Single-radio systems are likely to lead to network congestion and thus are not well-designed to support such real-time applications as voice and video. Furthermore, single-radio systems lose half their performance with every hop, so latency--and congestion--quickly becomes a big issue. A 50 percent per-hop reduction happens because the single radio must switch between sending traffic from client access to backhaul egress and from backhaul ingress to client access.
This increased latency directly affects throughput; a 54-Mbit/second backhaul becomes 24 Mbits/s at the next hop and then 12 Mbits/s at the hop after that.
While these are theoretical throughput rates, simple math shows that only limited throughput is available to each wireless client in a single-radio architecture. For example, if you have five access points (APs), each connected to 20 wireless clients, and all APs and clients share the same 802.11b channel (5 Mbits/s), only about 50 to 100 kbits/s are available per user--the same throughput as a dial-up connection. And as a result of using a single-radio device, all wireless clients and APs are forced to operate on the same channel. Network contention and RF interference result in predictably higher latency.
The following problems are associated with single-radio systems:
• The greater number of collisions in single-radio systems has an impact on the performance of all user connections to a node. A large number of wireless users connecting to the network simultaneously can reduce the entire network's performance, even if their individual connections are slower than the backhaul.
• A wireless user cannot transmit when a node in the vicinity is sending traffic to other nodes or other users.
• A single-radio node using IP to create and maintain a mesh network has an impact on network performance because the IP link state and routing table changes increase latency and reduce throughput. While some believe the impact on performance is no more than 5 or 10 percent, it is actually much greater. The total cost of ownership is affected by network complexity, in- creased support to fix problems, the addition of new wired lines to attempt to fix the network, truck rolls and more.
In contrast, dual-radio systems dedicate one radio to client access and another to backhaul ingress and egress. Client congestion is relieved to some degree, but the backhaul is shared between ingress and egress, and therefore the radio must switch back and forth between them. While this is an improvement, latency continues to increase from node to node (or hop to hop).
Multiradio systems dedicate separate radios--even multiple separate radios--in each node to each function: client access, backhaul ingress and backhaul egress. The multiradio design dramatically improves system performance and has been proven to reduce latency and improve throughput.
Theoretically, an IP routing protocol should provide a performance benefit as a network becomes more populated, stabilizes and packets are optimized on a per-path basis. However, in a wireless mesh network, preferred and secondary paths change more frequently than in a wired network. As a result, wireless mesh networks require faster convergence than a large-scale wireless IP network infrastructure can provide. IP and the larger group of supporting protocols are affected by CSMA/CA as well as by their own mechanisms to support network congestion, among other things.
