Fast roaming is a crucial capability, enabling police, fire and emergency medical personnel to access critical information, transmit medical-monitoring data, gain access to video surveillance from any location and share investigation details at the scene of a crime.
IP and data congestion
Internet Protocol is often used to describe the entire protocol stack used for network communications. For example, the two common--and competing--IP standards for voice-over-IP are H.323 and Session Initiation Protocol. IP implementations used for video streaming, including video on demand, video surveillance and IPTV, are HTTP over TCP, MPEG-2 (H.264), MPEG-4, H.323, Real Time Streaming Protocol and more.
The protocols themselves may or may not provide mechanisms to ensure timely delivery, flow and congestion control, or quality-of-service guarantees.
Within IP, TCP is the most commonly implemented protocol to handle IP network congestion. TCP is a good example to look at in terms of the effect a wireless network may have on an IP.
Two unique characteristics of 802.11 multihop wireless networks affect TCP performance. The first is contention for the shared wireless channel on a given radio; consistent link-layer contention, such as that from a moderate or large number of wireless client devices, causes packets to drop.
Second, while improving channel use through spatial reuse (simultaneous scheduling of noninterfering trans- missions) is desirable, TCP's window- ing mechanism limits its effectiveness. In addition, traffic load has a great impact on link contention and spatial channel reuse.
TCP commonly increases its average window to cope with buffer overflow. When the buffer gets full--and it gets full fast, especially if latency is already high--more packets drop and retransmit. The problem becomes self-perpetuating, until there is nothing but a trickle of throughput because of degraded spatial reuse and increased packet loss.
Link-layer contention in multihop wireless networks causes packets to drop at the first sign of network overload or contention. Link-layer contention in- creases as the load increases, ultimately saturating the network connection to every intermediate node along the forwarding path.
The impact of TCP in a multihop scenario can result in a throughput reduction of 4 to 21 percent. Link-layer contention can also affect other protocols--as well as their mechanism to correct for such conditions--and thus can have a catastrophic effect on network performance.
Architecting the wireless mesh
Wireless mesh networks must be carefully architected to achieve optimal performance. The CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol is at the media-access control layer of wireless network devices. CSMA/CA works on a "listen before talk" scheme and can have a severe impact on the performance and reliability of wireless mesh networks. Users must therefore carefully consider the impact of CSMA/CA on single-, dual- and multiradio architectures.
