- Lisa Phifer, Core Competence
In the first part of this series, we discussed how to make the business case for integrated wired and wireless...
LANs. In this tip, learn how to make the necessary upgrades to the wired network to prepare for wireless network integration.
Today's 802.11n WLAN products hold distinct advantages over Fast Ethernet for enterprise network access. Pulling cable to offices and cubicles can take weeks and cause costly damage to walls and ceilings, while activating new wireless "ports" can be done without structural change or delay.
Furthermore, in hard-to-cable venues like cafeterias and courtyards, 802.11n can be more desirable than Fast Ethernet for LAN traffic aggregation. For example, two 40 MHz-wide 5 GHz channels can backhaul half a gigabit of application traffic. Better yet, wireless mesh APs can automatically find the best backhaul path to the wired network's core, relaying traffic through neighboring APs to bypass temporary outages.
However, as enterprise networks aim for integrated wireless LAN, incorporating more wireless access and backhaul links, traffic flows and loads will change. New mobile applications like WLAN voice and video will introduce quality of service (QoS) challenges. Identifying and planning network infrastructure updates to address these needs can help any wireless network integration go more smoothly.
Re-engineering traffic to avoid bottlenecks with an integrated WLAN
Large, distributed enterprise networks must be engineered to aggregate and route traffic in ways that enable sufficient capacity, quality, security and availability.
Before transitioning from wired to wireless network access, estimate the traffic load that high-throughput 802.11n APs and new mobile applications will impose upon wired and wireless backhaul links, distribution and core-layer LAN switches, and inter-office routers and firewalls. Predict current and future traffic loads, not just from the wireless access layer to the wired distribution/core but also between wireless clients. Use this traffic load analysis to identify potential bottlenecks that may require capacity upgrades.
For example, a contemporary 802.11n AP is capable of supporting data rates up to 450 Mbps. If fully utilized, a dual-radio AP could forward 2 x 300 Mbps = 600 Mbps across its backhaul link to a distribution layer switch. This clearly exceeds the forwarding capacity of 100 Mbps Fast Ethernet, requiring either 802.11n wireless or Gigabit Ethernet backhaul. However, those 802.11n APs may not be fully utilized at first. Legacy and distant wireless clients operate at slower data rates, and total load depends on client density and application mix. These factors must be identified and combined to analyze whether and when backhaul links and aggregation devices will become saturated.
In addition, wireless/mobile applications can significantly change traffic patterns. Traffic previously exchanged between wired devices, traversing access and distribution layer Ethernet switches, may now flow directly between wireless clients, traversing a single AP or AP mesh. Or, depending upon WLAN architecture, traffic may be hair-pinned through an upstream WLAN controller, creating a bottleneck as data rates increase. For latency-sensitive voice traffic originating in branch offices, traversing the Internet to a central WLAN controller might be a deal-breaker. Such factors must be evaluated to derive the best WLAN design, which is likely to be (to some extent) decentralized. This in turn will change how much and where traffic flows through wired network segments.
Providing QoS for WLAN voice and video on an integrated wireless network
Casual wireless Internet access could be accomplished with best-effort delivery, but emerging applications like WLAN voice and video require more thoughtful handling – not just over the air, but as traffic enters, traverses and exits wired network segments.
For example, VoIP operates well on a dedicated or lightly utilized AP, but adding bandwidth-hungry data or video clients to the same AP can easily starve voice calls. QoS mechanisms are usually required for multimedia applications to coexist peacefully. Over the air, Wi-Fi Multi Media (WMM) can map each application onto one of four access categories. Voice gets top priority to minimize latency and jitter, while video gets second billing to ensure consistently high throughput. Data apps are then split into best effort and background, ensuring that mission-critical apps aren't starved by bulk file transfers. Wireless APs use these WMM categories to give high-priority traffic more frequent airtime access.
Once wireless traffic hits access layer APs and switches, WMM categories must be mapped onto wired network QoS using 802.1q VLAN traffic segmentation, 802.1p LAN frame priority, and DiffServ IP packet marking. Traffic filtering and bandwidth management features on wireless APs and controllers can also be used to regulate and shape traffic flow between wireless and wired network segments. Also, within networks that distribute video streams to wireless clients, unicast-to-multicast traffic conversion at the wired network edge can result in better airtime utilization.
Planning wired equipment upgrades for wireless network integration
Before wireless can stop being a nice-to-have luxury, wired network equipment must be readied to handle wireless demands. But few companies can afford to rip and replace network equipment in one fell swoop. Those upgrades must be budgeted and scheduled over time, resulting in an incremental network infrastructure migration.
As previously noted, some Fast Ethernet switches may need to be replaced by Gigabit Ethernet switches to deliver sufficient backhaul bandwidth. Another possibility is to retire selected switches, replacing them with wireless backhaul. This "overlay" approach can be easier than upgrading actively used Fast Ethernet switches because installing new mesh APs need not disrupt existing cabling and port assignments. Over time, as wired access declines, older switches can be eliminated, transitioning any residual Ethernet clients to another wired or wireless port.
Finally, wired switches must be upgraded to deliver power to wireless APs via 802.3af or 802.3at Power over Ethernet (PoE). 802.11n APs use multiple-input multiple-output (MIMO) antennas and sophisticated signal processors that consume more electricity than legacy 802.11abg APs. In some cases, 802.11n APs exceed the 13 watts delivered by wired switch ports that implement 802.3af PoE. Fortunately, shortfalls are rapidly diminishing as vendors ship new, more power-efficient 802.11n APs. However, as 802.11n APs move from 2x2 to 3x3 and eventually 4x4 MIMO, power draw will increase. Over time, new wired LAN switches that implement 802.3at should be deployed to quench this growing thirst for power.
Completing the picture
In this tip, we focused on wired network equipment upgrades required to get the most from wireless network investments. Wired networks don't run themselves, however, and neither will integrated wired-wireless networks. Our next tip will explore the management and security upgrades needed to make integrated networks operate efficiently and effectively.
About the author: Lisa A. Phifer is vice president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment, and the use of emerging technologies and best practices.
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