This Common Application Guide details how to setup a T3SU 300 for subrate T-3 Internet Access to an ISP.
There is currently no standard in place for dividing up an unchannelized T-3. Consequently, each manufacturer has standards different from one another. Block sizes range as follows:
The T3SU 300 only supports the ADTRAN (75 kbps) and Larscom (3.158 Mbps) block sizes. When using a T3SU 300 for subrate T-3 applications the Internet Service Provider (ISP) must configure their router for one of these two modes. If the ISP is using a Cisco router they must set the "dsu mode" and "dsu bandwidth" to match the T3SU 300 configuration. If the ISP is using a Cisco with a fairly recent IOS loaded it may include the "dsu mode" for ADTRAN, which supports 75 kbps blocks. The link below is a Cisco pdf that details the needed commands on pages 10-14:
The following instructions detail how the T3SU 300 needs to be configured for subrate T-3:
1 - From the main menu go to Configuration > DS-3 Network > and set the following:
NOTE: The DS3 framing can be either C-Bit parity or M13 and needs to match how your T-3 provider has provisioned the circuit. The Line length can be short or long and needs to be set according to how many feet of coax is between the T3SU 300 and the provider’s T-3 handoff equipment.
2 - Next go to option 5 to set the Multiplexing Mode. Unless the ISP is using a router that supports the ADTRAN 75 kbps block size you will need to set the T3SU up to use the Larscom block size of 3.158 Mbps. If the is block size is currently set to "N x 75 kbps" choose option 1 and then select 2 in order to change it to "N x 3.158 Mbps". After you see the block size change to "N x 3.158 Mbps" you must choose option 2 (Apply settings).
NOTE: You will lose access to the T3SU 300 when changing the Blocksize if you are using telnet. When you lose your connection, simply telnet back in.
3 - Navigate back to the main configuration screen and go to DTE Ports > DTE Port 1 > and set up the HSSI port. You will need to assign the proper number of blocks to match the subrate service from your ISP. The screen shot below shows an example setup for a 25.26 Mpbs subrate T-3:
You must enable the port and apply the settings for the configuration to take effect. You should see the "OPERATION SUCCESSFUL" message at the bottom of the screen when applying the new settings. If you have problems with HSSI port settings not saving, make sure that the Remote auto-configuration is disabled under the DS3 Network Configuration menu. By default this is set to "Slave" and will prevent DTE ports from being configured. NOTE: If TA is set to "Ignored" you will see a Port Status of "ERROR" when the HSSI cable is not plugged in or the router HSSI interface is not active.
4 - Escape out to the main configuration menu and select Save Configuration. You should see the "OPERATION SUCCESSFUL" message at the bottom of the screen when the configuration is saved.
This concludes the provisioning on the T3SU 300. They key is having the ISP match the Larscom mode (unless ADTRAN mode is supported) and appropriate bandwidth configuration on their end ("dsu mode" and "dsu bandwidth" commands on Cisco IOS).