First Look: Cisco's 1250 802.11n AP

Responsible IT decision makers are wary of pre-standard technologies, having discovered the hard way that "pre-standard" is often code for "proprietary, premature, and/or prone to vendor lock-in." Still, sometimes you have to walk on the wild side, so when Cisco Systems offered to let us test an early version of its 1250 802.11n Draft 2.0 access point in our Syracuse University Real-World Labs, we were excited to get a glimpse of the future of enterprise WLANs.

From a performance standpoint, our testing revealed speed increases of four to six times what an 802.11a/g infrastructure can provide. And using an 802.11n access point for even legacy a/b/g clients delivered a measurable performance advantage thanks to the ability of a multiple input, multiple output (MIMO) approach to maintain high-bandwidth data rates for a larger portion of an AP's coverage area. For a/b/g voice-over-WLAN phones, this reliability translates into higher-quality calls and fewer dead spots.

We believe 802.11n's bug-shakedown period will be a less rocky version of what occurred when 802.11g was an IEEE draft: 802.11g's Wi-Fi Alliance interoperability certification occurred after the standard was finalized; in contrast, products based on 802.11n Draft 2.0 are being certified today, before the standard's ratification by the IEEE. As of this writing, 225 products, albeit predominantly oriented toward small and home offices, have received the seal of interoperability from the Wi-Fi Alliance. Notable names on that list include Apple, Aruba Networks, Atheros, Broadcom, Cisco, Intel, and Meru Networks--all big players with an economic interest in ensuring that the current crop of 802.11n Draft 2.0 products are backward and forward compatible.This isn't to say maximum performance won't increase as subsequent versions of 802.11n are endorsed by the Wi-Fi Alliance. But it does mitigate the risk that current devices will be incompatible with future versions. Companies don't buy WLAN devices based on an IEEE standard but on the Wi-Fi Alliance's endorsement of interoperability and the independent certification that 802.11n Draft 2.0 has received. Also, Cisco is part of Intel's "Connect With Centrino" interoperability testing program, which gives an added comfort level.

SETTING A 5-GHZ STRATEGY

The 5-GHz frequency has been underused compared with the 2.4-GHz band, but with 802.11n that's poised to change. In the early days of WLANs, light user loads and a focus on maximizing coverage made the superior propagation characteristics of 2.4 GHz a clear choice over 5 GHz's more limited range. Now those WLANs have grown from scattered hotspots to pervasive coverage blankets with many microcells supporting a multitude of users and high-bandwidth applications, and the focus has shifted from coverage to capacity. And when you're talking capacity, nothing beats the massive amount of spectrum available in the 5-GHz band, which encompasses 21 non-overlapping channels when an AP implements full Dynamic Frequency Selection 2 (DFS2) support, compared with 2.4 GHz's modest three nonoverlapping channels.

Interference is another differentiator. WLANs in the 2.4-GHz band must contend with cordless phones, microwaves, and other 802.11b/g devices, but 5 GHz is relatively vacant except for cordless phones and a few 802.11a networks. Although the 802.11n standard supports either frequency, 5 GHz is superior when channel bonding comes into play because it supports many nonoverlapping 40-MHz channels, while 2.4 GHz supports only one.

In addition to increasing speeds with wider 40-MHz channels, 802.11n boasts more OFDM subcarriers, MAC Layer packet aggregation, and MIMO. Core to 11n's increased performance, MIMO allows 802.11n to use environmental multipath reflections. Multipath has caused dead spots for 802.11a/b/g devices because they had a limited number of antennas to sort out reflections. But in 11n, multiple antennas and multiple radios help boost reliability and allow multiple data streams, or "spatial streams," to be sent concurrently. As for nomenclature, a MIMO device with two transmit and three receive antennas supporting two spatial streams would be referred to as "2x3:2" MIMO, following a "TxR:S" convention. At its projected maximum, 802.11n using 4x4:4 MIMO will support 600-Mbps raw data rates. Current Draft 2.0 products top out at 300 Mbps using 2x3:2 or 3x3:2 MIMO.FAST TRACKWe won't say this review was without hiccups--in fact, we experienced two show-stopping issues in the beginning of our evaluation period, involving how Cisco's 1250 handled an older Intel 2915 a/b/g WLAN client in high-traffic and abrupt-disconnection situations. But a few glitches are to be expected when testing prerelease software, and Cisco responded quickly. After receiving updated controller code, we had no problems; an official version of that code train was released as 4.2.99.0.

We benchmarked the Cisco 1250 AP against Cisco's 1240AG device using Ixia's Chariot High Performance script in both open air and Azimuth Systems' Adept-n isolated environment. We used a Cisco 4402 controller and found the 1250 AP as easy to configure as any Cisco LWAPP access point. Our WLAN clients included a Lenovo T61 with an internal Intel 4965AGN chipset and an IBM T43 featuring Intel's 2915ABG silicon.

The Cisco 1250's maximum TCP Layer performance was 154.9 Mbps using a 40-MHz channel in the 5-GHz band--more than six times the performance of our 11a baseline of 22.8 Mbps. For a smaller 20-MHz channel size, performance was 84.8 Mbps and 96.7 Mbps for 5 GHz and 2.4 GHz, respectively; even after spectrum analysis of our testing site, we can't explain why 2.4 GHz is faster. That anomaly aside, Cisco's 1250 provided performance three to four times greater than peak 802.11a/g throughput.

We also ran peak assessments with AES encryption enabled and found that Cisco delivered line-rate performance in all but 40-MHz channel sizes, where throughput dipped to 144.5 Mbps, about a 7% loss. Cisco told us that optimizing its hardware encryption engine is a goal of the latest software release, and we expect this gap to narrow. We also measured coexistence performance when both an 11n and an 11a/g client were associated to the same radio on the 1250. With both clients, overall cell throughput dropped to 60 Mbps, with an 80% to 20% speed split at 5 GHz and a 90% to 10% split for 2.4 GHz.

We performed additional mobile rate vs. range testing inside a 50,000-square-foot warehouse, with cinder block and Sheetrock walls. We selected this location for its RF isolation and lack of a production WLAN. Then we upped the ante with a rotating Sherline Products digitally controlled turntable, which introduced movement in the client notebook's antenna orientation and systematically varied multipath conditions. We kept rotation speed at 1 RPM. After enabling AES encryption, we again used Chariot for traffic generation with bidirectional TCP pairs to assess overall performance with strong security; we took multiple test runs and averaged the results at distances of 15, 75, 80, and 130 feet. Across the entire location set, 802.11n's performance was four times better than 802.11g and better than 802.11a by three times and five times for 20-MHz and 40-MHz channel sizes, respectively.We also saw increased reliability of an 802.11a/g client's connection across the farther distance locations, at 80 and 130 feet. In the 2.4-GHz band, the 1250 access point maintained 20.8 Mbps as a minimum throughput, whereas the 1240 stepped performance down to 14 Mbps at the edge. In the 5-GHz band, minimum performance was 15.7 Mbps vs. 11 Mbps for the 1250 and 1240, respectively. So even if your clients are mainly legacy a/b/g, an 802.11n infrastructure will still deliver a performance boost thanks to MIMO. If voice over WLAN is on your agenda, this will translate into much higher quality and coverage.

Our last measured performance metric, increased range, is one of the most heavily touted advantages of 802.11n. Although we won't burst that bubble--our testing and experience with Cisco's 1250 found slight enhancements over Cisco's 1240--understand that boosting range is, in itself, not a reason to upgrade.

We used AirMagnet's Survey Pro 5.1 with a Cisco CB21AG client card to perform walkabouts, measuring signal strength values around our 390-by-150-foot test facility. In the 5-GHz band, the 1250 outshone the 1240AG in a few areas but exhibited a similar max range. In the 2.4-GHz band, we found coverage between the 1240 and 1250 nearly identical, with a measurable improvement over 5 GHz only at the very extremes, where numerous heavy walls provided challenging conditions.

Cisco recommends a 1:1 access point replacement strategy of 802.11n to 802.11a/g APs and employing the same microcellular approach as previous AP generations. Although in theory one could install a slightly fewer number of 11n APs to deliver the same aggregate performance as a denser 11a/g setup, we recommend continuing today's microcellular deployment approach to future-proof for tomorrow's bandwidth requirements.

THE POWER PROBLEMOf all the controversies within the 802.11n marketplace, nothing quite rivals the "he said, she said" vendor debates around Power over Ethernet (PoE) support in 802.11n access points. Because MIMO requires multiple radio chains to work its magic, the power requirements of dual-band 802.11n APs generally exceed that which 802.3af PoE can provide, forcing vendors to come up with a variety of creative remedies.

Cisco's solution involves injecting additional wattage onto an access point's wired connection, either through 1250-specific power injectors or the enhanced PoE capabilities available in its Catalyst 3750-E and 3560-E switches. Cisco also offers the option to run the 1250 platform with local AC power or a single radio module on PoE.

On the hardware front, the Cisco 1250 has a utilitarian design with a hardened white exterior accommodating two modular LWAPP radios when fully loaded. Although radio choices are limited to 2.4-GHz or 5-GHz 802.11n Dra2.0 modules supporting MIMO 2x3:2 (two transmit, three receive, with two spatial streams), additional options sporting enhanced MIMO capabilities may be available in the future. For our tests, we used three omnidirectional antennas on each radio module, but Cisco also offers a variety of compatible high-gain directional antennas.

Bottom line, at $1,299 for the dual-radio version, the 1250 is a bit costlier than current offerings, but the price/performance ratio is weighted in 11n's favor, especially if you've invested in 11n clients or are piloting voice over WLAN. Discounts will keep the price under the $1,000 mark, but the highly recommended, more-powerful 1250 PoE injectors add cost. Despite a few quirks with beta code, our experience with the product was positive, and we were impressed with its performance.