Everywhere i have read that the best WAN connections someone should use is Frame Relay and as backup ISDN connection.
If we only need 128Kbps WAN connection and i have available 2 channels PRI what should i use
Frame Relay 128Kbps and as backup one channel PRI or
Use the 2 channels of PRI and as backup 64Kbps of Frame Relay.
This is a real scenario. For making this scenario less complicate , we should consider that cost is irelevant.
Wow, frame with backup ISDN? Sounds like the "good old days"...
Frame-Relay is a "shared bandwidth" type connection. Typically you have a link-speed and a CIR (Committed Information Rate). Lets say you have a link speed of 512K and a CIR of 128K, that means that the provider guarantees you at least 128K of bandwidth, but that you can burst to 512K. However, with it being a shared medium, it is possible that you won't even get the 128K. Typically this is charged at a set-rate per month for usage within the CIR, and you might possibly get charged extra for bursting above that too much.
ISDN is more of a dedicated bandwidth connection. Each "B" channel you fire up is 64K worth of data, a typical ISDN is 2 B channels plus one "D" channel for signalling, which gives you 128K if properly configured. You cannot go above this rate, and typically you won't get any less than this rate as the bandwidth is not shared. ISDN is typically charged on a "per minute" or "per hour" basis and you can get long-distance charges involved, so it can be quite expensive. Check with your provider as to what the charges are and be careful!!
So, if you need the 128K guaranteed, it depends on how much you trust your frame-provider. It also depends on how you get your frame, what the line rate and CIR is as to how "good" this is. If its 128K line with a lower CIR, the ISDN would probably be faster...
That said, in today's world, frame might still be popular, but things like T1's or fractional T1's are quite cheap too. That or MPLS. MPLS is similar to Frame, but has quite a few better options than it. Many people these days will do a MPLS main circuit, and use a VPN based backup over the internet for example...