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The Impact of Multiple SSIDs on Wi-Fi Performance

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Back to the future with this Airheads Online article from June 2007

 

 

Introduction

 

A Virtual AP is a logical entity that resides within a physical Access Point (AP). To a client, the VAP appears as an independent access point with its own unique SSID.

 

There are multiple approaches to implementing virtual APs. One of the implementation uses a single BSSID and advertises all the SSIDs supported by the system on the same beacon. Some of the issues with this approach are

  • Incompatible with most 802.11 clients deployed.
  • Does not support different capability sets for each SSID

The de-facto industry standard is to use multiple BSSIDs. Only one SSID is advertised per
beacon and multiple beacons are used to advertise the SSIDs corresponding to the virtual APs configured. This solution is compatible with most 802.11 clients and also allows the SSIDs to support different capability sets. This solution however results in an increase in management traffic. The remainder of this document discusses the later solution. The term Virtual AP is used synonymously with BSSID throughout this document.

 

Effects of Virtual Access Points on the available throughput

 

Every VAP appears as an independent AP to the client. The VAPs emulate the operations of a
physical AP at the MAC level. All wireless management traffic that would be transmitted by one physical AP would also be transmitted by the VAP. For example, a physical AP can broadcast 3 SSIDs (using virtual APs). This AP would also transmit the management traffic of 3 independent APs, one for each VAP it supports.

 

The actual bandwidth supported by an 802.11 AP is constant (11 Mbps for 802.11b, 54 Mbps for 802.11g and 54 Mbps for 802.11a) independent of the number of the VAPs. Since the bandwidth available per 802.11 channel is fixed and the bandwidth required for management traffic requirement is on a per Virtual AP basis, definition of multiple VAPs results in a proportional decrease in the data bandwidth. This is further explained using the example below


Net = Net bandwidth available on an AP (11 Mbps for a 802.11b AP and 54
Mbps for a 802.11a AP)
Mgmt = Net bandwidth required per AP per SSID (per virtual AP)
VAP = Number of virtual APs configured
Data = Net data throughput available for data traffic
Data = Net – (Mgmt * VAP)

 

As can be seen, the data bandwidth decreases as the number of VAPs configured increases.
Large number of VAP definitions can result is very low data throughputs especially in an
80.211b/g environment and in extreme cases can result in on the air traffic congestion.

 

NOTE: The throughput of an 802.11 cell is not only affected by the traffic to and from the APs and stations in the WLAN but also neighboring APs and stations whose transmit coverage area includes the 802.11 cell in question.

 

The effects of virtual APs on the WLAN network largely depends on the 802.11 band used. This is attributed to the coverage area and the channels supported by the 802.11 b/g versus the 802.11 a band. 802.11 a has a large number of channels to choose form and the coverage area is smaller (100 feet as compared to 250-300 feet for b/g). Smaller cell sizes and larger number of channels results in lower chances of having neighboring cells of the same channel whose traffic can result in interference in any given area. As a result with the 802.11 a band, the effect of management traffic on the overall throughput is minimal when multiple VAPs are defined as compared to the b/g band.

 

Effects of virtual APs on 802.11 b/g deployments

 

Consider the following 802.11b/g deployment scenario. The maximum number of non-overlapping
channels available are 3 - channels 1, 6, 11. An ideal deployment for data capacity recommends placing APs at a distance of 30 - 45 feet from each other. In such a deployment, any 802.11 b/g client can hear at least 3 APs on the same channel


Figure 1.1: 802.11b/g AP deployment in a given coverage area. Channels used are 1,6,11 (color coded).

 

This is because the coverage area for an 802.11b/g environment is about 300 feet at maximum
power, the 802.11b/g client’s or AP’s packets can be heard over 300 feet at lower traffic rates. In addition most clients also transmit at the highest tx-power levels resulting in large coverage areas. In deployments of this size, there could be anywhere from 10 to 100 802.11b clients.

 

The effects are more pronounced in real world deployments with multiple floors and signals from neighboring offices bleeding into the coverage area. A client would now hear other APs on the same channel from neighboring WLAN deployments apart from the APs on its own valid WLAN network. As a result the client would hear 4 – 6 APs at any time. The bandwidth utilization for control traffic shoots up to 55% for 12 SSIDs at a data rate of 1Mbps assuming that there are at least 6 APs in the RF vicinity of each client.

 


Figure 1.2: The graph shows the effects of multiple virtual AP declarations (1,6,10) on 802.11 b/g channel throughput with an increase in BSSIDs.

 

 

Effects of virtual APs on an 802.11a network 
 
From the previous article it can be seen that multiple BSSIDs have a pronounced effect on an 802.11b/g network. This is largely attributed to the facts that the 802.11b/g band offers limited non-overlapping RF and channel coverage area for an 802.11b/g band is large (around 300 feet).

 

The same problems also affect the 802.11a network but the effects are less pronounced because the

  • 802.11a band offers a larger number of overlapping channels to choose from allowing the neighboring APs to be on distinct channels. This greatly reduces the number of APs that can be heard by a client. 
  • Coverage area for the 802.11a band is smaller which also helps alleviate the pains of a multi-ssid deployment. Since the client’s RF range is smaller, the client will not be able to listen to the APs that are further away which it would have otherwise heard if the band used was the 802.11 b/g band.

Example:

Figure 2.1: 802.11a AP deployment. Note that for the same coverage area (shown in Figure 1) an 802.11a deployment can accommodate more APs on different channels than 802.11b/g greatly reducing the possibilities of the traffic from cells on the same channel bleeding over.

 

 

Figure 2.2: Effects of multiple virtual AP declarations on the 802.11a throughput for different number of clients in the APs range.

 

The lesson learned here is that any wireless LAN design and deployment should be sensitive to the impact of SSIDs on the environment.

 

 

When to use Virtual Access Points (VAPs)

 

A single SSID is sufficient to provide basic connectivity. A WLAN deployment however is seldom basic and simple. The WLAN deployments are required to support different types of devices from multiple vendors.

  • The devices support different authentication and encryption methods. Depending on the level of encryption supported the devices have to be access-restricted to protect the integrity of the network and other wireless users.
  • Different devices have different network access requirements. A WiFi phone needs limited access to the call servers and other phones whereas for a data device like a laptop the access required depends on the access privileges of the host using the system. The network access needs to be restricted to prevent excess access privileges.

Solutions

Two possible solutions that address these requirements are discussed below. While both solutions use VAPs to address the requirements of a heterogeneous network, the methodology used largely influences the security aspect of the solution and the number of VAPs defined.

 

Solution 1. Using a unique VAP for each device class and user class.

 

In this case a unique VAP and SSID are defined for each encryption method and for each user class based on access privileges. Each of these SSIDs could optionally map to a unique VLAN on the wired network to restrict network access based on VLANs.

 

The inherent problems with this solution is that

  • It requires the definition of too many VAPs and VLANs
  • Each VAP definition increases the management traffic eventually choking the WLAN with management traffic
  • Security is now enforced based on the VAP association and not the identity of the user accessing the network.

Solution 2. Using VAPs for basic service separation and using firewalls to further segregate the users based on their access-privileges.

 

The advantage of this solution is two fold. It restricts the number of VAPs defined to a bare minimum. Since the access privileges are now based on the user/device identity and is firewall based, the network is secured from malicious attacks. This solution however requires the firewall capabilities to be integrated with the WLAN system.

 

Example:

  • Network Requirements: 
    Support for visitors with no encryption enforced. These users would have no access to the intranet and will be able to access the internet alone.
  • Voice handsets that support only WEP encryption and require specific RF settings 
  • Employee access with dynamic key exchange (WPA, WPA-2), advanced authentication like 802.1x, VPN and access based on their department - Sales, Marketing, Engineering, Administration

 

 SSID Encryption Description
Guest Open (no encryption)  Guest users can access the internet and have no access to the intranet. This SSID is required since it is not recommended to use 
the same SSID for encrypted and non-encrypted users 
Voice WEP shared keys  Voice needs limited access to the network (access to call servers only). They need to be on a different SSID since they have different DTIM requirements 
Employee  WPA, WPA2 dynamic keys and using advanced auth methods like 802.11i, 802.1x, VPN Access to the network is limited by the authentication profile and not by the SSID

 

 

Analysis of the Solutions

The difference between the two solutions might seem minimal but from the previous analysis Solution 1 can consume significantly higher bandwidth than Solution 2.

 

Solution 1 requires the definition of multiple additional SSIDs on the network which results in an increase in the wireless management traffic and a decrease in the actual data throughput on the network. Additionally SSIDs are used for user classification and access rights policing. Thus users are assigned access rights not by their identities but by their SSID association which could give a malicious spoofer privileged access into the network. The solution requires Employee A in the sales department to associate with the “Sales” SSID for the right network access privileges. Associating with the “Employee” SSID could result in Employee A gaining access to a privileged set of servers not accessible to the Sales user group. This is because the rights are assigned by the SSID and not Employee A’s identity or authentication profile. 
 
Solution 2 is the Aruba recommended solution. In this case the virtual APs are defined for basic service separation based on the radio configuration. User differentiation and access privileges are granted based on the individual user’s identity and authentication profile. Limiting the number of SSIDs has a direct bearing on the APs bandwidth as the wireless management traffic is kept in check. Also this solution improves security as privileges are granted based on the user’s identity.  Employee A from the Engineering department and Employee B from the Sales department would both associate with the Employee SSID but the Aruba system would assign different the access privileges based on the user’s identity and authentication profile. This ensures that the user will always be assigned the right access permissions depending on the user’s identity.  When users associate with an SSID supported a weak encryption, the rights of the users could be further limited to a subset of their actual rights to protect the integrity of the network. 
 
Conclusion 
 
Virtual APs address some of the basic wireless design requirements successfully only when used judiciously. The Virtual APs should be defined for basic service separation based on the radio configuration and not for user classification and access policing. Advanced and more secure methods like firewalls definitions should be used to ensure that user groups are assigned the right access policies depending on their encryptions and/or authentication methods. Employee /student access based on department or categories should be differentiated using firewall policies, which is more scalable and secure. Virtual APs should not be used to enforce security. 
 
In scenarios where multiple virtual APs have to be define like hosted services, hot spots, air ports, hospitality services where the same WLAN network is used by multiple vendors, the 802.11a  band should be used as the effects VAP definitions and the effects of AP management traffic on the data throughput is less when compared to a 802.11b/g network. 
 
Judicious use of virtual APs helps improve and secure the connection on the wireless side by the encryption method with acceptable bandwidth loss. In conclusion, Virtual Access Points should not be used as the means to secure the network or classify users by their access rights but should be used to group users by their basic service sets and RF requirements. 

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‎02-03-2012 04:25 PM
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bodinayaked

OZ, cannot see the figures..

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