TCP/IP (transmission control protocol Internet protocol) uses a layered approach to networking. When VoIP is the network transport service, UDP is substituted for TCP. UDP (user datagram protocol) is one of the many protocols included in TCP/IP, so VoIP can be made to run on any network type.
The Internet isn't the only network that supports VoIP. Any network — private or public — that runs the TCP/IP suite can run VoIP. Quality of service varies from one network type to another, and the Internet service provider (ISP) you choose can affect the quality of service you experience.
ISPs make the Internet go round
An ISP is a company in the business of providing Internet access to consumers and businesses. It's common to rank the quality of ISPs based on their tier level. Tier-0 is a logical ring formed by all the tier-1 ISPs. Tier-1 ISPs are considered the largest and usually the best type of Internet access providersbecause there is only one "hop" between the tier-1 ISP network and the end-user's network.
 | In geek-speak, a hop is a connection between networks. Thus, if a packet travels from your network to your ISP's network, that is one hop. If the packet then travels to a larger carrier (perhaps someone from whom your ISP leases lines), that is another hop. It isn't unusual for packets traveling through the Internet to go through 15, 20, or more hops from source to destination. |
National and international ISPs are all tier-1 and typically have multiple, ultra-high-bandwidth pipes connecting them into the core of the Internet. Regional ISPs tend to be tier-2. Smaller ISPs that acquire their Internet access through a tier-2 ISP are tier-3 ISPs. A few ISPs operate at the tier-4 and tier-5 levels.
When it comes to VoIP, each hop adds transmission overhead that may diminish the overall throughput of the call. On the one hand, if the network types involved in providing the end-to-end connection are using strictly dedicated bandwidth transports, you may not notice degradation. On the other hand, if you are using an ISP (regardless of its tier) that uses switched transports somewhere in the end-to-end connection, you are going to experience some degradation. This is because switched lines don't pass packets as well as dedicated lines. (That's why VoIP networks are designed using dedicated transports for on-net traffic and use switched lines only when it is necessary to go off-net to the public switched telephone network [PSTN].)
Depending on the number of hops over the Internet and the types of network lines between the VoIP caller's network and the VoIP receiver's network, there may be delay; it's inevitable when using the Internet. Having multiple hops doesn't necessarily equate with a poor connection, but it can be a significant factor.
 | How you choose to access the Internet and whether you then use the Internet to support VoIP are choices you need to make. In general, for companies with more than four or five locations, the Internet isn't a good choice for VoIP, although it may be a good choice for transferring computer data. |
Examining protocol layers
VoIP runs with and requires the use of TCP/IP. The good news is that the way that TCP/IP works for data networking is the same way that it works for VoIP, with the exception that VoIP utilizes UDP instead of TCP and also requires that some additional information be packaged with the data packet.
In the process of being transmitted from source to receiver, VoIP data follows the same process to construct and transmit packets as is followed by other TCP/IP data packets, as shown in Figure 1. The difference is that UDP is used instead of TCP at the transport layer; this is probably the most significant difference between computer data packets and VoIP telephony packets.
Figure 1: Differences in TCP/IP implementation for VoIP and data packets.
Because VoIP packets are constructed pretty much the same way as data packets, VoIP can run on any type of data network that utilizes TCP/IP. Because most corporate networks already handle TCP/IP traffic, the fact that VoIP packets can travel on the same network means that converting to VoIP can be relatively painless.
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