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Wednesday TECHtionary.com TECH-Tip – How Many Degrees of Freedom in a MIMO?The animated TECH-Tip tutorial is available at http://www.techtionary.com Summary The ability of MIMO-Multiple Input Multiple Output RF antennas to increase the spatial area for the receiver to capture a useable signal is called DoF-Degrees of Freedom. A DoF of one is LOS-Line of Sight where there is no gain as a result of spatial (space) multiplexing. Far-field, or Rayleigh distance (where the signal level varies inversely as the square of the frequency wavelength) is the region where the radiation pattern is independent of distance from the transmitting antenna. The ability of MIMO-Multiple Input Multiple Output RF antennas to increase the spatial area for the receiver to capture a useable signal is called DoF-Degrees of Freedom. A DoF of one is LOS-Line of Sight where there is no gain as a result of spatial (space) multiplexing. DoF (antenna spacing) is increased by separating the spacing of the antennas which enhances Far-field performance. MIMO antennas can achieve other types of array gain which includes: CCIR-Co-channel Interference Rejection is a type of signal rejection of adjacent frequency signals. CCI may come from nearby AP-Access Points or frequency harmonics (multiples) from other high power RF sources. Co-channel/cochannel Interference is a limiting factor in network design. That is, wireless system capacity (throughput) is the number of cells multiplied by the cell processing speed divided by the Co-Channel Interference. Diversity or distance Gain is the gain realized by utilizing multiple paths to increase the probability that any interfered path does not reduce performance. According to Intel, "diversity gain refers to techniques at the transmitter or receiver to achieve multiple looks at the fading channel. Diversity may be exploited in the spatial (antenna), temporal (time), or spectral (frequency) dimensions." There are three RF-Radio Frequency radiation fields in free space as a result of an antenna radiating (transmitting) power. - Near-field often referred to as the reactive near-field space is nearest to the transmitting antenna where reactive field dominates. - Fresnel zone or radiating near-field is the space between the reactive near-field and the far-field regions. This is the space where radiation fields dominate and where the angular field distribution depends on distance and obstructions from the transmitting antenna. - Far-field or Rayleigh distance is the space where the radiation pattern is independent of distance from the transmitting antenna. Thursday TECHtionary.com TECH-Tip Is Your WiFi Roaming Secure? Cisco FSR-Fast Secure Roaming The animated TECH-Tip tutorial is available at http://www.techtionary.com Summary FSR-Fast Secure Roaming is designed for latency-sensitive applications such as WVoIP, transaction processing, or other low-latency (no delay real-time applications). Details With fast secure roaming, authenticated client devices can roam securely at L2-Layer 2 Datalink from one access point to another without any perceptible delay during reassociation. FSR is designed for latency-sensitive applications such as WVoIP, transaction processing, or other low-latency (no delay real-time applications). WDS provides fast, secure handoff services to access points, without dropping connections, for fewer than 150ms roaming within a LAN-Local Area Network subnet. Cisco FSR-Fast Secure Roaming reduces to as little as 150 milliseconds reconnection of WiFi devices from one AP-Access Point to another. Designed for use with WVoIP-Wireless Voice Over Internet Protocol, FSR provides fast and secure handoff (also known as handover see Background below) services without dropping connections. Shown in the online animation, heres how FSR works: 1 - Access Point must now 802.1x authenticate with the Cisco WDS-Wireless Domain Service AP-Access Point (1) to establish a secure session. 2 - Initial client 802.1x authentication goes to the central AAA-Authentication, Authorization Accounting server (~500 ms-milliseconds). 3 - Client roams to new AP (2). WDS will send the client's base key to AP (2). 4 - Overall re-authentication has been reduced to 100-150 ms because the WDS has processed authentication, WAN link is not used. Not to be confused with MobileIP at Layer 3 Network, FSR is a Layer 2 Datalink function. In addition, FSR support many of the key security systems such as CCKM-Cisco Client Key Management, LEAP, MIC-Message Integrity Check (also known as MIChael), TKIP-Temporal Key Integrity Protocol. FSR is supported by Cisco Aironet 1200 and 1100 Series AP-Access Point. We conclude with a site planning TECH-Tip. The 802.1b standard provides for three non-interfering channels: 1, 6, and 11. AP-Access Points within range of each other should always be set to non-interfering channels to maximize the capacity and performance of the WiFi network. This approach makes designing your WiFi network for WVoIP and other wireless devices easy. About TECHtionary.com 303-444-6226 TECHtionary http://www.techtionary.com is the World's First and Largest Animated (rich media) Library/Magazine on Technology - Web Hosting Magazines Editors Choice for Technical Help. TECHtionary has more than 2,660+ free tutorials on data, internet, wireless, VoIP-Voice over Internet Protocol (internet telephony), PBX systems, central office switching, protocols, telephony, telecommunications, networking, routing, IPTV, WiMax, power systems, broadband, WiFi-Wireless Fidelity and other technologies. TECHtionary.com provides "just enough just-in-time" critical success information. TECHtionary produces web infomercials proven to "increase revenues, decrease customer support costs and increase customer satisfaction." Tom Cross CEO of TECHtionary is the Security and Emerging Technology Columnist for TMCnet, Technology Columnist for Telecommunications Magazine and member of the Technical Board of Advisors for the VOIP-Security Alliance. |