Traditional Quality of Service options don't really apply to cable and DSL broadband connections. This doesn't mean you can't apply some VoIP Quality of Service (QoS) policies, rules and even Class of Service settings in your router or firewall to prioritize voice traffic leaving your site. Once voice traffic leaves the user's device, it is dependent upon the Internet. By applying a policy or rule, you are only prioritizing your voice traffic to get it out the door before lower-priority traffic. That's where it ends, so the concept of end-to-end VoIP QoS doesn't really apply. Unlike managed networks -- like multiprotocol label switching (MPLS) services used by large enterprises -- small and mid-size businesses like small or home office environments rely on public Internet traffic that lacks end-to-end prioritization and QoS.
Comparing cable and DSL VoIP performance can be frustrating because it really depends on the facilities, and different parts of the country still have bandwidth constraints. If you're using DSL, you could be fighting with numerous other users for bandwidth. DSL changes available bandwidth based on the loop , and that loop is sensitive to weather, temperature and line conditions that impact speed. The same battle for bandwidth can happen with cable and is affected by heavy time-of-day use after business hours.
As an interconnect, I always prefer cable over DSL for any VoIP or SIP application. But, my first choice is always fiber, hands down. Business should seek fiber because fiber is always better than a coaxial cable or a hybrid of coaxial cable and fiber. Fiber is diverse, and the light spectrum offers more bandwidth over a small form factor that is not prone to power transients, like power surges, and lightning like coaxial cables. We normally elect to use fiber first, with cable as an alternative route for overflow and load balancing.
Whatever you do, test, test, test and use trial periods of 15 days to 30 days to determine if your medium -- cable, DSL or fiber like Verizon's FiOS -- will meet your needs for VoIP traffic.
Dig Deeper on VoIP QoS and Performance
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