Performance Issues

When 24 lines are digitally encoded on a single circuit and one fails, they all fail. This is a concern to many users because failures do happen. 90% of all errors occur at the local loop because of their exposure to environmental damage.

Maintenance Issues

Whenever a T1 line experiences problems, troubleshooting follows this sequence:

• The user experiences some downtime that affects data throughput.
• The user notifies the carrier.
• For testing, the carrier asks all users to get off the circuit.
• The carrier tests the line, often without detecting problems.
• The carrier returns the circuit to the customer.
• Once the circuit is returned, the customers may see problems again.

Error Detection

Errors can be caused by a spike of electrical energy from lightning or interference from other electrical devices. Other types of errors can be caused by physical damage to the wires usually at the local loop. There can be equipment failures such as the digital repeaters or multiplexers. Unfortunately, the process of finding the point of error is tenuous.        

ESF – A Step to Correct the Problem

In the 1980’s, AT&T implemented extended superframe format to provide error detection on T1 circuits. ESF uses the 8Kbps of overhead normally assigned for the framing format, thereby not affecting data bandwidth. The reasons to use ESF are to increase T1 circuit availability and improve performance.

            Extended superframe format extends the superframe from 12 frames of information to 24 frames. The 8 Kbps of overhead is now subdivided into three functions.

                Using this shared facility, the carrier has the option of performing three functions to support customer needs:

• Framing synchronization
• Error Detection
• In-service monitoring and diagnostics