An examination of the loop-start process reveals a problem if it is used a two-way interface between two switches, as when a PBX or Central Office \Voice Frame Relay Access Device (CO\VFRAD) is connected to central office lines. After a loop-start line has been seized by me calling switch, there is a periodof up to 4 seconds before the calling switch applies ringing voltage for the first time. During those 4 seconds the called switchdoesn’t know that the line is in use and could attempt to make a call on the same trunk. This condition, called ‘glare,’ prevents theline from being used for either call attempt. Glare is important when both sides are automatic; loop start can be used for 2-way calling if one side is operated by an attendant who can deal with the condition.
To avoid glare, the ‘ground start’ (GS) interface is used between switches on two-way trunks. It has a more positive process to seize the line: one of the leads is grounded; ringing may be optional. The CPE side grounds the Ring lead, the network side grounds the Tip lead. In this context, the CPE is the PBX facing the CO\VFRAD, which acts like network.
If you look back at E&M-IV signaling, you’ll see a strong resemblance to the basic ground start circuit. Each side applies battery, through a current detector, to a lead that the other side may ground. Unlike the E&M interface, GS is not fully symmetrical. Each side of the interface (FXS or FXO) is most often supplied in a different module, especially when ringing voltage is to be supplied by the FXS.
In the ground-start idle state the CO\VFRAD battery (+ side) is grounded by contact G. Contacts B and S, the ones used to ground the tip and ring leads, are open.
The current detector on each side of the interface registers when the other side grounds the lead and draws current from the battery. This ground signal is applied at seizure, so the called switch knows immediately that this line is not available. Grounding a lead is a positive indication of a new call; ringing voltage is not absolutely essential.
The procedure for the customer to originate a call is to close contact S, grounding the ring lead. At the same time the hookswitch places the phone circuit on the loop (tip and ring leads). Contact S draws current through the ring lead only, from the battery in the network side (in the CO\VFRAD).
When the FXS CO\VFRAD port (like the CO switch) recognizes the request for service, and is ready to receive dialed digits, it closes contact B to supply loop current and may open contact A. It may supply dial tone. The caller then dials the called number.
If the circuit has what’s called ‘floating battery,’ the more modern type, contact A is closed only during idle. This ground completes the circuit with the ring lead so contact S can draw current. ‘Conventional battery’ in the CO\CO\VFRAD or switch is grounded at all times: contact G is always closed. Only one switch will exist in any circuit Interface.
When the FXS CO\VFRAD port or switch originates the connection on the interface it is ‘terminating a call’ (because the call originated somewhere else). The alerting action is to close contact B, grounding the tip lead, and apply ringing voltage to the ring lead (on top of the battery voltage). Note that the 2 s on, 4 s off cadence of ringing may mean that the tip line is grounded for some seconds before a.c. ringing voltage appears.
The PBX may answer immediately, on sensing the grounded tip lead, or it may react only to ringing voltage. In either case, the PBX (FXO port) answers by going on-hook. The loop current trips the ringing generator off, cuts through the talk path, and opens the battery grounding contact A (if using a floating battery).
After recognizing a new call, the GS interface operates like loop-start until both sides go back on-hook. The disconnect process depends on which side goes first and whether the battery is floating or conventional.
If the terminal (PBX) disconnects, it opens the loop by going on-hook (breaking the contacts of the hookswitch). If it is a conventional battery on the other side of the interface (in the FXS CO\VFRAD or CO switch), the PBX waits until ground is removed from the tip lead, then a short guard time, before returning the line to idle.
If the switch or FXS CO\VFRAD disconnects first, it opens contact B, stopping the loop current. A floating battery circuit will also disconnect the negative battery side from the ring lead for about half a second. The current detector in the PBX waits 350 ms to confirm that the open loop is not just a transient, then may wait a short guard time or immediately return the port to idle.
Ground start has other useful features that weren’t imagined when it was introduced in the 1920s: The ground is removed by each side when it returns the line to idle. Positive indication of a disconnection before each new call helps automatic call distributors deal with high volumes of calls into hunt groups. Disconnect supervision prevents certain types of call fraud. Each end can tell when the other end has hung up. In England ground is called earth which leads to earth calling as the name for this interface type. But they’ve already heard the Joke about Earth calling Flash Gordon.
This introduction is meant to teach you about the functions and technology of a Central Office.
Analog to Digital Bandwidth
The Telephone Network
A Topology of Connection
Network Hierarchy (pre 1984)
Network Hierarchy (post 1984)
North American Numbering Plan
The Subscriber Extension
Local Access and Transport Areas
Wiring Connections: Hooking Things Up
Types of Communication
Lines Vs. Trunks
Foreign Exchange Signal