusively for LWAPP control traffic, and has been reworked from the standard 802.11e parameters for CWMIN, CWMAX, and so on, to provide more robust transmission but higher latencies.
Similarly, the 802.11e parameters for CWMIN, CWMAX, and so on, for the gold queue have been reworked to provide lower latency at the expense of slightly higher error rate and aggressiveness. The purpose of these changes is to provide a channel more conducive to voice applications.
Frames destined for Ethernet are queued as FIFO, up to the maximum available transmit buffer pool (256 frames). With 4.0.155.0 support for Layer 3 IP Differentiated Services Code Point (DSCP), marking of the packets has been added.
In the controller to RAP path for the data traffic, the outer DSCP value is set to the DSCP value of the incoming IP frame. If the interface is in tagged mode, the controller sets the 802.1Q VLAN ID, and derives the 802.1p UP (outer) from 802.1p UP incoming and the WLAN default priority ceiling. Frames with VLAN ID 0 will not be tagged (see Figure 25).
Figure 25 Controller to RAP Path
For LWAPP control traffic the IP DSCP value is set to 46, and the 802.1p user priority is set to 7. Prior to transmission of a wireless frame over the backhaul, regardless of node pairing (RAP/MAP) or direction, the DSCP value in the outer header is used to determine a backhaul priority. The following sections describe the mapping between the four backhaul queues the AP uses and the DSCP values shown in Table 3.
Table 3 Backhaul Path QoS
2, 4, 6, 8-23
Bronze
26, 32-63
Gold
None
Platinum
All others, including 0
Silver
Note
For a MAP to the client path, there are two different procedures, depending on whether the client is a WMM client or a normal client. If the client is a WMM client, the DSCP value in the outer frame is examined, and the 802.11e priority queue is used (see Table 4).
Table 4 MAP to Client Path QoS
2, 4, 6, 8-23
Bronze
26, 32-45, 47
Gold
46, 48-63
Platinum
All others, including 0
Silver
If the client is not a WMM client, the WLAN override (as configured at the controller) determines the 802.11e queue (bronze, gold, platinum, or silver), on which the packet is transmitted.
For client towards Access Point, there are modifications made to incoming client frames in preparation for transmission on the mesh backhaul or Ethernet. For WMM clients, Figure 26 illustrates the way in which the outer DSCP value is set from an incoming WMM client frame.
Figure 26 MAP to RAP Path
The minimum of the incoming 802.11e user priority and the WLAN override priority is translated using the information listed in Table 5 to determine the DSCP value of the IP frame. For example, if the incoming frame has as its value a priority indicating the gold priority, but the WLAN is configured for silver priority, the minimum priority of silver is used to determine the DSCP value.
Table 5 802.11e User Priority to DSCP Mapping
User Priority
LWAPP Header0
0
1
8
2
16
3
24
4
32
5
40
6
48
7
56
In the event that there is no incoming WMM priority, the default WLAN priority is used to generate the DSCP value in the outer header. In the event that the frame is an originated LWAPP control frame, the DSCP value of 46 is placed in the outer header.
Now that the DSCP value is determined, the rules described earlier for the backhaul path from RAP to MAP are used to further determine the backhaul queue on which the frame is transmitted. Frames transmitted from the RAP to the controller are not tagged. The outer DSCP values are left intact, as they were first constructed.
Bridging Backhaul Packets
Bridging services are treated a little differently from regular controller-based services. There is no outer DSCP value in bridging packets because they are not LWAPP encapsulated. Therefore, the DSCP value in the IP header as it was received by the AP is used to index into the table as described in the path from AP to AP (backhaul).
Bridging Packets From and To a LAN
Packets received from a station on a LAN are not modified in any way. There is no override value for the LAN priority. Therefore, in bridging mode the LAN must be properly secured. The only protection offered to the mesh backhaul is that non-LWAPP control frames that map to the platinum queue are demoted to the gold queue.
Packets are transmitted to the LAN precisely as they are received on ingress at entry Ethernet to the mesh.
The only way to integrate QoS between Ethernet ports on AP1500 and 802.11a is by tagging Ethernet packets with DSCP. The AP1500 will take the Ethernet packet with DSCP and will place it in the appropriate 802.11e queue.
The 1500 does not tag DSCP itself:
•
•
The Ethernet devices, like video cameras, should have the capability to mark the bits with DSCP value to take advantage of QoS.
Doppler Effect
Doppler has no measurable impact on the UDP throughput up to a velocity of 36,000km/h. For higher velocities, the throughput first decreases to 1Mbit/s. Connections are lost at a velocity of > 92,000 km/h, as shown in Figure 27.
Figure 27 Doppler Effect
Design Example
This section provides an example of a design for WLAN coverage in an urban or suburban area, adhering to the compliance conditions for United States domain.
Cell Planning and Distance
The starting point is the RAP-to-MAP ratio. There is currently no hard limitation of MAPs per RAPs, but the current recommended maximum number is 20 MAPs per RAP. For the backhaul, there is a typical cell size radius of 1000 feet. One square mile in feet is 5280^2 square feet, so the number of cells comes out to be nine, and you can cover one s
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