AT&T MERLIN LEGEND Installation & Maintenance Manual page 45

System Description
Loop-Start Lines/Trunks
LS lines/trunks are the standard for home and small business
Key systems. They are less expensive in some areas but have
certain limitations:
They do not protect against "glare." (Glare occurs when a
person tries to make an outside call on a line/trunk at the
same time an incoming call is being received on that
line/trunk.)
They have higher cable losses than GS lines/trunks
They cannot assure secure toll restriction.
Ground-Start Lines/Trunks
GS lines/trunks are outside lines/trunks used by some
businesses (such as hotels or motels) because the improved
signaling of GS allows more secure toll restriction.
In addition, GS lines/trunks prevent glare and provide cable
losses less than or equal to 4.5 dB.
The following types of outside lines/trunks come in either GS or
LS form:
basic lines
WATS (wide area telecommunications service)
800 service (In-WATS)
foreign exchange (FX)
1-14 Control Unit