CSIs can be characterised in numerous ways.

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In getting to grips with the various types of VoIP providers and networks, it is important to know and understand what underpins each network in terms of infrastructure. The Carrier Services Infrastructure (CSI) is essentially a highway system through which all communications (both public and private) travel. Whichever network type you opt for in running your VoIP solution, it will always be a subnetwork of a much larger CSI.

They may have a large australia telegram bandwidth capacity or a limited bandwidth capacity. They might be single-channel or multichannel, switched or dedicated, circuit-switched or indeed packet-switched.

Five CSIs currently exist. They are: Wireless, PSTN switched, OS dedicated, DS dedicated and HFC dedicated.

The Public Switched Telephone Network (PSTN) dates back to the 1870s. As its name suggests, it is a switched network. In terms of VoIP transports, it uses PRI line or DSL using a POTS line. Its VoIP service options include VoIP over PRI (primary rate interface) and VoIP over broadband DSL (VoDSL).

The digital service (DS) network was incorporated in 1964 and is a dedicated network. In terms of VoIP transports, it uses DS1 (T1) or DS3 (T3) and its VoIP service option is VoIP over PDN (private dedicated network) channels.

Optical Carriers (OC) saw inception in the 1980s. Like DS, they are a dedicated network. They use OC3 or OC12 VoIP transports and in terms of VoIP service options, they offer VoIP over PDN channels. The OC networks are sometimes also used in the provisioning of other dedicated transports such as DS3 or DS1.

The 1980s also saw the introduction of Hybrid fibre-cable networks (HFCs). Again, this is a dedicated network and one that uses Cable fibre for its VoIP transport. It offers VoIP over broadband cable modems.

The early 2000s saw the introduction of Wireless CSI. Wireless is a switched as opposed to a dedicated network. In terms of VoIP transports, it uses frequency spectrum channels. The VoIP service options on offer here include VoIP soft phone for pocket PC, VoIP over Wi-Fi (VoWiFi), and VoIP over WiMax (VoWiMax). With the wireless CSI, whilst it may ultimately use lines at its core, there are no lines in the physical sense for the customer. Instead, there are dedicated frequency spectrums and frequency channels.

CSIs themselves are not owned by any single carrier – all carriers own a portion of each one. They each lease what they do own to/from other carriers at wholesale rates and then resell to their customers. Most carriers are able to lease network transports from all five of the CSIs. Each CSI contains different types of network lines/services.

Dedicated, packet-switched networks are the optimum solution for VoIP. For businesses with multiple disparate locations, this would mean the principal use of transports from the DS and OC CSIs. In addition, wireless transports are routinely used in addressing the requirements of remote services.