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ADSL - the cheapest form of DSL - is generally not appropriate for a business, since it is usually limited to 128 Kb/second sending into the internet. You would not want to run a public web server on an ADSL connection if your goal is to make a good public impression.
SDSL is the same speed in both directions - pricing is based on the bit rate of the connection - anywhere from 128 Kb/sec to perhaps 1.5 Mb/second (the speed of a T1 line). The speed possible is limited by your distance from the CO, the quality of the copper wire pair, and your budget. SDSL is usually less expensive than a comparable T1 circuit.
For a more detailed description of the DSL options, follow this link.
A dedicated ISDN connection is what IDSL uses to transport message traffic. The main differences between IDSL and dedicated ISDN are regulatory issues regarding pricing and availability. To get IDSL, a DSL provider must have installed a DSLAM in the phone company Central Office. ISDN only requires that the traffic is routed over another ISDN circuit to your ISP.
If you only use the Internet for a portion of the day and don't want to run any public servers, a dialup ISDN connection could be enough for your needs.
Unlike an analog modem, ISDN connects immediately - no 30 seconds of modem squawking to get online....
Many companies use dialup ISDN as a backup in the event their DSL or T1 circuits go out of service.
A T1 circuit can be of any length, as the signal is digital instead of analog. Since the signal is digital, it can be run through repeaters and amplified as necessary to achieve the necessary distance.
A T1 consists of 24 channels with a bandwidth of 64Kb/sec. Each of the 24 channels can either support a phone call or be a data channel.
There are two basic ways to configure a T1 - the T1 needs to have the ability to send and receive control information to and from the central office - things like "Incoming call, ring the phone", "Outgoing call - here is the phone number", "Phoneset Off Hook", etc...
The T1 can either be configured with 23 Clear channels which get the entire 64 Kb/sec for sending data, and 1 channel used to send and receive the control information (23B+D), or the control information can be sent "In channel" and you get 24 channels, but each channel can only send and receive 56 Kb/second (also called D4 framing). An advantage to 23B+D is that the D control channel can control the signaling for more than one T1, so additional T1s can have 24 full B channels.
A Channelized T1 means that the 24 circuits do not have to all perform the same function. A router has the ability to use some of the channels for voice calls, some for the internet access, others for a VPN connection back to the home office. A T-1 that is not channelized means that you must use the entire T-1 all for the same purpose.
When a T1 is installed, it is not necessary to use all 24 channels. Most of the monthly cost is the bandwidth charge for what you send and receive. Installation of a T1 typically costs from $500-$1000 - more if you are a long way from the C/O... once the circuit is in place, you can "turn up" additional channels as your needs grow without having to have more installation charges (until you fill up all 24 channels).
You'll find that T1 circuits from competing local carriers (CLECs) are much lower cost and more flexible than your "Ma Bell" phone company, although competition is starting to affect the incumbant phone companies by making them offer more competitive pricing.
On the other hand, it's not likely in most installations that you actually need that entire T1 all of the time, so you can save a lot of money by committing to a specific maximum data transfer amount during the month, and to control the spikes in your usage.
A fairly common way to price a T-1 is based on burstable access... the T1 has the capability to go full speed at any time, but what you pay per month is based on how much you use and how often you use the entire bandwidth. This is somewhat similar in concept to demand billing for electricity - you are billed not only on the kwh you use, but also what your peak demand is on the system.
It's very important when ordering a T1 that you get, read and understand the Service Level Agreement (SLA) that will part of the contract that you sign. The SLA sets out the agreement of what the company is guaranteeing about the performance of the link. Typically, the SLA will state a minimum guaranteed rate, and a burstable maximum.
Frequently, this is quoted based on the 95th percentile. This means that during the month, your usage is measured at specific intervals. The top 5% of the samples when you heavily used the connection are discarded... the sample below the top 5% establishes the rate that you are billed for the month for using the circuit. bandwidth shaping tools exist to help you (or your ISP) manage the spikes so that one user downloading a bootleg copy of Revenge of the Clones doesn't end up costing you $1,000s in bandwidth charges.
If your goal is to tie together your branch locations with the main office, or you need to share your T1 connection with different types of connections (connecting terminals to IBM mainframes), and you need a reliable consistent connection, then you're the type of business that should be buying Frame relay.
Frame Relay traffic is a shared bandwitdh connection that is provided by your local phone company (and possibly a CLEC or IXC). It does not use the Internet to route traffic between your installations, although it can be configured to route traffic into and out of the internet.
If all your business needs is to provide internet access for the computers in your office and you only need TCP/IP communications, then you don't need frame relay - just acquire a point to point T1 circuit that routes into your local ISP, CLEC or IXC's network to enter the Internet through their gateway. You definitely want a firewall to isolate your internal computer systems from the access to the internet when using a point to point circuit.
A better option is to select one network carrier and establish a VPN (Virtual Private Network) with them. They will set up basically your own private internet.
The VPN uses that single backbone's equipment. This permits your company's WAN to be TCP/IP based, but doesn't expose you to the snooping risks of the shared Internet backbone.
VPNs also support dialin access, so employees can access their employer's network from home, as if they are sitting at their desk (although typically at much slower speeds). If your company's locations are not all serviced by one company's POPs, creating a workable VPN gets much more difficult.