A review of Problems Surrounding the Use of 802.11b Compliant Radio Products for Outdoor Point-to-Multipoint Communications and Metropolitan Area Networks and existing solutions.
The Wireless LAN 802.11 standard
Wireless LAN technology is currently one of the fastest growing segment of the wireless industry. With data throughputs of 11 mbps and upcoming increased data rates of 22 mbps in the 2.4 GHz band and 54 mbps in the 5.3 GHz band, 802.11 WLAN offers plenty of capacity for advanced networking applications in both home, office and even campus based environments. According to Cahners In-Stat Group the WLAN equipment market-consisting of Nics, access points and bridges will grow from $624 million in 1999 to well over $3 billion in 2002.
The 802.11 standard is constantly improving to meet the challenges of data, voice and even video networking. There are currently over 8 IEEE committees and sub-committees developing various enhancements to the standard on subject as diverse as HDTV (high definition TV) wireless streaming in the home, security of wireless links, harmonization of the standard with the European Hyperlan WLAN technology and improvements of the QOS (quality of service) for audio and video streaming. Despite these changes, the WLAN standard was primarily designed for indoor use within a home or office environment, not for outdoor operation. It is nonetheless very attractive to use this technology outdoors since it is very low cost, there are plenty of vendors offering standard compliant products and the data throughput is much better than the data pipes offered by Telcos, Cable and cellular operators.
Unfortunately, there are many problems associated with using the 802.11 technology outdoors and well intended WLAN resellers are quick to recommend its deployment outdoors to clients and this despite a lack of understanding of its limitations in such environments.
It is the objective of this article to explain the shortfalls of WLAN networking in outdoor environments and discuss techniques which have been developed to resolve them without loosing the benefits of WLAN technology.
THE HIDDEN NODE PROBLEM
The IEEE 802.11 standard for wireless LAN's was designed to extend wired Ethernet practices to a wireless Local Area Network. Because they were designed for local area networks, these network protocols function much like wired Ethernet. If a station has a packet to send, it first listens to see if another station is busy transmitting, and then transmits its packet. This assumes, of course, that the transmitting station is able to hear other stations on the network.
A common problem which occurs when 802.11 compliant equipment is used outdoor is the "hidden node" problem. This problem occurs when a wireless node cannot hear one or more of the other nodes, therefore the media access protocol cannot function properly. When this happens, multiple nodes will attempt to transmit their data over the shared medium simultaneoulsy, causing signal interference with one another. Much like a broadcast storm on a wired LAN segment can bring traffic to a standstill, hidden transmitter nodes interfering with one another will have a very detrimental effect on the performance of every wireless node in the network. This interference can cause overall performance of the entire wireless network to drop by as much as 50%. When using streaming video this number can easily increase to 70% due to the continuous nature of the transmission.
The 802.11b standard only addresses the hidden transmitter problem using a media access protocol called CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) and special packets called Request to Send (RTS), Clear to Send (CTS), and Acknowledge (ACK). 802.11b uses these special packets to alert every node on the network that a transmission node has data to send, that a transmission is about to take place, and that the transmission has ended by broadcasting the packets across the entire network. This time and bandwidth consuming process is required for each and every transmission by each and every wireless node. Unfortunately, it still doesn't address the hidden transmitter problem because it is possible for more than one node to initiate this process at the same time because they cannot "hear" each other directly. In fact, at the 23rd annual conference on Local Computer Networks, researchers from Colorado State University indicated, "Our simulations indicate that hidden terminals can have a very detrimental effect on the performance of the IEEE 802.11b MAC protocol."1 In an indoor environment, the RTS/CTS mechanism along with CSMA/CA may be sufficient, but it is simply not suited for long-range, high-speed wireless communications between buildings. Extensive test by SmartSight Networks R&D team has also demonstrated that the RTS/CTS mechanism is entirely inefficient when streaming video in an environment with ‘hidden nodes’. This is because the RTS/CTS protocol was developed for non continuous data transmission and does not respond fast enough for continuous data transmission.
THE FALLBACK OF 802.11b INTEROPERABILITY
The 802.11b standard integrates a complex protocol for authentication of new devices on the wireless network and for roaming between stations. In addition , , due to interoperability requirements, the radio preamble sent at the beginning of every 802.11 packet is sent at the lowest radio speed. For small packets, this preamble makes up a large percentage of the transmission time. As many small packets are sent to support roaming, authentication, acknowledgement of packets, etc., utilization drops to a fraction of what would be available if larger chunks of data were being transmitted.
THE SOLUTION
Since interoperability, high quality and low cost of 802.11b devices are strong incentive for using this technology in outdoor environments, several research organizations have studied ways to resolve the ‘Hidden Node’ and ‘effective bandwidth’ problems. One company, Karlnet Inc., has been particularly successful in its efforts. Its TurbocellTM software is today used by several OEMs including Orinoco (a division of Agere Systems formerly Lucent) with standard 802.11b radio modules.
TurboCell resolves the ‘Hidden Node’ problem by centralizing control of the wireless network at the TurboCell access point or base station. The TurboCell access point uses a highly-optimized polling technique to tell remote wireless stations when they can transmit. To avoid problems associated with pure polling schemes, TurboCell also employs a "free for all" period to enable stations that have data available but are low in the polling queue to transmit without much delay. The "free for all" period allows a station that may not have transmitted for a long period of time to begin transmitting once again and move to a higher priority in the polling scheme. The determination of polling intervals based on a complex combination of factors is finely tuned and the result of years of research into wireless performance in production environments.
For performance-critical outdoor Point-to-Multipoint environments optimal bandwidth usage is crucial. The TurboCell protocol remedy problems associated with roaming, authentication and long preamble by aggregating many small packets together into a single SuperPacket for transmission over the wireless link. In combination with the highly optimized adaptive polling technique employed by the TurboCell Base Station, actual throughput performance of the 802.11 network (or networks using other radios) is much closer to optimal performance.
USING OUTDOOR 802.11b DEVICES WITH VIDEO STREAMING
The leader in outdoor wireless digital video surveillance, SmartSight Networks, has partnered with Karlnet Inc., to embed the Turbocell protocol in its S1000w wireless video transmitters. The company is also fine-tuning the technology for continuous high quality MPEG4 video streams (video streaming is much more demanding than file transfer in wireless applications). The technology will allow to support dozens of domes and cameras in a wireless environment using high quality 802.11b (2.4 GHz) and 802.11a (5.3 GHz) compliant components.
The support for Turbocell technology is a major breakthrough for video surveillance in areas such as campus , highway and city-wide environments.
Digital technology, lower camera prices, size reduction and the use of the personal computer for video control have all contributed towards "scalability".
Scalability, a new industry buzz word, is defined as a system that can be scaled to any size from four cameras. This means that a system can be sized to suit the application - without change to the basic components.
Digital technology is now used for signal processing, transmission and archiving of a closed circuit video system. Processing functions are automatic and digital transmission technology allows for the observation and control of closed circuit video systems over LAN's, WAN's or Virtual Private Networks (VPN's).
Monitoring and control of remote site components is currently possible by converting a device's analog signal to a digital signal for digital processing. As we continue towards a totally digital environment, capturing and displaying of information will become fully digital. This will mean the expansion of traditional video into other uses and functions: information technology, for example.
While the number of components and devices can be scaled down, where networks are used systems are becoming larger. The platforms and communications between sites extend from small remote sites to large corporate campuses (as part of a larger, sophisticated national or international network). A single monitor and transmission device can remotely access a site thousands of miles away. Digital video management allows all cameras to go to a control center, where software is setup over the LAN, for example. This means that with password access and web browser software it is possible to view the cameras from a desktop or laptop computer anywhere - adding convenience and cost effectiveness to the many other advantages of digital systems.
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