After deciding what is important, you can sketch out what the wireless LAN will look like. Broadly speaking, there are two major ways of deploying a wireless LAN, and the choice depends broadly on whether you decide to use security at the link layer. This section describes and analyzes four different major architectures for wireless LANs. To a certain extent, this section presents four fairly rigid examples. As the market for wireless LAN network hardware matures, equipment may incorporate features from multiple topologies, allowing you to mix and match the features that best suit your needs.
Topology 1: The Monolithic Single-Subnet Network
In the beginning, there was one topology. Access points were simple bridges, and served only to attach wireless stations to the single wired network they were connected to. Without much networking intelligence in the access point, wireless networks needed to be designed around the trivial bridging engines in access points. Networks that supported mobility were correspondingly simple. When access points are simple bridges without any sophisticated knowledge of, say, VLANs or routing, they must all attach to the same IP subnet. As long as a station stays on the same IP subnet, it does not need to reinitialize its networking stack and can keep its TCP connections open.
Equipment limitations dictated the resulting network architecture. Every AP was attached to a single network. While the network provided mobility, it was often difficult to build, especially on large campuses. In addition to modifying backbone network configuration, administrators had to set aside new IP address ranges and route appropriately. The architecture was developed to shield wired networks from the danger of wireless networks in the time before the development of strong security protocols. These days, the high configuration overhead and management cost of building two parallel networks has driven this topology nearly to extinction on any network larger than a few access points.
Figure 21-2 shows the typical early wireless LAN deployment topology. All the APs are connected to a single monolithic network. The network is a single link-layer domain, and every station connected to the network is given an IP address on the IP subnet. For this reason, the monolithic architecture may be referred to as the single-subnet wireless LAN, the walled garden architecture, or occasionally the VPN architecture. (It should also be noted that most home networks take the single subnet approach, although typically with only one access point.)[*] The guiding principle of Figure 21-2 is that the access points in use cannot provide any services other than link-layer mobility, so they must all be connected to the same logical link layer. Other design decisions underlying this topology help augment the access control of the wireless device and lower management overhead by taking advantage of existing services, each of which will be considered in turn.
[*] Very large homes may require multiple APs. Generally speaking, an AP should be good for coverage over 3,000-5,000 square feet, which is sufficient for all but the largest homes.
Figure 21-2. The single subnet wireless LAN deployment topology
In Figure 21-2, the network linking all the access points, which is often called the access point backbone, is a single IP subnet. To allow users to roam between access points, the network should be a single IP subnet, even if it spans multiple locations, because IP does not allow for network-layer mobility. (Mobile IP is the exception to this rule; see the sidebar earlier in this chapter.) Network-layer mobility is supplied by the use of a switching infrastructure that supports linking all the access points together, and an IP addressing scheme that does not require anything beyond link-layer mobility.
In Figure 21-2, the backbone network may be physically large, but it is constrained by the requirement that all access points connect directly to the backbone router (and each other) at the link layer. 802.11 hosts can move within the last network freely, but IP, as it is currently deployed, provides no way to move across subnet boundaries. To the IP-based hosts of the outside world, the VPN/access control boxes of Figure 21-2 are the last-hop routers. To get to an 802.11 wireless station with an IP address on the wireless network, simply go through the IP router to that network. It doesn't matter whether a wireless station is connected to the first or third access point because it is reachable through the last-hop router. As far as the outside world can tell, the wireless station might as well be a workstation connected to an Ethernet.
If it leaves the subnet, though, it needs to get a IP new address and reestablish any open connections. The purpose of the design in Figure 21-2 is to assign a single IP subnet to the wireless stations and allow them to move freely between access points. Multiple subnets are not forbidden, but if you have different IP subnets, seamless mobility between subnets is not possible.
Older access points that cooperate in providing mobility need to be connected to each other at layer 2. One method of doing this, shown in Figure 21-3 (a), builds the wireless infrastructure of Figure 21-2 in parallel to the existing wired infrastructure. Access points are supported by a separate set of switches, cables, and uplinks in the core network. Virtual LANs (VLANs) can be employed to cut down on the required physical infrastructure, as in Figure 21-3 (b). Rather than acting as a simple layer-2 repeater, the switch in Figure 21-3 (b) can logically divide its ports into multiple layer-2 networks. The access points can be placed on a separate VLAN from the existing wired stations, and the "wireless VLAN" can be given its own IP subnet. Frames leaving the switch for the network core are tagged with the VLAN number to keep them logically distinct and may be sent to different destinations based on the tag. Multiple subnets can be run over the same uplink because the VLAN tag allows frames to be logically separated. Incoming frames for the wired networks are tagged with one VLAN identifier, and frames for the wireless VLAN are tagged with a different VLAN identifier. Frames are sent only to ports on the switch that are part of the same VLAN, so incoming frames tagged with the wireless VLAN are delivered only to the access points.
By making the access point backbone a VLAN, it can span long distances. VLAN-aware switches can be connected to each other, and the tagged link can be used to join multiple physical locations into a single logical network. In Figure 21-4, two switches are connected by a tagged link, and all four access points are assigned to the same VLAN. The four access points can be put on the same IP subnet and act as if they are connected to a single hub. The tagged link allows the two switches to be separated, and the distance can depend on the technology. By using fiber-optic links, VLANs can be made to go between buildings, so a single IP subnet can be extended across as many buildings as necessary.
Figure 21-3. Physical topologies for 802.11 network deployment
Figure 21-4. Using VLANs to span multiple switches
Tagged links can vary widely in cost and complexity. To connect different physical locations in one building, you can use a regular copper Ethernet cable. To connect two buildings together, fiber-optic cable is a must. Different buildings are usually at different voltage levels relative to each other. Connecting two buildings with a conductor such as copper would enable current to flow between (and possibly through) the two Ethernet switches, resulting in expensive damage. Fiber-optic cable does not conduct electricity and does not pick up electrical noise in the outdoor environment, which is a particular concern during electrical storms. Fiber also has the added benefit of high speeds for long-distance transmissions. If several Fast Ethernet devices are connected to a switch, the uplink is a bottleneck if it is only a Fast Ethernet interface. For best results on larger networks, uplinks are typically Gigabit Ethernet.
For very large organizations with very large budgets, uplinks do not need to be Ethernet. One company I have worked with uses a metro-area ATM cloud to connect buildings throughout a city at the link layer. With appropriate translations between Ethernet and ATM, such a service can be used as a trunk between switches.
Address assignment through DHCP
Within the context of Figure 21-2, there are two places to put a DHCP server. One is on the access point backbone subnet itself. A standalone DHCP server would be responsible for the addresses available for wireless stations on the wireless subnet. Each subnet would require a DHCP server as part of the rollout. Alternatively, most devices capable of routing also include DHCP relay. The security device shown in Figure 21-2 includes routing capabilities, and some firewalls and VPN devices include DHCP relay. With DHCP relay, requests from the wireless network are bridged to the access point backbone by the access point and then further relayed by the access controller to the main corporate DHCP server. If your organization centralizes address assignment with DHCP, take advantage of the established, reliable DHCP service by using DHCP relay. One drawback to DHCP relay is that the relay process requires additional time and not all clients will wait patiently, so DHCP relay may not be an option.
Static addressing is acceptable, of course. The drawback to static addressing is that more addresses are required because all users, active or not, are using an address. To minimize end-user configuration, it is worth considering using DHCP to assign fixed addresses to MAC addresses.
As a final point, there may be an interaction between address assignment and security. If VPN solutions are deployed, it is possible to use RFC 1918 (private) address space for the infrastructure. DHCP servers could hand out private addresses that enable nodes to reach the VPN servers, and the VPN servers hand out routable addresses once VPN authentication succeeds.
This is the oldest of the architectures in this chapter, and pre-dates all the work done on link-layer security in the past several years. It is generally used on networks where link-layer security is not a priority, either because security is secondary to providing services (as in the case of an ISP) or because security is provided through higher-layer protocols with VPN technology. Security trade-offs in wireless network design are discussed in more detail in Chapter 22.
Depending on the existing backbone, using this topology may require prohibitive work on the backbone, or it may be relatively easy. For maximum mobility, every access point must be attached to the wireless VLAN that snakes throughout the campus. If a network is built on a switched core, it may be relatively easy to create a VLAN that spans multiple switches across several wiring closets. However, there may be fundamental limitations on what is possible. If buildings are separated by routers, it may not be possible to build a single VLAN that spans an entire campus, and it may be necessary to settle for disjointed islands of mobility. Even worse, many older networks are not built around switched cores that allow easy VLAN extensions everywhere.
Furthermore, there is a practical limitation on the network diameter of a VLAN. 802.1D, the bridging standard, recommends that VLANs be built with a maximum diameter of seven switch hops. Depending on the physical topology, it may be impossible to build a single VLAN that can span the desired coverage area within the recommended limit. Alternatively, it may be possible to do so, but only with extensive modifications to the network core.
Performance of this design can vary greatly because it incorporates a single choke point. One of the most important aspects of making this design perform well is limiting the effect of pushing all the traffic through a single logical path. All the backbone devices must have sufficient capacity to handle the load from the entire wireless network.
Wireless LAN protocols are based on collision avoidance, and can sustain much higher loads than the collision-detection protocols used on wired LANs. Depending on the number of users associated with a particular access point, it may be reasonable to assume that the radio link is saturated. Maximum throughput rates vary slightly from product to product, but 6 Mbps is a reasonable maximum rate for 802.11b, with 802.11a and 802.11g both weighing in at 27-30 Mbps.
Avoiding congestion is much easier with the slow speeds of 802.11b. With only a 6 Mbps potential load per access point, a full duplex Fast Ethernet links to the access point backbone should be able to handle slightly over 30 APs. While 30 APs is not a monstrous network, it is enough to provide blanket coverage over a large open space for low-bandwidth applications. Upgrading to Gigabit Ethernet on the choke point vastly increases the number of APs that can be attached. Depending on the breakdown between upstream and downstream traffic, it is possible to connect 200-300 APs without worrying about backbone network congestion. Of course, gigabit choke point devices cost significantly more than Fast Ethernet choke point devices.
802.11a and 802.11g, with their potentially higher speeds, could pose more of a problem. With several times the speed, only a few access points can saturate a Fast Ethernet choke point. Assuming a favorable breakdown between upstream and downstream transmission, full duplex Fast Ethernet can connect six APs, which is not enough to cover many midsized offices. Dual-band APs that do both 802.11a and 802.11g present a double whammy because each radio may offer a high load.
Table 21-3 summarizes the discussion of backbone technology and the number of APs required to saturate the link. It is meant only as a "back of the envelope" estimate. Each backbone technology is divided by the AP-offered load to estimate the number of APs required to saturate the link. It does not take into account any protocol overhead or realistic split between upstream and downstream traffic. It is meant as a rough guide to select an appropriate uplink technology from your wireless subnet.
802.11b (~6 Mbps)
802.11a or 802.11g (~30 Mbps)
Dual-band a/b (~36 Mbps)
Dual-band a/g (~60 Mbps)
Half-duplex Fast Ethernet
Full-duplex Fast Ethernet
Full-duplex Gigabit Ethernet
This is the most varied of the architectures in terms of client integration. In the case of a service provider, it is likely that little or no client work is required. No security of any sort is applied, so there is nothing to configure. If extensive higher-layer security is applied on top of this architecture, however, there is extensive desktop integration to be done.
Topology 2: "E.T. Phone Home" or "Island Paradise"
Some organizations are simply too large to build a single access point network. The classic example is a major research university with multiple buildings distributed over several square miles. Configuring a single access point network to snake through the entire campus is simply out of the question, not least because large campuses depend on routed networks for broadcast isolation.
Network administrators compromised by dividing the wireless network into several "islands" of connectivity. In the university environment, an island often corresponds to a building or department, and it takes its IP addressing and routing information from that department's address allocation. Separating wireless LANs into islands also serves a valuable political purpose. Different departments can each build their own wireless network, complete with its own security policies and network service goals. Islands can also be built more quickly because no coordination is required between them. Many islands can be built simultaneously.
Piecemeal deployments look like multiple instances of the single subnet of Figure 21-2. The topology provides seamless mobility between the access points connected to the access point backbone network. In networks that cannot support a single VLAN for the access point backbone, a frequent compromise is to limit mobility to local areas where it is most useful. For example, in a multi-building campus, a typical goal is to provide seamless mobility within individual buildings, but not roaming between buildings. Each building would have a wireless LAN that looked something like Figure 21-2, and all the access point backbone networks would ultimately connect to a campus backbone.
In Figure 21-5 (a), there are several "islands" of connectivity, and each island provides mobility within itself. Inter-island roaming cannot be provided by 802.11 itself, but requires additional technology such as Mobile IP or a special client. 802.11 allows an ESS to extend across subnet boundaries, but does not support a seamless roaming operation.
Figure 21-5. Noncontiguous deployments
If you must break the campus into disjointed coverage areas, be sure to preserve the mobility that is most important to your users. In most cases, mobility within a building is important. Most buildings are built around a switched core, and can support an island of connectivity.
The single-subnet architecture achieved mobility by creating a single subnet for all access points, and keeping all the users on the that restricted subnet. This architecture borrows the same philosophy, but is designed to work with networks that are not able to create a single subnet.
At the most basic level, this architecture provides portability. Users can move between islands without restriction, but need to reestablish any open network connections as they move between islands. Connection reestablishment may be handled in a variety of ways, some of which may be transparent to the user. Many universities simply accept the limitations of portability, and instruct users to close any applications that use network resources before moving. If portability limitations are problematic, it may be possible to achieve mobility between IP networks by using client software or tunneling protocols.
This topology looks much like the first topology, except that it is replicated in several pieces. Most likely, the islands of connectivity connect to the network core through firewalls. Mobility between islands may be achieved by using a tunneling protocol that ensures that a user attaches to the same logical location on the network, no matter what their physical location.
Figure 21-6 shows how mobility can be grafted on to a collection of scattered networks. In Figure 21-6 (a), clients are given a local IP address that is tied to location. The local networks are represented by Net X and Net Y. Upon connection, clients are issued addresses from the IP space assigned to the X and Y networks. However, the client also initiates a connection to a central concentration point. Clients logically attach to the concentrator, and receive an address from a network logically attached to the concentrator, which is denoted by Net Z in the diagram. Packets sent from the client use its central anchor point address, Z, as the source, but they are bundled into a tunnel for transmission. Replies are routed back to Z, but the concentrator maintains a mapping of addresses on network Z to location-based addresses. Note that Figure 21-6 (a) does not specify any particular tunneling method. Mobile IP works in essentially this way, and a few specialized IPsec clients work this way as well.
Although the approach of Figure 21-6 (a) is conceptually straightforward, it requires changing the software on all wireless devices. In addition to the administrative challenge of loading new software on any wireless device and the potential instability of changing the network stack, it is likely that vendors of this software would not be able to support every operating system platform. Even if the major operating systems were supported, many embedded devices could not be. Figure 21-6 (b) offers an alternative approach where the tunneling is moved into the network. In Figure 21-6 (b), access points do not connect to a backbone network for the purpose of delivering traffic. The backbone network is used only to connect APs to the traffic concentration point. Any frames or packets from the client are delivered through the tunnel to the concentrator device, where they are sent on to the rest of the network. It does not matter where clients attach to the network because traffic is always routed to the traffic concentration point.
In both of the cases in Figure 21-6, the key is that client IP addresses become location-independent. IP addresses on local networks are used for the purpose of connectivity, but the logical point of attachment to the network is through a defined anchor point, just as in the previous topology.
Tunneling approaches work to unite disjointed coverage areas. In the first topology, mobility was all-or-nothing. Figure 21-5's disjointed coverage areas force network architects to design mobility around areas that are most important to users, subject to the constraints of the local network design. By using a tunneling approach, the network can be reunited into a single mobility cloud, but without the need to re-engineer the entire network backbone. There is, however, the difficulty of configuring any tunneling and working out the overlay topology.
Figure 21-6. Mobility through tunneling
One of the advantages of this architecture is that it is easy to use it with IPsec. IPsec is a suite of strong, trusted encryption protocols that have been widely used in hostile network environments, and that trust has allowed IPsec to be used to protect a great deal of sensitive information traversing the Internet. Many organizations that have a need to protect private personal information make extensive use of IPsec.
One drawback to relying on network-layer security is that it gives malicious attackers a foothold on your network. If association to the network is not protected, then attackers may obtain a network address and start launching attacks against against other clients or the network infrastructure outside the firewall. Strong firewall protection is a must to contain any attacks originating on the untrusted network. Host security is also extremely important because devious attackers would also likely attempt to subvert host security on the clients to hijack VPN tunnels, so personal firewall software is a must.
IPsec was designed with a point-to-point architecture in mind. When used between major sites, traffic is inherently point-to-point. However, LANs are not meant to be point to point networks. (Just ask anybody who has experience with ATM LAN Emulation!) Applications that make use of multicast will probably not work with IPsec without modification or network reconfiguration.
Providing connectivity through isolated islands gives this topology a distinct advantage over the first topology. Rather than one gateway device that handles all traffic from the wireless LAN, each island gateway must be capable of fowarding only that island's traffic. Multiple choke points between wireless and wired networks allow each choke point to be a smaller, and therefore less expensive, device.
This architecture is frequently used with IPsec, often with an existing VPN termination device. One problem that can occur is that VPN devices are often sized for remote user termination. If LAN users suddenly start using IPsec, the existing VPN termination device may prove inadequate. A centrally located VPN device must be able to provide encryption for the entire wireless LAN traffic load; each 802.11b access point may offer a traffic load of up to 6 Mbps each, while an 802.11a or 802.11g access point may serve up a load approaching 30 Mbps.
There are many different tunneling options available for this broad topology. Tunneling always imposes a network overhead because it requires encapsulation. An additional challenge that wireless LAN devices must face is the need for fragmentation in tunneling protocols. Many of the LAN backbones used to connect access points do not support jumbo frames, so any tunneling protocol that runs over Ethernet must incorporate fragmentation and reassembly. Beyond the fragmentation overhead, any tunneling protocol requires additional header information. Depending on the protocol selected, fragmentation overhead may be nontrivial.
Running user traffic across a network backbone may diminish the service quality. Large networks may not be able to provide consistent low-latency forwarding performance between the access points and the concentration device, especially if the tunneling mechanism is implemented over a best-effort protocol like IP. In the case of user data traffic, any service quality diminishment is likely to be negligable. If the wireless network must be used to support voice protocols, however, the impact of tunneling may be more substantial.
Compared to the single-subnet architecture, this topology integrates much better with networks that cannot support a single VLAN everywhere. At worst, this architecture requires creating several miniature single-subnet backbones. If tunneling functions are moved into the network, though, it is possible to extend networks out to remote locations without any backbone work.
VPN software is typically used with this approach, which requires configuring client software on any machine that will use the wireless network. In some organizations, this may not represent a large burden, especially if most of the users already have VPN software. However, many organizations limit the number of users given remote access privileges to limit the amount of client integration work necessary, or prevent remote access devices from being overwhelemed with the load. If you work for such an organization, there is a significant client software installation burden with a widespread wireless deployment. As mentioned previously, personal firewall software is mandatory to protect each client from link-layer attacks. Give preference to VPN clients that include personal firewall software, especially if the personal firewall policies can be centrally managed.
Topology 3: Dynamic VLAN Assignment
Both the single-subnet and island topologies are designed around the limitations of the first access points to hit the market. Early access points attached all users to the same network, and did very little to enforce different privileges on different groups of users. This topology was the first to embrace the wired world of VLANs and make them available to user groups. Instead of building a second parallel network, this topology extends the existing network, complete with any security systems and filters, into the wireless realm.
802.1X is the cornerstone of dynamic VLAN assignment. It plugs the wireless network neatly into an existing authentication infrastructure. Authentication servers have user profiles and privileges, and can map that privilege information on to the wireless LAN. For example, Figure 21-7 shows a RADIUS server handing out VLAN assignments to the access point. As part of the RADIUS access accept message, it includes an attribute that assigns an authenticated user to a particular VLAN. Based on that information, the access point tags any frames from the user on to the appropriate VLAN.
The advantage of doing authentication at the link layer, rather than a higher layer, is that users can be placed on a particular network with the privileges associated with that network from the start. When the access point receives the Access Accept message from the RADIUS server, it sends an 802.1X EAP Success message to the client. Network card drivers on the client interpret the EAP Success message as the equivalent event to a "link up" message, and send their DHCP request and begin initializing the network stack. By the time the network stack has begun to initialize, the network has already automatically configured itself to restrict the user to a particular set of access rights.
Figure 21-7. Dynamic VLAN topology
At the highest level, mobility in this topology is identical to the first topology. Users are attached to a consistent VLAN throughout the network, and thus can maintain the same IP address regardless of location. With the same IP address, any transport-layer state or application state remains valid throughout the life of the connection.
However, the underlying implementation of mobility offers several advantages over the single-subnet architecture. The first set of advantages have to do with the use of authentication services. Attributes from the RADIUS server ensure that users are always attached to the same VLAN, and hence, they stay attached to the same logical point on the network.
In addition to aiding mobility, providing consistent VLAN attachment can make other services work better. Providing mobility at the link layer reduces the apparent mobility to higher-layer protocols, and hence, the amount of work required of them. IPsec tunnels stay up consistently because the IP address does not change. Likewise, Mobile IP location updates are not necessary because the IP address is maintained.
Because the VLAN assignment is based on 802.1X and RADIUS, security in this topology is based on dynamically generated keys at the link layer, either through dynamic WEP, WPA, or CCMP. Dynamic key generation enables the second benefit of using authentication services. Once users have been identified, they can be separated into groups for different security treatment.
To separate traffic in the air between user groups, access points use multiple key sets. Upon authentication, every user is given a default (broadcast) key, and a key mapping (unicast) key. Broadcast domains are defined by the stations in possession of the same broadcast key. In Figure 21-8, the two users on the left are part of the same user group, and share the same broadcast key. When one sends, say, an ARP request, the other responds. Users who are part of a different broadcast domain are not able to decrypt and process the frame because they have a different broadcast key. Although user groups share the same radio capacity, they are not members of the same user group and remain separated over the radio network.
Figure 21-8. Broadcast separation by keys
Furthermore, the separation of user groups by VLAN allows the application of differentiated services, as shown in Figure 21-9. One common use of user identification and differentiation is to offer guest services. Internal users are identified and authenticated against a user database, and then connected to the internal network. Guest users do not have accounts on the main user database and cannot authenticate to the network. After failing to do so, they are attached to a different logical network. Guest networks may have "splash pages" that require a click-through agreement to not abuse the network; some organizations may also wish to require payment for guest access.
Figure 21-9. Differentiated user services
An additional advantage to link-layer security is that multicast is well-integrated into the security protocol. LAN protocols often make heavy use of multicast or broadcast frames, and the use of multicast LAN frames can only increase. Wireless networks are attractive because of their flexibility and location-independence. Protocols that assist in the automatic discovery and configuration of new devices usually rely heavily on multicast frames.
One downside to this topology relates to bureaucratic requirements around security. At the time this book was written, link-layer security could not comply with FIPS-140, the U.S. federal government's network security standard, because of a subtle flaw with the dynamic key derivation algorithms in 802.11i. Although the encryption mode used by CCMP is approved, a small change to the key derivation algorithm is likely to be required before 802.11i-based networks can meet the FIPS-140 bar.
This architecture does not necessarily require a choke point. Switching frames at the network edge eliminates the requirement for an oversized packet forwarding device. Wireless LANs are access networks, so by definition, a wireless LAN should not be able to overload a well-built network core. One of the downsides of this architecture, however, is that it is best deployed around a big, fast switched core.
Redesigning a network to use VLAN information dynamically can often impose a substantial redesign of the the network backbone. What is required depends on how the wireless LAN connects to the network core. When a wireless LAN connects to the core to attach users to multiple networks, it typically uses an 802.1Q tagged link. Wireless LAN products vary in how widely tagging is used, and to what extent the tags must be pushed across the network. In broad terms, there are two major ways to push VLAN information out to access points.
Direct core connection
When the connection to the network core is made directly, the access points must connect directly to the network core, usually through an 802.1Q-tagged link.
Note that the connection to the core is the logical connection from the access points. With some products, the access points must connect directly to the core, which means that every switch port used to connect to an access point must support any VLAN used by wireless users. Direct core connections for every access point imposes a huge backbone engineering requirement, and may even rule out the use of this topology. If the VLANs do not exist in every closet where APs connect, they must be extended everywhere before the wireless deployment can even begin.
Direct connections to the core may also pose a security risk. Most APs authenticate users, but the APs themselves do not authenticate. An attacker who replaces an AP with his own device may have a direct connection to the network core.
Indirect (tunneled) core connection
Instead of requiring every access point to connect directly to the core, some products allow the use of tunneling protocols to avoid significant changes to the backbone. Users connect to an access point, but the AP tunnels the user's frames to a remote location before they are placed on to the core network. Tunneling can be accomplished between access points, or between an access point and an aggregation device. The tunneling protocol may be proprietary, or it may be based on a simple encapsulation standard like the Generic Routing Encapsulation (GRE), IP in IP, or the Point-to-Point Protocol over Ethernet (PPPoE).
In Figure 21-10 (a), there are two APs on separate VLANs. After user authentication completes, the AP is responsible for connecting the user on to the appropriate VLAN. If the AP is directly attached, then the connection is easy. When the AP is not directly attached to the VLAN the user must be connected to, the tunnel is built between APs. AP2 locates the VLAN the user should be attached to, and sends user frames through the tunnel to AP1. AP1 then sends frames out on to the network normally. The user's logical attachment remains AP1, no matter what the physical location is. Depending on the implementation, it may be necessary to prevent tunneling across long distances. If the two networks are separated by state lines, or even an ocean, tunneling traffic is likely to result in user dissatisfaction.
In Figure 21-10 (b), the attachment is centralized at the core of the network rather than being distributed at the edge. Frames received by the access points are shuttled up through the tunnel to the concentrator, where they are placed on the appropriate network. VLAN information is only relevant at the end of a frame's journey through the wireless LAN system. Until the frames reach the concentrator, they do not carry VLAN tags. The advantage of a remote tunneling system is that users can be attached to VLANs that are not locally present. The VLANs need to be made available only to the concentrator.
Of the two methods, tunneled connections tend to impose less of a backbone engineering requirement because tags can be distributed on a more local basis. In the direct connect case, every port connected to an access point must carry the complete set of VLANs users may want to work with. The backbone impact is just as great as the first topology for each VLAN. In contrast, indirect connections can span wider areas by operating outside of a spanning tree domain, and configuration of individual switch ports may be easier.
802.1X supplicants are now built-in to the most common client operating systems. Windows 2000, Windows XP, and Mac OS X 10.3 all have 802.1X supplicant software built into the operating system. Provided that you wish to use one of the authentication protocols supported by the operating system's supplicant, there is no client installation to worry about. Furthermore, the built-in supplicant configuration can often be assisted by the use of large-scale system administration tools to distribute the required certificates or configuration information.
Topology 4: Virtual Access Points
A straightforward application of 802.1X and VLAN assignment leads directly to the previous topology. However, it works best when the network has only one class of userand that is hardly realistic. Most networks are now built to connect employees to internal resources while simultaneously giving guests access to the Internet. Supporting multiple classes of user is becoming much more common, but it creates additional work for security architects. Different logical networks must be run in parallel, often with vastly different security models.
Figure 21-10. Core connections for dynamic VLAN products
One method of building multiple logical networks is to build multiple physical networks, and manage each separately. Competition for network administrator time, access point locations, power and network connections, and radio resources makes building multiple physical networks unproductive. Instead, architects are turning to virtual access points, which enable multiple logical networks to be built on a single physical infrastructure. The physical network owner is responsible for maintaining the infrastructure as a common carrier, and serving as a transit network to other existing networks.
Several years ago, airports woke up to the possibility of using 802.11 as a network medium to connect business travelers (and their wallets) to the Internet. In the first wave, airports worked with specialist integrators to build a single wireless LAN that looked something like the first topology in this chapter. It could be used by business travelers, but was not at all suited to use by anybody else. Many applications of wireless networking went unheeded. Wireless networks are ideal for providing connections that may need to be moved and changed on a regular basis, such as retail kiosks or even the airline equipment at gates. It is easy to understand why a credit-card processing service or an airline would feel that a network designed for road warriors did not provide adequate security.
With a network designed around virtual APs, there is one set of physical infrastructure, owned by the building owner. The building owner is responsible for frequency coordination throughout the building. From a monetary perspective, the single physical network is also the only game in town, and the building owner can charge for access to the network. In office buildings designed for multiple tenants, the owner may choose to use the wireless network as an amenity to make tenancy more attractive.
 Newer leases for multitenant buildings are increasingly being written so that the owner retains control of the electromagnetic spectrum and can construct a building-wide network without working around tenant equipment, although the FCC takes a dim view of these provisions.
Virtual APs may also make a great deal of sense for the network users. With one organization handling physical installation, there is no overlapping installation effort, and it lessens turf wars over radio spectrum. A virtual AP can offer the same services as a dedicated AP, but a virtual AP is often cheaper to install because of the shared infrastructure. The most advanced virtual APs look exactly like multiple standalone APs. I expect that the management of virtual AP systems offer the ability to extend management infrastructure to the users, so that there is a low-level administration interface plus the ability to configure virtual networks for every customer subscribing to a multitenant network service.
Figure 21-11 shows what a network built on virtual access points would look like. In essence, it allows the network administrator to create several copies of the dynamic VLAN topology on one set of physical infrastructure, with each virtual network administered. In the figure, there are three distinct networks to be extended by the wireless LAN. Network A is a typical corporate network. Users who wish to gain access to it must have accounts on the corporate RADIUS server. Network B is a hot-spot service provider. For device-independence, many service providers use web-based authentication systems that trap user requests until users have identified themselves and made appropriate arrangements to pay for network access. Finally, Network C is designed to support voice over IP, and has an IP PBX system. One set of access points is deployed to support all three networks. One SSID identifies Network A. That SSID has a security configuation that requires 802.1X authentication against the RADIUS server on Network A, and it may be that systems attaching to the network have appropriate client software installed, such as antiviral protection. SSID A may support several different VLANs on Network A, depending on how the RADIUS server is configured. Network A is also configured to support strong encryption. Network B is supported by a second SSID that is configured for web-based authentication. Once users authenticate through the web system, they are allowed Internet access. Network B has no encryption because the service provider does not want to restrict subscriber computing platforms or require special client software beyond a web browser. SSID C is deployed to support voice over IP. Traffic on SSID C is likely prioritized over the other two because of the tighter quality of service requirements for voice traffic. How devices authenticate against SSID C depends on the handsets in use. Many VoIP handsets do not yet support 802.1X, leaving network administrators to rely on MAC filtering and static WEP for security.
Figure 21-11. Virtual access points
This topology provides essentially the same mobility as the previous topology. VLANs can be dynamically instantiated at the edge to connect users, so client stations are dynamically attached to the correct point on the network. As in the previous case, additional protocols may be used to extend mobility across more than a single VLAN domain.
With virtual access points, limiting mobility may be important. This architecture is designed around providing service, and it may be that service should not be ubiquitous. If an office building were to provide connectivity for tenants, the owners may choose to limit where tenants can connect. Rather than connecting anywhere in the building, the service may be limited to a particular floor or wing. If an airport were to deploy a network using virtual access points, the public hot spot service providers would likely be restricted to the public areas, while the airport operational network was available through more of the facility. Different products provide alternate approaches to limiting mobility. As with many other network control functions, your preference should be for centrally-administered access controls.
Due to the tie-in with the link layer, this topology is often used in conjunction with 802.1X and RADIUS. 802.1X should not be a requirement. Each of the virtual networks should have its own security configuration, which would allow every customer to enforce their own security policy. Different customers may have different requirements, and a network built on virtual access points should accommodate any reasonable security policy. For example, most hot spot providers are running web-based login systems. While a web-based login system may be good enough for some applications, legal requirements would impose much stricter handling on a system that accessed personally-identifiable information. Virtual AP-based networks need to accommodate both types of access simultaneously.
Performance is not constrained by a choke point anywhere. With the wireless network connecting directly into a larger network core, the only performance constraint is congestion on the core.
The biggest problem facing a multitenant network is that the network owner has to design a network with enough capacity for all the users. In the case of a private network owned and operated by one organization for its own purposes, it may be possible to estimate the network requirements. With a network service provided to others, the estimates obtained by consulting with users may be somewhat murky.
One additional item worthy of note is that analyzers may report overloaded channels because a single AP acts as multiple virtual APs. Provided that the network has been designed around the total throughput required by all users, it is acceptable to have multiple networks over the air.
Like any other AP, a virtual AP connects a radio network to a wired network. In the case of a virtual AP, both the radio and fixed networks may be created over a shared physical infrastructure. Rather than one wired network, or a set of networks owned by one organization, there may be a need to connect to several customer networks on the back end. When all the networks belong to one organization, they probably have similar security requirements and can be connected easily to wireless networks. Mapping wireless networks on to the wired networks of several different (and possibly competing) customers may require additional measures to ensure security and traffic separation.
As with the other topologies, the client software load depends a great deal on the security protocols. At the easy end of the spectrum, a network can be deployed using web-based authentication without requiring any client software. At the most difficult, the network can use several security protocols, each with its own client software requirement. One of the advantages to virtual AP-based networks is that the virtual APs may be used to create several networks, each with its own special security configuration.
Blurring the Boundaries Wi Fi Switching
One of the most talked-about developments in wireless networks in the past few years is the emergence of the new "Wi-Fi switch" architecture. In broad terms, Wi-Fi switches move network functions from access points into an aggregation device that provides control and management functions.
Control and management functions are quite important to a wireless network. Wireless networks often require a great deal of network administration support because they are composed of a herd of small devices. Wi-Fi switches can help administrators build larger networks by providing management support. Rather than managing many autonomous access points, network administrators can work with a handful of switch devices that control access points.
By reducing the time and effort required to control an AP, Wi-Fi switches can make it possible to have more APs on a wireless network. More APs usually means a denser coverage blanket with smaller coverage areas for each AP, and less bandwidth contention.
Wi-Fi switches can be used in any of the topologies discussed in this chapter, although devices from particular manufacturers may adapt better or worse to certain topologies. Different vendors have taken different approaches to easing management pain, as well as providing network-wide mobility, so choose one that fits into your network and supports your selected topology.
Introduction to Wireless Networking
Overview of 802.11 Networks
11 MAC Fundamentals
11 Framing in Detail
Wired Equivalent Privacy (WEP)
User Authentication with 802.1X
11i: Robust Security Networks, TKIP, and CCMP
Contention-Free Service with the PCF
Physical Layer Overview
The Frequency-Hopping (FH) PHY
The Direct Sequence PHYs: DSSS and HR/DSSS (802.11b)
11a and 802.11j: 5-GHz OFDM PHY
11g: The Extended-Rate PHY (ERP)
A Peek Ahead at 802.11n: MIMO-OFDM
Using 802.11 on Windows
11 on the Macintosh
Using 802.11 on Linux
Using 802.11 Access Points
Logical Wireless Network Architecture
Site Planning and Project Management
11 Network Analysis
11 Performance Tuning
Conclusions and Predictions