Xirrus XR-4000 Wireless Array

The traditional approach to widespread Wi-Fi coverage, employing multiple single- or dual-radio access points (APs) and a central WLAN controller, requires an equally large number of cable runs and Ethernet switch ports — all of which adds to the cost and complexity of an installation. California-based Xirrus is in the business of delivering a more efficient solution, based on modular AP/WLAN controller arrays arranged in a distinctive circular chassis, using directional antennas for increased coverage.

In October 2011, Xirrus updated the design of its Wi-Fi arrays with the XR series, which made a number of improvements including a more compact chassis design, multi-state (2.4GHz- and 5GHz-capable) 802.11n radios available in 300Mbps and 450Mbps models, new high-gain directional antennas and faster multi-core controller CPUs. The 13in. diameter, 8-slot XR-4000 has two Gigabit Ethernet ports for backhaul, while the 17in., 16-slot XR-6000 has four. All of Xirrus's XR arrays include a multi-gigabit controller, a firewall and a wireless threat sensor.

In November, Xirrus added two smaller arrays — the 7in., 2-slot XR-1000 and the 10in., 4-slot XR-2000 to complete the XR portfolio.

Xirrus claims that its Wi-Fi arrays can deliver efficient wireless connectivity in environments ranging from a few users in a remote location to an auditorium accommodating several thousand people. In this review, we examine the mid-range, 8-slot XR-4000, which can support up to 640 users.

Design
The Xirrus XR-4000 resembles nothing so much as a large dinner-plate-sized (13in. diameter) smoke alarm — and like an alarm, it's primarily designed for ceiling or wall mounting (a variety of accessories are available for the purpose). The front of the enclosure has activity LEDs for the eight APs, the two Gigabit Ethernet interfaces and power. At the back are the pair of RJ-45 GbE ports (one of which supports power over Ethernet), a third console port for use during installation, a cooling fan and a mounting bracket attachment.

The XR-4000 supports up to eight modular multi-state (2.4GHz or 5GHz) integrated access points (IAPs), arranged around a central controller board; our XR-4820 review model has eight 300Mbps IAPs

With the cover open, Xirrus's unique Wi-Fi architecture becomes even more apparent. Our XR-4820 review unit had eight multi-state (2.4GHz or 5GHz) modular APs running in 300Mbps mode, giving a total Wi-Fi bandwidth of 2.4Gbps. These plug into 2.5Gbps PCI Express slots on the controller board, whose central element is a Cavium 64-bit, quad-core Network Services Processor (NSP) that manages the Wi-Fi throughput, applies firewall rules and carries out intrusion detection and prevention. The Cavium NSP is accompanied by an FPGA that handles encryption/decryption duties.

The XI Series IAPs come in 2-antenna 300Mbps or 3-antenna 450Mbps versions; as well as providing 2.4GHz or 5GHz Wi-Fi connectivity, these flexible devices can be configured to work in RF monitor mode using an omnidirectional antenna

The XI Series APs themselves comprise a small PCB with an Atheros Wi-Fi chip and up to three distinctive high-gain directional antennas that include an integrated metallic reflector for minimising interference between adjacent APs in the array. Our review unit had the 300Mbps (2x2 MIMO) XI-N300 modules. For maximum throughput, you can specify — or upgrade to — the 450Mbps (3x3 MIMO) XI-N450 units. If necessary, the latter can be installed and licensed for 300Mbps operation, and then software-upgraded to 450Mbps mode as required.

One of the APs will usually be configured to work in Monitor mode (using an omnidirectional antenna), acting as a wireless threat sensor and performing spectrum analysis, packet capture, intrusion detection and prevention.

Installation
The XR-4000 can be set up using a command-line interface (CLI) via the Console port, or using the web management interface (WMI) via one of the Ethernet ports. We (or rather, the Xirrus engineer who walked us through the install) used the CLI/Console route, plugging one of the Gigabit Ethernet connections into the company's wired LAN to obtain an IP address via DHCP.

After entering a country code (to ensure the AP radios use the correct channels and transmission powers), a presupplied licence key and some basic network parameters, the Wi-Fi array was automatically configured and became operational.

We then had access to the web management interface's rich panoply of features, which we'll outline below.

Features
The WMI is built around an expandable left-hand menu pane with four main headings: Status, Configuration, Tools and Log Messages. Content associated with menu selections is displayed in the main body of the page.

Status
The Status section has sub-sections for Array, Network, RF Monitor, Stations and Statistics, plus System and IDS Event Logs. Shown below is the Summary page for the Xirrus XR-4000 array, displaying the status of the two gigabit Ethernet interfaces and each of the eight integrated APs:

Status summary for the XR-4000's eight integrated access points (IAPs): IAP1 is in dual-frequency 802.11a/b/g/n RF monitor mode, 3 and 7 are in 2.4GHz b/g/n mode and 2, 4, 5, 6 and 8 are in 5GHz a/n mode

The other Array status pages are Information (including firmware version, serial numbers for IAP modules, MAC addresses, licensing information and the controller's current temperature), Configuration (Running, Saved, Lastboot, Factory) and Admin History.

The Network status pages provide details on the array's network interface settings, any neighbouring Xirrus arrays, spanning tree status, routing, ARP (Address Resolution Protocol), DHCP leases, client connection tracking and NAT, other CDP (Cisco Discovery Protocol) compliant devices on the network and network assurance testing. Here's the Connection Tracking page:

This status page provides details of client session connections

The RF Monitor status pages give you detailed statistics (packets/sec, bytes/sec, 802.11 busy, other busy, SNR, noise floor, error rate, average RSSI, average data rate) on each of the array's IAPs and across the entire collection of 802.11a/b/g/n Wi-Fi channels. shown here is the IAP RF status page:

Radio frequency status for the XR-4820's eight IAPs (note that IAP1 is in Monitor mode)

The other RF Monitor pages cover Intrusion Detection, Channel History and Radio Assurance. The Intrusion Detection page (shown below) lists all detected access points and allows you to classify them as Aprroved, Known, Blocked or Unknown. Channel History gives you a rolling readout of statistics for a selected channel, with 5-second intervals for 2.4GHz channels and 10 seconds for 5GHz channels.

The Intrusion Detection status page lets you classify detected access points as Approved, Known, Blocked or Unknown

The main Stations status page shows client devices that are visible to the array — either associated, unassociated or both. You can opt to see extra identification, security and connection information by clicking checkboxes at the top, and also select specific clients to either deny them access or temporarily deauthenticate them. Subsidiary pages in this section provide a location map of clients (to which you can add a custom overlay such as an office floorplan — see image below), and display the received signal strength indicator (RSSI), signal-to-noise ratio (SNR) and noise floor for each client at each of the array's IAPs. You also get a Max by IAP page showing the maximum number of clients that have been linked to each IAP for various time periods (hour, day, week, month, year) and a Station Assurance page that identifies clients with connectivity problems.

Clients connected to the XR-4820 array, overlain on the ZDNet UK office floorplan

The final Status section, apart from the system and intrusion detection logs, is Statistics, which provides bytes, packets, errors and retries data for the array's IAPs, both in summary and in detail per IAP. There are also statistics pages for each Gigabit Ethernet interface, any assigned VLANs, WDS (Wireless Distribution System, for linking multiple arrays without a wired backbone) links, intrusion detection, firewall filters for blocking or allowing traffic, and associated clients.

Summary statistics for the XR-4820's eight IAPs; you can click on the IAPs to get pages of more detailed information

Configuration
There are no fewer than 11 sections under the WMI's Configuration banner, although you won't necessarily have to delve into all of them. First up is an Express Setup subsection, whose purpose is self-explanatory. On this single page, you can specify a name for the array and enter various admin contact details, configure SNMP settings (whether to enable v2, for example) and the two Gigabit Ethernet ports (whether to enable DHCP, for example), enter SSID details (including network name and security settings), change the default admin settings (new username, password, privilege levels and so on), and set the time and date. Having done all this, you hit the Execute button at the foot of the screen, and the IAPs are automatically configured — an LED lights up as each one switches on. To make the setup permanent, you click 'Save changes to flash' at the top left of the Express Setup screen.

Express Setup gets the XR-4000 up and running quickly

The ten remaining Configuration subsections are where you delve into the nitty gritty of the array: Network, Services, VLANs, Security, SSIDs, Groups, IAPs, WDS (Wireless Distribution System), Filters and Clusters. We haven't the space to consider each of these in detail, but here's a selection of things you can do to optimise the Wi-Fi experience for your users.

To ensure that radios on the array do not become overloaded with clients, you can enable Load Balancing. This causes a maxed-out radio to send an 'AP full' message to would-be clients, encouraging them to associate with other, less congested, radios. A more aggressive load balancing setting causes an overloaded radio to simply ignore all requests.

If you want to ensure that clients do not use unreasonable amounts of bandwidth, you can impose traffic (packets/sec) limits at the SSID level, and for also individual attached clients. You can also create firewall filters to block certain types of traffic (such as video streaming) if necessary.

If you're only managing a single Xirrus array, as in this review, then the basic Web Management Interface is fine. For larger multi-array installations you'll want to consider the Xirrus Management System, which is available as a preconfigured appliance, a Windows Server-based application or a cloud-based deployment.

Performance
To get a feel for the coverage of the XR-4000 array, we took signal strength measurements on its 2.4GHz and 5GHz channels at various points around the ZDNet UK office, and compared them to those for our in-house Wi-Fi network. The latter uses a collection of six Cisco 1242AG access points spread over three floors.

In location 1 the Xirrus array delivers better signal strength (in dBm) than the in-house Cisco APs in both the 2.4GHz and 5GHz bands — no surprise, since there's only one Cisco AP on the fourth floor

A floorplan of the 4th floor at CBS Interactive, showing two test locations (labelled 1 and 2, both meeting rooms) and the Xirrus XR-4820 array (large red circle)

In location 2, the Xirrus array also generally delivers better signal strength, although one in-house 5GHz channel comes out on top

Our test client (a Samsung Galaxy Tab tablet) received signals in both the 2.4GHz and 5GHz bands in the two locations on the same floor as the Xirrus array (above), and acceptable signals in one location on the floor below as well (below). All in all, the eight-AP Xirrus array delivers impressive coverage from a single location.

A floorplan of the 3rd floor at CBS Interactive, showing one test location (3)

The Xirrus array's 2.4GHz signal holds up well in location 3, a floor below, but the 5GHz signal is noticeably poorer; there are more in-house Cisco APs on this floor, resulting in better signal strength in both the 2.4GHz and 5GHz bands

Conclusion
The Xirrus XR-4000 is a very flexible, configurable and manageable Wi-Fi array that provides a compelling alternative to traditional solutions with multiple separate APs and varying degrees of manageability. Xirrus's modular architecture and pricing — $2,000 for the XR-4000 chassis, $200 per XI-N300 integrated access point and $200 per IAP for the ArrayOS-N300 software ($5,200 or around £3,367 all told) — allows businesses to cater for their current requirements while retaining a clear upgrade path as the organisation grows.

Specifications

LAN
LAN standards supported	1000Base-T
LAN ports	2
Management
Web-based management	yes
Firmware in flash memory	yes
Management software	Xirrus Management System (optional)
SNMP version supported	v2
Power
Power source	Power over Ethernet
Power switch	no
Power consumption	45 W
Security
Network Address Translation	yes
SPI firewall	yes
DoS attack prevention	yes
Packet filtering by port	yes
Packet filtering by DNS name	yes
Packet filtering by IP address	yes
Packet filtering by MAC address	yes
WAN
WAN protocols supported	DHCP, NAT, TCP/IP
Wireless
Wireless standards supported	802.11a, 802.11b, 802.11g, 802.11n
Wireless security standards	WEP, WPA, WPA2, RADIUS
Antenna type	internal
MIMO	yes
Number of antennas	2
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