I think you may be allocating too many channels for your systems if you assign 2 erlangs for 64kbps data rates. My primary reasoning is that the Rx direction is not nearly as loaded as the Tx direction. And also that you are not using the 64 kbps on the Rx or Tx side continuously. The traffic loading will most likely be less than 100%. As an example, data packets will be assigned an order in a particular channel based on service session duration, queueing, average packet length, service priority, etc… so the system tries to keep the channel occupied without having services wait too long before being serviced. But there is a good chance that the sytem will not have enough requests to keep this channel busy all the time. So when allocating erlangs for data at a specific data rate the channel usage has to be taken into consideration.