Specifications

Performance Tuning, and Monitoring 165
Remedy:
The IBM 9783 unit cannot balance the individual circuits in the PBX.
In this case the PBX may offer adjustments that equalize station gain with
trunk gain.
If none of the above methods succeeds, then perform these steps:
1. Perform an echo return loss measurement on the PBX (refer to 5.2.4.1, “Echo
Return Loss” on page 165).
2. Have your telephone company measure the dB level of your outside lines to
check if they are within specification. (Your telephone company can tell you the
specifications.)
3. If you are using loop start outside lines, you can often improve the dB level by
ordering a trunk. Contact your local telephone company.
For echo cancellation to work, the echo return loss must be 6 dB or higher. Most
circuits have a higher ERL, usually between 12 and 22.
5.2.4.1 Echo Return Loss
If the source impedance is equal to the load, maximum power will be transferred
and will reflect no power to the source. If the impedances do not match, maximum
transfer will not take place, and any energy not transferred is reflected back.
Echo return loss (ERL) is a consideration in 2- to 4-wire hybrids. To minimize
echo, the return loss must be as high as possible. In a typical measurement on a
4-wire trunk card, you might expect to see an ERL of approximately 18. This
reading is with the call set up and terminated on the station side of the PBX. The
higher the reading, the more protection there is against transmitted energy being
returned to the source.
The IBM 9783 unit requires at least 6 dB of ERL to handle echo correctly. (The
level is between 9 and 6 dB, with possible problems occurring at 9 dB and 6 dB
being the absolute lowest figure.) Usually ERL can be minimized by adjusting Tx
and Rx levels, but if the return loss is already too low, echo cancellation may not
operate.
Figure 56 shows how to measure ERL on a PBX. The IBM 9783 unit must be
disconnected. The audio pairs must be attached to T & R, and T1 & R1. The trunk
type on the ERL test device (for example, a Sage) must be set to 4-wire E&M,
600W terminated. The station (telephone) is taken off-hook, and by direct trunk
select or access code connects to the trunk (4-wire E&M) under test.
Figure 56. Echo Return Loss