Core Issue
The maximum distance is established by a total allowable loop resistance, including the phone or terminal equipment, of 600 ohms. Cisco FXS interface meets Operations, Process & Systems (OPS) Class B specifications. Specifically, it can source greater than 20ma to a resistive distance of up to 1100 ohms. The homework question is translating impedance to actual distance in meters in a production environment; which depends on the gauge of the wire and how the wire is routed and punched-down. It depends on the type of phone and equipment hooked on the line, as well as to how well that equipment works.
Resolution
The formula to calculate DC loop resistance is:
LRi = CRi * SLi
Where
LRi represents the DC loop resistance for the cable section.
CRi represents the Conductor resistance per unit length for the cable section. This is used to gauge the 24 rj11 cable.
SLi represents the length of the cable section.
For example:
LRi =600ohms
CRi = 173 ohm/loopkm
SLi = ?
= 600 ohm/ 173 ohm/loopkm
= (600 / 173 ) loop KM
= 3.47KM
= 3470 Meters
The Foreign Exchange Office (FXO) Analog Voice Port Maximum Distance is the maximum distance determined by the PBX or other equipment that provides battery and connects to the FXO voice port.
Refer to Cisco VG 224 Analog Phone Gateway, Planning Your Installation and Ringing and Idle Voltages on Cisco FXS Interfaces for more information.
