Understanding VoIP and CSIs (Carrier Services Infrastructures) - dummies

Understanding VoIP and CSIs (Carrier Services Infrastructures)

By Timothy V. Kelly

As you begin to discover the various VoIP network types (such as a DSL or a T1 line), it’s essential to know about each network type’s underlying CSI, or carrier services infrastructure. Whatever network type you may choose to use for your VoIP (voice over Internet protocol), it’s always a subnetwork of a larger CSI.

A CSI is like a highway system that lays out all the many roads that enable people to drive to their destinations. Within our highway system, you could characterize some roads as being large or wide, some as small or narrow, and some roads as being between these two extremes. Roads may be further broken down by type of surface, that is, asphalt, concrete, gravel, or dirt.

Similarly, you could characterize a CSI as having great amounts of bandwidth capacity or limited bandwidth capacity; as single-channel or multichannel; as switched or dedicated; or as circuit-switched or packet-switched. Five CSIs exist, through which all public and private communications travel:

  • PSTN switched
  • DS dedicated
  • OC dedicated
  • HFC dedicated
  • Wireless

Table 1 summarizes the overall state of CSIs.

Table 1: Carrier Services Infrastructure Types and VoIP Services



Network Type

VoIP Transports

VoIP Service Options

Public switched telephone network (PSTN)



PRI line

DSL using POTS line

VoIP over PRI

VoIP over broadband DSL (VoDSL)

Digital service (DS)



DS1 (T1), DS3 (T3)

VoIP over private dedicated network channels

Optical carrier (OC)



OC3, OC12

VoIP over private dedicated network channels

Used to provision other dedicated transports such as DS1, DS3

Hybrid fiber-cable (HFC)



Cable fiber

VoIP over broadband cable modem




Frequency spectrum channels

VoIP soft phone for pocket PC

VoIP over WiFi (VoWiFi)

VoIP over WiMax (VoWiMax)

VoIP runs best in a dedicated, packet-switched carrier services network. For a company with multiple locations, this means primarily using transports coming out of the DS and OC CSIs. Wireless transports may be used to augment or support the routine need for remote telephony services.

Carrier service companies are constantly adding and upgrading network transport lines and equipment in all five of the CSIs. They also grow by merging with carriers that are more heavily vested in another CSI than they are. This is important to understand if you’re running VoIP in a multilocation network. If you have private, dedicated transports, you’re not so much concerned with how much of the dedicated line is owned by one or more carrier providers as you are with the underlying requirement that it be dedicated to your VoIP network 24 hours a day, 365 days a year. At the same time, if you can acquire dedicated lines that are owned from point A to point B by one carrier company, chances are that the single owner may be more apt to resolve maintenance problems than a dedicated line owned by several carrier companies.

Just like the highway system, CSIs aren’t owned by any one carrier because all carriers own a portion of each CSI. What they don’t own they must lease from other carriers at wholesale and resell to the customer. Most carrier service companies can lease network transports from all five CSIs.

All five of the CSIs relate to the telecommunications industry, but each CSI contains different types of network lines and services. In the case of the wireless CSI, which ultimately uses lines at its core, there are no lines for the customer in the physical sense of the word. But there’s a frequency spectrum and frequency channels, just like the various station channels that operate on radio.