The range of carrier services offered by solution providers can vary, and many are being displaced by new technologies and protocols.

CRN asked a number of VARs, carriers and master agents what are the most important types of services for VARs to know -- top sellers, technologies to watch, services with real sales traction. Here's what they came up with.

The key to Multiprotocol Label Switching (MPLS) is efficiency, because, properly deployed, MPLS can converge different access technologies, such as frame relay, internet protocol and Ethernet, as well as boost the performance and scaleability of IP routing.

A Session Initiated Protocol, or SIP, trunk, essentially allows a company to replace its its traditional fixed PSTN lines but still realize PSTN connectivity using the Internet. Again, the motivation is efficiency and cost savings, because a SIP trunk can eliminate the need for PSTN gateways and other connectivity technology.

Virtual Private LAN services let organizations provide Ethernet-based multipoint connectivity over IP and MPLS networks.

Layer 2 Ethernet is still a strong seller, solution providers say, especially as a replacement for traditional private line services.

Sometimes referred to as "carrier hotels," colo services minimize costs of a company's data center infrastructure because they can access compute resources from a third party location. Telecom/carrier services are usually part of the offering.

A core technology in the carrier channel, an integrated services digital network offers digital transmission of video, voice, data and other forms of network connectivity over the PSTN.

They're commoditized in most geographies, but various types of services that might be described as "business connectivity" -- T1 lines, DS3, etc. -- are offered by most of the country's CLECs.

Frame Relay is one of the most frequently used wide area network (WAN) protocols, though according to solution providers, they are continuing to be displaced by MPLS, VPN and various broadband services.

You might call them the basics: Local and long distance service as well as VoIP service and Internet access that are some of the bread-and-butter carrier services deployments frequently found in the channel.

Hosted versions of various services, especially VoIP, are also increasingly relied-upon.

Not so much a carrier services as a professional service or consulting opportunity, many VARs offer telecom expense management (TEM) services or software that help companies sort out their carrier costs and billing plans in efficient manners.