ork: Hubs in a network create some strategies are not properly implemented, it will become challenges for administrators. The collision domain, evident both with the quality and timeliness of voice and the number of ports connected by hubs without data traffic. a switch or router in between, should be kept as low There are three technologies that work well with as possible. The effective (half-duplex) bandwidth VoIP: ATM, Frame Relay, and point-to-point (PPP) cir- available on a shared collision domain is approxi- cuits. These technologies all have good throughput, low mately 35 percent of the total bandwidth available. latency, and low jitter. ATM has the added benefit of Too many access lists: Access lists slow down enhanced QoS. Frame Relay and PPP links are more a router. While they are appropriate for voice net- economical but lack some of the traffic-shaping features works, care must be taken not to apply them to un- of ATM. necessary interfaces. A good practice is to model the Of the three technologies, Frame Relay is the most traffic beforehand, with access lists applied only to difficult WAN circuit to use with VoIP. Congestion in the appropriate interface in the appropriate direc- Frame Relay networks can cause frame loss, which can tion, not all interfaces in all directions. significantly degrade the quality of VoIP conversations. With Frame Relay, proper sizing of the CIR is critical. In Additional concerns when implementing VoIP include: a Frame Relay network, any traffic exceeding the CIR is Network Address Translation (NAT): Due to limita- marked DE, and will be discarded at the carrier's option tions in the H.323 VoIP standard, VoIP conversations if it experiences congestion in its switches. It is very rarely work across NAT boundaries. It is important to important that voice packets not be dropped. Therefore, route voice streams around routers or firewalls run- CIR should be sized to average traffic usage. Usually, 25 ning NAT or use a H.323-friendly NAT. CHAPTER 3.3 VolP Technology VPN: The encr