Technology has divided pretty much along two lines, so to speak. They are the traditional TDM or Time Division Multiplexing standards based on circuit switching and the newer IP or Internet Protocol standards based on packet switching. Even this division is blurred because TDM networks can transport IP services and IP networks can transport TDM services.
Core network technology may not matter unless you have a specific need. For instance, Metro Ethernet can offer you level 2 switching capability so that you can connect multiple sites around town as if they are on the same LAN.
A better approach may be to go after the lowest cost service that meets your requirements. Let's take a look at some of what's available.
T1 lines are still the best choice for many businesses that need dedicated Internet connections, point to point data exchange, or outside lines for PBX telephone systems. Yes, T1 is a TDM standard. But connecting a T1 line to a packet network is as simple as installing a managed router often supplied free by the carrier. Otherwise a plug-in CSU/DSU card may interface your router to the T1 demarcation point.
T1 has a variant called T1 PRI or ISDN PRI. This is a high capacity digital telephone trunk line that gives you up to 23 outside phone lines plus Caller ID. ISDN PRI is usually a much better deal than individual analog phone lines for more than 6 - 12 lines.
One huge advantage of T1 lines is that they are almost universally available in the U.S. This is valuable for businesses located in smaller towns or rural areas where T1 service may be all you can get. But heavy competition in metro areas also makes T1 a good choice for downtown and suburban companies. Line lease rates have plunged in recent years. Bonding lines to multiply bandwidth keeps T1 being the preferred choice for many businesses. T1 lines can also be used as SIP trunks to support enterprise VoIP phone systems using IP PBX.
DS3 and SONET are the larger TDM technology services. They use fiber optic rather than twisted pair copper for delivery. Availability is generally limited to metro areas when fiber is already "lit" or easily installed.
The direct competitor to T1 and DS3 is now Metro Ethernet, also known as Carrier Ethernet. This is inherently a packet based technology and directly compatible with Ethernet LANs. Ethernet can be delivered over copper at 10 to 45 Mbps bandwidths, but higher speeds require fiber. Where available, Ethernet WAN service is often much lower is cost than other services. It can be priced at half the cost per Mbps compared to equivalent TDM services.
MPLS or Multi Protocol Label Switching is an IP network technology that is quickly replacing older Frame Relay networks for meshed connections to multiple locations. The "multi" in MPLS means it is capable of transporting any protocol, including TDM services. Most often it is used to establish a secure private network among many business locations.
Still wondering what will work best for your company. The easy way to find out is to get a list of competitive telecom services available for your location and recommendations from experts who know the industry and aren't tied to any one particular carrier. You may be surprised at how many good options you have.
