CISCO MEDICAL GRADE NETWORK (MGN 2.0 Wireless Architectures)
DAS Subsection. November 18, 2009
Cisco Systems, Inc. has most recently upgraded their description of the design and requirements for a "wireless medical grade network". This has gone into much greater detail than in the past and covers a wealth of information, all of which serves as a baseline for the design and deployment purposes of a healthcare wireless network. For the past many years it has been my opinion that 802.11a/b/g and now 802.11n; should not be integrated into any DAS design and/or DAS implementation. The most recent update of November 18, 2009 via Cisco Systems seems to validate somewhat this from a technical perspective. This is directly from www.cisco.com public document and I will provide some "comments" throughout. The following is "quoted" and is directly from this design guide and not from Integra Systems, Inc. Integra Systems, Inc. before the end of this year article forthcoming in IT Horizons, part of www.aami.org is in line with Cisco Systems, Inc.design guideline document as stated.
FROM CISCO SYSTEMS (MGN Guide V2.0 Wireless Architecture)
"Prior to deploying Distributed Antenna Systems (DAS) or leaky coax systems (Leaky coaxial is traditionally deployed for such environments as tunnels whereby the propagation of the public safety, cellular and PCS signals in this environment does make sense, however in healthcare this not traditionally used. Passive or non-radiating cable is used.) It is necessary to determine how such alternate antennas affect the collision domain, cell size, cell throughput and the performance of your applications. It is necessary to know what frequencies, antenna types, and 802.11 radios they support.
When deploying DAS, the access points are centralized in a single location or co-located in a single enclosure or equipment closet. Unfortunately, aggregating access points in one location and using distributed antennas violates several of the technical requirements for optimal WLAN functionality and coverage. When multiple antennas are connected to a single access point, degradation or impairment of most of Cisco’s advanced features in the Cisco Unified Wireless Network should be expected because it is impossible to reuse channels to maximize throughput when the access points are aggregated and the channels are shared across the entire antenna system. In active DAS systems, passive antennas are at the end of regular coax. In passive DAS systems, leaky cable (see previous) is distributed around the floor. When deploying DAS with the Cisco Unified Wireless Network, the signal strength also decreases and creates poor application performance. The use of a radiating cable
or a leaky (see previous) coaxial type of deployment also leads to performance degradation because this introduces loss in the antenna system on both the transmitter and the receiver. To avoid this situation, Cisco recommends the use of one diversity antenna to a single access point radio. And in the case of an 802.11n radio, the antenna needs to be a MIMO antenna solution. This is necessary to provide better wireless coverage and application performance. Outside of this recommended design, only best effort Cisco TAC support can be provided to customers. Such approaches to the use of DAS systems should be carefully considered given the added risk involved in the overall wireless architecture design. The Cisco WLAN controllers enable Radio Resource Management (RRM), which provides advanced and automated management capabilities and enhanced performance. RRM allows Cisco’s Unified WLAN Architecture to continuously analyze the existing RF environment, automatically adjusting the power levels of access points and channel configurations to help mitigate channel interference and signal coverage problems. RRM also helps to increase system capacity and provide automated self-healing functionality to compensate for RF dead zones and AP failures. RRM and the Cisco Location solution are designed around antennas with well-defined RF characteristics. The addition of any third-party antenna results in non-standard (i.e., not tested or validated) results.
Some RRM adjustments may be required to achieve optimal performance, especially
for customers who have deployed the Cisco Location Solution or VoWLAN services. For more technical information, refer to the Cisco Application Bulletin that highlights the limitations of deploying DAS with the Cisco Unified Wireless Network.
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Cisco Medical-Grade Network (MGN) 2.0—Wireless Architectures
Environmental and Physical Considerations
The motivations for DAS architectures and the main benefit is the ability to offer services via multiple, discrete wireless systems for more pervasive coverage. Healthcare providers deploy DAS solutions to manage several wireless services, including clinical data, computerized physician order entry (CPOE), paging, public cellular services, nurse-call systems, patient telemetry, and future monitoring applications. The system can also distribute two-way radio for security and maintenance, as well as Wi-Fi and cellular phone access for patients, visitors, and staff. Hospitals are also looking to centralize management of these wireless systems. Hospitals are also delivering Wi-Fi from the DAS architecture
to limit ceiling penetration as they may have infection control mechanisms in place, making access above the ceiling tiles cost prohibitive.
DAS Benefits
• Convergence—Ability to carry multiple wireless access technologies, such as cellular and Wi-Fi, simultaneously and allow the building planner to provide coverage for many diverse services, such as cellular phone, two-way radios, paging, Wi-Fi, Medical Telemetry systems, etc.
• Time savings—IT managers can deploy both wireless technologies with a single cabling at the same time.
• Physical security for network equipment—The access points are secured in a locked wiring closet rather than in ceilings or walls near users’ desks, reducing the likelihood of damage or theft.
• Network maintenance—Easier configuration, maintenance, and replacement of access points since they are located in a single, easily accessible location.
• Potentially better compliance with JCAHO ICRA requirements during access point replacement if mounted above ceiling tiles.
DAS Limitations
• Unpredictable performance—Performance, such as capacity and throughput, are highly dependent on site design and installation and output power. Connecting antennas to aggregated access points with coax cabling significantly reduces the output power at each antenna. For example, a long coax cable that runs between access points and antennas introduces loss in the antenna system on both the transmitter and the receiver.
• Performance degradation for 802.11a and 802.11n—Performance is degraded network wide, however the edge of the network may provide acceptable performance because it is impossible to reuse channels to maximize throughput when the access points are aggregated and the channels are shared across the entire antenna system. The reduced output problem is worse with 802.11a or 802.11n installations because the 5GHz frequency attenuates much more quickly. The signal strength to transmit large files or support voice quality of service may also be lower.
• Performance degradation for MIMO 802.11n—The 802.11n standard uses multiple antennas with MIMO technology. It is unclear how newer technologies that rely on multiple antennas can be leveraged with a DAS system since this limits user capacity. There is currently no upgrade path to support 802.11n because most DAS solutions are based on one antenna. Also, the high attenuation caused by the coax DAS system and distortion issues created by aggregating access points makes it difficult to provide even acceptable capacity.
• Radio Resource Management—RRM may not perform as expected due to the change in access point site design and increased network complexity. Some RRM adjustments need to be made to achieve optimal results.
• Location—Effectively Location Based Services is either not supported or significantly degraded with most DAS solutions. Without additional hardware such as ZigBee sensors, DAS and RTLS are typically mutually exclusive.
• Voice—Voice QoS may be lower due to lower signal strength and lack of diversity antenna support. VoWLAN deployments also require a denser population of access points than most data-oriented application deployments and therefore must be designed properly to maintain QoS and minimize delay and jitter.
Note Cisco does not certify, endorse or provide support for Wi-Fi deployments over any distributed antenna system. The DAS vendor or integrator is solely responsible for the support of the DAS products and for any RF-related issues. This includes location accuracy, RF coverage, roaming issues related to RF, multipath issues, and scalability. While Cisco Technical Assistance Center (TAC) and Cisco field teams do not provide support for RF issues that arise in a Cisco WLAN used over a DAS, they will, however, provide support for non-RF related issues if they arise.
