Challenges for resizing Wireless LAN

More users, larger traffic, and more multimedia applications overwhelmed the enterprise's wireless LAN. High-density WLAN is facing the challenge of resizing.

As IT manager at the University of Tennessee, Knoxville, USA, Philippe Hanset has been a headache for Slingbox, which refers to various devices for video transmission over the network) and the interaction between campus Wireless LAN.

A vendor has been diligently selling its Slingbox to new students. This product can be installed in any dormitory with cable TV services. A wireless LAN can help them transmit video streams to laptops anywhere on campus. Imagine a scenario where hundreds of laptops of students can enable video streams like "American Idol" anytime, anywhere. Hanset admitted, "This will bring huge challenges ." However, he deliberately weakened the severity of the problem.

Similar deployment is a challenge that wireless LAN administrators often face. They must continue to expand the scale while supporting more users and provide services for more demanding applications. In large-scale Wireless LAN deployment, higher education has become a test center for these problems.

Early Wireless LAN focused on increasing the number of access points covering certain areas. But today, many wireless administrators are focusing on capacity expansion. This side focuses on a wide range of content, prompting people to have a deeper understanding of the range of access point capabilities, and pay more attention to the expansion of background systems, servers and networks.

The rise of high-density Wireless LAN

Brad Noblet was once an IT Director at Dartmouth and Harvard University, and is currently an independent consultant at BN Consulting. "We are used to 20 to 50 wireless users in each region, and the 20 to 50 users may be out of 50 or 100 feet," he said ." The assumption is that few users only need email or Web search services.

At present, low-density Wireless LAN is making high-density Wireless LAN, which poses a new challenge to network administrators. Brandeis is a university of Higher Learning in Waltham, Massachusetts. John Turner, head of network and system at the University, said: "We first deployed this wireless LAN three years ago, and there were very few wireless clients at that time. But now, everyone has a laptop ."

The expansion challenge also includes providing sufficient wireless and wired bandwidth to your applications. "As the number of users increases, these scaling problems become more and more obvious and we must take action," Noblet said ." Such actions include many clients connecting to the Access Point and obtaining IP addresses, then verifying and obtaining sufficient application bandwidth, wireless and wired), and limiting their behavior as netizens.

Configure access points based on capacity

Noblet urges network administrators to configure access points for performance or capacity, rather than the number of access points. He found that some access point configurations do not have any restrictions on client Association. If a large number of users gather around the access point, they will find that the connection speed is extremely slow or there is no connection at all. "The real problem is that you need to understand the throughput performance of a specific data stream," Noblet said ."

However, everyone agrees that capacity planning at the access point level is more artistic than scientific. Dan McCarriar, Assistant Director of network services at Carnegie Mellon University CMU, said: "when talking about this topic, I want to emphasize that in our deployment environment, we are not a vendor but an IT professional who knows users and application scenarios the most."

At present, CMU has completed half of the deployment work in 802.11, using two manufacturers' devices, of which Aruba's equipment is used in the teaching area, and Xirrus's equipment is used in the dormitory area. The Xirrus package includes a wireless LAN controller and 4, 8, or 16 Wi-Fi receivers installed in an ultra-large "Smoke alarm" container called an array, the partition antenna in the device can prevent interference. Therefore, CMU only needs to insert a single array into a high-density area, without complicated microcell planning and management.

Synchronize with DHCP

In some cases, the DHCP server cannot be synchronized with the inbound client, but the DHCP server of the CMU can be synchronized. The key is to design a centralized IT infrastructure for these types of services, so that both wired and wireless clients can be used and can be quickly and conveniently expanded.

In the future, Turner plans to bind location and mobility services with DHCP to create a more seamless mobile experience in the campus. "The DHCP server does not know that someone is disconnected," he said. We can do something between the central Wireless LAN controller and DHCP, so that we don't need to keep the addresses for those who will never go back ."

Hanset pointed out that the University of Tennessee encountered a slightly different DHCP problem: some students' laptops or rogue access points in the dormitory themselves assume the role of a DHCP server, DHCP lease is provided for the Client requesting the request. The school blocks these hosts on the switch port or the Aruba Wireless LAN controller.

Another expansion problem is that after the client obtains the IP address, it will keep the address as long as possible according to its own needs. Therefore, these addresses in the same subnet cannot be re-allocated to the new client. In some cases, the address may be exhausted.

CMU currently runs a flat Wireless LAN. Scott Ambrose, Network Design and Development Manager, plans to collect a large amount of statistics, covering the average number of devices in the network and the number of peak users, and the locations of access points. All of this data will be used to plan the size and quantity of partitions, and how to assign available IP addresses to each partition.

Multimedia Scaling

The use of multimedia is advancing rapidly, and 802.11n is expected to make this explosive growth further. All these universities are configuring multicast support for wireless LAN to minimize bandwidth requirements. In fact, users can adjust to a single multicast stream, similar to watching a broadcast TV), rather than receiving their own independent unicast streams. "You must understand your applications and service content," Noblet said. Only in this way can we determine whether unicast or multicast transmission is used ."

"As long as possible, we will enable multicast," said Hanset of the University of Tennessee. However, to further improve performance, the University also disabled the lowest data transmission rate (1 Mbps and 2 Mbps) of the wireless LAN. Therefore, each broadcast packet is sent at a rate of 5.5Mbps ."

McCarriar from Carnegie University said that each item should be tested. He warned: "The multicast support provided by the vendor has become a common phenomenon ."

11n is not a panacea

Some may think that 802.11n is a magic weapon to solve high-density Wireless LAN, but they are wrong. All users mentioned in this article are using or evaluating 11n, which is expected to provide 5 to 6 times higher throughput than the existing 802.11a/B/g wireless LAN. Capacity increase is certainly a good thing, but all people think that even with 11n, it still requires careful and accurate end-to-end capacity planning.

Philip Hanset, IT manager at the University of Tennessee, said: "This standard can ensure a faster speed, but IT cannot prevent a person from occupying the majority of bandwidth. I hope there is a fair share mechanism in the wireless LAN to prevent this phenomenon, especially in high-density wireless networks ."

At the same time, the throughput of 11n may be seriously affected by the bandwidth insufficiency of the uplink connection. Tests show that the 11n access point of an Aruba can provide a speed of 80 to 90 Mbps in a general network environment, but the bandwidth can be increased to 160 Mbps when the LAN uses a gigabit connection device.

In addition, in many migration processes, enterprises simply replace the 11a/B/g Access Point with 11n devices, and forget that the 11n product can use a 5 GHz frequency. At this frequency, the channel becomes more and the radio noise is greatly reduced. Therefore, some adjustments are necessary.

However, the signal transmission capability in the 5 GHz band is not as good as that in the 2-4 GHz band. A school has installed a 11n Access Point and hopes that it can achieve better performance in the 5 GHz band, however, after two walls, the signal disappears. 11n is not a panacea, and it is not that easy to solve the high-density expansion problem.

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