A traditional voice network relies upon 64 kbps circuit-switched connections between the originator and the recipient of a call. While this dedicated bandwidth helps to ensure the quality of a call, it is also somewhat wasteful. At many points in any conversation, voice traffic is not crossing the circuit, since natural silences occur in human speech (although it certainly depends on who you are talking to). Even so, the circuit is connected, and the bandwidth is not available for other users.
In contrast, when a voice conversation is passed across a network using packet switching, a dedicated circuit is not created. Instead, the voice “data” becomes the payload of a packet or frame, which is subsequently packet-switched across the network according to the technology or protocol used. For example, with VoIP the voice data is the payload of an IP datagram, which is subsequently switched or routed across a network just like any other IP data traffic. Unless explicitly configured to handle VoIP traffic differently (via mechanisms like queuing), routers will treat the packet in the same way as any other IP packet – routing it from the source node to the destination across the “best” possible path. The initial benefit of this method is clear – voice traffic only uses network resources as required, and does not necessarily require the “reservation” of bandwidth resources (or a dedicated circuit) when voice traffic is not being transferred. This is an advantage, but it also presents some challenges. For example, because voice traffic is time-sensitive, techniques like QoS and compression need to be considered and implemented to ensure that packets arrive at their destination in a timely manner.
Through the implementation of technologies like VoIP, companies can also reduce costs, using existing WAN links to transfer packet-based voice traffic between locations, rather than expensive tie trunks. While existing WAN links may have enough excess capacity to handle this additional traffic, it is quite likely that they will need to be upgraded to support the additional traffic that would result from adding packet-switched voice traffic to the network. Although most voice networking vendors (including Cisco) are careful to remind you of the administrative benefits of managing a single converged network rather than separate voice and data networks, the reality is that moving to a single converged network usually requires significant staff training and more importantly, proper planning.
Examples of typical organization goals associated with implementing a VoIP network solution include:
- Reduce costs associated with traditional PSTN connections and long distance changes.
- Lower overall total cost of ownership
- Improve user productivity
- Reduce reliance on a single vendor for equipment or services
- Enable new IP-based voice applications to be deployed
- Move towards a single managed network for voice and data
