Artech House - Voice.over.802.11.pdf - VirtuaLitera

188 Voice over 802.11vendors can also engineer their platforms to be NEBS compliant. These measuresallow softswitch to match the “five 9s” of reliability demonstrated by Class 4. Anumber of softswitch solutions have achieved “five 9s.”Building high availability into the switching component of a Vo802.11network is simply a matter of good engineering. The main components of engineeringa Class 4 or Class 5 switch for HA are redundancy, no SPOF, hotswitchover, preservation of calls, in-service upgrades, component reliability,reproducible quality, and NEBS compliance. The same is true for a softswitchednetwork.In an HA system, two or more systems are loosely coupled to each otherwith the help of redundancy software. The reliability provided can be furtherclassified as asymmetric or symmetric based on whether the systems act asactive/standby (idle) or run in a parallel load sharing/balancing mode. Anactive/standby type of a system has further categories such as 1+1 redundancy orN+K redundancy based on number of active nodes and the number of standbynodes that are available. Cluster mode is another such HA architecture in whichapplications can run in either load-sharing or failover mode. The reliability ofHA systems can be further enhanced by hardening some of the hardware componentsof the individual system constituting an HA system. Typical candidatesfor such treatment are network interface cards, disk controllers, disks, and powersupply [2].HA computing utilizes the redundant resources of clustered (two or more)processors (Figure 11.3). Such solutions address redundancy for all componentsof a system, processors, main and secondary memory, network interfaces, andpower/cooling. Although hardware redundancy may be effectively addressed byclustered (redundant) hardware resources, the class of errors detected is lesscomprehensive and the time required to recover from errors is much longer thanthat of fault-tolerant machines. Still, fault recovery in tens of seconds, frommost common equipment failures, can be achieved at less than half the cost oftraditional fault-tolerant computer systems. HA systems are often configured asdual-redundant pairs [3].Carriers require high system availability and are concerned with the effectsof possible softswitch downtime. Carriers demand low MTBF and employ trafficoverload control, the shedding of call processing capacity in the event ofcomponent failures, and quick failure detection and recovery mechanisms. Thesoftswitch answer is to architect redundant softswitch hardware nodes at differentlocations throughout the network, which contributes to the overall networkreliability.In addition to COs posing a SPOF, the copper wires that go to the wiringpedestal in a residential neighborhood are also a SPOF. The wiring pedestal is aSPOF. The fiber-optic cable that runs from the wiring pedestal to the CO is also