The Drawbacks of ATM
Several years ago there was a big ATM boom. Since that time, switched 100Mbps Ethernet and Gigabit Ethernet have taken the front row in high speed networks.
A lot of corporations whom originally invested in ATM wished that they hadn't after they installed it as they found various incompatibilities amongst manufacturers, proprietary methods and worst of all, a throughput level much lower than they had ever expected!!!
If you look into the basics of ATM "it's Merits", you'll find three (3) major points that hold ATM over and above all other protocols:
1. The first protocol with unlimited bandwidth expandability.
2. A single protocol for both LAN and WAN.
3. The capability to transmit voice, video and data simultaneously.
These are the major three (3) points that were to catapult ATM over and above any other protocol on the market at that time.
Let's look at these three points to see how they really stand up to their advertisements.
1. The first protocol with unlimited bandwidth expandability.
When ATM was first released, there were numerous 25Mbps and some 155Mbps expensive systems. These systems were all limited in capability and ILMI bugs and performance problems were galore, not to mention that the final specifications for the ATM protocol management were not released yet as there were two groups pushing in opposite directions for credit based management as well as rate based management. Both were entirely different and a concensus was not to be easily found at the very beginning. Manufacturers of one were not completely compatible with the other and sniffers were outrageously priced even with their limited capability. Problems flourished and only a small hand of specialists were able to offer any help what-so-ever.
Anyway, back to the bandwidth. When ATM was originally developed, there were several operational modes:
a. CBR: Constant Bit Rate
b. VBR: Variable Bit Rate
c. ABR: Available Bit Rate
d. UBR: Unspecified Bit Rate